Interactive Fly, Drosophila

The corepressor CtBP (carboxyl-terminal binding protein) is involved in transcriptional pathways important for development, cell cycle regulation, and transformation. CtBP binding to cellular and viral transcriptional repressors is regulated by the nicotinamide adenine dinucleotides NAD+ and NADH, with NADH being two to three orders of magnitude more effective. Levels of free nuclear nicotinamide adenine dinucleotides, determined using two-photon microscopy, correspond to the levels required for half-maximal CtBP binding and are considerably lower than those previously reported. Agents capable of increasing NADH levels stimulate CtBP binding to its partners in vivo and potentiate CtBP-mediated repression. It is proposed that this ability to detect changes in nuclear NAD+/NADH ratio allows CtBP to serve as a redox sensor for transcription (Zhang, 2002).

C-terminal-binding protein directly activates and represses Wnt transcriptional targets in Drosophila

Regulation of Wnt transcriptional targets is thought to occur by a transcriptional switch. In the absence of Wnt signaling, sequence-specific DNA-binding proteins of the TCF family repress Wnt target genes. Upon Wnt stimulation, stabilized β-catenin binds to TCFs, converting them into transcriptional activators. C-terminal-binding protein (CtBP) is a transcriptional corepressor that has been reported to inhibit Wnt signaling by binding to TCFs or by preventing -catenin from binding to TCF. This study shows that CtBP is also required for the activation of some Wnt targets in Drosophila. CtBP is recruited to Wnt-regulated enhancers in a Wnt-dependent manner, where it augments Armadillo (the fly β-catenin) transcriptional activation. CtBP is required for repression of a subset of Wnt targets in the absence of Wnt stimulation, but in a manner distinct from previously reported mechanisms. CtBP binds to Wnt-regulated enhancers in a TCF-independent manner and represses target genes in parallel with TCF. The data indicate dual roles for CtBP as a gene-specific activator and repressor of Wnt target gene transcription (Fang, 2006).

CtBP has previously been identified as a repressor of Wnt signaling, as measured by TCF reporter genes in cultured cells. Consistent with this, CtBP was identified in an overexpression screen via its ability to suppress Wg and Arm action in the developing eye. In wing imaginal discs, CtBP overexpression also inhibited the Wg target Senseless (Sens). Consistent with this overexpression data, the reduction of CtBP in cultured cells via RNAi is also consistent with a role for CtBP in repressing some Wnt targets (Fang, 2006).

The working model for CtBP repression of Wnt target gene expression holds that CtBP binds to the same area of the nkd and CG6234 loci as TCF, but this binding is TCF-independent. Consistent with this, knock down of CtBP and TCF or gro synergistically derepresses nkd expression. No synergism was seen with TCF/gro double depletions. The RNAi and ChIP data together favor a model where CtBP acts in parallel with TCF/Gro to repress nkd expression in the absence of Wg stimulation. Because CtBP has no detectable ability to bind nucleic acids, it is assumed that unknown DNA-binding protein(s) recruit CtBP to the WRE (Fang, 2006).

The existing models for CtBP antagonism of Wnt signaling cannot explain the data. TCF-independent recruitment of CtBP to WREs is not consistent with work suggesting direct binding of CtBP to TCF. The alternative mechanism, where a CtBP/APC complex diverts Arm/β-catenin away from TCF, also is inconsistent with the results. In this model, the activation of nkd expression after CtBP RNAi treatment would be dependent on TCF and arm. Because the derepression of nkd occurred when both CtBP and TCF were depleted and was not affected when arm was also inhibited, this model is not favored to explain the effects of CtBP depletion on nkd expression. These distinct mechanisms for CtBP repression are not mutually exclusive and may all occur in some contexts (Fang, 2006).

There is a qualitative difference in the amount of derepression found between the two Wg targets studied in Kc cells. Depletion of CtBP and TCF/gro causes a large (20- to 30-fold) increase in nkd basal expression, but has a much more modest (<3-fold) effect on CG6234. These differences may reflect a fundamental difference in the way TCF/Gro and CtBP act on various Wnt targets in unstimulated cells, but it is equally likely that the surrounding cis-elements in these targets have a strong influence on the degree of derepression that can be observed (Fang, 2006).

In addition to defining a novel mechanism for CtBP repression of Wg targets, strong evidence is provided for CtBP playing a role in Wg-mediated transcriptional activation. In the wing imaginal discs, loss of CtBP resulted in a lag in Wg-dependent activation of Sens and a reduction in Dll expression. In cultured Kc cells, CtBP depletion caused a two- to three-fold reduction in the ability of Wg to activate CG6234 expression. The ability of Gal4-Arm chimeras to activate a Gal4 reporter gene is also highly dependent on CtBP levels. In all these contexts, CtBP is not absolutely required for Wg signaling, but is necessary for maximal activation of Wg/Arm transcriptional activation (Fang, 2006).

The positive effect of CtBP on Wg signaling is direct, as judged by ChIP. Assuming that ChIP is measuring the degree of occupancy of CtBP on the chromatin, and not simply antigen accessibility, Wg stimulation promotes the association of CtBP with the CG6234 WRE. This increase in CtBP binding is not observed in TCF-depleted cells. Gal4-Arm recruits endogenous CtBP to a UASluc reporter. Taken together, these data support a model where TCF/Arm recruits CtBP to Wg targets. No binding between Arm and CtBP has been detected by co-immunoprecipitation, suggesting that another factor(s) may act as an adaptor between CtBP and the Arm bound to TCF (Fang, 2006).

Arm has transcriptional activation activity in both the N- and C-terminal portions of the protein. CtBP overexpression or RNAi depletion greatly effects the activity of the N-terminal half of Arm but has no effect on the C-terminal portion. Consistent with this, the N-terminal portion can recruit CtBP to a reporter gene, but not the C-terminus. Other factors that have been linked to the N-terminal portion of Arm include Lgs and Pygo and the ATPases Pontin and Reptin. It may be that CtBP acts in concert with one or more of these factors (Fang, 2006).

CtBPs have strong sequence similarity with D2-hydroxyacid dehydrogenases. hCtBP1 is a functional dehydrogenase and point mutations blocking CtBP1 dehydrogenase activity inhibit its ability to interact with binding partners and act as a transcriptional corepressor. However, another group found that similar mutations had no effect on the ability of CtBP to repress transcription. In this report, mutation of two residues (D290A and H312T) predicted to be essential for catalytic activity had no effect on the ability of fly CtBP to potentiate Gal4-Arm transcriptional activation. Further complicating the issue is data from experiments expressing the fly CtBP fused to Gal4DBD in mammalian cells. In some cells, Gal4-CtBP activated a UAS reporter, while the same reporter was repressed in other cell lines. Interestingly, conversion of CtBP's catalytic histidine to glutamine abolished transcriptional activation, but not repression. The heterologous nature of these experiments and the differences in the assays employed may explain the discrepancy between these studies, and further experiments will be needed on endogenous targets to determine how much dehydrogenase activity of CtBP contributes to repression and activation of Wnt targets (Fang, 2006).

Although CtBP is required for maximal activation of CG6234 expression and a Gal4-Arm-dependent reporter gene, Wg activation of nkd does not appear to require CtBP. The basis for this gene-specific requirement for CtBP is not clear. CtBP is recruited to the nkd WRE in a Wg-dependent manner, similar to what was observed for CG6234. It may be that CtBP is required for nkd activation, but this is masked by its role in repressing nkd expression. This hypothesis could be tested if were possible to separate CtBP's activator and repressor activities (Fang, 2006).

The requirement for CtBP in Wnt transcriptional activation may have been previously overlooked due to its well-characterized role as a co-repressor. For example, mouse embryos that lack CtBP2 have axial truncations and reduced Brachyury (T) expression that is reminiscent of Wnt3a mutants. These results suggest that the activating role for CtBP in Wnt signaling that was identified is evolutionarily conserved (Fang, 2006).

Protein Interactions

Human CtBP attenuates transcriptional activation and tumorigenesis mediated by the adenovirus E1A protein. The E1A sequence motif that interacts with CtBP, Pro-X-Asp-Leu-Ser-X-Lys (P-DLS-K), is present in the repression domains of two unrelated short-range repressors in Drosophila (Knirps and Snail) and is essential for the interaction of these proteins with Drosophila CtBP (dCtBP). A P-element-induced mutation in dCtBP exhibits gene-dosage interactions with a null mutation in knirps, which is consistent with the occurrence of Knirps-dCtBP interactions in vivo. These observations suggest that CtBP and dCtBP are engaged in an evolutionarily conserved mechanism of transcriptional repression, which is used in both Drosophila and mammals (Nibu, 1998a).

The pre-cellular Drosophila embryo contains 10 well characterized sequence-specific transcriptional repressors, which represent a broad spectrum of DNA-binding proteins. Two of the repressors, Hairy and Dorsal, are known to recruit a common co-repressor protein, Groucho. Evidence is presented that three different repressors, Knirps, Krüppel and Snail, recruit a different co-repressor, dCtBP. Mutant embryos containing diminished levels of maternal dCtBP products exhibit both segmentation and dorsoventral patterning defects, all of which can be attributed to loss of Krüppel, Knirps and Snail activity. In contrast, the Dorsal and Hairy repressors retain at least some activity in dCtBP mutant embryos. dCtBP interacts with Krüppel, Knirps and Snail through a related sequence motif, PXDLSXK/H (also termed P-DLS-R). This motif is essential for the repression activity of these proteins in transgenic embryos. It is proposed that dCtBP represents a major form of transcriptional repression in development, and that the Groucho and dCtBP co-repressors mediate separate pathways of repression (Nibu, 1998b).

A Gal4-Knirps fusion protein containing the C-terminal third of the Knirps protein (amino acid residues 255-429) has been shown to be able to repress a modified eve stripe 2-lacZ reporter gene in transgenic embryos. The fusion protein contains the Knirps P-DLS-K motif, and mutations in this sequence (PMDLSMK to AAAASMK) inactivate its repression activity. These results suggest that dCtBP is an important component of Knirps-mediated repression, but do not exclude the possibility that additional sequences in Knirps are also important for repression. To address this issue of sufficiency, the function of the P-DLS-K motif was examined in the context of the full-length, wild-type protein. Knirps is normally expressed in two domains, one anterior to eve stripe 1 and the other in the presumptive abdomen, spanning eve stripes 4, 5 and 6. The posterior border of stripe 3 is thought to depend on repression by Knirps. Ectopic expression of knirps with the eve stripe 2 enhancer results in the loss of stripe 3 expression and dominant lethality. It has been suggested that the endogenous stripe 3 pattern is repressed by the diffusion of ectopic Knirps products from stripe 2. A mutant form of Knirps that lacks the P-DLS-K motif does not repress stripe 3 expression. The mutant protein is identical to native Knirps except for four changes in the P-DLS-K motif (PMDLSMK to AAAASMK). The mutant protein is expressed at the same levels as the wild-type protein, but does not mediate efficient repression. Moreover, while the ectopic expression of the wild-type Knirps protein results in embryonic lethality, transgenic strains that misexpress similar levels of the mutant protein are fully viable. These results suggest that P-DLS-K represents the primary repression motif in the Knirps protein, although high levels of the mutant protein cause weak and variable disruptions in the stripe 3 pattern (Nibu, 1998b).

The mechanism by which dCtBP mediates transcriptional repression is unknown. However, the current study provides evidence against a previously proposed mechanism for Krüppel (Sauer, 1995). Krüppel activity is shown to be lost in dCtBP mutants, and the C-terminal region of the protein contains an essential P-DLS-H repression motif. Moreover, preliminary studies suggest that ectopic expression of the native Krüppel protein causes patterning defects in early embryos, which are reversed when the P-DLS-H motif is mutagenized. These results strongly suggest that Krüppel-mediated repression depends on the recruitment of the dCtBP co-repressor. The earlier study provided evidence that repression depends on the direct interactions of Krüppel with the beta-subunit of the TFIIE general transcription factor (Sauer, 1995). It is conceivable that this mechanism of repression is employed in other tissues at later stages in the Drosophila life cycle, although it is noted that a recent study provides strong evidence that a mammalian Krüppel-like protein also employs a CtBP co-repressor (Nibu, 1998b and references).

The BTB/POZ (broad complex Tramtrack bric-a-brac/Pox virus and zinc finger) domain is an evolutionarily conserved protein-protein interaction motif. Many BTB-containing proteins are transcriptional regulators involved in a wide range of developmental processes. However, the significance of the BTB domain in development has not been evaluated. Evidence is presented that overexpression of the Tramtrack69 (Ttk69) protein not only blocks neuronal photoreceptor differentiation but also promotes nonneuronal cone cell specification in early Drosophila eye development. The Ttk69 BTB domain is critical for mediating interaction with the Drosophila homolog of C-terminal-binding protein (dCtBP) in vitro, and dCtBP minus mutations genetically interact with ttk69. Furthermore, the C-terminal region downstream of the DNA-binding zinc fingers has been shown to be essential for Ttk69 function. A dCtBP consensus binding motif in the C terminus appears to contribute to Ttk69 activity, but it cannot be fully responsible for the function of the C terminus (Wen, 2000).

