Phospholipid scramblase involved in autophagy and cytoplasm to vacuole transport (Cvt) vesicle formation (PubMed:7593182, PubMed:8224160, PubMed:8663607, PubMed:10735854, PubMed:14504273, PubMed:24440502, PubMed:27050455, PubMed:33106658, PubMed:33439214). Cycles between the preautophagosomal structure/phagophore assembly site (PAS) and the cytoplasmic vesicle pool and supplies membrane for the growing autophagosome (PubMed:10662773, PubMed:11689437, PubMed:17329962, PubMed:17426440, PubMed:18829864, PubMed:22826123, PubMed:24905091, PubMed:33439214). Lipid scramblase activity plays a key role in preautophagosomal structure/phagophore assembly by distributing the phospholipids that arrive through ATG2 from the cytoplasmic to the luminal leaflet of the bilayer, thereby driving autophagosomal membrane expansion (PubMed:33106658, PubMed:32883836, PubMed:33439214). Required for mitophagy (PubMed:18818209). Also involved in endoplasmic reticulum-specific autophagic process and is essential for the survival of cells subjected to severe ER stress (PubMed:17132049). Recruits vesicle-tethering proteins TRS85 and YPT1 to the autophagosome formation site (PubMed:23129774). Recruits also ATG23 and ATG8 to the PAS (PubMed:14504273). a 1,2-diacyl-sn-glycero-3-phosphocholine(in) = a 1,2-diacyl-sn-glycero-3-phosphocholine(out) a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) a 1,2-diacyl-sn-glycero-3-phosphoethanolamine(in) = a 1,2-diacyl-sn-glycero-3-phosphoethanolamine(out) a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate)(in) = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate)(out) Homotrimer; forms a homotrimer with a central pore that forms a path between the two membrane leaflets (PubMed:33106658). Interacts with ATG23 and ATG27 to form a cycling complex for trafficking to the PAS (PubMed:14504273, PubMed:17178909, PubMed:27050455). Interacts (via N-terminus) with ATG11, required for recruitment of ATG9 to the PAS for the Cvt pathway during nutrient-rich conditions (PubMed:17178909, PubMed:17192412, PubMed:19371383, PubMed:31356628). Interacts (via N-terminus) with ATG17; required for recruitment to the PAS during autophagy and starved conditions (PubMed:19371383). Interacts with ATG2 and ATG18; required for the retrieval of ATG9 from the PAS to the cytoplasmic pool (PubMed:11382760, PubMed:14723849, PubMed:24905091, PubMed:24440502). Interacts with ATG41 (PubMed:26565778). Interacts with the conserved oligomeric Golgi (COG) complex subunits COG3 and COG4 (PubMed:20065092). Interacts with TRS85 (PubMed:23129774). The vast majority of ATG9 exists on cytoplasmic mobile vesicles (designated ATG9 vesicles) that were derived from the Golgi apparatus in a process involving ATG23 and ATG27 (PubMed:22826123, PubMed:22509044). The peripheral pool of ATG9 partially colocalizes within tubulovesicular clusters adjacent to mitochondria (PubMed:20855505, PubMed:26565778). It appears that membrane that contains ATG9 is delivered to the autophagosome from the Golgi-endosomal system rather than from the ER or mitochondria (PubMed:20444982, PubMed:20861302). Within the PAS, localizes at the edge of the isolation membrane (PubMed:22144692, PubMed:23549786). ATG9 cycling is regulated by at least the conserved oligomeric Golgi (COG) complex, ARP2, HOG1, PIK1, SEC2, SEC9, SSO1 and SSO2 (PubMed:18287533, PubMed:20444978, PubMed:21784249, PubMed:21576396, PubMed:22977244). Forms a homotrimer with a solvated central pore, which is connected laterally to the cytosol through the cavity within each protomer (By similarity). Acts as a lipid scramblase that uses its central pore to function: the central pore opens laterally to accommodate lipid headgroups, thereby enabling lipid flipping and redistribution of lipids added to the outer leaflet of ATG9-containing vesicles, thereby enabling growth into autophagosomes (By similarity). Phosphorylated by ATG1; phosphorylation is required for autophagy and cytoplasm to vacuole transport (Cvt) vesicle formation (PubMed:24440502). Phosphorylation by ATG1 regulates ATG18 interaction and preautophagosome elongation (PubMed:24905091). Phosphorylation at Ser-122 is required for selective autophagy by regulating anterograde trafficking and interaction with ATG23 and ATG27 (PubMed:27050455, PubMed:33443148). Phosphorylation at Ser-122 prevents ubiquitination by the SCF(MET30) complex (PubMed:33443148). Ubiquitinated by the SCF(MET30) complex in normal conditions, leading to its degradation by the proteasome, thereby preventing inappropriate induction of autophagy (PubMed:33443148). Ubiquitination by the SCF(MET30) complex is prevented by phosphorylation at Ser-122 (PubMed:33443148). Belongs to the ATG9 family.