The Transcriptional Coactivator SAYP Is a Trithorax Group Signature Subunit of the PBAP Chromatin Remodeling Complex

Abstract

SWI/SNF ATP-dependent chromatin remodeling complexes (remodelers) perform critical functions in eukaryotic gene expression control. BAP and PBAP are the fly representatives of the two evolutionarily conserved major subclasses of SWI/SNF remodelers. Both complexes share seven core subunits, including the Brahma ATPase, but differ in a few signature subunits; POLYBROMO and BAP170 specify PBAP, whereas OSA defines BAP. Here, we show that the transcriptional coactivator and PHD finger protein SAYP is a novel PBAP subunit. Biochemical analysis established that SAYP is tightly associated with PBAP but absent from BAP. SAYP, POLYBROMO, and BAP170 display an intimately overlapping distribution on larval salivary gland polytene chromosomes. Genome-wide expression analysis revealed that SAYP is critical for PBAP-dependent transcription. SAYP is required for normal development and interacts genetically with core-and PBAPselective subunits. Genetic analysis suggested that, like BAP, PBAP also counteracts Polycomb silencing. SAYP appears to be a key architectural component required for the integrity and association of the PBAP-specific module. We conclude that SAYP is a signature subunit that plays a major role in the functional specificity of the PBAP holoenzyme.Gene expression control is one of the most fundamental biological processes and, to a large extent, occurs at the transcriptional level. The transcription of a single protein-encoding eukaryotic gene involves a stunning plethora of regulating factors comprising some 100 or so distinct polypeptides (16,23,28). These can be classified as sequence-specific DNA-binding transcription factors that initiate the recruitment of positive or negative coregulatory complexes and the basal transcription machinery. Coactivators include a variety of proteins performing distinct functions during the transcription cycle such as the opening up of chromatin structure, mediating posttranslational histone modifications or bridging between activators and the basal transcription machinery. It has become clear that the diversity among gene-specific activators and repressors is complemented by functional specification among coregulatory complexes and even the core transcription machinery. One important class of coregulators is formed by the ATP-dependent chromatin-remodeling factors (remodelers).Remodelers are large multisubunit complexes defined by the presence of an ATPase "engine" subunit (2, 3, 21, 24, 32). These proteins act like DNA translocases and use the energy derived from ATP hydrolysis to change the DNA-histone contacts, thus remodeling chromatin structure (4, 41). Based on the identity of their central ATPase, four major classes of remodelers have been recognized: SWI/SNF, ISWI, CHD/Mi2, and Ino80/Swr1 (17). Different remodelers are not exchangeable; rather, each executes unique biological functions. An early example of functional diversification was our finding that the Drosophila SWI/SNF class Brahma (BRM) remodelers, but not the ISWI remodelers, act as chromatin-specific coac...

Transcription coactivator SAYP combines chromatin remodeler Brahma and transcription initiation factor TFIID into a single supercomplex

“…1C Lower) indicated that SAYP was associated only with the PBAP subfamily of the Brahma complex (3). This is in agreement with the results reported by (14).…”

“…3B) and found that these patterns overlap considerably. Together with the data by (14), this fact demonstrates that SAYP shares its chromosomal sites with both TFIID and Brahma.…”

“…SAYP homologs in various metazoans have an evolutionarily conserved core containing the SAY domain, which is involved in transcription activation, and 2 PHD fingers (13). Recently, SAYP was found to be associated with the chromatinremodeling Brahma complex of the PBAP subfamily (14). Here, we show that SAYP interacts both with Brahma and with TFIID, assembling them into a stable supercomplex named BTFly (Brahma and TFIID in one assembly).…”

“…(Lower) Shown are Western blot analysis of the same preparation (Ca), control IP with preimmune IgG (Cb), and nuclear extract for the presence of OSA and several components of TFIID (Cc). Chalkley et al (14) provided evidence for genetic interactions of SAYP with subunits of PBAP (but not BAP). We confirmed their findings and also revealed a decrease in the viability of flies carrying a combination of e(y)3 u1 and brm 2 (Fig.…”

Two Subunits Specific to the PBAP Chromatin Remodeling Complex Have Distinct and Redundant Functions during Drosophila Development

“…While the manuscript was in preparation, Chalkley et al (14) reported that the PBAP complex contains a third specific subunit, Supporter of activation of Yellow protein (SAYP), that is required for the stability of Bap180 and Bap170. Although homologues of this subunit have not been reported in purified vertebrate PBAF complexes, genome-wide gene expression changes in S2 cells after RNAi knockdown of SAYP, Bap170, and Bap180 were strongly correlated, suggesting that the three proteins act as a functional unit.…”