Figure 1: The mechanistic features of yeast autophagy.

Initiation of autophagy requires the formation of the Atg1 kinase complex at the PAS to allow the recruitment and activation of other Atg proteins. In yeast there is typically one PAS per cell, but the precise nature of this site is not well defined; the PAS may literally become a phagophore, making it a dynamic structure that continuously reforms. Next, the PtdIns3K complex translocates to the PAS to begin the nucleation process that will catalyze further movement of additional Atg proteins to this site. Ubl proteins such as Atg8–PE participate in cargo recognition during selective types of autophagy, and also play a role in determining the size of the autophagosome. Membrane delivery to the phagophore allows expansion and maturation into an autophagosome. This process also employs the transmembrane protein Atg9; however, the mechanism for phagophore expansion is poorly understood. Upon autophagosome completion, Atg4 deconjugates Atg8–PE on the surface of the autophagosome and the resulting vesicle fuses with the vacuole. In yeast, the inner vesicle is released into the lumen as an autophagic body. Within the vacuole, the contents of the autophagic body are released following lysis by the putative lipase Atg15. Finally, the cargo is degraded by resident hydrolases, and the resulting macromolecules are then released back into the cytosol via permeases.