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A unifying feature of many neurodegenerative disorders is the accumulation of polyubiquitinated protein inclusions in dystrophic neurons, e.g. containing α-synuclein, which is suggestive of an insufficient proteasomal activity. We demonstrate that α-synuclein and 20 S proteasome components co-localize in Lewy bodies and show that subunits from 20 S proteasome particles, in contrast to subunits of the 19 S regulatory complex, bind efficiently to aggregated filamentous but not monomeric α-synuclein. Proteasome binding to insoluble α-synuclein filaments and soluble α-synuclein oligomers results in marked inhibition of its chymotrypsin-like hydrolytic activity through a non-competitive mechanism that is mimicked by model amyloid-Aβ peptide aggregates. Endogenous ligands of aggregated α-synuclein like heat shock protein 70 and glyceraldehyde-6-phosphate dehydrogenase bind filaments and inhibit their anti …