The Atg8 and Atg12 ubiquitin‐like conjugation systems in macroautophagy

Abstract

As a lysosomal/vacuolar degradative pathway that is conserved in eukaryotic organisms, autophagy mediates the turnover of long‐lived proteins and excess or aberrant organelles. The main characteristic of autophagy is the formation of a double‐membrane vesicle, the autophagosome, which envelops part of the cytoplasm and delivers it to the lysosome/vacuole for breakdown and eventual recycling of the degradation products. Among the approximately 30 autophagy‐related (Atg) genes identified so far, there are two ubiquitin‐like proteins, Atg12 and Atg8. Analogous to ubiquitination, Atg12 is conjugated to Atg5 by Atg7—an E1‐like protein—and Atg10—an E2‐like protein. Similarly, Atg7 and Atg3 are the respective E1‐like and E2‐like proteins that mediate the conjugation of Atg8 to phosphatidylethanolamine. Both Atg12–Atg5 and Atg8 localize to the developing autophagosome. The Atg12–Atg5 conjugate facilitates the lipidation of Atg8 and directs its correct subcellular localization. Atg8–phosphatidylethanolamine is probably a scaffold protein that supports membrane expansion and the amount present correlates with the size of autophagosomes.