14-3-3 Binding to Ataxin-1(ATXN1) Regulates Its Dephosphorylation at Ser-776 and Transport to the Nucleus

Spinocerebellar ataxia type 1 (SCA1) is a lethal neurodegenerative disorder caused by expansion of a polyglutamine tract in ATXN1. A prominent site of pathology in SCA1 is cerebellar Purkinje neurons where mutant ATXN1 must enter the nucleus to cause disease. In SCA1, phosphorylation of ATXN1 at Ser-776 modulates disease. Interestingly, Ser-776 is located within a region of ATXN1 that harbors several functional motifs including binding sites for 14-3-3, and splicing factors RBM17 and U2AF65. The interaction of ATXN1 with these proteins is thought to be regulated by the phosphorylation status of Ser-776. In addition, Ser-776 is adjacent to the NLS in ATXN1. Although pS776-ATXN1 is enriched in nuclear extracts of cerebellar cells, the vast majority of 14-3-3 is in the cytoplasmic fraction. We found that dephosphorylation of cytoplasmic pS776-ATXN1 is blocked by virtue of it being in a complex with 14-3-3. In addition, data suggest that binding of 14-3-3 to cytoplasmic ATXN1 impeded its transport to the nucleus, suggesting that 14-3-3 must disassociate from ATXN1 for transport of ATXN1 to the nucleus. Consistent with this hypothesis is the observation that once in the nucleus pS776 is able to be dephosphorylated. Evidence is presented that PP2A is the pS776-ATXN1 phosphatase in the mammalian cerebellum. In the nucleus, we propose that dephosphorylation of pS776-ATXN1 by PP2A regulates the interaction of ATXN1 with the splicing factors RBM17 and U2AF65.