nejire: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

Gene name - nejire

Synonyms - CREB-binding protein - CBP

Cytological map position - 8F--9A

Function - transcriptional coactivator

Keywords - chromatin modification, histone acetyltransferase

Symbol - nej

FlyBase ID: FBgn0261617

Genetic map position -

Classification - CREB-binding protein

Cellular location - nuclear



NCBI links: Precomputed BLAST | Entrez Gene | UniGene |
BIOLOGICAL OVERVIEW

CREB-binding protein or CBP (in Drosophila, the protein termed Nejire) is a transcriptional coactivator that interacts with a large number of developmentally important transcription factors. CBP and p300 are highly related proteins: mammalian CBP was originally identified by its interaction with CREB (cAMP response-element-binding protein) and p300 was originally identified as a target of the adenoviral E1A oncoprotein. CBP is recruited to DNA by several transcription factors, including CREB (Drosophila homolog: CrebB-17A) and cFos (Drosophila homolog: Fos-related antigen/Kayak). Transcriptional coactivators are considered to be accessory proteins that interact with transcription factors and are required for proper transcription factor function.

Since inactivation of only a single copy of CBP causes severe developmental defects in Rubenstein-Taybi syndrome (Petrij, 1995), it has been proposed that CBP is a limiting integrator of multiple signal transduction pathways at the level of gene activation. Recently, CBP/p300 were reported to have intrinsic histone acetyltransferase (HAT) activity (Ogryzko, 1996 and Bannister, 1996). The identification in Tetrahymena of the first transcription-associated HAT has allowed a major advance in understanding the connection between histone acetylation and gene transcription. HAT has been found to share significant homology to the Saccharomyces cerevisiae adapter GCN5 (Brownell, 1996). GCN5 acetylates specific lysines in a pattern associated with transcriptionally active chromatin, and HAT activity is required for the activation of GCN5-responsive genes. These observations establish the causal link between histone acetyltransferase activity and activation of transcription. HAT activity displayed by multiple families of proteins is associated with gene activation through the modification of chromatin structure. PCAF, a mammalian homolog of GCN5, and CBP are found in a complex with a third family of HATs, the hormone receptor coactivators SRC and ACTR (Chen, 1997 and Spencer, 1997). Importantly, the in vitro substrate specificity of each of these HATs is distinct, suggesting that multiple HATs can act in concert at or near single promoters.

Drosophila CBP was isolated by screening Drosophila libraries under low-stringency hybridization conditions with a C. elegans CBP probe. The hemizygous nej mutant embryos die at stage 9 or 10 during embryogenesis, although some embryos survive to hatching. The most severe phenotype in nef hemizygotes is the twisting of the embryo that occurs at germband elongation.

Drosophila CBP is a coactivator of Cubitus interruptus in Hedgehog signaling. Ci is a known target of the Hh signal transduction pathway; in turn, Ci targets wingless. The expression of wingless is strikingly reduced at the posterior margin of each parasegment in CBP mutants. In addition, engrailed expression, which is maintained by Wg protein, is significantly lower in such mutants than in wild type. These observations suggest the Drosophila CBP might contribute to the functioning of some transcription factors involved in the activation of the Wingless-Engrailed signaling pathway. Ci protein physically interacts with Drosophila CBP. A series of deletion mutants of ci indicates that a region of Ci between amino acids 1020 and 1160 is required for a phosphorylation independent interaction with Drosophila CBP. This region is part of the Ci transactivation domain, C-terminal to five putative Protein kinase A (PKA) sites. Drosophila CBP expression augments transactivation by CI up to a maximum of 62 fold. The dominant gain-of-function ciD mutant phenotype, in which the longitudinal vein 4 of the adult wing is shortened, some posterior row hairs are missing, and the posterior wing margin is flattened, can be explained by the inappropriate expression of ci in the posterior compartment of the wing imaginal disc, where it is usually repressed by Engrailed. A subset of the ciD wing defects is suppressed by the haploinsufficiency of Drosophila CBP. Thus Drosophila CBP is required for the activation of Cubitus interruptus target genes such as patched, and CBP is required for the activator function of Ci but not for the repressor function. Drosophila CBP binds to dCREB2, the Drosophila homolog of CREB, in a phosphorylation-dependent manner, whereas the CBP-Ci interaction is phosphorylation-independent (Akimaru, 1997a).