Transcriptional repressor proteins play essential roles in controlling the correct temporal and spatial patterns of gene expression in Drosophila embryogenesis. Repressors such as Knirps, Krüppel, and Snail mediate short-range repression and interact with the dCtBP corepressor. The mechanism by which short-range repressors block transcription is not well understood; therefore, a detailed structure-function analysis of the Knirps protein has been undertaken. To provide a physiological setting for measurement of repression, the activities of endogenous or chimeric Knirps repressor proteins were assayed on integrated reporter genes in transgenic embryos. Two distinct repression functions have been identified in Knirps. One repression activity depends on dCtBP binding, and this function maps to a C-terminal region of Knirps that contains a dCtBP binding motif. In addition, an N-terminal region was identified that represses in a CtBP mutant background and does not bind to the dCtBP protein in vitro. Although the dCtBP protein is important for Knirps activity on some genes, one endogenous target of the Knirps protein, the even-skipped stripe 3 enhancer, is not derepressed in a CtBP mutant. These results indicate that Knirps can utilize two different pathways to mediate transcriptional repression and suggest that the phenomenon of short-range repression may be a combination of independent activities (Keller, 2000).

Knirps has been shown to repress transcription when bound adjacent to either basal promoters or activators within enhancer elements. These studies of Knirps activity when the protein binds close to the basal promoter reveals additional properties of the endogenous protein. (1) Repression by Knirps does not appear to be sensitive to phasing effects, as shown by equivalent activity of constructs with Knirps binding sites offset by 5 bp at -70 and -75 bp. (2) In this series of genes, the transcriptional repression activity appears to be directed at the basal promoter element, because the repression weakens as the distance from Knirps sites to the basal promoter is increased while the distance to the enhancer element is held constant. (3) While Knirps repression is limited to a relatively short distance, there is a measurable interval (from 100 to 130 bp) over which Knirps activity is attenuated but not entirely abolished (Keller, 2000).

This intermediate level of repression might be useful in adjusting the amount of repression imposed on a target gene or setting a target gene threshold, as has been demonstrated for the Drosophila Giant short-range repressor. With Giant, a less-than-twofold difference in posterior versus anterior protein levels is sufficient to switch a gene from on to off. Thus, two features of short-range repressors may allow for flexibility in genetic regulatory circuits: (1) short-range repressors allow modular enhancers to act independently, by avoiding regulatory cross talk, and (2) the exquisite distance dependence may contribute to the differential response of endogenous target genes to repressor gradients (Keller, 2000).

This study demonstrates that the Knirps protein contains two functionally distinct repression activities. The C-terminal region appears to mediate repression through recruitment of the dCtBP protein: it consists of a region contained within residues 202 to 358 (minimally, residues 248 to 291 and 313 to 358 including the PMDLSMK dCtBP binding motif. In contrast, the N-terminal repression region (minimally, residues 139 to 330) appears to function independently of dCtBP. Although this region contains some of the amino acid residues that are present in the dCtBP binding constructs, the two activities are clearly distinct based on dCtBP dependence. The N-terminal region does not bind to dCtBP and it can repress in a mutant embryo that lacks maternal dCtBP. Any residual amounts of dCtBP from maternal or zygotic expression are likely to be very low, because the loss of maternal dCtBP expression causes a loss of activity of Snail, Knirps, and Krüppel on a number of target genes, producing severe embryonic defects and early developmental arrest (Keller, 2000).

Knirps repression domains have been defined in the context of Gal4 fusion proteins, but several lines of evidence suggest that the native Knirps protein can also repress target genes independently of dCtBP. Most compellingly, an eve stripe 3 lacZ reporter gene that is derepressed in a knirps mutant background is not derepressed in a CtBP mutant. In addition, a frameshift mutation (kni^14F) that produces a protein lacking the dCtBP interaction motif retains partial activity, perhaps via the N-terminal repression activity that has been defined in this study. Finally, a study of ectopically expressed Knirps protein that lacks a dCtBP binding motif found that the protein has weak repression on eve stripe 3 (Keller, 2000).

A region of the Knirps protein containing an alanine-rich tract had been identified in earlier studies as a repression domain in cell culture studies but is neither necessary nor sufficient for repression in the embryo. The repression function of 189-254 protein may be specific to transfection assays, similar to findings for the non-Groucho binding region of the Engrailed repressor protein (Keller, 2000).

It is not yet clear whether repression by the N-terminal and C-terminal regions of Knirps contribute to quantitative or qualitative differences in repression, or if these two aspects of repression are indeed entirely separable. The eve stripe 3 enhancer is clearly repressed in the region of kni expression in the absence of maternal dCtBP, yet in previous experiments, ectopically expressed Knirps was able to repress the eve stripe 3 element effectively only when the dCtBP binding motif of the protein was still intact. The most likely explanation for these apparently contradictory results is that dCtBP contributes to a portion of the Knirps-mediated repression of stripe 3. Endogenous Knirps is abundant enough to repress expression of eve stripe 3 in dCtBP mutant embryos, but the levels of ectopically produced Knirps protein are apparently insufficient to repress effectively when binding to dCtBP is abolished. dCtBP may also have an effect on Knirps protein stability or targeting, which might contribute to the reduced activity of the mutant protein. Previous studies have indicated that in the absence of dCtBP, repression of a synthetic rho lacZ reporter gene by endogenous Knirps is reduced. However, close examination of the data indicates that some anterior repression is apparently present, consistent with the idea that Knirps retains a measurable level of activity in the dCtBP mutant (Keller, 2000).

The Gal4-Knirps chimeras containing only the N-terminal repression domain appear to have higher levels of activity on lacZ reporters than does full-length Knirps protein lacking the dCtBP binding motif. A test was performed to see whether this difference might be attributed to masking of the N-terminal repression region by the C terminus in the absence of dCtBP. The data indicate that this model is not correct; Gal4-Knirps chimeras containing the N-terminal repression domain linked to a C-terminal region lacking a dCtBP binding activity are highly effective repressors. Gal4-Knirps chimeras may be inherently more effective repressors if one role of dCtBP is to facilitate dimerization of Knirps proteins. With chimeras, this function would be provided by the Gal4 DNA binding domain, because Gal4 binds DNA as a dimer. Alternatively, autoinhibition of the Knirps DNA binding domain, similar to that seen with Ets-1, AML-1, and Pitx2, may be relieved by dCtBP binding, but Gal4 chimeras would not be subject to such regulation. However, the effective regulation of eve stripe 3lacZ in a CtBP mutant argues for a simpler quantitative effect model. Loss of dCtBP binding might simply reduce the total repression activity of Knirps protein, so that the low levels of misexpressed Knirps would be unable to effect repression. The Gal4-Knirps repressor utilizing only one repression region might be more functional due to increased effectiveness of dimerized repressor proteins or to a greater sensitivity of the lacZ reporters used (Keller, 2000).

Multiple repression activities in a protein may allow for qualitative or quantitative effects on gene expression. Qualitatively, a repressor may operate selectively in distinct tissue types or on different promoters. Loss of maternal dCtBP protein does not affect eve stripe 3 regulation, but it does abolish repression of the eve stripe 4+6 enhancer element, suggesting that this element is dCtBP dependent. Quantitatively, dual activities may increase the overall level of repression, much as transcriptional activators have been suggested to employ multiple paths to achieve synergistic activation (Keller, 2000).

Examples of both qualitative and quantitative effects are seen with the ZEB repressor, a protein that contains two repression domains. One domain blocks activation by Myb and Ets factors of lymphocyte-specific promoters, while the second domain, which contains a conserved CtBP binding motif, blocks the activity of the muscle cell-specific MEF2C factor. In contrast to these activator-specific effects, a quantitative contribution of multiple repression domains has been observed with the murine ZEB homolog deltaEF-1. When CtBP binding residues are mutated in deltaEF-1, repression of a MyoD-activated promoter is impaired but not abolished (Keller, 2000 and references therein).

Other repressor proteins may also possess both CtBP-dependent and dCtBP-independent activities. In Drosophila, the Krüppel protein contains a C-terminal dCtBP binding repression domain and an N-terminal repression domain. The latter domain has only been characterized in cell culture assays, but genetic evidence indicates that Krüppel can repress hairy in a CtBP mutant, possibly by means of this N-terminal domain. The Wnt signaling pathway transcription factor Tcf-3 can interact with both the Groucho and CtBP proteins through separate repression domains in Xenopus laevis, and the CtBP-binding portion of XTcf-3 has potent repression activity in the frog embryo. The Rb retinoblastoma protein has been shown to interact with both histone deacetylases and CtBP, although the physiological relevance of the CtBP interactions is not yet clear. Net, an Ets protein family member that can repress transcription of the c-fos promoter, has also been shown to possess two independent repression domains, one of which interacts with CtBP1. Loss of the CtBP binding motif from Net reduces the repression activity of the protein in cell culture assays. Finally, the BKLF transcription factor, which can interact with CtBP2 to repress transcription in Drosophila cell culture, contains an additional CtBP-independent activity detectable in NIH 3T3 cells (Keller, 2000).

dCtBP and its homologs appear to be able to mediate repression directly when recruited to promoters by a heterologous DNA binding domain, both in cell culture systems and in the embryo. The dCtBP corepressor has homology to alpha-hydroxy acid dehydrogenases and contains a conserved NAD-binding domain. The protein binds to NAD, but no dehydrogenase activity has been detected in vitro, and mutation of a conserved histidine in the putative active site does not compromise the repression activity of a chimeric CtBP2 protein in cell culture assays. The dCtBP protein may contain other uncharacterized enzymatic activities. Recently it was reported that the Sir2 transcriptional repressor possesses ADP ribosylation activity, and furthermore, that NAD is important for histone deacetylase activity of the protein. Some evidence suggests that CtBP may function through histone deacetylase pathways, but pair-rule gene repression by gap proteins such as Knirps and Krüppel is not compromised by mutations in the Rpd3 histone deacetylase (Keller, 2000).

The physiological relevance of CtBP binding is not yet known for a number of proteins that have been found to interact in yeast two-hybrid assays, but genetic evidence from Drosophila clearly indicates that dCtBP is an important repression cofactor. These data demonstrate that for at least one Knirps target gene, another pathway of repression is also utilized. A considerable body of evidence, including genetic and biochemical data, indicates that repressors may have multiple lines of communication with the transcriptional machinery, just as transcriptional activators have been found to contain multiple activation domains that act on multiple targets. Further genetic and biochemical characterization of Knirps will help elucidate the pathways utilized by this short-range repressor (Keller, 2000).

Trithorax (Trx) is a member of the trithorax group (trxG) of epigenetic regulators; these proteins are required to maintain active states of Hox gene expression during development. A trithorax acetylation complex (TAC1) has been purified that contains Trx, dCBP, and Sbf1. Like CBP, TAC1 acetylates core histones in nucleosomes, suggesting that this activity may be important for epigenetic maintenance of gene activity. dCBP and Sbf1 associate with specific sites on salivary gland polytene chromosomes, colocalizing with many Trx binding sites. One of these is the site of the Hox gene Ultrabithorax (Ubx). Mutations in either trx or the gene encoding dCBP reduce expression of the endogenous Ubx gene as well as of transgenes driven by the bxd regulatory region of Ubx. Thus Trx, dCBP, and Sbf1 are closely linked, physically and functionally, in the maintenance of Hox gene expression (Petruk, 2001).

Notch signal transduction involves the presenilin-dependent intracellular processing of Notch and the nuclear translocation of the intracellular domain of Notch, NICD. NICD associates with Suppressor of Hairless [Su(H)], a DNA binding protein, and Mastermind (Mam), a transcriptional coactivator. In the absence of Notch signaling, Su(H) acts as a transcriptional repressor. Repression by Su(H) is relieved by the activation of Notch. In the Drosophila embryo, this transcriptional switch from repression to activation is important for patterning the expression of the single-minded (sim) gene along the dorsoventral axis. The mechanisms by which Su(H) inhibits the expression of Notch target genes in Drosophila has been investigated. Hairless, an antagonist of Notch signaling, is required to repress the transcription of the sim gene. Hairless forms a DNA-bound complex with Su(H). Furthermore, it directly binds the Drosophila C-terminal Binding Protein (dCtBP), which acts as a transcriptional corepressor. The dCtBP binding motif of Hairless is essential for the function of Hairless in vivo. It is proposed that Hairless mediates transcriptional repression by Su(H) via the recruitment of dCtBP (Morel, 2001).

