α-synuclein inhibits Snx3-retromer retrograde trafficking of the conserved membrane-bound proprotein convertase Kex2 in the secretory pathway of Saccharomyces cerevisiae.

We tested the ability of alpha-synuclein (α-syn) to inhibit Snx3-retromer-mediated retrograde trafficking of Kex2 and Ste13 between late endosomes and the trans-Golgi network (TGN) using a Saccharomyces cerevisiae model of Parkinson's disease. Kex2 and Ste13 are a conserved, membrane-bound proprotein convertase and dipeptidyl aminopeptidase, respectively, that process pro-α-factor and pro-killer toxin. Each of these proteins contains a cytosolic tail that binds to sorting nexin Snx3. Using a combination of techniques, including fluorescence microscopy, western blotting and a yeast mating assay, we found that α-syn disrupts Snx3-retromer trafficking of Kex2-GFP and GFP-Ste13 from the late endosome to the TGN, resulting in these two proteins transiting to the vacuole by default. Using three α-syn variants (A53T, A30P, and α-synΔC, which lacks residues 101-140), we further found that A53T and α-synΔC, but not A30P, reduce Snx3-retromer trafficking of Kex2-GFP, which is likely to be due to weaker binding of A30P to membranes. Degradation of Kex2 and Ste13 in the vacuole should result in the secretion of unprocessed, inactive forms of α-factor, which will reduce mating efficiency between MATa and MATα cells. We found that wild-type α-syn but not A30P significantly inhibited the secretion of α-factor. Collectively, our results support a model in which the membrane-binding ability of α-syn is necessary to disrupt Snx3-retromer retrograde recycling of these two conserved endopeptidases.