The Function and Trafficking of Atg8 during Autophagosome Formation.

Abstract

Eukaryotic cells rely on autophagy to remove excess or damaged organelles and proteins. In this pathway, cytoplasmic materials are delivered to the lysosomes via double-membrane vesicles, the autophagosomes. The formation of autophagosomes, which involves the expansion and deformation of the precursor membrane sac, the phagophore, is catalyzed by the core autophagy machinery proteins at the phagophore assembly site (PAS). Previous studies have gradually discovered the order of assembly of the core autophagy machinery proteins at the PAS. In contrast, we know little about what these proteins do after PAS assembly. In this study, I first focused on how Atg8, one of the core machinery proteins, functions in autophagosome formation and demonstrated that (1) the amount of Atg8 at the PAS controls the size of autophagosomes produced and that (2) each round of autophagosome formation involves the recruitment of Atg8 to the phagophore and the subsequent deconjugation and release of Atg8 from this site. By tracing the trafficking of Atg8 in live cells, I established a temporal dissection of the autophagosome formation process. This allowed the examination of events at late stages of autophagosome formation and led to the further discovery that defects in Atg8 release not only arrest the existing autophagosome formation processes, but also prevent the regeneration of the PAS, which is necessary for sustained autophagosome formation. In addition, the data suggest that the release of Atg8 happens after the departure of Atg9 from the PAS, and that deconjugation of Atg8 is important in maintaining its correct localization. Furthermore, I developed two statistical methods for calculating the sizes of intracellular vesicles from sizes of their sections obtained through transmission electron microscopy. The methods were used to estimate the size of autophagic bodies, which is used in turn to estimate the area density of Atg8 molecules on the phagophore.