Two central issues in polyglutamine-induced neurodegeneration are the influence of the normal function of the disease protein and modulation by protein quality control pathways. By using Drosophila, we now directly link host protein function and disease pathogenesis to ubiquitin pathways in the polyglutamine disease spinocerebellar ataxia type 3 (SCA3). Normal human ataxin-3--a polyubiquitin binding protein with ubiquitin protease activity--is a striking suppressor of polyglutamine neurodegeneration in vivo. This suppressor activity requires ubiquitin-associated activities of the protein and is dependent upon proteasome function. Our results highlight the critical importance of host protein function in SCA3 disease and a potential therapeutic role of ataxin-3 activity for polyglutamine disorders.