InteractiveFly: GeneBrief

SAGA factor-like TAF6: Biological Overview | References


Gene name - SAGA factor-like TAF6

Synonyms -

Cytological map position - 21E2-21E2

Function - chromatin factor

Keywords - component of histone acetyltransferase (SAGA) complex, required for SAGA coactivator function

Symbol - Saf6

FlyBase ID: FBgn0031281

Genetic map position - 2L:813,314..815,951 [-]

Classification - histone fold domain protein

Cellular location - nuclear



NCBI links: Precomputed BLAST | EntrezGene
BIOLOGICAL OVERVIEW

This paper describes SAF6, a novel histone fold domain-containing protein that replaces TAF6 in the Drosophila histone acetyltransferase (HAT) SAGA that is required for SAGA-dependent gene expression. The histone acetyltransferase complex SAGA is well characterized as a coactivator complex in yeast. In this study of Drosophila SAGA (dSAGA), three novel components are described that include an ortholog of Spt20, a potential ortholog of Sgf73/ATXN7, and a histone fold protein, SAF6 (SAGA factor-like TAF6). SAF6, which binds directly to TAF9, functions analogously in dSAGA to TAF6/TAF6L in the yeast and human SAGA complexes, respectively. Moreover, TAF6 in flies is restricted to TFIID. Mutations in saf6 disrupt SAGA-regulated gene expression without disrupting acetylated or ubiquitinated histone levels. Thus, SAF6 is essential for SAGA coactivator function independent of the enzymatic activities of the complex (Weake, 2009).

In Drosophila, the histone acetyltransferase (HAT) Gcn5 is the catalytic subunit of two separate high-molecular-weight complexes: ATAC and SAGA (Kusch, 2003; Muratoglu, 2003). Studies in Saccharomyces cerevisiae have shown that SAGA also has ubiquitin protease activity specific for monoubiquitinated histone H2B (ubH2B). In addition to its enzymatic activities, SAGA has transcription coactivator activities mediated through its interactions with transcription activators and the TATA-binding protein (TBP) (Baker, 2007). Drosophila SAGA (dSAGA) includes the orthologs of most components of the yeast SAGA (ySAGA) complex. In addition, dSAGA contains subunits that are unique to the fly complex, such as the WD repeat-containing protein WDA (Weake, 2009).

SAGA is essential for development in multicellular organisms, and Gcn5 is required for viability in both mice and Drosophila (Xu, 2000; Carre, 2005). Furthermore, mutations that disrupt the HAT activity of dSAGA, such as ada2b and wda, result in lethality in flies (Qi, 2004; Pankotai, 2005; Guelman, 2006). Moreover, mutations that specifically affect the ubiquitin protease activity of dSAGA are lethal, and result in defects in axon targeting in the larval eye-brain complex (Weake, 2008). To characterize dSAGA more fully, attempts were made to identify orthologs of all ySAGA components, as there are subunits present in the ySAGA and human SAGA complexes for which orthologs have not yet been identified in flies (Rodriguez-Navarro 2009). In addition, the apparent orthologs of some ySAGA subunits do not appear to function analogously in the Drosophila complex. Specifically, it has been unclear from previous studies whether TAF6, which is a shared subunit of both the SAGA and TFIID coactivator complexes in yeast, is in fact a subunit of dSAGA (Kusch, 2003; Guelman, 2006). In the human SAGA complexes, TAF6 is replaced by TAF6L (PAF65) (Ogryzko, 1998; Martinez, 2001; Nagy, 2009). However, the Drosophila homolog of TAF6L is encoded by a gene that is expressed exclusively in primary spermatocytes (Hiller, 2004). Thus, it is unlikely that this testis-specific TAF6L is a subunit of SAGA in the majority of Drosophila cell types (Weake, 2009).