The mechanism by which Hairless might regulate transcription was investigated. Sequence analysis of Hairless identifies a putative binding site for the Drosophila C-terminal Binding Protein (dCtBP). This site is located at the very C terminus of the Hairless protein. In Drosophila and mammals, CtBP is a transcriptional corepressor. It was therefore tested whether Hairless binds to dCtBP. The full-length Hairless protein, H[1-1076], interacts with dCtBP in a yeast two-hybrid assay. In contrast, a truncated version of Hairless in which the last 15 amino acids had been deleted, H[1-1061], does not bind to dCtBP. This shows that the Hairless-dCtBP interaction strictly depends on the conserved C-terminal part of Hairless that contains the dCtBP binding site. Furthermore, a small C-terminal peptide, H[1052-1076], is sufficient to bind dCtBP. Finally, a specific interaction between Hairless and dCtBP is also observed in vitro with a GST pull-down assay. H[1-1076], but not H[1-710] or H[1-1061], is efficiently retained by a GST-dCtBP fusion protein. This in vitro interaction indicates that the Hairless-dCtBP interaction is likely to be direct. It is concluded that the conserved C-terminal part of Hairless contains a motif necessary and sufficient to bind dCtBP (Morel, 2001).

To test the functional significance of this binding site, an in vivo assay was used. The expression of a truncated version of Hairless that does not bind dCtBP, H[1-1061], does not lead to an increased density of sense organs and does not rescue the loss of Hairless function. Thus, the last 15 amino acids of Hairless are required for the activity of the protein. Interestingly, flies overexpressing both H[1-1061] and Su(H) display a wild-type phenotype. This shows that H[1-1061] is unable to cooperate with Su(H) to block Notch signaling. Nevertheless, H[1-1061] expression suppresses the loss-of-bristle phenotype associated with increased levels of Su(H). Since H[1-1061] binds Su(H), it is possible that H[1-1061] proteins form nonproductive complexes with Su(H). Accordingly, the residual activity of the mutant H^RP1 protein, which carries a 68 amino acid C-terminal deletion, might result from its ability to sequester Su(H) without actively repressing transcription. These results therefore suggest that Hairless requires the binding of dCtBP to repress the expression of Notch target genes (Morel, 2001).

In summary, these findings indicate that Hairless links Su(H) to the dCtBP corepressor. It is therefore proposed that Hairless antagonizes Notch signaling activity by recruiting dCtBP to repress Notch target gene expression. The activation of the Notch receptor would then lead to a competition between NICD and Hairless to assemble DNA-bound regulatory complexes of opposite activities (Morel, 2001).

Two corepressors have been identified in the early Drosophila embryo: Groucho and dCtBP. Both proteins are recruited to the DNA template by interacting with short peptide motifs conserved in a variety of sequence-specific transcriptional repressors. Once bound to DNA, Groucho appears to mediate long-range repression, while dCtBP directs short-range repression. The short-range Krüppel repressor is converted into a long-range repressor by replacing the dCtBP interaction motif (PxDLSxH) with a Groucho motif (WRPW). The resulting chimeric repressor causes a different mutant phenotype from that of the native Krüppel protein when misexpressed in transgenic embryos. The different patterning activities can be explained on the basis of long-range silencing within the hairy 5' regulatory region. The analysis of a variety of synthetic transgenes provides evidence that Groucho-dependent long-range repressors do not always cause the dominant silencing of linked enhancers within a complex cis-regulatory region. A 'hot chromatin' model is suggested, whereby repressors require activators to bind DNA (Nibu, 2001a).

Complex enhancers direct stripes and bands of gene expression in the early Drosophila embryo. These enhancers are typically 300 bp-1 kb in length and contain clustered binding sites for transcriptional activators and repressors. Different enhancers can work independently of one another within a common cis-regulatory region to direct composite patterns of gene expression. For example, the seven-stripe even-skipped (eve) expression pattern is activated by five separate enhancers located 5' and 3' of the transcription unit. The ability of these enhancers to function in an autonomous fashion depends on short-range transcriptional repressors that work over distances of <100 bp to inhibit, or quench, upstream activators. The binding of the Krüppel repressor to the stripe 2 enhancer does not interfere with the activity of the stripe 3 enhancer since Krüppel mediates repression only when positioned near upstream activators. Consquently, Krüppel quenches Bicoid activators within the stripe 2 enhancer without interfering with the D-Stat activators bound to the stripe 3 enhancer (Nibu, 2001a).

There are several short-range repressors in the early embryo, including Krüppel, Snail, Knirps and Giant. Most or all of these repressors interact with a common corepressor protein, dCtBP, which is the Drosophila homolog of a human protein that was found to attenuate the oncogenic activities of the adenovirus E1A protein. dCtBP is maternally expressed and ubiquitously distributed throughout early embryos. A variety of studies suggest that the dCtBP corepressor protein is recruited to the DNA template by interacting with a conserved sequence motif contained in most or all sequence-specific short-range repressors: PxDLSxK/R/H. There is emerging evidence that mammalian CtBP proteins also function as corepressors, although it is not known currently whether the mammalian repressors (e.g. bKLF, Ikaros and ZEB-1) only function over short distances (Nibu, 2001a).

A number of repressors can work when positioned far from upstream activators and the core promoter. For example, the binding of the Hairy repressor to a modified rhomboid lateral stripe enhancer (NEE) can cause the dominant silencing of a linked mesoderm-specific enhancer, even when the two enhancers are separated by >1 kb in the 5' cis -regulatory region. Hairy interacts with a second ubiquitous corepressor protein, Groucho. Hairy-Groucho interactions depend on a conserved sequence motif at the Hairy C-terminus: WRPW. These studies suggest that the dCtBP corepressor protein mediates short-range repression, while Groucho mediates long-range repression. The present study provides additional support for this possibility (Nibu, 2001a).

The long-range action of the Groucho corepressor poses a potential problem with regard to enhancer autonomy in complex promoter regions. In principle, the binding of a Groucho-dependent repressor could result in the dominant silencing of all enhancers located in the 5' and 3' regulatory regions of a target gene. This imposes a potentially severe constraint on the evolution of complex patterns of gene activity. To investigate this issue, the activities have been examined of chimeric repressor proteins that contain the DNA-binding domains of the short-range Krüppel or Snail repressors and the Groucho interaction sequences in the long-range Hairy repressor. These chimeric repressors were expressed in specific regions of transgenic embryos using defined, heterologous enhancers. The Krüppel-Hairy fusion protein causes altered patterns of segmentation gene expression that are consistent with the notion that Hairy-Groucho interactions convert Krüppel into a long-range repressor. However, the abnormal rhomboid expression pattern obtained with a similar Snail-Hairy fusion protein suggests that it does not function as a dominant silencer, but instead causes the local repression of a single enhancer. The subsequent analysis of a number of synthetic transgenes provides direct evidence that the long-range Hairy repressor does not always cause the dominant silencing of linked enhancers (Nibu, 2001a).

A comparison of the altered patterns of hairy expression obtained with the twi-Krüppel and twi-Krüppel-hairy transgenes provides evidence that dCtBP and Groucho mediate short- and long-range repression, respectively. The twi-Krüppel transgene causes the repression of hairy stripe 6, but not stripe 5. Previous studies have shown that the stripe 6 enhancer contains optimal, high-affinity Krüppel operator sites that can be occupied by the low levels of Krüppel produced in ventral regions by the twi-Krüppel transgene. These low levels appear to be insufficient to bind the low-affinity sites within the hairy stripe 5 enhancer and, consequently, the native Krüppel protein works as a short-range repressor to inhibit stripe 6 expression without affecting stripe 5 expression. In contrast, the twi-Krüppel-hairy transgene leads to the repression of both stripes 5 and 6. The binding of the Krüppel-Hairy fusion repressor to the stripe 6 enhancer appears to cause the dominant silencing of the neighboring stripe 5 enhancer over a distance of ~2.5 kb in the hairy 5' regulatory region. An implication of these observations is that different repression domains exert distinct influences on embryonic patterning. Replacing the PxDLSxH motif (native Krüppel) with WRPW (Krüppel-Hairy) changes the regulatory activity of the Krüppel repressor (Nibu, 2001a).

The Snail-Hairy fusion protein represses the rhomboid lateral stripes, but fails to repress the amnioserosa pattern. In contrast, the same Hairy repression domain permits Krüppel to function as a dominant silencer within the hairy 5' regulatory region. There are several possible explanations for the failure of the Snail-Hairy repressor to silence rhomboid expression in the amnioserosa. Perhaps there is competition between dCtBP bound to the Snail moiety and Groucho bound to the Hairy moiety within the fusion protein. The Krüppel-Hairy fusion protein was mutagenized to eliminate the dCtBP motif (PEDLSMH), whereas the Snail-Hairy fusion protein retains both dCtBP sequences. Previous studies suggest that the conversion of the weak dCtBP interaction motif near the Hairy C-terminus, PLSLVIK, into an optimal motif, PLDLSIK, disrupts the repressor function of an otherwise normal Hairy protein. This result was taken as evidence that the dCtBP and Groucho corepressors interfere with one another when bound to closely linked motifs within the Hairy C-terminus. An argument against this explanation for the behavior of the Snail-Hairy fusion protein stems from the observation that the binding of Hairy to a modified NEE is sufficient to repress a linked mesoderm enhancer (twist PE), but not a similarly spaced race enhancer. Similarly, the binding of Hairy to a modified race enhancer fails to silence the mesoderm enhancer (Nibu, 2001a).

It is proposed that Hairy can only bind active or 'open' enhancers. The NEE is activated by the maternal Dorsal nuclear gradient and, consequently, it might contain activator proteins in both ventral and lateral regions of early embryos. As a result, the binding of Hairy to the modified h-NEE-h enhancer can lead to the dominant silencing of a linked mesoderm enhancer (twist PE). In contrast, there is no Dorsal activator in dorsal regions of the early embryo, thereby rendering the h-NEE-h enhancer in a closed or condensed state. This absence of activator might preclude the binding of Hairy so that the race enhancer is not silenced. Similarly, the race enhancer is probably activated by transcription factors that are restricted to dorsal regions, such as Zen and Smads. These activators are absent in ventral regions and, consequently, Hairy may be unable to bind the h-race-h enhancer and silence linked enhancers such as the twist PE (Nibu, 2001a).

The altered pattern of hairy expression caused by the Krüppel-Hairy fusion protein can be interpreted in the context of this 'hot chromatin' model. There is evidence that hairy stripes 5, 6 and 7 are activated by a posterior gradient of the Caudal activator. The binding of the Krüppel-Hairy fusion protein to the optimal Krüppel operator sites in the stripe 6 enhancer would be expected to silence the neighboring stripe 5 enhancer due to the open conformation of the stripe 6 enhancer in those regions of the embryo where stripe 5 is expressed. Thus, the Caudal activator might bind to both enhancers in the position of stripe 5, thereby rendering the stripe 6 enhancer accessible to the Krüppel-Hairy fusion protein (Nibu, 2001a).

The dependence of repressors on activators might restrain long-range repressors and permit enhancer autonomy. This dependence might reflect the inherent properties of activators and repressors. Some activators recruit enzymes that decondense chromatin, and this may be essential for the binding of repressors in vivo. Short-range repression has been put forward as an important mechanism for enhancer autonomy. It is suggested that a second mechanism involves the reliance of repressors on activators for binding to target enhancers (Nibu, 2001a).

There are at least three short-range gap repressors in the precellular Drosophila embryo: Krüppel, Knirps, and Giant. Krüppel and Knirps contain related repression motifs, PxDLSxH and PxDLSxK, respectively, which mediate interactions with the dCtBP corepressor protein. Giant might also interact with dCtBP. The misexpression of Giant in ventral regions of transgenic embryos results in the selective repression of eve stripe 5. A stripe5-lacZ transgene exhibits an abnormal staining pattern in dCtBP mutants that is consistent with attenuated repression by Giant. The analysis of Gal4-Giant fusion proteins has identified a minimal repression domain that contains a sequence motif, VLDLS, which is conserved in at least two other sequence-specific repressors. Removal of this sequence from the native Giant protein does not impair its repression activity in transgenic embryos. It is proposed that Giant-dCtBP interactions might be indirect and mediated by an unknown bZIP subunit that forms a heteromeric complex with Giant (Nibu, 2001b).

The minimal Giant repression domain spans amino acid residues 60-133. Alignment of this sequence with the Drosophila database identifies significant homology with the zinc finger repressor, Odd-skipped (Odd). Odd represses the expression of engrailed within the even-numbered parasegments and thereby defines which of the Ftz-expressing cells activate engrailed. Giant and Odd share the following sequence: VLDLSxxxxSxExP. A third transcriptional repressor in the early embryo, Tailless, also contains the VLDLS motif. Tailless is important for repressing segmentation gene expression in the anterior and posterior poles. It is unclear whether this sequence participates in Giant-dCtBP interactions, even though it is related to the dCtBP motif (PxDLSxR/K/H). Perhaps VLDLS helps recruit an unknown corepressor protein that mediates the residual repression activity of Gal4-Giant fusion proteins in dCtBP mutants (Nibu, 2001b).

