Phospholipid scramblase involved in autophagy and cytoplasm to vacuole transport (Cvt) vesicle formation. Cycles between the preautophagosomal structure/phagophore assembly site (PAS) and the cytoplasmic vesicle pool and supplies membrane for the growing autophagosome. 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. Required for mitophagy. Also involved in endoplasmic reticulum-specific autophagic process and is essential for the survival of cells subjected to severe ER stress. Different machineries are required for anterograde trafficking to the PAS during either the Cvt pathway or bulk autophagy and for retrograde trafficking. Recruits vesicle-tethering proteins TRS85 and YPT1 to the autophagosome formation site. Recruits also ATG23 and ATG8 to the PAS. 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. Interacts with ATG23 and ATG27 to form a cycling complex for trafficking to the PAS. Interacts (via N-terminus) with ATG11, required for recruitment of ATG9 to the PAS for the Cvt pathway during nutrient-rich conditions. Interacts (via N-terminus) with ATG17; required for recruitment to the PAS during autophagy and starved conditions. Interacts with ATG2 and ATG18; required for the retrieval of ATG9 from the PAS to the cytoplasmic pool. Interacts with ATG41. Interacts with the conserved oligomeric Golgi (COG) complex subunits COG3 and COG4. Interacts with TRS85. 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. The peripheral pool of ATG9 partially colocalizes within tubulovesicular clusters adjacent to mitochondria. It appears that membrane that contains ATG9 is delivered to the autophagosome from the Golgi-endosomal system rather than from the ER or mitochondria. Within the PAS, localizes at the edge of the isolation membrane. ATG9 cycling is regulated by at least the conserved oligomeric Golgi (COG) complex, ARP2, HOG1, PIK1, SEC2, SEC9, SSO1 and SSO2. Forms a homotrimer with a solvated central pore, which is connected laterally to the cytosol through the cavity within each protomer. 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. Phosphorylated by ATG1; phosphorylation is required for autophagy and cytoplasm to vacuole transport (Cvt) vesicle formation. Phosphorylation by ATG1 regulates ATG18 interaction and preautophagosome elongation. Phosphorylation at Ser-122 is required for selective autophagy by regulating anterograde trafficking and interaction with ATG23 and ATG27. Phosphorylation at Ser-122 prevents ubiquitination by the SCF(MET30) complex. Ubiquitinated by the SCF(MET30) complex in normal conditions, leading to its degradation by the proteasome, thereby preventing inappropriate induction of autophagy. Ubiquitination by the SCF(MET30) complex is prevented by phosphorylation at Ser-122. Belongs to the ATG9 family.