Yeast autophagosomes: de novo formation of a membrane structure Takeshi Noda, Kuninori Suzuki, Yoshinori Ohsumi  Trends in Cell Biology  Volume 12, Issue 5, Pages 231-235 (May 2002) DOI: 10.1016/S0962-8924(02)02278-X

Fig. 1 The topology of autophagy. (a) A cup-shaped membrane sac elongates to form the autophagosome, enveloping the neighboring cytosol. The outer membrane of the autophagosome fuses with the vacuolar membrane, and the inner membrane structure – the autophagic body – is delivered into the vacuolar lumen. The autophagic body is disintegrated and the contents degraded. The multivesicular body is topologically similar to the autophagosome. (b) Depiction of conventional vesicular transport from the endoplasmic reticulum (ER) to the Golgi body. In contrast to autophagy, the topology of the organelle is maintained throughout the process. Trends in Cell Biology 2002 12, 231-235DOI: (10.1016/S0962-8924(02)02278-X)

Fig. 2 The nucleation–assembly–elongation (NAE) model. (a) When nutrients are rich and the cells grow, the nutrient-sensing protein kinase Tor is active and autophagy is not induced. Instead, Cvt vesicles are formed. This process is called nucleation. (b) When nutrients are poor and the cells are starved, Tor is inactive and autophagy is induced. In addition to the nucleation process, autophagosome formation requires the assembly of new membrane and its elongation to form larger structures. The preautophagosomal structure (PAS) is the organizing center of both structures. Trends in Cell Biology 2002 12, 231-235DOI: (10.1016/S0962-8924(02)02278-X)

Fig. 3 The lipidation cycle of an Aut7 molecule. Aut7/Apg8 is a ubiquitin-like protein without a transmembrane region. It undergoes a ubiquitination-like reaction through the serial actions of E1 (Apg7) and E2 (Apg3/Aut1) enzymes and, as a result, its C-terminus is conjugated with phosphatidylethanolamine (PE) via an amide bond. The PE-conjugated Aut7 behaves like an integral membrane protein. PE is then removed from Aut7 by the Aut2/Apg4 protease, and Aut7 reverts to the free form, which is soluble or peripherally bound to membranes. Trends in Cell Biology 2002 12, 231-235DOI: (10.1016/S0962-8924(02)02278-X)