Phase separation promotes Atg8 lipidation for autophagy progression

Upon starvation, the autophagy-initiating Atg1 complex undergoes phase separation to organize the pre-autophagosomal structure (PAS) in yeast, from which autophagosome formation is considered to proceed. However, the physiological roles of the PAS as a liquid droplet remain unclear. Here we show that core Atg proteins are recruited into early PAS droplets that are formed by phase separation of the Atg1 complex with different efficiencies in vitro. The Atg12-Atg5-Atg16 E3 ligase complex for Atg8 lipidation is the most efficiently condensed in the droplets via specific Atg12-Atg17 interaction, which is also important for the PAS targeting of the E3 complex in vivo. In vitro reconstitution experiments reveal that E3-enriched early PAS droplets promote Atg8 lipidation and incorporate Atg8-coated vesicles to the interior, thereby protecting them from Atg4-mediated delipidation. These data suggest that the PAS utilizes its liquid-like property to function as an efficient production site for lipidated Atg8 and pool membrane seeds to drive autophagosome formation.