To further characterize the subunit composition and function of dSAGA, affinity purification and MudPIT (multidimensional protein identification technology) analysis were used. These efforts revealed proteins in SAGA, including the products of genes CG17689, CG9866, and CG3883. Sequence analysis shows that CG17689 encodes an ortholog of Spt20/p38IP, and CG9866 encodes a potential ortholog of Sgf73. The third subunit, encoded by an uncharacterized gene, CG3883, is a novel histone fold domain (HFD)-containing protein that was named SAF6 (SAGA factor-like TAF6). SAF6 is homologous but not orthologous to TAF6, and can functionally substitute for TAF6 within the majority of dSAGA complexes, binding directly to the HFD of TAF9. In contrast, TAF6 in flies is restricted to TFIID. Hence, SAF6 provides a means by which the functions of the TAF octamer in SAGA can be examined, independent of TFIID. TAF function within SAGA, as examined through SAF6, is critical for Drosophila development, since saf6 mutant animals die as second instar larva. However, loss of SAF6 did not affect global levels of acetylated or ubiquitinated histones, and is therefore unlikely to affect the integrity or enzymatic activities of the dSAGA complex. Instead, SAF6 is essential for the coactivator function of SAGA in regulating gene expression, independent of the catalytic activities of the complex (Weake, 2009).

To identify candidates for dSAGA subunits, SAGA was isolated using tandem Flag-HA affinity purification from S2 cell nuclear extracts with the SAGA-specific subunits Ada2B, WDA, and Ada1, and the shared ATAC/SAGA subunit Sgf29 as bait proteins. The composition of affinity-purified SAGA was determined by MudPIT. Peptides from three novel proteins were consistently identified in affinity-purified SAGA: CG17689, CG9866, and CG3883. These three polypeptides were present at levels similar to those of the SAGA subunits Gcn5, Spt3, and TAF9, and were not found in control purifications from cells expressing a nonspecific tagged bait protein, or in samples from cells lacking a tagged protein. Moreover, none of these polypeptides were identified in purifications of the Gcn5-containing ATAC complex (Suganuma, 2008; Weake, 2009).

It was then asked whether these polypeptides had similarity to any subunits of the ySAGA complex. The uncharacterized CG17689 gene (FBgn0036374) encodes a 1873-amino-acid polypeptide with a predicted molecular mass of 201 kDa. Psi-BLAST searches revealed that CG17689 shares specific sequence similarity with SPT20 (p38IP) in mammals, which has been identified recently as a subunit of mammalian SAGA. Thus, CG17689 will be referred to as Spt20. The uncharacterized CG9866 (FBgn0031420) gene encodes a 971-amino-acid polypeptide with a predicted molecular mass of 104.2 kDa. Iterative Psi-BLAST searching identified weak similarity between the N-terminal region of CG9866 and the first nonglobular Sgf11-related domain of S. cerevisiae Sgf73 and human ATXN7. However, in contrast to Sgf73 and ATAXN7, CG9866 does not contain a recognizable SCA7 domain. Although it shares one homologous domain with Sgf73/ATXN7, it is unclear whether CG9866 also regulates the histone deubiquitination activity of SAGA-like yeast Sgf73. For this reason, CG9866 has not been named until functional similarity between this protein and Sgf73 can be established (Weake, 2009).

The third novel polypeptide identified in affinity-purified SAGA, CG3883 (FBgn0031281), has a predicted molecular mass of 79.3 kDa, and shares no discernable overall similarity with any of the known ySAGA subunits. However, closer examination of CG3883 using sequence similarity searches revealed the presence of a HFD with significant similarity to the H4-like HFDs that are confined to the N termini of TAF6 and TAF6L. However, there is no significant sequence conservation between CG3883 and TAF6 or TAF6L outside of this region. Due to the similarity between the HFDs of CG3883 and TAF6, CG3883 was named (Weake, 2009).

