Purification and characterization of mSin3A-containing Brg1 and hBrm chromatin remodeling complexes

Alteration of nucleosomes by ATP-dependent remodeling complexes represents a critical step in the regulation of transcription. The human SWI/SNF (hSWI /SNF) family is composed of complexes that contain either Brg1 or hBrm as t he central ATPase; however, these separate complexes have not been compared functionally. Here we describe the establishment of cell lines that expres s epitolpe-tagged Brg1 and hBrm and a characterization of the complexes ass ociated with these two ATPases. We show that Brg1 fractionates into two com plexes that differ in activity and subunit composition, whereas hBrm is fou nd in one complex with lower activity than the Brg1 complexes. These three complexes can remodel nucleosomal arrays, increase restriction enzyme acces sibility, and hydrolyze ATP in a DNA-dependent manner. The three complexes differ markedly in their ability to remodel mononucleosomal core particles. We also show that the hBrm complex and one of the Brg1 complexes contain c omponents of the mammalian Sin3 (mSin3) complex. In addition, we have found that Brg1, hBrm, and BAF155 can interact specifically with mSin3A in vitro , showing a direct association of hSWI/SNF complexes with proteins involved in gene repression. These unexpected functional characteristics indicate t hat these hSWI/SNF complexes play diverse regulatory roles.