Ald6p Is a Preferred Target for Autophagy in Yeast, Saccharomyces cerevisiae*

Macroautophagy is the process of intracellular bulk protein degradation induced by nutrient starvation and is generally considered to be a nonselective degradation of cytosolic enzymes and organelles. However, it remains a possibility that some proteins may be preferentially degraded by autophagy. In this study, we have performed a systematic analysis on the substrate selectivity of autophagy in yeast, Saccharomyces cerevisiae, using two-dimensional PAGE. We performed a differential screen on wild-type and Δatg7/apg7 autophagy-deficient cells and found that cytosolic acetaldehyde dehydrogenase (Ald6p) decreased under nitrogen starvation. As assessed by immunoblot, Ald6p was reduced by greater than 82% after 24 h of nitrogen starvation. This reduction was dependent on Atg/Apg proteins and vacuolar proteases but was not dependent on the proteasome or the cytoplasm to vacuole targetting (Cvt) pathway. Using pulse-chase and subcellular fractionation, we have also demonstrated that Ald6p was preferentially transported to vacuoles via autophagosomes. Δatg7 Δald6 double mutant cells were able to maintain higher rates of viability than Δatg7 cells under nitrogen starvation, and Ald6p-overexpressing cells were not able to maintain high rates of viability. Furthermore, the Ald6pC306S mutant, which lacks enzymatic activity, had viability rates similar to Δald6 cells. Ald6p enzymatic activity may be disadvantageous for survival under nitrogen starvation; therefore, yeast cells may preferentially eliminate Ald6p via autophagy.