Protein Kinase C gamma Mutations Drive Spinocerebellar Ataxia Type 14 by Impairing Autoinhibition

Spinocerebellar ataxia type 14 (SCA14) is a neurodegenerative disease caused by germline mutations in the diacylglycerol (DG)/Ca2+-regulated protein kinase C gamma (PKCγ), leading to Purkinje cell degeneration and progressive cerebellar dysfunction. The majority of the approximately 50 mutations identified in PKCγ cluster to the DG-sensing C1 domains. Here, we use a FRET-based activity reporter to show that ataxia-associated PKCγ mutations enhance basal activity by compromising autoinhibition. Although impaired autoinhibition generally leads to PKC degradation, the C1 domain mutations protect PKCγ from phorbol ester-induced downregulation. Furthermore, it is the degree of disrupted autoinhibition, not changes in the amplitude of agonist-stimulated activity, that correlate with disease severity. Specifically, a SCA14 mutation in which phenylalanine 48 in the C1A domain is deleted had high basal activity both in cells and in vitro, yet was unresponsive to agonist stimulation. Validating that the pathology arises from disrupted autoinhibition, we show that the degree of impaired autoinhibition correlates inversely with age of disease onset in patients: mutations that cause high basal activity are associated with early onset, whereas those that only modestly increase basal activity, including a previously undescribed mutation D115Y, are associated with later onset. Molecular modeling indicates that almost all SCA14 mutations that are not in the C1 domains are at interfaces with the C1B domain, and bioinformatics analysis reveals that mutations in the C1B domain are under-represented in cancer. Thus, clustering of SCA14 mutations to the C1B domain provides a unique mechanism to enhance PKCγ basal activity while protecting the enzyme from downregulation.