BAF60a Mediates Critical Interactions between Nuclear Receptors and the BRG1 Chromatin-Remodeling Complex for Transactivation

Abstract

Nuclear hormone receptors are ligand-dependent transcriptional regulators that modulate chromatin structure. However, the precise molecular mechanisms by which receptors recruit chromatin-remodeling activity are not fully elucidated. We show that in the absence of its ligand-binding domain, the glucocorticoid receptor (GR) is able to interact with both nuclear receptor coactivators and the BRG1 chromatin-remodeling complex in vivo. Individually, the GR makes direct interactions with BRG1-associated factor 60a (BAF60a) and BAF57, but not with BRG1, BAF155, or BAF170. Further, BAF60a possesses at least two interaction surfaces, one for GR and BRG1 and a second for BAF155 and BAF170. A GR mutant, GR(R488Q), that fails to interact with BAF60a in vitro has reduced chromatin-remodeling activity and reduced transcriptional activity from the promoter assembled as chromatin in vivo. Stable expression of a BAF60a truncation mutant, BAF60a4-140, caused chromatin-specific loss of GR functions in vivo. In the presence of the BAF60a mutant, the GR fails to interact with the BRG1 complex and consequently is also deficient in its ability to activate transcription from chromatin. Thus, in addition to previously identified BAF250, BAF60a may provide another critical and direct link between nuclear receptors and the BRG1 complex that is required for promoter recruitment and subsequent chromatin remodeling.Eukaryotic genes are highly organized into chromatin, which may restrict the access of regulatory factors to promoter control sequences (4,20,37,(46)(47)(48). The repetitive basic unit of chromatin is the nucleosome, which is an octamer of core histones 2A, 2B, 3, and 4. This histone octamer is wrapped by approximately 147 bp of DNA. In addition, the linker histone H1, which includes multiple subtypes in mammals, binds to the DNA between two adjacent nucleosomes, called linker DNA (16). The C-terminal histone fold domain of each core histone forms the structural body around which the DNA is wrapped (3,25). In contrast, the histone N-terminal tails, which are rich in basic residues, are thought to extend outward and interact with adjacent nucleosomes. This interaction allows nucleosomal arrays to self-associate into a higher-order chromatin fiber (26).Nuclear hormone receptors (NHRs) represent a superfamily of transcription factors with conserved structural and functional domains (28). As a member of the nuclear receptor superfamily, the domain structure of the glucocorticoid receptor (GR), which includes a DNA binding domain (DBD), a ligand binding domain (LBD), and a hypervariable N-terminal domain, has been extensively investigated (28). NHRs are potent transactivators with two transcriptional activation functions (AFs), namely, the ligand-independent AF-1 within the N-terminal domain and the ligand-dependent AF-2 centered at helix 12 of the LBD (18). The LBD has proven to be a particularly promiscuous interacting surface for a variety of coactivator and corepressor complexes that are critical for NHR transcriptional function (...

The SWI/SNF complex and cancer

“…Each has one or more alternative transcripts that add to the complexity of the SWI/SNF complex. BAF60A has been linked to lung cancer risk (Gorlov, 2005), binds to p53 and is necessary for steroid receptor function (Hsiao et al, 2003). Baf60C is expressed specifically in the heart and somites in the early mouse embryo (Lickert et al, 2004).…”

Chromatin Remodeling Complexes Interact Dynamically with a Glucocorticoid Receptor–regulated Promoter

“…work has shown that GR makes direct contacts not necessarily with BRG1 but with the BRG1-or BRM-associated factors, BAF 57 and BAF60a (Hsiao et al, 2003).…”

“…To establish the role of the SWI/SNF complex in the transcriptional regulation of the MMTV-LTR, we carried out transcription reporter assays in the human adrenal carcinoma cell line (SW13). The SW13 cell line expresses all the BAFs and the SWI/SNF complex can be reconstituted by transfecting BRG1 or BRM into this cell line (Muchardt and Yaniv, 2001;Hsiao et al, 2003). This model system has allowed us to assess the contribution of each chromatin remodeling protein to the hormone-dependent regulation of MMTV transcription.…”

Nuclear receptors and chromatin remodeling machinery

“…The interaction between GR and BRG1 appears to be mediated through BAF250 and/or BAF60a of SWI/SNF. Glucocorticoid receptor binding of BAF60a takes place in a ligand independent manner, but is required for efficient GR-mediated transcriptional initiation and remodeling of the MMTV promoter in vivo (Hsiao et al, 2003). By contrast, the interaction between GR and BAF250 seems to be dependent on the presence of ligand .…”

“…Recruitment of the SWI/SNF complex is not limited to the type I NRs. The remodeling complex has also been found to associate with transcription activation of RAR/ RXR and VDR/RXR indicating involvement of SWI/SNF in a wide range of NR-mediated events (Hsiao et al, 2003;Koszewski et al, 2003).…”