Reconstitution of a core chromatin remodeling complex from SWI/SNF subunits

Protein complexes of the SWI/SNF family remodel nucleosome structure in an ATP-dependent manner. Each complex contains between 8 and 15 subunits, several of which are highly conserved between yeast, Drosophila, and humans. We have reconstituted an ATP-dependent chromatin remodeling complex using a subset of conserved subunits. Unexpectedly, both BRG1 and hBRM, the ATPase subunits of human SWI/SNF complexes, are capable of remodeling mono-nucleosomes and nucleosomal arrays as purified proteins. The addition of INI1, BAF155, and BAF170 to BRG1 increases remodeling activity to a level comparable to that of the whole hSWI/SNF complex. These data define the functional core of the hSWI/SNF complex.     

Swapping function of two chromatin remodeling complexes

SWI/SNF- and ISWI-based complexes have distinct yet overlapping chromatin-remodeling activities in vitro and perform different roles in vivo. This leads to the hypothesis that the distinct remodeling functions of these complexes are specifically required for distinct biological tasks. By creating and characterizing chimeric proteins of BRG1 and SNF2h, the motor proteins of human SWI/SNF- and ISWI-based complexes, respectively, we found that a region that includes the ATPase domain specifies the outcome of the remodeling reaction in vitro. A chimeric protein based on BRG1 but containing the SNF2h ATPase domain formed an intact SWI/SNF complex that remodeled like SNF2h. This altered-function complex was active for remodeling and could stimulate expression from some, but not all, SWI/SNF responsive promoters in vivo. Thus, we were able to separate domains of BRG1 responsible for function from those responsible for SWI/SNF complex formation and demonstrate that remodeling functions are not interchangeable in vivo.