An Ikaros-containing chromatin-remodeling complex in adult-type erythroid cells

We have previously described a SWI/SNF-related protein complex (PYR complex ) that is restricted to definitive (adult-type) hematopoietic cells and tha t specifically binds DNA sequences containing long stretches of pyrimidines . Deletion of an intergenic DNA-binding site for this complex from a human beta-globin locus construct results in delayed human gamma- to beta-globin switching in transgenic mice, suggesting that the PYR complex acts to facil itate the switch. We now show that PYR complex DNA-binding activity also co purifies with subunits of a second type of chromatin-remodeling complex, nu cleosome-remodeling deacetylase (NuRD), that has been shown to have both nu cleosome-remodeling and histone deacetylase activities. Gel supershift assa ys using antibodies to the ATPase-helicase subunit of the NuRD complex, Mi- 2 (CHD4), confirm that Mi-2 is a component of the PYR complex. In addition, we show that the hematopoietic cell-restricted zinc finger protein Ikaros copurifies with PYR complex DNA-binding activity and that antibodies to Ika ros also supershift the complex. We also show that NuRD and SWI/SNF compone nts coimmunopurify with each other as well as with Ikaros. Competition gel shift experiments using partially purified PYR complex and recombinant Ikar os protein indicate that Ikaros functions as a DNA-binding subunit of the P YR complex. Our results suggest that Ikaros targets two types of chromatin- remodeling factors-activators (SWI/SNF) and repressors (NuRD)-in a single c omplex (PYR complex) to the beta-globin locus in adult erythroid cells. At the time of the switch from fetal to adult globin production, the PYR compl ex is assembled and may function to repress gamma-globin gene expression an d facilitate gamma- to beta-globin switching.