Ubiquitination of a new form of alpha-synuclein by parkin from human brain: Implications for Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characteriz ed by the progressive accumulation in selected neurons of protein inclusion s containing alpha -synuclein and ubiquitin. Rare inherited forms of PD are caused by autosomal dominant mutations in alpha -synuclein or by autosomal recessive mutations in parkin, an E3 ubiquitin Ligase, We hypothesized tha t these two gene products interact functionally, namely, that parkin ubiqui tinates alpha -synuclein normally and that this process is altered in autos omal recessive PD. We have now identified a protein complex in normal human brain that includes parkin as the E3 ubiquitin Ligase, UbcH7 as its associ ated E2 ubiquitin conjugating enzyme, and a new 22-kilodalton glycosylated form of alpha -synuclein (alpha Sp22) as its substrate. In contrast to norm al parkin, mutant parkin associated with autosomal recessive PD failed to b ind alpha Sp22. In an in vitro ubiquitination assay, alpha Sp22 Was modifie d by normal but not mutant parkin into polyubiquitinated, high molecular we ight species. Accordingly, alpha Sp22 accumulated in a nonubiquitinated for m in parkin-deficient PD brains. We conclude that alpha Sp22 is a substrate for parkin's ubiquitin Ligase activity in normal human brain and that Loss of parkin function causes pathological alpha Sp22 accumulation. These find ings demonstrate a critical biochemical reaction between the two PD-Linked gene products and suggest that this reaction underlies the accumulation of ubiquitinated alpha -synuclein in conventional PD.