Associates reversibly with the 19S proteasome and deubiquitinates proteasome substrates. Primary role in the nervous system appears to be maintenance of mono-Ub for reuse. May deubiquitinate specific substrates, including the neurodegenerative proteins tau and ataxin-3 to suppress their degradation, and the Wnt signaling regulator Dishevelled to regulate its interaction with partners. Mutations in mice cause ataxia and lead to abnormal NMJ structure and function, which is suppressed by reintroducing mono-Ub. This evidence concerns the gene MAPT and cerebellar ataxia.