The low levels of Giant produced by an twi-giant transgene are sufficient to repress the endogenous eve stripe 5 pattern but not stripe 2. The failure to repress stripe 2 is consistent with previous studies, which suggested that Giant might interact with a localized 'partner' in anterior regions of the early embryo. It is also possible that stripe 2 regulation depends on high concentration of the Giant protein. There are two alternative explanations for the sufficiency of low levels of Giant to repress stripe 5. First, the stripe 5 enhancer might contain optimal high-affinity Giant operator sites. Alternatively, Giant might interact with an unknown bZIP subunit, X, that is broadly expressed in the early embryo (Nibu, 2001b).

The second possibility, whereby Giant-X heterodimers regulate stripe 5 expression, is favored. Putative Giant operator sites in the stripe 5 enhancer lack obvious dyad symmetry, which might be expected for Giant-Giant homodimers. Moreover, the VLDLS motif is essential for the repression activity of Gal4-Giant fusion proteins but is dispensable in the context of the twi-giant transgene. For example, a deletion that removes the entire minimal repression domain (amino acids 60-133) does not significantly impair the ability of a twi-giant transgene to repress eve stripe 5 and hairy stripes 3, 4, and 5. Presumably, Gal4-Giant fusion proteins function as homomultimers, so that mutations in the repression domain attenuate or eliminate activity. In contrast, the same mutations might not disrupt the activities of a heterodimeric Giant-X complex because of the ability of subunit X to recruit dCtBP. Future studies will focus on the identification of subunit X and the corepressor(s) that interact with the conserved VLDLS motif (Nibu, 2001b).

The Giant protein is a short-range transcriptional repressor that refines the expression pattern of gap and pair-rule genes in the Drosophila blastoderm embryo. Short-range repressors including Knirps, Krüppel, and Snail utilize the CtBP cofactor for repression, but it is not known whether a functional interaction with CtBP is a general property of all short-range repressors. Giant repression activity was studied in a CtBP mutant and it has been found that this cofactor is required for Giant repression of some, but not all, genes. While targets of Giant such as the even-skipped stripe 2 enhancer and a synthetic lacZ reporter show clear derepression in the CtBP mutant, another Giant target, the hunchback gene, is expressed normally. A more complex situation is seen with regulation of the Krüppel gene, in which one enhancer is repressed by Giant in a CtBP-dependent manner, while another is repressed in a CtBP-independent manner. These results demonstrate that Giant can repress both via CtBP-dependent and CtBP-independent pathways, and that promoter context is critical for determining giant-CtBP functional interaction. To initiate mechanistic studies of the Giant repression activity, a minimal repression domain within Giant has been identified that encompasses residues 89-205, including an evolutionarily conserved region bearing a putative CtBP binding motif (Stunk, 2001).

The deletional analysis of Gal4-Giant chimeras indicates that Giant repression function can be localized to residues 89-205, an area of the protein that contains several tracts of highly conserved residues. Chimeras containing other portions of the Giant protein do not exhibit significant repression activity, suggesting that these regions cannot act autonomously to mediate repression, and might instead contribute to protein stability or expression. In particular, residues 266-322 appear to correlate with significantly higher repression activity of these proteins. The low levels of chimeric protein expression in the embryo precluded direct quantitation of each protein, thus this analysis is based primarily on those that did show significant activity (Stunk, 2001).

No significant physical interaction could be detected between Giant and CtBP in vitro, and the Giant protein lacks a perfect match to the consensus CtBP binding motif P-DLS-K/R/H found in the Knirps, Krüppel, and Snail proteins. However, a partial match is present: VLDLSRR (residues 98-104). The motif is evolutionarily conserved and is found within the minimal repression domain defined, consistent with a possible role in repression. Indeed, deletion of residues 89-107 inactivates the chimeric repressor. This region is clearly not sufficient for high-level repression, however, suggesting that other portions of the protein play important structural or functional roles. If CtBP directly contacts Giant in vivo, the lack of strong interaction in vitro may indicate that Giant must be posttranscriptionally modified to facilitate interaction with CtBP, perhaps via phosphorylation. Posttranslational modifications are known to play a role in CtBP binding in some instances; E1A-CtBP interactions have been shown to be regulated by acetylation of a conserved lysine residue in the CtBP binding motif. Alternatively, Giant may bind CtBP indirectly through a cofactor, much as BRCA1 has been suggested to bind CtBP through CtIP, or CtBP might be recruited via a heterodimeric basic-zipper partner of Giant. To determine whether CtBP-dependent and CtBP-independent repression activities are mediated by the same or distinct portions of the Giant protein, future studies will need to focus on identifying mutant proteins that are deficient in each of these activities (Stunk, 2001).

What characteristics of a regulatory region dictate CtBP-dependent or CtBP-independent repression? In considering which features of a gene determine CtBP-dependence or -independence, the structure of the basal promoter cannot be the deciding factor, for the same Kr promoter is regulated by distinct elements, some that exhibit CtBP-dependence and some that show CtBP-independence. Similarly, the eve gene is repressed by Knirps via CtBP-dependent and CtBP-independent regulatory elements. While the eve enhancers in question are kilobases apart, the Kr regulatory elements driving anterior and central domain (CD) expression are closely intertwined, and appear to share at least some of the same activator binding sites, suggesting that subtle differences in enhancer architecture or differences in levels of regulatory proteins interacting with those elements may dictate CtBP dependence. The Giant binding site in the Kr CD2 enhancer site was shown to be of higher affinity than the gt1 site in the eve stripe 2 enhancer. Thus, there may be a correlation between Giant binding site affinity and the requirement for CtBP, with elements containing Giant sites of lower affinity showing CtBP-dependence. A consensus has been derived for the Giant protein by aligning binding sites for Giant from eve, Kr, and the recently identified abdA iab-2 enhancer site. The consensus features an extended half-site inverted repeat TNTTAC, consistent with the dimeric nature of basic zipper proteins, and a central ACGT core common to recognition motifs for many basic zipper proteins. The higher affinity sequences from the CtBP-independent Kr CD element are closer to the consensus than those of the CtBP-dependent eve stripe 2 enhancer. Weaker sites may only be partially occupied, resulting in an overall lower level of Giant mediated repression. A loss of CtBP might further depress repression activity below a critical threshold, leading to the derepression observed. Repression of the lacZ reporter containing the giant CD1 site from Kr is CtBP-dependent, a result that contrasts with the CtBP independence of the CD itself, but this particular site may not be optimal, since it contains two mismatches. Full Giant activity may also be mediated on the native CD element through the additional high-affinity CD2 site (Stunk, 2001).

Other factors besides binding site affinity can affect Giant�s activity, and possibly its CtBP-dependence. Small alterations in the location of Giant binding sites is sufficient to strongly affect the ability of Giant to repress in transgenic embryo assays. Thus, location and affinity of Giant sites needs to be considered in studying CtBP-dependent repression. It is not believed that differences in the nature of the activators explain CtBP-dependence or -independence, because both AD and CD enhancers of Kr are activated by Bicoid protein, as is the eve stripe 2 enhancer. Detailed studies illuminating how the general properties of short-range transcriptional repressors are integrated into the design of promoter elements will promote an understanding of the control of complex developmentally regulated genes (Stunk, 2001).

Low-level ectopic expression of the Runt transcription factor blocks activation of the Drosophila melanogaster segmentation gene engrailed (en) in odd-numbered parasegments and is associated with a lethal phenotype. By using a genetic screen for maternal factors that contribute in a dose-dependent fashion to Runt-mediated repression, it is shown that there are two distinct steps in the repression of en by Runt. The initial establishment of repression is sensitive to the dosage of the zinc-finger transcription factor Tramtrack. By contrast, the co-repressor proteins Groucho and dCtBP, and the histone deacetylase Rpd3, do not affect establishment but instead maintain repression after the blastoderm stage. The distinction between establishment and maintenance is confirmed by experiments with Runt derivatives that are impaired specifically for either co-repressor interaction or DNA binding. Other transcription factors can also establish repression in Rpd3-deficient embryos: this indicates that the distinction between establishment and maintenance may be a general feature of eukaryotic transcriptional repression (Wheeler, 2002).

Transcriptional repressors can be classified as short- or long-range, according to their range of activity. Functional analysis of identified short-range repressors has been carried out largely in transgenic Drosophila, but it is not known whether general properties of short-range repressors are evident in other types of assays. To study short-range transcriptional repressors in cultured cells, chimeric tetracycline repressors were created based on Drosophila transcriptional repressors Giant, Drosophila C-terminal-binding protein (dCtBP), and Knirps. Giant and dCtBP are found to be efficient repressors in Drosophila and mammalian cells, whereas Knirps is active only in insect cells. The restricted activity of Knirps, in contrast to that of Giant, suggests that not all short-range repressors possess identical activities, consistent with recent findings showing that short-range repressors act through multiple pathways. The mammalian repressor Kid is more effective than either Giant or dCtBP in mammalian cells but is inactive in Drosophila cells. These results indicate that species-specific factors are important for the function of the Knirps and Kid repressors. Giant and dCtBP repress reporter genes in a variety of contexts, including genes that are introduced by transient transfection, carried on episomal elements, or stably integrated. This broad activity indicates that the context of the target gene is not critical for the ability of short-range repressors to block transcription, in contrast to other repressors that act only on stably integrated genes (Ryu, 2002).

The regulation of inducible promoters via chimeric tetracycline repressor (TetR) proteins has attracted considerable interest for use in ectopic expression systems in cell culture, microbes, plants, and whole animals. In these systems, a chimeric protein consisting of the Escherichia coli TetR protein fused to an activation domain binds to promoters containing Tet response elements (TREs). On addition of tetracycline or doxycycline, the chimeric protein is released from the promoter and the gene is inactivated. TetR DNA-binding domains with reverse specificity have been developed to permit activation of target genes on addition of the drug. Although this system can be highly regulated, low-level basal expression can be a problem in the case of potentially toxic gene products. To overcome this problem, higher specificity Tet DNA-binding domains have been recently developed. Many endogenous genes accomplished tight regulation by the coordinated action of repressors and activators. To mimic such composite systems, a Tet repressor can be combined with a Tet activator to give repression and activation in the absence and presence of doxycycline, respectively. Such combined Tet-based activation/repression systems have been developed for yeast and mammalian systems. Most of these systems use the KRAB repressor domain. Whether KRAB repressors can work in nonvertebrate cell types has not been reported, however. In this study, a panel of transcriptional repressors has been created based on well characterized short-range repressors from Drosophila. The chimeric proteins show reproducible repression activity in the Tet system in a variety of cell types and on stably integrated or transiently introduced reporter genes. Compared with the mammalian Kid repressor, these repressors may be the preferred alternative for regulation of expression in some cell types and with certain transgene configurations (Ryu, 2002).

CtBP and Hairy

hairy is a Drosophila pair-rule segmentation gene that functions genetically as a repressor. To isolate protein components of Hairy-mediated repression, a yeast interaction screen was carried out and a Hairy-interacting protein was identified, the Drosophila homolog of the human C-terminal-binding protein (CtBP). Human CtBP is a cellular phosphoprotein that interacts with the C-terminus of the adenovirus E1a oncoprotein and functions as a tumor suppressor. dCtBP also interacts with E1a in a directed yeast two-hybrid assay. dCtBP interacts specifically and directly with a small, previously uncharacterized C-terminal region of Hairy. dCtBP activity appears to be specific to Hairy in the Hairy/Enhancer of split [E(spl)]/Dpn basic helix-loop-helix protein class. A P-element insertion was identified within the dCtBP transcription unit that fails to complement alleles of a known locus, l(3)87De (Poortinga, 1998).

To target protein interactions with specific conserved regions of the Hairy protein, a two-hybrid screen was carried out using a LexA-tagged Hairy partial protein bait. This strategy also allowed the circumvention of the reporter system repression that was encountered when using full-length Hairy protein as a bait. A VP16-tagged Drosophila library constructed from 0-4 h embryonic mRNAs was screened with a bait that encodes the Hairy Orange domain through to the C-terminus (h-C, amino acids 93-343. In directed yeast two-hybrid assays, this fragment, h-C28, interacts weakly with full-length Hairy, but more strongly with Hairy partial proteins and with E(spl)m, another member of Hairy-class bHLH proteins. It does not interact with Dpn and interacts poorly, if at all, with E(spl)m3, -m5, -m8, -mbeta -mgamma. h-C28 does not show interaction with proteins from other HLH classes (i.e. Scute, Emc). The region of Hairy required for interaction with h-C28 was mapped using a series of Hairy deletions and partial proteins fused to LexA. h-C28 interacts strongly with a 25 amino sequence immediately upstream of, but not including, the C-terminal WRPW motif. This identifies a previously undefined protein interaction domain within Hairy. dCtBP also interacts with itself (Poortinga, 1998).

