Tor-Mediated Induction of Autophagy via an Apg1 Protein Kinase Complex

TLDR

Autophagy is a nutrient‑starvation‑induced membrane trafficking pathway to the vacuole/lysosome, and in yeast the Tor kinase represses its induction through an as‑yet‑unknown mechanism. We demonstrate that starvation or rapamycin activates Apg1 kinase, while Tor‑dependent hyperphosphorylation of Apg13 reduces its binding to Apg1, and this Apg1–Apg13 interaction is essential for autophagy but not for the Cvt pathway, and distinct Apg1‑associated proteins (Apg17 and Cvt9) selectively mediate autophagy or Cvt, revealing a nutrient‑dependent switch between the two vesicular transport systems.

Abstract

Autophagy is a membrane trafficking to vacuole/lysosome induced by nutrient starvation. In Saccharomyces cerevisiae, Tor protein, a phosphatidylinositol kinase-related kinase, is involved in the repression of autophagy induction by a largely unknown mechanism. Here, we show that the protein kinase activity of Apg1 is enhanced by starvation or rapamycin treatment. In addition, we have also found that Apg13, which binds to and activates Apg1, is hyperphosphorylated in a Tor-dependent manner, reducing its affinity to Apg1. This Apg1–Apg13 association is required for autophagy, but not for the cytoplasm-to-vacuole targeting (Cvt) pathway, another vesicular transport mechanism in which factors essential for autophagy (Apg proteins) are also employed under vegetative growth conditions. Finally, other Apg1-associating proteins, such as Apg17 and Cvt9, are shown to function specifically in autophagy or the Cvt pathway, respectively, suggesting that the Apg1 complex plays an important role in switching between two distinct vesicular transport systems in a nutrient-dependent manner.