These observations have been used to explain how two pathways can interfere with each other. PKA phosphorylates dCREB2 and this signaling function appears to repress Hh target genes such as dpp. Since Drosophila CBP binds to dCREB2 in a phosphorylation dependent manner, a limited amount of Drosophila CBP might be recruited to PKA-phosphorylated dCREB2, resulting in a decrease in Ci activity due to a diminished availability of Drosophila CBP. Therefore, a limited amount of Drosophila CBP might be recruited to PKA-phosphorylated dCREB2, resulting in a decrease in Ci activity, explaining the antagonistic actions of PKA and Hedgehog (Akimaru, 1997a).

The role of Drosophila CBP in the regulation of Hh target genes fails to explain the primary developmental effect of Drosophila CBP mutation observed at germband elongation: the clockwise or counter-clockwise twisting of the embryo, just behind the cephalic furrow, often with the posterior side down. The ventral and cephalic furrows appear normal, but the mesodermally derived internal tissues and a block of ectodermal cells are often missing. On the basis of this phenotype, the Drosophila CBP mutant was named nejire, which means 'twist' in Japanese. Mesoderm formation is crucial event that takes place during early embryogenesis. To initiate the differentiation of the mesoderm in Drosophila, multiple zygotic genes such as twist (twi) and snail (sna), which encode a basic-helix-loop-helix and a zinc finger transcription factor, respectively, are required. The transcription of these genes is induced by maternal Dorsal protein, a transcription factor involved in establishment of dorsal-ventral polarity, that is homologous to the NF-kappa B family of proteins. Drosophila CBP mutants, devoid of both maternal and zygotic nejire expression, fail to express twi and therefore generate twisted embryos. This is explained by results showing that dCBP is necessary for Dorsal-mediated activation of the twi promoter. In vitro Dorsal has been shown to bind the N-proximal portion of Drosophila CBP in a phosphorylation independent manner. A region of Dorsal lying between amino acids 186 and 356 (a part of the Rel homology domain) is required for interaction with CBP. Further studies indicated that a CBP-Dorsal complex is formed on the twist promoter (Akimaru, 1997b).

CBP not only exerts an effect on DNA transcription by acting as a histone transacetylase, but it can also acetylate transcription factors. In addition CBP can exert an inhibitory effect on transcription. The interaction of Drosophila CBP with Pangolin demonstrates both these points. T-cell factor (TCF), a high-mobility-group domain protein, is the transcription factor activated by Wnt/Wingless signaling. When signaling occurs, TCF binds to its coactivator, beta-catenin/Armadillo, and stimulates the transcription of the target genes of Wnt/Wingless by binding to TCF-responsive enhancers. Inappropriate activation of TCF in the colon epithelium and other cells leads to cancer. It is therefore desirable for unstimulated cells to have a negative control mechanism to keep TCF inactive. Drosophila CREB-binding protein binds to Drosophila TCF (Pangolin). dCBP mutants show mild Wingless overactivation phenotypes in various tissues. Consistent with this, Drosophila CBP loss-of-function suppresses the effects of armadillo mutation. Moreover, Drosophila CBP is shown to acetylate a conserved lysine in the Armadillo-binding domain of Pangolin, and this acetylation lowers the affinity of Armadillo binding to Pangolin. Although CBP is a coactivator of other transcription factors, these data show that CBP represses TCF. Indeed, point mutations have been found in the HAT and CBP2 domains of the remaining p300 gene in colon and gastric carcinomas that have lost one copy of this gene, indicating that p300/CBP has a tumor-suppressor function (Muraoka, 1996). This is consistent with data showing the Drosophila CBP represses Pangolin to antagonize Armadillo-mediated activation of Wingless target genes (Waltzer, 1998).


PROTEIN STRUCTURE

Amino Acids - 3190

Structural Domains

The deduced amino-acid sequence of Drosophila CBP predicts a protein of 332K relative molecular mass, which includes all the motifs found in the mammalian CBP/p300 homologs (Akimaru, 1997a).


nejire: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 20 January 99

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