Sumoylation inhibits alpha-synuclein aggregation and toxicity

Posttranslational modification of proteins by attachment of small ubiquitin-related modifier (SUMO) contributes to numerous cellular phenomena. Sumoylation sometimes creates and abolishes binding interfaces, but increasing evidence points to another role for sumoylation in promoting the solubility of aggregation-prone proteins. Using purified alpha-synuclein, an aggregation-prone protein implicated in Parkinson's disease that was previously reported to be sumoylated upon overexpression, we compared the aggregation kinetics of unmodified and modified alpha-synuclein. Whereas unmodified alpha-synuclein formed fibrils, modified alpha-synuclein remained soluble. The presence of as little as 10% sumoylated alpha-synuclein was sufficient to delay aggregation significantly in vitro. We mapped SUMO acceptor sites in alpha-synuclein and showed that simultaneous mutation of lysines 96 and 102 to arginine significantly impaired alpha-synuclein sumoylation in vitro and in cells. Importantly, this double mutant showed increased propensity for aggregation and cytotoxicity in a cell-based assay and increased cytotoxicity in dopaminergic neurons of the substantia nigra in vivo. These findings strongly support the model that sumoylation promotes protein solubility and suggest that defects in sumoylation may contribute to aggregation-induced diseases.