To confirm that SAF6 is a bona fide subunit of dSAGA, tagged SAF6 was purified from S2 cell nuclear extract by tandem Flag-HA affinity chromatography. MudPIT analysis of affinity-purified SAF6 revealed the presence of all known subunits of SAGA. In contrast, no peptides for ATAC-specific subunits were identified. It was then asked whether SAF6-purified SAGA had comparable HAT activity to SAGA purified through other SAGA-specific subunits, such as WDA (Guelman, 2006). HAT assays were performed using Gcn5-normalized SAF6- and WDA-purified SAGA complexes using HeLa core histones as substrate. SAF6-purified SAGA exhibited a similar level of HAT activity and histone preference to WDA-purified SAGA (Guelman, 2006). While these studies were conducted in S2 cells, the broad temporal expression of SAF6 suggests that it is a primary component of SAGA in flies. It is concluded from these results that SAF6 is indeed a bona fide subunit of dSAGA (Weake, 2009).

Due to the similarity between the HFDs of SAF6 and TAF6, it was asked whether SAF6 might be present instead of TAF6 within dSAGA complexes. Studies in S. cerevisiae have shown that TAF6, TAF9, TAF12, and Ada1 within ySAGA combine to form a TAF octamer complex with marked similarities to the histone octamer. Peptides corresponding to orthologs of TAF9, TAF12, and Ada1 were consistently identified with a high percent of sequence coverage in purifications of dSAGA. However, TAF6 is consistently underrepresented or absent from these same purifications. When comparing cNSAF (complex-specific spectral abundance factor) values for TAF6 and its presumed binding partner, TAF9, across five different SAGA purifications, TAF6 detection is significantly lower than TAF9. TAF6 cNSAF values are also significantly lower than the mean SAGA subunit cNSAF value (Weake, 2009).

Only one SAGA-specific subunit, WDA, consistently copurifies more than a few peptides from TAF6. The higher coverage of TAF6 peptides in WDA-purified SAGA raised the possibility that a subset of SAGA complexes might contain TAF6 in addition to, or instead of, SAF6. Thus, to determine whether TAF6 is a bona fide subunit in any fraction of dSAGA complexes, tagged TAF6 was purified from S2 cell nuclear extract by tandem Flag-HA affinity chromatography and the resulting complex was analyzed. TAF6 is a known subunit of TFIID and, as expected, high numbers of peptide spectra corresponding to the other TAF components of TFIID were identified in the TAF6 purification. However, very low levels of SAGA-specific subunits were identified in the TAF6 purification. A comparison of the dNSAF (distributive normalized SAF) values from SAGA-specific purifications with values from the TAF6 purification reveals a low level of TFIID that copurifies with SAGA. No nucleases were used in these purifications; therefore, it is possible that the limited amount of TFIID in the SAGA purifications results from co-occupancy of these coactivator complexes at some genomic loci. Alternatively, there may be a weak interaction between these complexes. Notably, TAF6 is not present at levels higher than any of the other non-SAGA TAFs in SAGA-specific purifications, indicating that its presence is likely to result from being a component of TFIID. It was then asked whether a paralog of TAF6, TAF6L, is present in dSAGA instead of TAF6, as it replaces TAF6 in the human SAGA complexes (Ogryzko, 1998; Martinez, 2001; Nagy, 2009). Drosophila TAF6L is encoded by the testis-specific gene meiosis I arrest (mia) (Hiller, 2004). Consistent with the spermatocyte-specific expression of TAF6L in flies, no peptides for TAF6L were detected in SAGA isolated from S2 cells. Thus, neither TAF6 nor TAF6L are stable subunits of the majority of dSAGA complexes in S2 cells; therefore, it is concluded that the majority of dSAGA complexes contain SAF6 instead of TAF6 (Weake, 2009).

The replacement of TAF6 by SAF6 within SAGA is likely to have functional consequences for the DNA-binding activity of SAGA, because sequence analysis indicates that the sequence conservation between SAF6 and TAF6 sequences is limited to the HFD and does not include the region of TAF6 that contains DNA-binding activity (Shao, 2005). Thus, the replacement of TAF6 by SAF6 in dSAGA is likely to convey differential specificity of DNA binding by SAGA relative to TFIID in flies. One particular aspect of this differential DNA-binding specificity might involve the downstream core promoter element (DPE), which is present at many TATA-less promoters in Drosophila and other metazoans, but not in yeast. TAF6 binds directly to the DPE (Burke, 1997). Whereas yeast promoters lack DPEs and contain TAF6 as a common subunit of both TFIID and SAGA, metazoans such as Drosophila, in which a subset of promoters contains DPE motifs, confine TAF6 to TFIID. Analysis of the promoters of SAGA-regulated genes confirms that these are less likely to contain a DPE motif compared with the rest of the genome, supporting the hypothesis that, without TAF6, dSAGA is not preferentially targeted to DPE-containing promoters (Weake, 2009).