The precellular Drosophila embryo contains approximately 10 well characterized transcriptional repressors. At least half are short-range repressors that must bind within 100 bp of either upstream activators or the core transcription complex to inhibit (or quench) gene expression. The two long-range repressors can function over distances of 1 kilobase or more to silence transcription. Previous studies have shown that three of the five short-range repressors interact with a common corepressor protein, dCtBP. In contrast, the two long-range repressors, Hairy and Dorsal, recruit a different corepressor protein, Groucho. Hairy also was shown to interact with dCtBP, thereby raising the possibility that Groucho and dCtBP are components of a common corepressor complex. To investigate this issue, wild-type and mutant forms of Hairy were misexpressed in transgenic embryos. Evidence is presented that Hairy-mediated repression depends on the Groucho interaction sequence (WRPW) but not the weak dCtBP motif (PLSLV) present in the native protein. Conversion of the PLSLV motif into an optimal dCtBP interaction sequence (PLDLS) disrupts the activity of an otherwise normal Hairy protein. These results suggest that dCtBP and Groucho mediate separate pathways of transcriptional repression and that the two proteins can inhibit one another when both bind the same repressor (Zhang, 1999).

The removal of the weak dCtBP interaction motif (PLSLV) does not impair Hairy-mediated repression of Sxl, fkh, hkb, and tll. If anything, removal of this motif augments Hairy function. This observation suggests that the binding of dCtBP somehow interferes with Groucho-mediated repression. Additional support for this view stems from the observation that the PLDLS/WRPW protein, which contains an optimal dCtBP motif, is inactive and fails to repress any of the target genes that were examined. The simplest interpretation of these results is that the dCtBP and Groucho corepressors interfere with one another when both are bound to Hairy. Such antagonistic interactions are supported by previous genetic studies, which suggest that lowering the dose of maternal dCtBP products can partially suppress the embryonic phenotypes of hairy mutants. The P-SLV-K and WRPW motifs are separated by just nine amino acid residues within the C terminus of the Hairy protein. When dCtBP and Groucho both bind, they might be unable to interact with additional corepressors or with their target proteins in the core transcription complex (Zhang, 1999).

Drosophila C-terminal binding protein (dCtBP) and Groucho have been identified as Hairy-interacting proteins required for embryonic segmentation and Hairy-mediated transcriptional repression. While both dCtBP and Groucho are required for proper Hairy function, their properties are very different. As would be expected for a co-repressor, reduced Groucho activity enhances the hairy mutant phenotype. In contrast, reduced dCtBP activity suppresses it. dCtBP can function as either a co-activator or co-repressor of transcription in a context-dependent manner. The regions of dCtBP required for activation and repression are separable. mSin3A-histone deacetylase complexes are altered in the presence of dCtBP and dCtBP interferes with both Groucho and Mad transcriptional repression. Similar to CtBP's role in attenuating E1A's oncogenicity, it is proposed that dCtBP can interfere with corepressor-histone deacetylase complexes, thereby attenuating transcriptional repression. Hairy defines a new class of proteins that requires both CtBP and Groucho co-factors for proper function (Phippen, 2000).

Members of the widely conserved Hairy/Enhancer of split family of basic Helix-Loop-Helix repressors are essential for proper Drosophila and vertebrate development and are misregulated in many cancers. While a major step forward in understanding the molecular mechanism(s) surrounding Hairy-mediated repression was made with the identification of Groucho, Drosophila C-terminal binding protein (dCtBP), and Drosophila silent information regulator 2 (dSir2) as Hairy transcriptional cofactors, the identity of Hairy target genes and the rules governing cofactor recruitment are relatively unknown. The chromatin profiling method DamID was used to perform a global and systematic search for direct transcriptional targets for Drosophila Hairy and the genomic recruitment sites for three of its cofactors: Groucho, dCtBP, and dSir2. Each of the proteins was tethered to Escherichia coli DNA adenine methyltransferase, permitting methylation proximal to in vivo binding sites in both Drosophila Kc cells and early embryos. This approach identified 40 novel genomic targets for Hairy in Kc cells, as well as 155 loci recruiting Groucho, 107 loci recruiting dSir2, and wide genomic binding of dCtBP to 496 loci. DamID profiling was adapted such that tightly gated collections of embryos (2-6 h) could be used, and 20 Hairy targets related to early embryogenesis were found. As expected of direct targets, all of the putative Hairy target genes tested show Hairy-dependent expression and have conserved consensus C-box-containing sequences that are directly bound by Hairy in vitro. The distribution of Hairy targets in both the Kc cell and embryo DamID experiments corresponds to Hairy binding sites in vivo on polytene chromosomes. Similarly, the distributions of loci recruiting each of Hairy's cofactors are detected as cofactor binding sites in vivo on polytene chromosomes. Fifty-nine putative transcriptional targets of Hairy were identified. In addition to finding putative targets for Hairy in segmentation, groups of targets were found suggesting roles for Hairy in cell cycle, cell growth, and morphogenesis, processes that must be coordinately regulated with pattern formation. Examining the recruitment of Hairy's three characterized cofactors to their putative target genes revealed that cofactor recruitment is context-dependent. While Groucho is frequently considered to be the primary Hairy cofactor, it is associated with only a minority of Hairy targets. The majority of Hairy targets are associated with the presence of a combination of dCtBP and dSir2. Thus, the DamID chromatin profiling technique provides a systematic means of identifying transcriptional target genes and of obtaining a global view of cofactor recruitment requirements during development (Bianchi-Frias, 2004).

The 59 putative Hairy targets identified correspond to bands of Hairy immunostaining on polytene chromosomes, suggesting that the polytene chromosome staining faithfully represents Hairy binding. Polytene chromosomes are functionally similar in transcriptional activity and display factor/cofactor binding properties similar to chromatin of diploid interphase cells, despite their DNA endoreplication (Bianchi-Frias, 2004).

Since the microarray chips used contained roughly half of Drosophila cDNAs, the actual number of Hairy targets was estimaed to be approximately twice that number (i.e., 118 targets). This predicted number of Hairy targets is close to the approximately 120 strongly staining sites observed on polytene chromosomes. Of the 59 putative Hairy targets identified in both the Kc cell and embryo DamID experiments, 58 correspond to bands of Hairy staining on the polytene chromosomes, suggesting that polytene chromosome staining is representing Hairy binding sites without regard to tissue specificity. It is not yet clear what is limiting Hairy accessibility in different tissues or why Hairy's access does not appear to be limited in salivary glands. It may be that polytene chromosome organization necessitates a looser chromatin structure or that the large number of factors that seem to be endogenously expressed in salivary glands affects accessibility. Ultimately, additional confirmation of the DamID and polytene staining correspondence will require microarray tiling chips containing overlapping genomic DNA fragments; however, such genomic DNA tiling chips are currently unavailable (Bianchi-Frias, 2004).

DNA methylation by tethered Dam has been shown to spread up to a few kilobases from the point where it is brought to the DNA. It was of concern in the beginning that Hairy targets might be missed if the DNA fragments of 2.5 kb or less that were recovered for probes were far away from the start of the transcribed region, especially since the Drosophila microarray chip used was generated using full-length cDNAs. Indeed, Hairy has been described as a long-range repressor; it is likely to bind at a distance from the transcription start site. However, the targets identified by DamID in both Kc cells and in embryos correspond closely to the Hairy staining pattern on polytene chromosomes. As is the case for Hairy, the distribution of DamID-identified loci that recruit the long-range repression-mediating Groucho corepressor corresponds well with the distribution of Groucho binding sites on polytene chromosomes. These results suggest that there is a higher-order structure to the promoter that is allowing factors that bind far upstream of the transcription start site to have physical access to the transcribed region (i.e., DNA looping) or that Hairy does not bind as far away from the transcription start site as it had been proposed to do (Bianchi-Frias, 2004).

Hairy is needed at multiple times during development, where it has primarily been associated with the regulation of cell fate decisions. During embryonic segmentation, ftz has long been thought to be a direct Hairy target. However, the order of appearance of ftz stripes is not inversely correlated with those of Hairy, as would be expected if ftz stripes are generated by Hairy repression. While it was not possible to assess ftz as a direct Hairy target using DamID, no evidence was found for ftz being a direct Hairy target based on the association of Hairy with polytene chromosomes. Indeed, the evidence suggesting that ftz is a direct target of Hairy is based on timing, i.e., that there is not enough time for another factor to be involved. Since the half-life of the pair-rule gene products is very short (less than 5 min), it is possible that additional factors could be acting and that the interaction between Hairy and ftz is indirect (Bianchi-Frias, 2004).

Interestingly, one of the Hairy targets identified in embryos is the homeobox-containing transcriptional regulator, prd. Pair-rule genes have been split into two groups: primary pair-rule genes mediate the transition from nonperiodic to reiterated patterns via positional cues received directly from the gap genes, whereas secondary pair-rule genes take their patterning cues from the primary pair-rule genes and in turn regulate the segment polarity and homeotic gene expression. The transcriptional regulator prd was originally categorized as a secondary pair-rule gene since its expression is affected by mutations in all other known pair-rule genes. However, prd stripes were subsequently shown to require gap gene products for their establishment, and the prd locus has the modular promoter structure associated with primary pair-rule genes. Thus prd has properties of both primary and secondary pair-rule genes and is a good candidate to directly mediate Hairy's effects on segmentation. Hairy can specifically bind to C-box sequences in the prd promoter and interacts genetically with prd. Further experiments will be required to determine if Paired in turn binds to the ftz promoter, such that the order of regulation would be Hairy > prd > ftz (Bianchi-Frias, 2004).

In addition to identifying potential targets for Hairy in segmentation, targets were identified that implicate Hairy in other processes including cell cycle, cell growth, and morphogenesis. The group of targets implicating Hairy in the regulation of morphogenesis includes: concertina, a G-alpha protein involved in regulating cell shape changes during gastrulation; kayak, the Drosophila Fos homolog involved in morphogenetic processes such as follicle cell migration, dorsal closure, and wound healing; pointed and mae, both of which function in the ras signaling pathway to control aspects of epithelial morphogenesis; egh, a novel, putative secreted or transmembrane protein proposed to play a role in epithelial morphogenesis, and Mipp1, a phosphatase required for proper tracheal development (Bianchi-Frias, 2004).

Hairy has been thought to be involved mostly in the regulation of cell fate decisions. However, mosaic experiments in the eye imaginal disc have suggested that Hairy may also play a role in the regulation of cell cycle or cell growth. Consistent with this, another group of Hairy targets implicates Hairy in the regulation of cell cycle or cell growth; this group includes stg, the Drosophila Cdc25 homolog; dacapo, a cyclin-dependent kinase inhibitor related to mammalian p27^kip1/p21^waf1; IDGF2, a member of a newly identified family of growth-promoting glycoproteins, and ImpL2, a steroid-responsive gene of the secreted immunoglobulin superfamily that functions as a negative regulator of insulin signaling. Consistent with a role for Hairy in growth signaling, mammalian HES family proteins have been linked to insulin signaling (Bianchi-Frias, 2004).

Since cells that are dividing or proliferating cannot simultaneously undergo the cell shape changes and cell migrations required for morphogenetic movements, Hairy may be required to transiently pause the cell cycle in a spatially and temporally defined manner, thereby allowing the cell fate decisions regulated by the transcription cascade to be completed. Since Hairy is itself spatially and temporally expressed, Hairy must be only one of several genes necessary to orchestrate these processes. While much progress has been made in understanding the regulatory networks governing pattern formation, cell proliferation, and morphogenesis, and while it is clear that they must be integrated, the details surrounding their coordination have not yet been elucidated. Thus, the putative Hairy targets identified are consistent with known processes involving Hairy and suggest that in addition to regulating pattern formation, Hairy plays a role in transiently repressing other events, perhaps in order to coordinate cell cycle events with the segmentation cascade. Further experiments will be needed to determine how these different roles for Hairy fit together (Bianchi-Frias, 2004).

The numbers of loci that recruit Groucho, dCtBP, and dSir2 cofactors are consistent with the breadth of interaction they have been shown to exhibit. One hundred and fifth-five loci were identified that recruit Groucho and, as expected, roughly twice as many sites were found on polytene chromosomes. Although Groucho was the first Hairy cofactor identified and its interaction site is often described as Hairy's 'major' repression motif, Groucho is associated with only a minority of Hairy targets in Kc cells. Groucho's dominance as a cofactor during segmentation may reflect a preference for Groucho in the reporter assays used previously to assess corepressor activity, or it may be more heavily recruited to Hairy's targets during segmentation. In the future it will be interesting to determine the loci that recruit Groucho in early embryos and, because Groucho binds a number of other repressors, which, if any, of these factors recruits Groucho as its major cofactor (Bianchi-Frias, 2004).

