Macroautophagy is mediated by a unique organelle, the autophagosome, which encloses a portion of cytoplasm for delivery to the lysosome. Autophagosome formation is dynamically regulated by starvation and other stresses and involves complicated membrane reorganization. Since the discovery of yeast Atg-related proteins, autophagosome formation has been dissected at the molecular level. In this review we describe the molecular mechanism of autophagosome formation with particular focus on the function of Atg proteins and the long-standing discussion regarding the origin of the autophagosome membrane.

During the process of autophagy, cytoplasmic materials are sequestered by double-membrane structures, the autophagosomes, and then transported to a lytic compartment to be degraded. One of the most fundamental questions about autophagy involves the origin of the autophagosomal membranes. In this study, we focus on the intracellular dynamics of Atg9, a multispanning membrane protein essential for autophagosome formation in yeast. We found that the vast majority of Atg9 existed on cytoplasmic mobile vesicles (designated Atg9 vesicles) that were derived from the Golgi apparatus in a process involving Atg23 and Atg27. We also found that only a few Atg9 vesicles were required for a single round of autophagosome formation. During starvation, several Atg9 vesicles assembled individually into the preautophagosomal structure, and eventually, they are incorporated into the autophagosomal outer membrane. Our findings provide conclusive linkage between the cytoplasmic Atg9 vesicles and autophagosomal membranes and offer new insight into the requirement for Atg9 vesicles at the early step of autophagosome formation.

Autophagy is a bulk proteolytic process that is indispensable for cell survival during starvation. Autophagy is induced by nutrient deprivation via inactivation of the rapamycin-sensitive Tor complex1 (TORC1), a protein kinase complex regulating cell growth in response to nutrient conditions. However, the mechanism by which TORC1 controls autophagy and the direct target of TORC1 activity remain unclear. Atg13 is an essential regulatory component of autophagy upstream of the Atg1 kinase complex, and here we show that yeast TORC1 directly phosphorylates Atg13 at multiple Ser residues. Additionally, expression of an unphosphorylatable Atg13 mutant bypasses the TORC1 pathway to induce autophagy through activation of Atg1 in cells growing under nutrient-rich conditions. Our findings suggest that the direct control of the Atg1 complex by TORC1 induces autophagy.