Evaluation of Stp2p-dependent alpha-Synuclein Toxicity in Yeast: Role of GAPDH?

Parkinson's disease (PD) is linked to alpha-synuclein misfolding and aggregation in the substantia nigra par compacta cells of the brain. Discovering and characterizing factors that regulate alpha-synuclein misfolding and toxicity have high therapeutic potential. A recent study (Willingham et al., 2003) demonstrated that over-expression of wild type (WT) alpha-synuclein in an STP2 deletion (stp2[delta]) S. cerevisiae strain is toxic to the yeast. We tested the hypothesis that this toxicity involves glyceraldehyde 3-phosphodehydrogenase (Gapdhp), a known anti-apoptotic protein. Surprisingly, we did not observe toxicity in stp2[delta] strains over-expressing WT alpha-synuclein or its familial mutants (A30P and A53T). Moreover, lack of Stp2p did not alter alpha-synuclein distribution in living yeast cells and no increase in cytoplasmic aggregation was seen. Neither the levels of alpha-synuclein nor Gapdhp changed in stp2[delta] cells. Furthermore, contrary to our prediction, genetic deletion of GAPDH3 (gapdh3[delta]) did not enhance growth in strains that over-expressed alpha-synuclein. We propose that the lack of alpha-synuclein toxicity in stp2[delta] may be due to our use of a different protein expression system: pYES2-based expression may not have produced enough alpha-synuclein to overwhelm protein quality-control systems and yield the toxicity that underlies PD pathogenesis.