A hallmark of most neurodegenerative diseases is the accumulation of toxic, misfolded proteins. A better understanding of how protein fate determination occurs may reveal novel therapeutic targets for degenerative brain diseases. While much has been learned about how the protein quality control system of neurons handles abnormal proteins, a major unresolved question remains: How is the determination to fold or degrade such proteins made by the chaperone and ubiquitin systems? The proposed studies will test the overall hypothesis that a specific E2 enzyme, Ube2w, regulates protein fate by rapidly monoubiquitinating misfolded proteins. Aim 1 will employ Ube2w knockout mice and a mouse model of tauopathy to investigate Ube2w's predicted neuroprotective role in vivo. Aim 2 will define the novel mechanism by which Ube2w rapidly attaches ubiquitin to substrates. Aim 3 will seek to establish a direct correlation between the unfoldedness of a substrate protein and the attachment of ubiquitin by Ube2w and other E2s. Together these aims will help elucidate the role of Ube2w in degrading proteotoxic proteins and may provide insight into its role in countering neurotoxicity.