CtBP was identified as a repressive co-factor, first on the basis of its binding to the C-terminal region of E1A, and in Drosophila by its association with the developmental repressors Hairy and Knirps. CtBP is an integral component in a variety of multiprotein transcriptional complexes. It has been shown to function as a context-dependent cofactor, having both positive and negative effects on transcriptional repression depending upon the repressor to which it is recruited. More than 40 different repressors have been shown to recruit CtBP. Consistent with this wide recruitment of CtBP, 496 loci that recruit dCtBP were found by DamID profiling and roughly twice that many sites on polytene chromosomes. A global protein-protein interaction study has shown that the binding partners for Groucho and dCtBP are largely nonoverlapping. This, along with the near exclusivity of Groucho and dCtBP binding as assayed by DamID and polytene chromosome staining, makes it unlikely that both cofactors work together as a general rule and strengthens the possibility that the binding of each of these factors assembles different protein complexes that are, for the most part, mutually exclusive (Bianchi-Frias, 2004).

dSir2 was only very recently identified as a corepressor for Hairy and other HES family members. 107 loci were identified by DamID profiling that recruit dSir2 and roughly twice that many sites on polytene chromosomes. Surprisingly, the distribution of loci recruiting dSir2 identified by DamID profiling, as well as dSir2′s staining on polytene chromosomes, shows regional binding specificity. This binding specificity may be a reflection of the different nuclear compartments in which these regions of the chromosomes are found. Sir2 has been described mostly as a protein involved in heterochromatic silencing rather than in euchromatic repression. The number of dSir2 euchromatic sites observed is similar to that of Groucho, suggesting that euchromatic repressors (in addition to HES family members) are likely to recruit Sir2. Consistent with this, a recent report has described a role for mammalian Sir2 in repressing the muscle cell differentiation program. The region-specific binding of dSir2 might reflect a difference in the types of factors it can associate with, or the association of dSir2 with particular chromosomal regions or nuclear domains (Bianchi-Frias, 2004).

Interestingly, dCtBP and dSir2 recruitment are largely overlapping, and this association continues outside of those loci where Hairy binds: 90% of dSir2-recruiting loci also recruit dCtBP. dCtBP and dSir2 are unique among transcriptional coregulators in that they both encode NAD⁺-dependent enzymatic activities. As NAD and NADH levels within the cell exist in closely regulated equilibrium, it is possible that dCtBP and dSir2 function as NAD/NADH redox sensors. In this way, the cell could use coenzyme metabolites to coordinate the transcriptional activity of differentiation-specific genes with the cellular redox state (Bianchi-Frias, 2004).

CtBP and Brinker

Responses to graded Dpp activity requires an input from a complementary and opposing gradient of Brinker (Brk), a transcriptional repressor protein encoded by a Dpp target gene. Brk harbours a functional and transferable repression domain, through which it recruits the corepressors Groucho and CtBP. By analysing transcriptional outcomes arising from the genetic removal of these corepressors, and by ectopically expressing Brk variants in the embryo, it has been demonstrated that these corepressors are alternatively used by Brk for repressing some Dpp-responsive genes, whereas for repressing other distinct target genes they are not required. These results show that Brk utilizes multiple means to repress its endogenous target genes, allowing repression of a multitude of complex Dpp target promoters (Hasson, 2001).

That putative Brk target genes are repressed in clones of cells with increased gro dosage strongly suggests that Brk is a Gro-dependent repressor. Accordingly, Brk's proposed repression domain (RD) harbours a potential Gro recruitment motif (FKPY), similar to the Gro-binding domains defined in the repressors Hairy (WRPW), Runt (WRPY) and Huckebein (FRPW), and identical to that in Even-skipped (Eve). Brk also contains a CtBP-binding domain (PMDLSLG. Brk is shown, in fact, to be able to interact physically with both Gro and CtBP, and the functional relevance of these associations to Brk's in vivo repressor capacity are addressed (Hasson, 2001).

To demonstrate Brk's ability to associate with the two corepressors in vitro, the protein's putative RD (amino acids 369-541) was fused to glutathione S-transferase (GST), and it was incubated with radioactively labelled Gro or CtBP. In GST pull-down assays, Brk's RD (Brk^RD) readily retains [³⁵S]methionine-labelled Gro. To test further the specificity of this interaction, three mutant derivatives of the Brk^RD, fused to the GST moiety, were generated in which the Gro recruitment domain (Brk^RDmutG; FKPY to FEAY), the core of the CtBP-binding motif (Brk^RDmutC; DLS to AAA) or both (Brk^RDmutC/G) were altered. Brk's binding to Gro is impaired by the modifications in the FKPY motif. Significantly, however, Gro associates with the GST- Brk^RDmutC construct as strongly as it does with the native GST-Brk^RD fusion. GST-Brk^RD also binds labelled CtBP in vitro and, although the binding of Brk to CtBP is weak in this assay, the specificity of the interaction is clearly evident: the association between the two proteins is abolished by mutations in the CtBP recruitment domain but is unaffected by alterations in the Gro recruitment motif (Hasson, 2001).

Brk has been reported to negate transcription by competing with activators, such as Mad/Medea, for overlapping DNA target sites, thereby preventing activators' access to target promoters. The direct interactions of Brk with Gro and CtBP, however, suggest that Brk acts in a more instructive manner. While in the former 'passive' mechanism Brk is expected to rely solely on its competitive DNA-binding activity, the latter 'active' mechanism predicts that it accommodates an innate RD that depends on the recruitment of corepressors (Hasson, 2001).

To establish whether Brk contains a functional RD that can silence gene expression, separable from its DNA-binding domain, an in vivo assay was employed that relies on repression of the sex-determining Sex-lethal (Sxl) gene by ectopic expression of the pair-rule gene hairy. Sxl is normally expressed only in female embryos whereas, in males, it is repressed by Deadpan (Dpn), an autosomally encoded Hairy-related repressor protein. When Hairy is expressed prematurely, under the hunchback (hb) promoter, it mimics Dpn's repressor function and eradicates Sxl transcription in the anterior of syncytial blastoderm female embryos. Because Sxl is essential for dosage compensation in females, this repression subsequently leads to female-specific lethality. A form of Hairy, lacking its own RD, is inert in this assay. However, fusion of heterologous RDs to the truncated Hairy protein restores its ability to repress Sxl. Indeed, the equivalent expression of a hb-Hairy-Brk^RD transgene results in an effective repression of Sxl in the anterior halves of female embryos and female-specific lethality ensues. Thus, the region in Brk spanning the Gro- and CtBP-binding domains promotes potent repression in embryos (Hasson, 2001).

The ability to selectively disrupt Brk binding to each individual corepressor allowed the exploration of the dependence of its repressor potential on Gro and/or CtBP in vivo. Since both Gro- and CtBP-mediated repression can be detected in the Sxl-repression assay, truncated Hairy was fused to the three derivatives of the Brk RD, mutated in the Gro, CtBP or both recruitment motifs and placed under hb promoter regulation. In female embryos expressing Hairy-Brk^RDmutC, Sxl is substantially repressed, although not as effectively as by Hairy-Brk^RD. Furthermore, this repression still leads to statistically significant female-specific lethality. Thus, blocking CtBP binding does not completely abolish activity of the Brk RD. In comparison, mutating the Gro recruitment domain causes only residual Sxl repression and no apparent female-specific lethality. Finally, Sxl expression is seen throughout female embryos expressing hb-Hairy-Brk^RDmutC/G, and no female-specific lethality is observed. Thus, Brk relies mainly on Gro for repressing Sxl. Nevertheless, since mutating the CtBP recruitment motif in Brk's RD attenuates Sxl repression, it is concluded that, for full potency as a negative transcriptional regulator, Brk requires both corepressors (Hasson, 2001).

These data indicate that the interactions between Brk and the corepressors Gro and CtBP are indispensable for maximal repression of Sxl in vivo. Whether Brk requires both cofactors for repression of its endogenous target genes was examined. For repression of distinct target genes, Brk requires Gro and/or CtBP differentially, presumably as a function of specific promoter topology and architecture (Hasson, 2001).

Brk competes with an activator for binding to an omb wing enhancer, suggesting that, for this promoter, Brk should act independently of corepressors. Consistent with this, omb-lacZ is not ectopically expressed in cells homozygous for gro^E48 (hereafter referred to as gro^- clones), nor is it affected by CtBP loss-of-function clones, generated using the l(3)87De-10 allele (CtBP^-), or by CtBP^-, gro^- double mutant clones). Thus, single and double mutant clones for gro and CtBP do not phenocopy the omb derepression seen in brk^- clones, implying that Brk can repress omb even in the absence of these corepressors. Repression of the Dpp target gene spalt (sal) is also independent of Gro and CtBP. Nonetheless, in gro overexpression clones, omb is repressed, suggesting that, even for the omb promoter, Gro reinforces Brk repressor function (Hasson, 2001).

To establish whether Brk represses vgQ via Gro, CtBP or both, vgQ-lacZ expression was monitored in gro^- and CtBP^- single, and CtBP^-; gro^- double mutant clones. In this instance, a mandatory requirement for Gro, but not for CtBP is found; in gro^- clones, vgQ is upregulated. Importantly, as is the case for brk^- clones, the cell-autonomous upregulation of vgQ is seen only in gro^- clones close to the periphery of the disc, suggesting that the observed effects are Brk dependent. In contrast, in CtBP^- mutant clones vgQ expression is downregulated, in the Brk territory but also outside it, at the centre of the disc, indicating that these effects are Brk independent and that CtBP is positively required for vg expression. CtBP^-;gro^- double mutant clones show a composite effect: ectopic expression and upregulation of vgQ in clones in the brk expression domain, and a phenotype resembling that of CtBP^- clones at the middle of the disc, where brk is not expressed. Thus, Brk repression of vgQ is Gro- but not CtBP-dependent (Hasson, 2001).

omb and vgQ expression is completely shut off in clones of cells overexpressing gro, whereas that of brk is only reduced, suggesting that Brk might be repressing its own transcription via a negative autoregulatory loop. To establish whether, in negating its own expression, Brk is assisted by Gro and/or CtBP, gro^- and CtBP^- single, or CtBP^-, gro^- double mutant clones were stained for brk-lacZ expression. brk is never ectopically expressed in any of the single mutant clones, whereas ectopic brk expression is clearly observable in double mutant clones. Thus, in the absence of one corepressor, repression is adequately mediated by the other, suggesting that negative autoregulation by Brk is robust, relying on either Gro or CtBP (Hasson, 2001).

Strikingly, the effects on brk expression are seen only in double mutant clones found at the periphery of the disc, but not at the center where Shn is active, supporting the notion that the effects are, indeed, Brk- but not Shn-dependent. Furthermore, the fact that double mutant clones at the middle of the disc do not ectopically express brk suggests that Shn-mediated repression of brk transcription must be taking place even in the absence of both corepressors (Hasson, 2001).

To be able to compare Brk's dependence on Gro and CtBP in the embryo, full-length Brk was expressed in its native form or with its corepressor-binding domains mutated, using UAS-brk transgenes driven by maternal GAL4 . This experimental design is inapplicable for studying Brk's targets in the wing, since ectopic expression of brk prevents proliferation and survival of imaginal disc cells, but is nevertheless effective in the embryo. Ectopic Brk represses zen and dpp in mid- to late-cellularizing embryos but not earlier, so endogenous Brk targets were analyzed in transgenic embryos at comparable stages of development. Ectopic expression of all three mutant forms of Brk in embryos brings about repression of zen to the same extent as does native Brk. This result suggests that Brk represses zen independently of corepressors. In contrast, Brk requires corepressors for negating transcription of both tolloid (tld) and dpp. Thus, abolishing Brk's interactions with Gro (Brk^mutG), but not with CtBP (Brk^mutC), completely relieves tld repression, indicating that Brk repression of tld is strictly Gro dependent, as is repression of pannier (pnr). Similarly, dpp is repressed in embryos expressing Brk^mutC, but is still transcribed in embryos expressing Brk with its Gro recruitment motif mutated. In the case of dpp, however, CtBP must also be contributing to Brk repression, since the level of dpp expression is significantly lower in Brk^mutG-expressing embryos, in comparison with wild-type embryos or embryos expressing Brk^mutC/G. Thus it is concluded that, for repression of dpp, Brk rests mainly on Gro, yet for maximal repressor activity it also requires CtBP. The data indicate that Brk utilizes different means of repression for silencing its downstream targets in the embryo, as in the adult (Hasson, 2001).

Gro and CtBP mediate gene silencing in qualitatively different ways. Gro potentiates long-range repressors that function at a distance and that are able to block, in a dominant fashion, complex modular promoters consisting of multiple enhancer elements. In contrast, CtBP-dependent short-range repressors inhibit activators only locally, thereby permitting enhancer autonomy in a compound promoter. By virtue of its ability to recruit both Gro and CtBP, together with its capacity to outcompete pMad and other activators from binding DNA, Brk is competent to repress a multitude of complex Dpp target promoters, which receive positive inputs from manifold signaling pathways. It is proposed that, for promoters with low-affinity Mad-binding sites, the driving repressor force is direct competition between Brk and pMad for DNA binding, whereas for Dpp target promoters that contain high-affinity Mad-binding sites, corepressors are essential for mediating Brk repression. For this latter class of promoters, Brk relies on one or both of its cognate corepressors, depending on the particular promoter topology (Hasson, 2001).