As discussed above, TAF6 forms part of a TAF octamer complex within ySAGA that bears remarkable similarities to the histone octamer (Selleck, 2001). Moreover, the HFDs of TAF6 and TAF9, which are most similar to those of histones H4 and H3, respectively, interact via hydrophobic contacts within this octamer complex to form a heterodimer. Structural studies have shown that the TAF6/TAF9 complex exists as a heterotetramer that is similar to the (H3/H4)2 heterotetrameric core of the histone octamer (Weake, 2009 and references therein).

Alignment of TAF6 and TAF6L from multiple species with Drosophila SAF6 shows that, within the HFD of SAF6, 18 of the 28 residues involved in heterodimer contacts between TAF6 and TAF9 are conserved with TAF6. Since SAF6 replaces TAF6 within dSAGA, it was asked whether the HFD of SAF6 could interact with TAF9. The HFDs of SAF6, TAF6, and TAF9 were coexpressed in Escherichia coli as pairwise combinations of GST- and Flag-tagged proteins. Heterodimer complexes were purified by sequential glutathione-Sepharose chromatography, followed by Flag-agarose chromatography. This approach showed that GST-TAF9 copurifies with both Flag-TAF6 and Flag-SAF6. Furthermore, GST-SAF6 reciprocally copurifies with Flag-TAF9. However, the HFDs of TAF9 and SAF6 do not copurify through these chromatographic steps in controls containing only a single HFD in the presence of either Flag or GST alone. Thus, the HFD of SAF6 is sufficient to interact with the HFD of TAF9 in vitro. This suggests that SAF6 could incorporate into the TAF octamer structure within dSAGA via interactions with the HFD of TAF9 (Weake, 2009).

The interaction of SAF6 and TAF9 suggests that SAF6 might incorporate into the TAF octamer within dSAGA and thus play a TAF-mediated role within the complex. Hence, SAF6 provides a means of examining TAF function within SAGA independent of TFIID. Mutations disrupting SAF6 might therefore affect the expression of SAGA-regulated genes and/or the catalytic activities of the complex if the TAF components within SAGA are critical for these processes (Weake, 2009).

To distinguish these possibilities, attempts were made to generate a mutation in saf6 that would disrupt expression of the protein. A nonlethal P-element insertion was identified in the 5' UTR of saf6: EY05869. Imprecise excision of EY05869 generated a 303-base-pair (bp) deletion [saf6δ303] that removed 15 nucleotides (nt) of the first exon of saf6 as well as a large region of the adjacent gene, CG3639. A genomic rescue construct was generated containing the adjacent uncharacterized gene, CG3639, together with its upstream and downstream regulatory regions, including 116 nt of the first exon of saf6. This construct [CG3639+] completely rescues CG3639 expression in larvae homozygous for the saf6δ303 deletion. The saf6δ303 deletion is a likely null allele because it removes the predicted promoter region, translation initiation codon, and part of the first exon. No SAF6 transcripts are detected by RT-PCR analysis in larvae homozygous for saf6δ303. The saf6δ303; CG3639+ genotype will be referred to hereafter as saf6 (Weake, 2009).

