Background: Atg32 is a transmembrane protein essential for mitochondria autophagy in yeast. Results: Atg32 harbors a module that is crucial for interactions with Atg8 and Atg11, and can even promote other organelle autophagy. Conclusion: Atg32 acts as a direct initiator at early stages of mitochondria autophagy. Significance: This might be a common molecular feature in mitochondria autophagy conserved from yeast to humans.

Zinc is an essential nutrient for all forms of life. Within cells, most zinc is bound to protein. Because zinc serves as a catalytic or structural cofactor for many proteins, cells must maintain zinc homeostasis under severely zinc-deficient conditions. In yeast, the transcription factor Zap1 controls the expression of genes required for uptake and mobilization of zinc, but to date the fate of existing zinc-binding proteins under zinc starvation remains poorly understood. Autophagy is an evolutionarily conserved cellular degradation/recycling process in which cytoplasmic proteins and organelles are sequestered for degradation in the vacuole/lysosome. In this study, we investigated how autophagy functions under zinc starvation. Zinc depletion induced non-selective autophagy, which is important for zinclimited growth. Induction of autophagy by zinc starvation was not directly related to transcriptional activation of Zap1. Instead, TORC1 inactivation directed zinc starvation-induced autophagy. Abundant zinc proteins, such as Adh1, Fba1, and ribosomal protein Rpl37, were degraded in an autophagy-dependent manner. But the targets of autophagy were not restricted to zinc-binding proteins. When cellular zinc is severely depleted, this non-selective autophagy plays a role in releasing zinc from the degraded proteins and recycling zinc for other essential purposes.

Autophagy is a bulk degradation process conserved from yeast to mammals. To examine the roles of autophagy in cellular metabolism, we generated uophay-defective () mutants in the X2180-1B strain background. We compared the growth of wild-type (WT) and cells in minimal (synthetic dextrose, SD) and rich (yeast extract/peptone/dextrose, YEPD) medium, and we found that mutations in the core autophagy machinery result in defects in the diauxic shift, the transition from fermentation to respiratory growth upon glucose depletion, specifically in SD. Furthermore, we confirmed that autophagy was induced prior to the diauxic shift, implying that it plays a role in this process. In YEPD, mutants grew normally, so we assumed that the insufficiency of certain nutrients in SD was responsible for the defects. We ultimately identified iron, which is a necessary cofactor for respiratory activity, as the nutrient required for the diauxic shift in mutants. Indeed, mutants exhibited defects in respiration, which was rescued by supplementation with iron. Based on these data, we hypothesized that autophagy is involved in iron recycling during the diauxic shift.ΔΔ or ΔΔ cells, which are unable to export iron from the vacuole, also exhibit defects in the diauxic shift, so iron released from the vacuole is important for the shift in SD medium. Finally, we observed that ΔΔ cells accumulated nearly twice as much vacuolar iron as ΔΔΔ cells, suggesting that autophagy is involved in iron recycling by the vacuolar transport and degradation of iron-containing cargos.