Most regulated gene expression in eukaryotes is the result of transcriptional regulation. Current models of transcriptional regulation invoke the targeted recruitment of enzymes that make covalent changes in histone and/or DNA structure, in ATPases that remodel histone-DNA interactions, and, in cases of gene activation, in RNA polymerase itself. A major challenge is to identify the diverse roles of chromatin modifiers in transcriptional regulation.
rotein) shares homology with members of the SNF2/SWI2 class of chromatin-remodeling enzymes (20
). SRCAP was originally identified in a yeast two-hybrid protein screen for proteins that interact with the histone acetyltransferase CBP (C
rotein). Subsequent studies using transiently transfected reporters have implicated SRCAP as a coactivator for CREB- and nuclear hormone receptor-mediated transcriptional activation (30
). The mechanism by which SRCAP activates transcription has not been completely elucidated. Results from a number of studies, including mammalian two-hybrid and coimmunoprecipitation experiments, indicate SRCAP may function in part by recruitment of CBP or other coactivators, such as the arginine methyltransferase CARM-1 and the nuclear hormone receptor coactivator GRIP-1 (20
). A role for the conserved ATPase/helicase homology domain in regulation of transcription has not been established, since this activity is dispensable for SRCAP activity in transient transfection assays, nor have the natural chromosomal promoters that utilize SRCAP to regulate transcription been identified.
In mammals, the protein most homologous to SRCAP is p400. p400 was identified through its interactions with the adenoviral protein E1A and appears to play a critical role in E1A-mediated transformation (16
). A role of p400 in the activation of transcription mediated by myeloid zinc finger protein (MZF-2A) has also been reported (34
). Both SRCAP and p400 have been isolated as parts of large multiprotein complexes implicated in the remodeling of chromatin, including the DMAP1 complex (11
), the TRRAP/TIP60 histone acetylase complex (4
), and the NCoR-1 histone deacetylase complex (46
). The yeast SWR1 complex (29
), which catalyzes histone variant exchange in vitro, contains several subunits homologous to subunits of a recently identified SRCAP complex (5
SRCAP-related proteins have been described for humans (20
), rats (accession number XP_341933
), fish (accession number CAG00637
), Drosophila melanogaster
), worms (8
), the protozoan Toxoplasma gondii
), and yeast (21
), suggesting that SRCAP represents an ancient member of the conserved SWI2/SNF2 family of ATPase/helicase proteins. In Drosophila
, the gene domino
encodes the sole homolog of SRCAP and p400, which is termed DOM. Similar to SRCAP and p400, DOM can be isolated as part of a TRRAP/TIP60 histone acetylase complex (26
is essential for organismal viability and has been implicated in several aspects of fly development, including hematopoeisis, wing development, and female fertility (39
). Clones of domino
mutant cells cannot be recovered in the adult wing, suggesting that domino
is essential for cell viability. Genetic interactions with mutations in several Polycomb
group genes suggest that domino
may play a role in homeotic gene silencing. domino
alleles also suppress heterochromatic position effect variegation, suggesting a role in heterochromatin-mediated silencing as well.
Here we identify SRCAP as a homolog of DOM and provide genetic evidence that SRCAP is at least partially functionally equivalent to DOM. We show that human SRCAP can complement the recessive female sterility of hypomorphic domino alleles and that this rescue depends on an intact ATPase homology domain. We find that SRCAP binds to chromosomes, colocalizes extensively with DOM, and is recruited to some sites of active transcription, such as steroid (ecdysone)-regulated loci, but not to activated heat shock loci. We show that SRCAP can recruit Drosophila CBP to ectopic chromosomal sites, providing the first in vivo evidence to suggest that SRCAP and CBP interact directly or indirectly on chromosomes. We also show that domino is required for proper Notch pathway activity, and that wild-type SRCAP—but not a SRCAP ATPase domain mutant—can substitute for DOM in Notch-dependent wing development. Thus, SRCAP is implicated in Notch signaling. We test this implication using a Notch-dependent reporter in mammalian cells and show that SRCAP is a potentiator of Notch-dependent gene activation. Taken together, these data implicate SRCAP and its Drosophila ortholog, DOM, in developmental gene activation pathways.