α-Synuclein Protofibrils Inhibit 26 S Proteasome-mediated Protein Degradation

Abstract

The impaired ubiquitin-proteasome activity is believed to be one of the leading factors that contribute to Parkinson disease pathogenesis partially by causing α-synuclein aggregation. However, the relationship between α-synuclein aggregation and the impaired proteasome activity is yet unclear. In this study, we examined the effects of three soluble α-synuclein species (monomer, dimer, and protofibrils) on the degradation activity of the 26 S proteasome by reconstitution of proteasomal degradation using highly purified 26 S proteasomes and model substrates. We found that none of the three soluble α-synuclein species impaired the three distinct peptidase activities of the 26 S proteasome when using fluorogenic peptides as substrates. In striking contrast, α-synuclein protofibrils, but not monomer and dimer, markedly inhibited the ubiquitin-independent proteasomal degradation of unstructured proteins and ubiquitin-dependent degradation of folded proteins when present at 5-fold molar excess to the 26 S proteasome. Together these results indicate that α-synuclein protofibrils have a pronounced inhibitory effect on 26 S proteasome-mediated protein degradation. Because α-synuclein is a substrate of the proteasome, impaired proteasomal activity could further cause α-synuclein accumulation/aggregation, thus creating a vicious cycle and leading to Parkinson disease pathogenesis. Furthermore we found that α-synuclein protofibrils bound both the 26 S proteasome and substrates of the 26 S proteasome. Accordingly we propose that the inhibitory effect of α-synuclein protofibrils on 26 S proteasomal degradation might result from impairing substrate translocation by binding the proteasome or sequestrating proteasomal substrates by binding the substrates. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.