Abstract
Autophagy is a catabolic process that recycles cytoplasmic contents and is crucial for cell survival during stress. The target of rapamycin (TOR) kinase regulates autophagy as part of two distinct protein complexes, TORC1 and TORC2. TORC1 negatively regulates autophagy according to nitrogen availability. In contrast, TORC2 functions as a positive regulator of autophagy during amino acid starvation, via its target kinase Ypk1, by repressing the activity of the calcium-dependent phosphatase calcineurin and promoting the general amino acid control (GAAC) response. Precisely how TORC2-Ypk1 signaling regulates calcineurin within this pathway remains unknown. Here we demonstrate that activation of calcineurin requires Mid1, an endoplasmic reticulum-localized calcium channel regulatory protein implicated in the oxidative stress response. We find that normal mitochondrial respiration is perturbed in TORC2-Ypk1-deficient cells, which results in the accumulation of mitochondrial-derived reactive oxygen species that signal to Mid1 to activate calcineurin, thereby inhibiting the GAAC response and autophagy. These findings describe a novel pathway involving TORC2, mitochondrial oxidative stress, and calcium homeostasis for autophagy regulation.