Sumoylation Protects Against β-Synuclein Toxicity in Yeast.

Aggregation of alpha - synuclein (alpha Syn) plays a central role in the pathogenesis of Parkinson's disease (PD). The budding yeast Saccharomyces cerevisiae serves as reference cell to study the interplay between alpha Syn misfolding, cytotoxicity and post-translational modifications (PTMs). The synuclein family includes alpha, beta and gamma isoforms. beta-synuclein (beta Syn) and alpha Syn are found at presynaptic terminals and both proteins are presumably involved in disease pathogenesis. Similar to alpha Syn, expression of beta Syn leads to growth deficiency and formation of intracellular aggregates in yeast. Co-expression of alpha Syn and beta Syn exacerbates the cytotoxicity. This suggests an important role of beta Syn homeostasis in PD pathology. We show here that the small ubiquitin-like modifier SUMO is an important determinant of protein stability and beta Syn-induced toxicity in eukaryotic cells. Downregulation of sumoylation in a yeast strain, defective for the SUMO-encoding gene resulted in reduced yeast growth, whereas upregulation of sumoylation rescued growth of yeast cell expressing beta Syn. This corroborates a protective role of the cellular sumoylation machinery against beta Syn-induced toxicity. Upregulation of sumoylation significantly reduced beta Syn aggregate formation. This is an indirect molecular process, which is not directly linked to beta Syn sumoylation because amino acid substitutions in the lysine residues required for beta Syn sumoylation decreased aggregation without changing yeast cellular toxicity. alpha Syn aggregates are more predominantly degraded by the autophagy/vacuole than by the 26S ubiquitin proteasome system. We demonstrate a vice versa situation for beta Syn, which is mainly degraded in the 26S proteasome. Downregulation of sumoylation significantly compromised the clearance of beta Syn by the 26S proteasome and increased protein stability. This effect is specific, because depletion of functional SUMO did neither affect beta Syn aggregate formation nor its degradation by the autophagy/vacuolar pathway. Our data support that cellular beta Syn toxicity and aggregation do not correlate in their cellular impact as for alpha Syn but rather represent two distinct independent molecular functions and molecular mechanisms. These insights into the relationship between beta Syn-induced toxicity, aggregate formation and degradation demonstrate a significant distinction between the impact of alpha Syn compared to beta Syn on eukaryotic cells.