PolyQ-expanded ataxin-3 binds with higher affinity to beclin 1 and deubiquitinates it less, resulting in increased proteasomal degradation of beclin 1. PolyQ-expanded ataxin-3 increases K48-linked ubiquitination of beclin 1. Reduced beclin 1 abundance has been reported in rodent MJD models and MJD patient brains. Here, BECN1 is linked to Spinocerebellar ataxia type 3.