If SAF6 is important for either of the SAGA enzymatic activities, the global levels of acetylated or ubiquitinated histones should be altered in the saf6 mutant. Histones were isolated from homozygous saf6 second instar larvae and compared with histones from OregonR second instar larvae by Western blotting. Mutant saf6 larvae show no significant increase in global levels of ubH2B, and exhibit similar levels of H3 Lys 9 acetylation (H3K9ac) both globally and at SAGA-regulated genes. This contrasts with other SAGA mutations: ada2b and wda significantly decrease H3K9ac, whereas nonstop and sgf11 mutations increase levels of ubH2B (Guelman, 2006; Weake, 2008). Furthermore, in contrast to wda embryos (Guelman, 2006), saf6 mutant animals show no decrease in H3K9ac levels, as observed by immunostaining of stage 15-16 embryos. This implies that SAF6 is not critical for catalytic activities of SAGA, and suggested that SAGA remains intact and properly targeted in the absence of SAF6. Confirming this, antibodies against Gcn5 and Ada3 coimmunoprecipitate the SAGA-specific subunit Ada2b in extracts prepared from saf6 embryos. Additionally, there is no significant loss of Gcn5 at SAGA-regulated genes in the saf6 mutant (Weake, 2009).

Studies in yeast have shown that SAGA has important functions as a coactivator that are independent of its enzymatic activity. At SAGA-regulated promoters, SAGA is required for recruitment of the general transcription machinery independent of the HAT Gcn5. Specifically, Spt3 within ySAGA interacts with TBP and is required for TBP recruitment to the promoters of some inducible genes. Furthermore, the ySAGA subunit Spt8 binds TBP directly, and activator-recruited SAGA has been implicated in transferring TBP to the TATA box. The current MudPIT data support a limited interaction between SAGA and TBP/TFIID in Drosophila (Weake, 2009).

The existence of a coactivator function of dSAGA that is independent of enzymatic activity is supported by observations that SAF6 is essential for Drosophila development. Mutant saf6 larvae die during the second larval instar stage and, in addition, no viable saf6-/- adult progeny are obtained when the saf6 allele is crossed to a deficiency spanning the SAF6 gene. In order to test for defects in SAGA coactivator function, saf6 mutants were examined for defects in SAGA-mediated gene regulation. RNA was isolated from saf6 and wda mutant stage 14-16 embryos, and the expression of SAGA-regulated genes was examined using quantitative RT-PCR. RNA was isolated from wild-type OregonR embryos for comparison. Mutant individuals of both genotypes show no apparent gross morphological defects during embryogenesis, despite the decrease in H3K9ac already observed by stage 16 of embryogenesis in wda individuals (Guelman, 2006). However, in both saf6 and wda mutants, expression of a subset of SAGA-regulated genes that include the JUN kinase basket and the Notch signaling pathway component liquid facets is down-regulated relative to the wild type. saf6 mutant animals have near wild-type levels of WDA transcripts, indicating that these defects in gene expression do not result from down-regulation of WDA. Transcripts corresponding to the gene adjacent to SAF6, CG3639, which is rescued in the saf6 mutant animals by a genomic expression construct, are similar in all three genotypes. Thus, while saf6 mutants fail to alter SAGA recruitment or histone acetylation, both globally and at SAGA-regulated genes, they show defects in SAGA-regulated gene expression similar to those observed for wda mutants. These data indicate that SAF6, and likely the TAF octamer in SAGA, primarily play a role in the coactivator functions of the SAGA complex and are not essential for the histone-modifying activities or recruitment of the complex (Weake, 2009).

Post-transcription initiation function of the ubiquitous SAGA complex in tissue-specific gene activation

The Spt-Ada-Gcn5-acetyltransferase (SAGA) complex was discovered from Saccharomyces cerevisiae and has been well characterized as an important transcriptional coactivator that interacts both with sequence-specific transcription factors and the TATA-binding protein TBP. SAGA contains a histone acetyltransferase and a ubiquitin protease. In metazoans, SAGA is essential for development, yet little is known about the function of SAGA in differentiating tissue. This study analyzed the composition, interacting proteins, and genomic distribution of SAGA in muscle and neuronal tissue of late stage Drosophila embryos. The subunit composition of SAGA was the same in each tissue; however, SAGA was associated with considerably more transcription factors in muscle compared with neurons. Consistent with this finding, SAGA was found to occupy more genes specifically in muscle than in neurons. Strikingly, SAGA occupancy was not limited to enhancers and promoters but primarily colocalized with RNA polymerase II within transcribed sequences. SAGA binding peaks at the site of RNA polymerase pausing at the 5' end of transcribed sequences. In addition, many tissue-specific SAGA-bound genes required its ubiquitin protease activity for full expression. These data indicate that in metazoans SAGA plays a prominent post-transcription initiation role in tissue-specific gene expression (Weake, 2011).

