BRG1/BRM-associated factor complex subunit diversity promotes temporally distinct gene expression programs in cardiogenesis
Chromatin remodeling complexes instruct cellular differentiation and lineage specific transcription. The BRG1/BRM associated factor (BAF) complexes are important for several aspects of differentiation. We show that the catalytic subunit Brg1 has a specific role in cardiac precursors (CPs) to initiate cardiac gene expression programs and repress non-cardiac expression. Using immunoprecipitation with mass spectrometry (IP-MS), we determined the dynamic composition of BAF complexes during mammalian cardiac differentiation, and identified BAF60c (SMARCD3) and BAF170 (SMARCC2) as subunits enriched in CPs and cardiomyocytes (CM). Baf60c and Baf170 co-regulate gene expression with Brg1 in CPs, but in CMs control different gene expression programs, although still promoting a cardiac-specific gene set. BRG1, BAF60, and BAF170 all modulate chromatin accessibility, to either promote accessibility at activated genes, while closing up chromatin at repressed genes. BAF60c and BAF170 are required for proper BAF complex composition and stoichiometry, and promote BRG1 occupancy in CM. Additionally, BAF170 facilitates expulsion of BRG1-containing complexes in the transition from CP to CM. Thus, dynamic interdependent BAF complex subunit assembly modulates chromatin states and thereby directs temporal gene expression programs in cardiogenesis.\n\nSignificance statementBRG1/BRM associated factors (BAF) form multi-subunit protein complexes that reorganize chromatin and regulate transcription. Specific BAF complex subunits have important roles during cell differentiation and development. We systematically identify BAF subunit composition and find temporal enrichment of subunits during cardiomyocyte differentiation. We find the catalytic subunit BRG1 has important contributions in initiating gene expression programs in cardiac progenitors along with cardiac-enriched subunits BAF60c and BAF170. Both these proteins regulated BAF subunit composition and chromatin accessibility and prevent expression of non-cardiac developmental genes during precursor to cardiomyocyte differentiation. Mechanistically, we find BAF170 destabilizes the BRG1 complex and expels BRG1 from cardiomyocyte-specific genes. Thus, our data shows synergies between diverse BAF subunits in facilitating temporal gene expression programs during cardiogenesis.