Sumoylation Protects Against ß-Synuclein Toxicity in Yeast.

ABSTRACT

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