SAGA has been purified from Drosophila and mammalian cells and was found to contain homologs of most of the yeast SAGA subunits, including the Gcn5 (see Drosophila Pcaf) and Ubp8 catalytic subunits. In metazoans, SAGA may have roles in both normal development and cancer (Koutelou, 2010). Individual loss of the SAGA subunits Gcn5, Ada2b, Ada3, WDA, Sgf11, and SAF6 results in developmental defects and larval lethality in flies (Weake, 2009 and references therein). Similarly, Gcn5 deletion in mice leads to defects in mesoderm development and embryonic lethality (Xu, 2000). However, catalytic site mutations in Gcn5 survive longer but suffer neural tube closure defects and exencephaly (Bu, 2007). Furthermore, loss of the ubiquitin protease in Drosophila SAGA (Nonstop) leads to defects in photoreceptor axon targeting followed by lethality at late larval stages (Weake, 2011).

A system was designed in which SAGA could be isolated from different cell types in Drosophila embryos so that its composition and localization pattern could be determined in different tissues. To this end, the GAL4/UAS system was used to express a Flag-HA tagged version of the SAGA-specific protein Ada2b (Ada2bH1F2) in muscle or neuronal cells using the mef2-GAL4 and elav-GAL4 drivers, respectively. Expression of Ada2bH1F2 under the control of its genomic enhancer sequences rescues viability of the lethal ada2b1 allele. Whereas mef2 is expressed in committed mesoderm, the somatic and visceral musculature, and cardiac progenitors, elav is expressed prominently in neuronal cells and transiently in glial cells of the embryonic CNS. Ada2bH1F2 is expressed at levels similar to those of endogenous Ada2b using this system. To enrich for cell populations of interest that express tagged Ada2b, muscle and neuronal cells were labelled using GFP, and these cells were isolated using fluorescence-activated cell sorting (FACS). To determine whether the purified cells exhibit the characteristic gene expression profiles of each cell type, GFP-labeled neuronal and muscle cells were isolated from late stage embryos by FACS. RNA isolated from these tissues was compared with RNA extracted from whole embryos using cDNA microarrays, and genes were identified that are differentially expressed in muscle or neurons using significance analysis of microarrays. The differentially expressed genes identified using this approach were compared with ImaGO terms that describe the expression pattern of individual genes during Drosophila embryogenesis as determined by in situ hybridization. Genes identified as being expressed preferentially in muscle relative to neurons were enriched for ImaGO terms including embryonic/larval muscle system and dorsal prothoracic pharyngeal muscle. In contrast, genes identified as being expressed preferentially in neurons were enriched for ImaGO terms such as ventral nerve cord and dorsal/lateral sensory complexes. It is concluded that cells isolated using the FACS approach are enriched for the cell types of interest (Weake, 2011).

This study examined SAGA composition and localization in muscle and neuronal cells of late stage Drosophila embryos. Surprisingly, extensive colocalization of SAGA with Pol II was observed at both promoters and coding regions in muscle cells. Notably, genes at which SAGA was not detected in this assay have low levels of Pol II bound. It is suggested that SAGA might be important for recruitment and/or retention of high levels of Pol II at the promoter-proximal pause site in flies, and perhaps, therefore, more generally in higher eukaryotes. SAGA has been previously observed on the coding sequence of a small number of individual transcribed genes in yeast. Recently, low levels of Ada2b were detected on the 3' region of several different genes during larval development (Zsindely, 2009). It is noted that although some SAGA is present across the coding region of many genes, the peak of acetylated H3-Lys9 is restricted to the 5' region of the two genes that were examined: wupA and exba. A similar 5' bias of acetylated H3-Lys9 has been observed previously in genome-wide studies of histone modifications. It is speculated that the acetylation activity of SAGA in the 3' region of the gene is counteracted by histone deacetylases such as Rpd3S that have been shown to associate with the elongating form of Pol II (Weake, 2011).