Brk utilizes a self-reliant mechanism, which need not depend on tethered corepressors, by competing with activators over coinciding DNA-binding sites. In the absence of both Gro and CtBP, Brk represses not only omb and zen, but also sal, suggesting that the Brk-binding site(s) in the sal promoter overlap with those employed by activators. Transcription of both sal and vgQ requires activation by Mad, yet, although both promoters are exposed to identical levels of pMad, the sal expression domain is spatially more restricted than that of vgQ, presumably because activation of sal requires higher levels of pMad than that of vgQ. Hence, 'passive' competition-based repression should efficiently block activation of sal but may not be sufficient for promoters like vgQ, which are activated even by low amounts of Mad. For silencing such promoters, alternative mechanisms such as recruitment of corepressors have evolved and are employed (Hasson, 2001).

Brk represses its distinct endogenous target genes by recruiting Gro and/or CtBP differentially. For the silencing of many target promoters, Gro alone is sufficient (vg, tld and pnr) but, for fully repressing others, Brk depends on both corepressors. Thus, in the case of dpp and Sxl, when CtBP is lacking, a decrease in Brk's overall repressor capacity is apparent and, in the absence of Gro, repression is almost completely impaired. Importantly, for negating its own transcription, Brk can utilize either corepressor (Hasson, 2001).

The majority of activator and repressor binding sites in most Dpp-responsive enhancers have yet to be precisely mapped. It is nevertheless proposed that lengthy and complex promoters, which respond to several signaling inputs, will be found to be strictly silenced in a Gro-dependent manner. Thus, in repressing the vgQ enhancer, a composite cis-acting regulatory sequence with multiple elements that integrate information relayed by the dpp, wingless and EGF receptor signaling pathways, Brk is fully reliant on Gro, but not on CtBP. For other more simple promoters, short-range repression should be adequate and will be mediated by either corepressor, as exemplified by the robust Brk autoregulation, for which either Gro or CtBP is sufficient; CtBP and Gro are presumably interchangeable in this context, compensating for each other's absence (Hasson, 2001).

Significantly, the overexpression of gro results in ectopic omb repression, suggesting that, even for promoters that are switched off in a 'passive', competitive manner, excess Gro can over-potentiate Brk-mediated negative transcriptional regulation. Thus, Gro and/or CtBP might reinforce Brk repression of those promoters on which it initially acts by competing with activators for binding to DNA, via recruitment of histone deacetylases and alterations to chromatin structure, or by some other mechanism (Hasson, 2001).

In the embryonic midgut of Drosophila, Wingless (Wg) signaling elicits threshold-specific transcriptional response, that is, low-signaling levels activate target genes, whereas high-signaling levels repress them. Wg-mediated repression of the HOX gene Ultrabithorax (Ubx) is conferred by a response sequence within the Ubx B midgut enhancer, called WRS-R. It further depends on the Teashirt (Tsh) repressor, which acts through the WRS-R without binding to it. Wg-mediated repression of Ubx B depends on Brinker, which binds to the WRS-R. Brinker binds to a site distinct from that occupied by the Wg effector, the Pangolin/Armadillo activator complex. Brinker thus acts at short range to block the activity of this complex. Furthermore, Brinker blocks transcriptional activation by ubiquitous Wg signaling. Brinker binds to Tsh in vitro, recruits Tsh to the WRS-R, and mutual physical interactions are found between Brinker, Tsh, and the corepressor dCtBP. This suggests that the three proteins may form a ternary repressor complex at the WRS-R to quench the activity of the nearby-bound Pangolin/Armadillo transcription complex. Finally, brinker and tsh produce similar mutant phenotypes in the ventral epidermis, and double mutants mimic overactive Wg signaling in this tissue. This suggests that Brinker, which was initially discovered as an antagonist of Dpp signaling, may have a widespread function in antagonizing Wg signaling (Saller, 2002).

Most likely, Brinker uses a mechanism called quenching to block Pangolin/Armadillo. Quenching involves interaction of repressors (and the corepressors they recruit) with activators bound to nearby sites. Brinker is known to be able to quench target genes by recruiting the corepressor Groucho, which is involved in multiple quenching processes. groucho antagonizes wg, and TCF factors can bind to Groucho proteins directly, so Pangolin may thus be able to recruit Groucho unassisted. However, these findings do not rule out the possibility that Pangolin relies on cooperation with Brinker to achieve Groucho recruitment (Saller, 2002 and references therein).

Brinker can bind to the corepressor dCtBP, so Brinker may recruit dCtBP instead of, or in addition to, Groucho. Recall that Tsh plays a critical role in the Wg-mediated repression in the midgut. Moreover, Tsh can bind to Brinker as well as to dCtBP, so it seems plausible that Tsh plays a pivotal role in assisting Brinker in the recruitment of dCtBP. Like Groucho, dCtBP is a corepressor with quenching activity. In addition, Tsh may itself be involved in the quenching process. It has been suggested that quenching may be based on obstruction of the interaction between the activation domain of a transcriptional activator and the general transcription machinery -- intriguingly, hypophosphorylated Tsh binds to the carboxy-terminal activation domain of Armadillo to modulate Wg signaling (Saller, 2002 and references therein).

CtBP and Teashirt

Drosophila teashirt functions as a region-specific homeotic gene that specifies trunk identity during embryogenesis. Based on sequence homology, three tsh-like (Tsh) genes have been identified in the mouse. Their expression patterns in specific regions of the trunk, limbs and gut raise the possibility that they may play similar roles to tsh in flies. By expressing the putative mouse Tsh genes in flies, evidence is provided that they behave in a very similar way to the fly tsh gene: (1) ectopic expression of any of the three mouse Tsh genes, like that of tsh, induces head to trunk homeotic transformation; (2) mouse Tsh proteins can rescue both the homeotic and the segment polarity phenotypes of a tsh null mutant; (3) following ectopic expression, the three mouse Tsh genes affect the expression of the same target genes as tsh in the Drosophila embryo; (4) mouse Tsh genes, like tsh, are able to induce ectopic eyes in adult flies; (5) all Tsh proteins contain a motif that recruits the C-terminal binding protein and contributes to their repression function. As no other vertebrate or fly protein has been shown to induce such effects upon ectopic expression, these results are consistent with the idea that the three mouse Tsh genes are functionally equivalent to the Drosophila tsh gene when expressed in developing Drosophila embryos (Manfroid, 2004).

Comparison of the organization of Tsh with Tsh-related proteins in mouse and humans suggests that common functional features are probably defined by the region encompassing the three zinc-finger motifs and by the presence of a motif known to interact with CtBP. Interestingly, mouse and Drosophila Tsh proteins display intrinsic transcriptional repression activity. The repression ability of Tsh proteins is partly due to their interaction with the co-repressor CtBP. In the visceral mesoderm, Tsh is recruited to the Ubx enhancer in a repressor complex containing Brk and CtBP, wherein Tsh does not seem to bind directly to DNA, but rather Brk is the DNA-binding partner. In the ectoderm, however, Tsh directly binds to the modulo enhancer and represses transcription in vivo. The association of CtBP with Tsh is dependent on the CtBP-interacting motif (PLDLS) located in the N-terminal part of Tsh, and this CtBP/Tsh complex contributes to the observed repression. An analogous motif (PIDLT) is found in the C-terminal part of the three mouse Tsh proteins. Despite the different context encompassing the PIDLT motif in the mouse proteins (C-terminal), this motif is functional and essential for the repressor function of mouse Tsh1. Although the role of this motif was addressed only for only mouse Tsh1, repression activity of Tsh2 and Tsh3 is equally potentiated by mouse Ctbp1, suggesting that mouse Ctbp1 is a co-repressor acting with all mouse Tsh proteins. Interestingly, the PIDLT motif lies within a region of the three mouse Tsh proteins where the sequence similarity is low and thus appears to be a highly conserved functional domain in a variable region. In addition, it is worth noting that, in mammalian cells, some repression activity persists in mouse Tsh1 after deletion of the CtBP-interacting motif, implying that other mechanisms of transcriptional repression are used by mouse Tsh. In contrast to Tsh, which contains a repressor domain rich in Ala, analysis of the mouse Tsh protein sequences fail to reveal a comparable feature or any known motif that could account for the mouse Tsh1DeltaPIDLT repressor activity (Manfroid, 2004).

CtBP and Hairless

The DNA-binding transcription factor Suppressor of Hairless [Su(H)] functions as an activator during Notch (N) pathway signaling, but can act as a repressor in the absence of signaling. Hairless (H), a novel Drosophila protein, binds to Su(H) and has been proposed to antagonize N signaling by inhibiting DNA binding by Su(H). In vitro, H directly binds two corepressor proteins, Groucho (Gro) and dCtBP. Reduction of gro or dCtBP function enhances H mutant phenotypes and suppresses N phenotypes in the adult mechanosensory bristle. This activity of gro is surprising, because it is directed oppositely to its traditionally defined role as a neurogenic gene. Su(H)-H complexes can bind to DNA with high efficiency in vitro. Furthermore, a H-VP16 fusion protein causes dominant-negative phenotypes in vivo, a result consistent with the proposal that H functions in transcriptional repression. Taken together, these findings indicate that 'default repression' of N pathway target genes by an unusual adaptor/corepressor complex is essential for proper cell fate specification during Drosophila peripheral nervous system development (Barolo, 2002b).

H is a novel protein, with no known vertebrate homologs. However, the H gene has been identified in three members of the order Diptera: Drosophila melanogaster, D. hydei, and the mosquito Anopheles gambiae. H is surprisingly poorly conserved among these three species: It shares 63% identity between D. melanogaster and D. hydei (diverged ~65 Mya), and 33% identity between Drosophila and Anopheles (diverged ~260 Mya). The rapid divergence of the H protein sequence readily allows the identification of short conserved motifs, which are presumably important for H function. Two such regions occur in a part of H that is required for its interaction with Su(H) in vitro (Barolo, 2002b).

Another conserved motif in the H protein is YSIxxLLG, which is perfectly conserved from Drosophila to Anopheles. This sequence resembles certain examples of the 'eh1' type of Gro-binding domain found in many transcriptional repressor proteins. Among eh1 domains, the 'octapeptide' motifs in the Pax 2/5/8 proteins, which have been shown to directly mediate repression by recruiting Gro-family corepressors, show the greatest similarity to this region of H. In addition, the extreme C-terminal sequence of H, PLNLSKH, includes a match to the consensus binding site for the CtBP corepressor, Px(D/N)LS. The PLNLS motif, fully conserved from Drosophila to Anopheles, exactly matches motifs found in four vertebrate CtBP-binding transcription factors. H also contains three lengthy alanine-repeat domains: AAAVAAAAAAAAA, AAAAAAAAAA, and AAVAAA AAAAAA. Alanine repeats and alanine-rich regions are common in transcriptional repression domains, and are found in many repressor proteins. However, these repeats are reduced or absent in the D. hydei and A. gambiae H proteins: this suggests that they may not make an essential contribution to H function (Barolo, 2002b).

A gel retardation experiment reported by Brou (1994), indicating that H can inhibit the binding of Su(H) to DNA in vitro, has strongly influenced interpretations of genetic studies of H, Su(H), and N. A DNA-binding-inhibition model of H function is indeed consistent with both loss- and gain-of-function genetic data demonstrating that H affects cell fate in a manner antagonistic to N signaling, including the N-stimulated transcriptional activation function of Su(H). However, the recent discovery of Su(H)-mediated transcriptional repression has forced a reconsideration of this simple model, since it makes incorrect predictions about the effect of H on a cell fate that is dependent on the repression function of Su(H). It is proposed that the genetic data on cell fate are instead consistent with a different role for H: facilitating transcriptional repression by Su(H) (Barolo, 2002b).

During the socket/shaft cell fate decision in adult mechanosensory bristle development, the cell that responds to N signaling takes the socket fate, while its sister cell, in which N signal transduction is blocked by the Numb protein, takes the shaft fate. Overexpression of Su(H), or loss of H function, during the socket/shaft decision causes both cells to adopt the socket fate; conversely, overexpression of H, or loss of Su(H) function, results in two shaft cells. Autorepression by Su(H) in shaft cells is important for maintaining the shaft cell fate. The corepressors Gro and dCtBP are important for specification of the shaft cell, a fate that is inhibited by N signaling and depends on both H activity and Su(H)-mediated repression. Reduction of gro or dCtBP function strongly enhances the effects of both reduction of H activity and loss of Su(H) repression, and suppresses the effects of reduced N signaling in the bristle lineage. It is therefore concluded that Gro and dCtBP, along with H and transcriptional repression mediated by Su(H), act in the opposite direction from the N signaling pathway during the socket/shaft cell fate decision, in that they promote the fate (shaft) that is inhibited by N signaling. The observation that both gro and dCtBP heterozygotes show a weak dominant (haploinsufficient) shaft-to-socket cell fate conversion phenotype is further confirmation of an important role for both corepressors in promoting the shaft cell fate. These results represent the first in vivo functional evidence for the involvement of Gro and dCtBP in transcriptional repression mediated by Su(H) (Barolo, 2002b).

