Ubiquitin-binding site 2 of ataxin-3 prevents its proteasomal degradation by interacting with Rad23.

Polyglutamine repeat expansion in ataxin-3 causes neurodegeneration in the most common dominant ataxia, spinocerebellar ataxia type 3 (SCA3). Since reducing levels of disease proteins improves pathology in animals, we investigated how ataxin-3 is degraded. Here we show that, unlike most proteins, ataxin-3 turnover does not require its ubiquitination, but is regulated by ubiquitin-binding site 2 (UbS2) on its N terminus. Mutating UbS2 decreases ataxin-3 protein levels in cultured mammalian cells and in Drosophila melanogaster by increasing its proteasomal turnover. Ataxin-3 interacts with the proteasome-associated proteins Rad23A/B through UbS2. Knockdown of Rad23 in cultured cells and in Drosophila results in lower levels of ataxin-3 protein. Importantly, reducing Rad23 suppresses ataxin-3-dependent degeneration in flies. We present a mechanism for ubiquitination-independent degradation that is impeded by protein interactions with proteasome-associated factors. We conclude that UbS2 is a potential target through which to enhance ataxin-3 degradation for SCA3 therapy.