SAGA localizes to different genes in muscle and neurons of late stage Drosophila embryos, and the number of genes bound by the complex in each tissue correlates with the number of transcription factors associated with the complex. These findings indicate that the differential localization of SAGA may be regulated by its association with different transcription factors in different cell types. A number of studies have found that transcription factor-binding sites tend to be clustered within the fly genome. This observed colocalization of transcription factors, together with the current data showing the association of SAGA with a large number of different transcription factors, indicates that multiple transcription factors might be involved in recruiting SAGA to its target genes (Weake, 2011).

SAGA is present at the promoter-proximal pause site together with Pol II at genes that are stalled or infrequently transcribed. The presence of SAGA together with paused Pol II is consistent with a role for SAGA in post-initiation deubiquitination of H2B, which has been shown in yeast to be important for phosphorylation of Ser-2 of the Pol II CTD and its subsequent transition into transcription elongation. In flies, phosphorylation of Ser-2 of the Pol II CTD by P-TEFb is also required for release of the paused polymerase into transcription elongation. Hence, the strong colocalization of SAGA with polymerase that has initiated transcription but is paused prior to elongation suggests a prominent function for SAGA in regulating tissue-specific gene expression at a step occurring post-initiation in metazoans. Consistent with the possibility, it was observe that the SAGA-bound genes that are most dependent on its ubiquitin protease activity for full expression are preferentially expressed in a specific tissue (Weake, 2011).

SAS-6 assembly templated by the lumen of cartwheel-less centrioles precedes centriole duplication

Centrioles are 9-fold symmetric structures duplicating once per cell cycle. Duplication involves self-oligomerization of the centriolar protein SAS-6, but how the 9-fold symmetry is invariantly established remains unclear. This study found that SAS-6 assembly can be shaped by preexisting (or mother) centrioles. During S phase, SAS-6 molecules are first recruited to the proximal lumen of the mother centriole, adopting a cartwheel-like organization through interactions with the luminal wall, rather than via their self-oligomerization activity. The removal or release of luminal SAS-6 requires Plk4 and the cartwheel protein STIL. Abolishing either the recruitment or the removal of luminal SAS-6 hinders SAS-6 (or centriole) assembly at the outside wall of mother centrioles. After duplication, the lumen of engaged mother centrioles becomes inaccessible to SAS-6, correlating with a block for reduplication. These results lead to a proposed model that centrioles may duplicate via a template-based process to preserve their geometry and copy number (Fong, 2014).


REFERENCES

Search PubMed for articles about Drosophila Saf6

Baker, S. P. and Grant, P. A. (2007). The SAGA continues: Expanding the cellular role of a transcriptional co-activator complex. Oncogene 26: 5329-5340. PubMed ID: 17694076

Bu, P., Evrard, Y. A., Lozano, G. and Dent, S. Y. (2007). Loss of Gcn5 acetyltransferase activity leads to neural tube closure defects and exencephaly in mouse embryos. Mol. Cell Biol. 27: 3405-3416. PubMed ID: 17325035

Burke, T. W. and Kadonaga, J. T. (1997). The downstream core promoter element, DPE, is conserved from Drosophila to humans and is recognized by TAFII60 of Drosophila. Genes Dev. 11(22): 3020-31. PubMed ID: 9367984

Carre, C., Szymczak, D., Pidoux, J. and Antoniewski, C. (2005). The histone H3 acetylase dGcn5 is a key player in Drosophila melanogaster metamorphosis. Mol. Cell. Biol. 25: 8228-8238. PubMed ID: 16135811

Guelman, S., et al. (2006). The essential gene wda encodes a WD40 repeat subunit of Drosophila SAGA required for histone H3 acetylation. Mol. Cell. Biol. 26: 7178-7189. PubMed ID: 16980620

