The Drosophila trithorax group proteins BRM, ASH1 and ASH2 are subunits ofdistinct protein complexes

The trithorax group gene brahma (brm) encodes an activator of Drosophila ho meotic genes that functions as the ATPase subunit of a large protein comple x. To determine if BRM physically interacts with other trithorax group prot eins, we purified the BRM complex from Drosophila embryos and analyzed its subunit composition. The BRM complex contains at least seven major polypept ides. Surprisingly, the majority of the subunits of the BRM complex are not encoded by trithorax group genes. Furthermore, a screen for enhancers of a dominant-negative brm mutation identified only one trithorax group gene, m oira (mor), that appears to be essential for brm function in vivo. Four of the subunits of the BRM complex are related to subunits of the yeast chroma tin remodeling complexes SWI/SNF and RSC, The BRM complex is even more high ly related to the human BRG1 and hBRM complexes, but lacks the subunit hete rogeneity characteristic of these complexes. We present biochemical evidenc e for the existence of two additional complexes containing trithorax group proteins: a 2 MDa ASH1 complex and a 500 kDa ASH2 complex. These findings s uggest that BRM plays a role in chromatin remodeling that is distinct from the function of most other trithorax group proteins.