Degradation of a-Synuclein by Proteasome*

Mutations in a-synuclein are known to be associated with Parkinson’s disease (PD). The coexistence of this neuronal protein with ubiquitin and proteasome subunits in Lewy bodies in sporadic disease suggests that alterations of a-synuclein catabolism may contribute to the pathogenesis of PD. The degradation pathway of a-synuclein has not been identified nor has the kinetics of this process been described. We investigated the degradation kinetics of both wild-type and A53T mutant 6XHis-tagged a-synuclein in transiently transfected SHSY5Y cells. Degradation of both isoforms followed firstorder kinetics over 24 h as monitored by the pulse-chase method. However, the t1⁄2 of mutant a-synuclein was 50% longer than that of the wild-type protein (p < 0.01). The degradation of both recombinant proteins and endogenous a-synuclein in these cells was blocked by the selective proteasome inhibitor b-lactone (40 mM), indicating that both wild-type and A53T mutant a-synuclein are degraded by the ubiquitin-proteasome pathway. The slower degradation of mutant a-synuclein provides a kinetic basis for its intracellular accumulation, thus favoring its aggregation.