A Cycling Protein Complex Required for Selective Autophagy

Survival of environmental stress conditions requires the maintenance of cellular homeostasis. To preserve this balance, cells utilize a degradative mechanism known as autophagy. During this process, in response to starvation or other stresses, bulk cytoplasm is non-specifically sequestered within double-membrane vesicles and delivered to the lysosome/vacuole for subsequent degradation and recycling. The cytoplasm to vacuole targeting (Cvt) pathway is a type of specific autophagy, which occurs constitutively during growing conditions. Here, we examine three autophagy-related (Atg) proteins, Atg9, Atg23 and Atg27, which exhibit a unique localization pattern, residing both at the pre-autophagosomal structure (PAS) and other peripheral sites. These proteins colocalize, interact with one another in vivo, and form a functional complex. Furthermore, all three proteins cycle between the PAS and the other sites, and depend upon one another for this movement. Our data suggest that Atg9, Atg23 and Atg27 play a role in Atg protein retrieval from the PAS. In addition, Atg9 and Atg27 are the only known integral membrane Atg proteins involved in vesicle formation; a better understanding of their function may offer insight into the mechanism of membrane delivery to the PAS, the site of double-membrane vesicle assembly.