Interaction of INI1 /hSNF5, a component of the SWI /SNF chromatin remodeling complex with the gene-specific activator, c -MYC
Cheng, Se-Wing Grace
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Chromatin organization plays a key role in the regulation of gene expression. Transcription can be regulated by packing DNA into chromatin, thereby preventing the binding of regulatory proteins and impeding elongation by RNA Polymerase II. The evolutionarily conserved SWI/SNF complex is one of several multi-protein complexes that mediate chromatin remodeling in an ATP-dependent manner. In yeast, the SWI/SNF and RSC complexes have been shown to be important for the transactivation of a set of inducible genes and for viability, respectively. BRG1 and hBRM (hSWI2/SNF2), ATPase components of mammalian SWI/SNF complexes, are implicated in hormone receptor activation and in cell cycle arrest through retinoblastoma protein. The yeast SNF5 homologue, INI1/hSNF5 is a core component of the mammalian SWI/SNF complex, along with BRG1, BAF170 and BAF155, and has been demonstrated to be a tumor suppressor.;To determine the role of INI1/hSNF5 in cellular functions, and to identify potential sequence-specific activators that require the SWI/SNF complex for activation, we used the yeast two-hybrid system to isolate proteins that interact with INI1/hSNF5. One of the several interacting clones obtained was c-MYC, a sequence-specific activator involved in cellular proliferation and apoptosis. We found that the interaction between c-Myc and INI1/hSNF5 is direct and is observed both in vivo and in vitro. This interaction requires the basic helix-loop-helix leucine zipper region of c-MYC and the Repeat 1 region of INI1. Based on transient reporter assays, c-MYC-mediated transactivation was inhibited by dominant negative mutants of INI1/hSNF5 and BRG1/hSNF2 which in turn, was reversed by the over-expression of the respective wild-type proteins. INI1/hSNF5 does not interact with Max in the two-hybrid system and does not appear to form an INI1/c-MYC/MAX ternary complex in vivo. Chromatin immunoprecipitation of a transiently transfected E-box-driven reporter plasmid in HeLa-TetON cells, show that c-MYC, Max, INI1/hSNF5 and BRG1 can associate with E-box. Our studies suggest that direct interaction of INI1/hSNF5 with c-MYC mediates the recruitment of the SWI/SNF complex to the c-MYC responsive promoters. We propose that SWI/SNF-dependent chromatin remodeling is required for activation of a subset of c-MYC target genes.