The itinerary of a vesicle component, Aut7p/Cvt5p, terminates in the yeastvacuole via the autophagy/Cvt pathways

Aminopeptidase I (API) is delivered to the yeast vacuole by one of two alte rnative pathways, cytoplasm to vacuole targeting (Cvt) or autophagy, depend ing on nutrient conditions, Genetic, morphological, and biochemical studies indicate that the two pathways share many of the same molecular components . The Cvt pathway functions during vegetative growth, while autophagy is in duced during starvation. Both pathways involve the formation of cytosolic v esicles that fuse with the vacuole, In either case, the mechanism of vesicl e formation is not known. Autophagic uptake displays a greater capacity for cytosolic protein sequestration. This suggests the involvement of an induc ible protein(s) that allows the vesicle-forming machinery to adapt to the i ncreased degradative needs of the cell. We have analyzed the biosynthesis o f Aut7p, a protein required for both pathways. We find Aut7p expression is induced by nitrogen starvation. Aut7p is degraded by a process dependent on both proteinase A and Cvt/autophagy components. Protease accessibility ass ays demonstrate that Aut7p is located within vesicles in strains defective in vesicle delivery or breakdown. Finally, the aut7/cvt5 mutant accumulates precursor API at a stage prior to vesicle completion. These data suggest t hat Aut7p is induced during autophagy and delivered to the vacuole together with precursor API by Cvt/autophagic vesicles.