In yeast, 15 Apg proteins coordinate the formation of autophagosomes. Autophagy is a bulk degradation process induced by starvation in eukaryotic cells [1,2]. Apg9 plays a direct role in the formation of the cytoplasm to vacuole targeting and autophagic vesicles, and it is the sole transmembrane protein in the autophagosome-forming machinery. It colocalises with Atg2 at the expanding edge of the isolation membrane where Atg2 receives phospholipids from the endoplasmic reticulum (ER). Atg9 is a lipid scramblase that translocates phospholipids between outer and inner leaflets of liposomes [2,3]. Phospholipids delivered by Atg2 are translocated from the cytoplasmic to the luminal leaflet by Atg9, driving autophagosomal membrane expansion [3].

1: Apg9p/Cvt7p is an integral membrane protein required for transport vesicle formation in the Cvt and autophagy pathways. Noda T, Kim J, Huang WP, Baba M, Tokunaga C, Ohsumi Y, Klionsky DJ; J Cell Biol 2000;148:465-480. PMID:10662773

2: The pre-autophagosomal structure organized by concerted functions of APG genes is essential for autophagosome formation. Suzuki K, Kirisako T, Kamada Y, Mizushima N, Noda T, Ohsumi Y; EMBO J 2001;20:5971-5981. PMID:11689437

3: Atg9 is a lipid scramblase that mediates autophagosomal membrane expansion. Matoba K, Kotani T, Tsutsumi A, Tsuji T, Mori T, Noshiro D, Sugita Y, Nomura N, Iwata S, Ohsumi Y, Fujimoto T, Nakatogawa H, Kikkawa M, Noda NN; Nat Struct Mol Biol. 2020; [Epub ahead of print] PMID:33106658