Ataxin-3 is a histone-binding protein with two independent transcriptional corepressor activities

The mechanisms of pathology for the family of polyglutamine disease proteins are unknown; however, recently it was shown that several of these proteins inhibit transcription suggesting that transcriptional repression may be a potential mechanism for pathology. In the present study we use cell transfections, in vitro binding, co-immunoprecipitations, and reporter assays to show that the polyglutamine disease protein, ataxin-3, interacts with the major histone acetyltransferases cAMP-response-element binding protein (CREB)-binding protein, p300, and p300/CREB-binding protein-associated factor and inhibits transcription by these coactivators. Importantly, endogenous ataxin-3 is co-immunoprecipitated with each of these coactivators in non-transfected cells. The C-terminal polyglutamine-containing domain of ataxin-3 inhibits coactivator-dependent transcription and is required for binding coactivators. The N-terminal domain of ataxin-3 inhibits histone acetylation by p300 in vitro and inhibits transcription in vivo. Histone binding and blocking access of coactivators to acetylation sites on histones appears to be the mechanism of inhibition. Together, our data provide a novel mechanism of transcriptional regulation by ataxin-3 that involves targeting histones, coactivators, and an independent mode of direct repression of transcription, and suggests that its physiological function and possibly pathological effects are linked to its interactions with these proteins.