Fong, C. S., Kim, M., Yang, T. T., Liao, J. C. and Tsou, M. F. (2014). SAS-6 assembly templated by the lumen of cartwheel-less centrioles precedes centriole duplication. Dev Cell 30: 238-245. PubMed ID: 25017693

Hiller, M., et al. (2004). Testis-specific TAF homologs collaborate to control a tissue-specific transcription program. Development 131: 5297-5308. PubMed ID: 15456720

Koutelou, E., Hirsch, C. L. and Dent, S. Y. (2010). Multiple faces of the SAGA complex. Curr. Opin. Cell Biol. 22: 374-382. PubMed ID: 20363118

Kusch, T., Guelman, S., Abmayr, S. M. and Workman, J. L. (2003). Two Drosophila Ada2 homologues function in different multiprotein complexes. Mol. Cell. Biol. 23: 3305-3319. PubMed ID: 12697829

Martinez, E., et al. (2001). Human STAGA complex is a chromatin-acetylating transcription coactivator that interacts with pre-mRNA splicing and DNA damage-binding factors in vivo. Mol. Cell. Biol. 21: 6782-6795. PubMed ID: 11564863

Muratoglu, S., et al. (2003). Two different Drosophila ADA2 homologues are present in distinct GCN5 histone acetyltransferase-containing complexes. Mol. Cell. Biol. 23: 306-321. PubMed ID: 12482983

Nagy Z, et al. (2009) The human SPT20-containing SAGA complex plays a direct role in the regulation of endoplasmic reticulum stress-induced genes. Mol. Cell. Biol. 29: 1649-1660. PubMed ID: 19114550

Ogryzko, V. V., et al. (1998). Histone-like TAFs within the PCAF histone acetylase complex. Cell 94: 35-44. PubMed ID: 9674425

Pankotai, T., et al. (2005). The homologous Drosophila transcriptional adaptors ADA2a and ADA2b are both required for normal development but have different functions. Mol. Cell. Biol. 25: 8215-8227. PubMed ID: 16135810

Qi, D., Larsson, J. and Mannervik, M. (2004). Drosophila Ada2b is required for viability and normal histone H3 acetylation. Mol. Cell. Biol. 24: 8080-8089. PubMed ID: 15340070

Rodriguez-Navarro, S. (2009). Insights into SAGA function during gene expression. EMBO Rep. 10: 843-850. PubMed ID: 19609321

Selleck W, et al. (2001). A histone fold TAF octamer within the yeast TFIID transcriptional coactivator. Nat. Struct. Biol. 8: 695-700. PubMed ID: 11473260

Shao H, et al. (2005). Core promoter binding by histone-like TAF complexes. Mol. Cell. Biol. 25: 206-219. PubMed ID: 15601843

Suganuma, T, et al. (2008). ATAC is a double histone acetyltransferase complex that stimulates nucleosome sliding. Nat. Struct. Mol. Biol. 15: 364-372. PubMed ID: 18327268

Weake, V. M., et al. (2008). SAGA-mediated H2B deubiquitination controls the development of neuronal connectivity in the Drosophila visual system. EMBO J. 27: 394-405. PubMed ID: 18188155

Weake, V. M., et al. (2009). A novel histone fold domain-containing protein that replaces TAF6 in Drosophila SAGA is required for SAGA-dependent gene expression. Genes Dev. 23: 2818-2823. PubMed ID: 20008933

Weake, V. M., et al. (2011). Post-transcription initiation function of the ubiquitous SAGA complex in tissue-specific gene activation. Genes Dev. 25(14): 1499-509. PubMed ID: 21764853

Xu, W., et al. (2000). Loss of Gcn5l2 leads to increased apoptosis and mesodermal defects during mouse development. Nat. Genet. 26: 229-232. PubMed ID: 11017084

Zsindely, N., et al. (2009). The loss of histone H3 lysine 9 acetylation due to dSAGA-specific dAda2b mutation influences the expression of only a small subset of genes. Nucleic Acids Res. 37: 6665-6680. PubMed ID: 19740772

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Biological Overview

date revised: 20 March 2012

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