Ataxin-3 with an altered conformation that exposes the polyglutamine domain is associated with the nuclear matrix

Spinocerebellar ataxia type-3 or Machado-Joseph disease (SCA3/MJD) is a mem ber of the CAG/polyglutamine repeat disease family, In this family of disor ders, a normally polymorphic CAG repeat becomes expanded, resulting in expr ession of an expanded polyglutamine domain in the disease gene product, Exp erimental models of polyglutamine disease implicate the nucleus in pathogen esis; however, the link between intranuclear expression of expanded polyglu tamine and neuronal dysfunction remains unclear. Here we demonstrate that a taxin-3, the disease protein in SCA3/MJD, adopts a unique conformation when expressed within the nucleus of transfected cells. The monoclonal antibody 1C2 is known preferentially to bind expanded polyglutamine, but we find th at it also binds a fragment of ataxin-3 containing a normal glutamine repea t, In addition, expression of ataxin-3 within the nucleus exposes the gluta mine domain of the full-length nonpathological protein, allowing it to bind the monoclonal antibody 1C2, Fractionation and immunochemical experiments indicate that this novel conformation of intranuclear ataxin-3 is not due t o proteolysis, suggesting instead that association with nuclear protein(s) alters the structure of full-length ataxin-3 which exposes the polyglutamin e domain. This conformationally altered ataxin-3 is bound to the nuclear ma trix, The pathological form of ataxin-3 with an expanded polyglutamine doma in also associates with the nuclear matrix, These data suggest that an earl y event in the pathogenesis of SCA3/MJD may be an altered conformation of a taxin-3 within the nucleus that exposes the polyglutamine domain.