Location and membrane sources for autophagosome formation - from ER-mitochondria contact sites to Golgi-endosome-derived carriers.

Recent advances have revealed much about the signaling events and molecular components associated with autophagy. Although it is clear that there are multiple points of intersection and connection between autophagy and known vesicular membrane transport processes between membrane compartments, autophagy is modulated by a distinct set of molecular components, and the autophagosome has a unique membrane composition. A key question that has yet to be resolved with regards to autophagosome formation is its membrane source. Various evidences have indicated that membranes from the endoplasmic reticulum (ER), mitochondria, Golgi, endosomes and the plasma membrane could all potentially act as a source of autophagosomal membrane in non-specialized macroautophagy. Recent investigations have generated advances in terms of the mitochondria's involvement in starvation-induced autophagy, and refined localization of autophagosome formation to ER-mitochondria contact sites. On the other hand, data accumulates on the delivery of membrane sources to the pre-autophagosome structure by Atg9-containing mobile carriers, which likely originated from the Golgi-endosome system. The answer to the question on the origin of membrane materials for autophagosome biogenesis awaits further reconciliation of these different findings.