title: alpha-Synuclein protofibrils inhibit 26 S proteasome-mediated protein degradation: understanding the cytotoxicity of protein protofibrils in neurodegenerative disease pathogenesis.
abstract: The impaired ubiquitin-proteasome activity is believed to be one of the leading factors that contribute to Parkinson disease pathogenesis partially by causing alpha-synuclein aggregation. However, the relationship between alpha-synuclein aggregation and the impaired proteasome activity is yet unclear. In this study, we examined the effects of three soluble alpha-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 alpha-synuclein species impaired the three distinct peptidase activities of the 26 S proteasome when using fluorogenic peptides as substrates. In striking contrast, alpha-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 alpha-synuclein protofibrils have a pronounced inhibitory effect on 26 S proteasome-mediated protein degradation. Because alpha-synuclein is a substrate of the proteasome, impaired proteasomal activity could further cause alpha-synuclein accumulation/aggregation, thus creating a vicious cycle and leading to Parkinson disease pathogenesis. Furthermore we found that alpha-synuclein protofibrils bound both the 26 S proteasome and substrates of the 26 S proteasome. Accordingly we propose that the inhibitory effect of alpha-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.
authors.affiliation: Department of Biochemistry and Molecular Genetics, School of Medicine, University of Colorado Denver, Aurora, CO 80045, USA.
authors.forename: Nan-Yan
authors.initials: NY
authors.lastname: Zhang
authors.forename: Zhiyong
authors.initials: Z
authors.lastname: Tang
authors.forename: Chang-Wei
authors.initials: CW
authors.lastname: Liu
mesh_terms: D000818:Animals; D002417:Cattle; D019636:Neurodegenerative Diseases / Q000378:metabolism* / Q000473:pathology; D046988:Proteasome Endopeptidase Complex / Q000302:isolation & purification / Q000378:metabolism*; D061988:Proteasome Inhibitors; D011485:Protein Binding; D017510:Protein Folding; D012995:Solubility; D025801:Ubiquitin / Q000378:metabolism; D051844:alpha-Synuclein / Q000302:isolation & purification / Q000378:metabolism* / Q000494:pharmacology*
chemical_list: D061988:Proteasome Inhibitors; D025801:Ubiquitin; D051844:alpha-Synuclein; D046988:Proteasome Endopeptidase Complex