Figure 1.

Morphology of macroautophagy and location/functions of UBL Atg proteins. The key morphological intermediates of macroautophagy are shown for mammals (a) and yeast (b). In either case, the process begins with the nucleation of the phagophore, the initial sequestering compartment (for simplicity, an omegasome is not depicted). This step and the subsequent expansion of the phagophore require the function of Atg8/LC3. Upon completion the phagophore becomes a double-membrane autophagosome. In mammals the autophagosome may fuse with a late endosome to form an amphisome, or may fuse directly with the lysosome to generate an autolysosome. In yeast, the autophagosome fuses with the vacuole, releasing the inner vesicle that is termed an autophagic body. The inner vesicle of the autophagosome is lysed and the cargoes are degraded, with the breakdown products being released into the cytosol for reuse. Atg8/LC3-II is present on both sides of the phagophore, and on the early autophagosome, but is released from the outer membrane by Atg4/ATG4B-dependent deconjugation during autophagosome maturation. Regulatory kinases including protein kinase A and TOR inhibit Atg1/ULK1 and Atg1/ATG13 during normal growth. See the text for details on the functions of the proteins involved in the UBL-protein conjugation systems.