Mammalian heterochromatin protein 1 (HP1) alpha, HP1beta and HP1gamma are closely related non-histone chromosomal proteins that function in gene silencing, presumably by organizing higher order chromatin structures. Here, we show by co-immunoprecipitation that HP1alpha, but neither HP1beta nor HP1gamma, forms a complex with the BRG1 chromatin-remodeling factor in HeLa cells. In vitro, BRG1 interacts directly and preferentially with HP1alpha. The region conferring this preferential binding has been mapped to residues 106-180 of the HP1alpha C-terminal chromoshadow domain. Using site-directed mutagenesis, we have identified three amino acid residues I113, A114 and C133 in HP1alpha (K, P and S in HP1beta and HP1gamma) that are essential for the selective interaction of HP1alpha with BRG1. Interestingly, these residues were also shown to be critical for the silencing activity of HP1alpha. Taken together, these results demonstrate that mammalian HP1 proteins are biochemically distinct and suggest an entirely novel function for BRG1 in modulating HP1alpha-containing heterochromatic structures.