Genetic analyses show that gro loss-of-function mutations enhance the effects of reduced H activity on two N-mediated cell fate decisions, the socket/shaft decision and the epidermal/SOP decision, while reduction of gro activity suppresses the effects of N loss of function on the socket/shaft and pIIA/pIIB cell fate decisions. In addition, gro has a weak haploinsufficient bristle loss phenotype, resembling an excess of N signaling. A role for gro in promoting the SOP cell fate is surprising, because gro was originally identified as a 'neurogenic' gene that acts to inhibit the SOP fate downstream of N signaling, in its capacity as a corepressor for bHLH transcriptional repressor proteins encoded by N target genes in the Enhancer of split gene complex [E(spl)-C]. In fact, gro was named after the phenotype of flies homozygous for gro¹, a weak hypomorphic allele: bushy tufts of bristles over the eyes caused by a failure of N-mediated lateral inhibition of the SOP fate. At least one E(spl)-C bHLH repressor gene appears to be directly repressed by Su(H) in SOPs; the proposal that Gro promotes the SOP fate by cooperating with H to repress N target genes in this cell is currently being tested. If proved, this would represent a novel and complex form of regulation, in which Gro inhibits the SOP fate in all but one cell of the proneural cluster by partnering with the E(spl)-C bHLH repressors, and simultaneously promotes the SOP fate in one neighboring cell by preventing the expression of its own partners (Barolo, 2002b).

The current results support the hypothesis that H antagonizes N signaling by acting as an adaptor molecule between the transcription factor Su(H) and the corepressor proteins Gro and dCtBP. This model entails an unusual mechanism of repression: DNA-binding transcriptional repressors that recruit CtBP or the Gro family of corepressors generally do so via direct protein-protein interactions, although evidence for CtBP recruitment by non-DNA-binding proteins has been reported. In mammalian cells, the corepressors SMRT and CIR bind directly to the Su(H) homolog CBF1 (Barolo, 2002b).

In contrast to a DNA-binding inhibition model for H function, an adaptor/corepressor model explains why H counters N^IC/Su(H)-mediated activation, but not Su(H)-mediated repression. Like previous views of H function, this model presumes competition between Su(H)-binding partners, in this case between N^IC-containing activation complexes and H/Gro/dCtBP repression complexes. N^IC activation complexes are likely to include the Mastermind (Mam) protein, and may also include the p300 coactivator. In the presence of N signaling, Su(H)/N^IC/Mam complexes presumably replace Su(H)/H/Gro/dCtBP complexes on target genes, and convert Su(H) from a repressor to an activator. Whether this occurs by simple affinity-based competition for binding to Su(H), or by a mechanism involving active impairment of the H/Su(H) interaction, is unknown. Under an adaptor/corepressor model, the H mutant phenotype results from derepression of Su(H)/N target genes in cells lacking N pathway activity, thus mimicking an increase in N signaling. The H overexpression phenotype may be explained by the displacement of N^IC-containing activation complexes by an excess of H-containing repression complexes, thus repressing N^IC/Su(H) target genes in cells that respond to the N signal (Barolo, 2002b).

It has recently become apparent that the transcriptional target genes of at least six major developmental signaling pathways are in many cases subject to 'default repression'; that is, binding sites for signal-regulated transcription factors, which mediate activation during signaling events, mediate repression in the absence of signaling (for review, see Barolo, 2002a). Each of these pathways uses a different mechanism to switch from repression to activation upon stimulation of the pathway, but in each case, the effect seems to be the same: restricting the expression of pathway target genes to cells that receive active signaling. The results of this study strongly suggest that H contributes to default repression in the N pathway by directly recruiting the corepressors Gro and dCtBP to Su(H), and that formation of H/Su(H) repression complexes is crucial for the establishment of two N-inhibited cell fates, the SOP and shaft cell fates. Default repression, therefore, appears to be as important as signal-dependent activation for proper cell fate specification in this developmental context (Barolo, 2002b).

Lateral inhibition, wherein a single cell signals to its neighbors to prevent them from adopting its own fate, is the best-known setting for cell-cell communication via the Notch (N) pathway. During peripheral neurogenesis in Drosophila, sensory organ precursor (SOP) cells arise within proneural clusters (PNCs), small groups of cells endowed with SOP fate potential by their expression of proneural transcriptional activators. SOPs use N signaling to activate in neighboring PNC cells the expression of multiple genes that inhibit the SOP fate. These genes respond transcriptionally to direct regulation by both the proneural proteins and the N pathway transcription factor Suppressor of Hairless [Su(H)], and their activation is generally highly asymmetric; i.e., only in the inhibited (non-SOP) cells of the PNC, and not in SOPs. The substantially higher proneural protein levels in the SOP put this cell at risk of inappropriately activating the SOP-inhibitory genes, even without input from N-activated Su(H). This is prevented by direct 'default' repression of these genes by Su(H), acting through the same binding sites Su(H) uses for activation in non-SOPs. Derepression of even a single N pathway target gene in the SOP can extinguish the SOP cell fate. Finally, crucial roles are defined for the adaptor protein Hairless and the co-repressors Groucho and CtBP in conferring repressive activity on Su(H) in the SOP. This work elucidates the regulatory logic by which N signaling and the proneural proteins cooperate to create the neural precursor/epidermal cell fate distinction during lateral inhibition (Castro, 2005).

Su(H) is known to act as a transcriptional repressor in another context during sensory organ development; namely, the socket/shaft sister cell fate decision in the bristle lineage. Auto-repression of Su(H) is necessary to prevent inappropriate high-level activation of the gene in the shaft cell, which in turn can cause this cell (which does not respond to N signaling) to adopt the N-responsive socket cell fate. The biochemical basis of transcriptional repression by Su(H) has been studied in some detail in this setting. Specifically, the Hairless (H) protein has been shown to act as an adaptor that recruits the transcriptional corepressor proteins Gro and CtBP to Su(H), thus conferring repressive activity (Castro, 2005).

Earlier work can be interpreted to suggest that a similar protein complex might mediate repression by Su(H) in the SOP. At several macrochaete and many microchaete positions on the adult fly, simultaneous reduction of the doses of Hairless and gro in an otherwise wild-type background leads to significant bristle loss; this is due to a failure of commitment to the SOP cell fate. A plausible interpretation of these findings is that H and Gro are normally part of a repressive Su(H)-containing complex in the SOP, and that reduction of their doses sufficiently compromises the repressive activity as to partially de-repress N pathway target genes like E(spl)m8, leading to failure of SOP specification. As a test of this model, it was thought that it might be possible to detect such de-repression of a suitable reporter gene. This expectation was borne out. Late third-instar wing discs from wild-type larvae or larvae heterozygous for null alleles of either Hairless or gro only rarely exhibit detectable activity of an E(spl)malpha-GFP reporter transgene in SOPs. By contrast, wing discs from larvae doubly heterozygous for null alleles of both Hairless and gro show substantial frequencies of ectopic GFP expression in SOPs. Moreover, the SOP expression observed in the double heterozygotes is considerably stronger than that detected rarely in a wild-type background. These results demonstrate that normal levels of Hairless and gro activity are required for the Su(H)-dependent repression of N pathway target genes in SOPs, and are consistent with the participation of a Su(H)-H-Gro-containing protein complex in this repression (Castro, 2005).

Broad overexpression of Hairless (including in proneural clusters) during lateral inhibition causes a 'neurogenic' phenotype; that is, the appearance of supernumerary bristles surrounding normal bristles. This phenotype is readily understood in light of the model described above; namely, that Hairless normally serves to recruit Gro and CtBP to Su(H) for its repressive activity in the SOP. Overexpression of Hairless in the N-responsive non-SOP cells of the PNC would be expected to elevate their levels of the repressive form of Su(H), causing repression of N pathway target genes that would normally be activated by the Su(H)-NIC-Mam complex. This in turn would result in a partial failure of lateral inhibition and the commitment of additional cells in the PNC to the SOP fate, giving rise to ectopic bristles in the adult (Castro, 2005).

A key prediction of the model is that the ability of Hairless to bind Gro (via the motif YSIHSLLG) and CtBP (via the motif PLNLSKH) should be required for the SOP fate-promoting activity of H. This prediction was tested by using an E(spl)malpha GAL4 driver to express different forms of H specifically in the non-SOP cells of the PNCs. The orbital region of the adult fly head is a particularly favorable territory in which to assay the production of supernumerary bristles by H overexpression. Expression of a wild-type UAS-Hairless transgene results in the appearance of an average of approximately four ectopic bristles in the orbital region. This activity is significantly impaired by mutating either the Gro recruitment motif (UAS-H[Gm]) or the CtBP-binding motif (UAS-H deltaC), suggesting that both co-repressors make a functional contribution. Loss of both motifs (UAS-H[Gm] deltaC) essentially abolishes the capacity of Hairless to promote ectopic bristle development in this assay. These results are strongly consistent with the interpretation that the SOP cell's requirement for Hairless activity is based on the recruitment by Hairless of Gro and CtBP to confer repressive activity on Su(H), thus preventing inappropriate expression of inhibitory N pathway target genes (Castro, 2005).

It is concluded that discrete transcriptional cis-regulatory modules, bearing binding sites for both Su(H) and the proneural proteins, direct the non-SOP-only expression pattern of E(spl)-C genes in PNCs. Mutation of the Su(H) sites in these modules results in an inversion of this pattern of activity, including both the loss of most non-SOP expression and the appearance of strong ectopic expression in SOPs. These observations reveal a dual role for Su(H) in the PNC: as a direct, N-activated transcriptional activator of E(spl)-C genes in non-SOP cells, and as a direct transcriptional repressor of the same genes in the SOP. The issue was addressed as to whether Su(H)-mediated repression of E(spl)-C genes in the SOP is important developmentally. The experiments with wild-type and Sm versions of an E(spl)m8 genomic DNA transgene demonstrate that it is. Failure to repress this single bHLH repressor gene is sufficient to extinguish the SOP fate (marked by Sens) at a frequency significantly greater than that observed with a repressible (wild-type) transgene. Evidence is provided that the Hairless protein is responsible for conferring repressive activity on Su(H) in the SOP, by recruiting the co-repressors Gro and CtBP. It is suggested that the Hairless null phenotype widespread, irreversible loss of the SOP fate in an E(spl)-C-dependent manner, offers the best indication of the developmental consequences of relieving Su(H)-mediated repression of all E(spl)-C genes in the SOP (Castro, 2005).

Notch signal transduction centers on a conserved DNA-binding protein called Suppressor of Hairless [Su(H)] in Drosophila species. In the absence of Notch activation, target genes are repressed by Su(H) acting in conjunction with a partner, Hairless, which contains binding motifs for two global corepressors, CtBP and Groucho (Gro). Usually these corepressors are thought to act via different mechanisms; complexed with other transcriptional regulators, they function independently and/or redundantly. This study investigated the requirement for Gro and CtBP in Hairless-mediated repression. Unexpectedly, it was found that mutations inactivating one or the other binding motif can have detrimental effects on Hairless similar to those of mutations that inactivate both motifs. These results argue that recruitment of one or the other corepressor is not sufficient to confer repression in the context of the Hairless-Su(H) complex; Gro and CtBP need to function in combination. In addition, this study demonstrates that Hairless has a second mode of repression that antagonizes Notch intracellular domain and is independent of Gro or CtBP binding (Nagel, 2005).

To test the repressive effects of Hairless in the absence of N^ICD, Hairless ability to inhibit transcription in the presence of Grainyhead (Grh) was tested. The Notch response (NRE) reporter contains binding sites for the transcriptional activator Grh that stimulate transcription fourfold in the absence of N^ICD and increase the stimulation seen in the presence of N^ICD. Addition of full-length Hairless inhibits these effects, reducing transcription in the presence of Grh alone by 50%. Furthermore, this inhibitory effect is dependent on Su(H), as indicated by a lack of repression of HDeltaS, and requires both CtBP and Gro, since Hairless proteins with either interaction domain mutated (HDeltaC, H*C, HDeltaG, H*G) lose most of their repressive activity. Again, the levels of activity with the single mutants are similar to the levels seen with the double-mutant forms of the protein (HDeltaGC, H*GC) and all resulted in >90% of the expression seen with Grh. These experiments suggest that Hairless has two modes of repression, one that operates by repressing the transcriptional machinery through its recruitment of global corepressors and a second that operates by directly antagonizing N^ICD (Nagel, 2005).

These data confirm therefore that both Gro and CtBP can function as corepressors with Hairless, and indeed both factors are necessary for full repression by Hairless on the NRE; preventing the interaction with one or the other factor severely compromises Hairless activity. This is in apparent contrast to the effects on vg^BE-LacZ, for which only Gro appears essential. Furthermore, the two cofactors appear to act together, since Hairless proteins lacking both interaction motifs retains a level of repression that is comparable to the results seen upon removing either alone (Nagel, 2005).

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