Phospholipid scramblase involved in autophagy by mediating autophagosomal membrane expansion (PubMed:22456507, PubMed:27510922, PubMed:29437695, PubMed:32513819, PubMed:33468622, PubMed:33850023, PubMed:32610138, PubMed:33106659). Cycles between the preautophagosomal structure/phagophore assembly site (PAS) and the cytoplasmic vesicle pool and supplies membrane for the growing autophagosome (PubMed:16940348, PubMed:22456507, PubMed:33106659). Lipid scramblase activity plays a key role in preautophagosomal structure/phagophore assembly by distributing the phospholipids that arrive through ATG2 (ATG2A or ATG2B) from the cytoplasmic to the luminal leaflet of the bilayer, thereby driving autophagosomal membrane expansion (PubMed:33106659). Also required to supply phosphatidylinositol 4-phosphate to the autophagosome initiation site by recruiting the phosphatidylinositol 4-kinase beta (PI4KB) in a process dependent on ARFIP2, but not ARFIP1 (PubMed:30917996). In addition to autophagy, also plays a role in necrotic cell death (By similarity). 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) Homotrimer; forms a homotrimer with a central pore that forms a path between the two membrane leaflets (PubMed:32610138, PubMed:33106659). Interacts (via cytoplasmic its C-terminus) with ATG2A (PubMed:32610138, PubMed:33106659). Interacts with SUPT20H (PubMed:19893488). Interacts (via the tyrosine-based sorting signal motif) with AP4M1; promoting association with the AP-4 complex (PubMed:29180427). Interacts with ARFIP1 and ARFIP2 (PubMed:30917996). Interacts with PI4K2A and PI4KB (PubMed:30917996). Interacts with ATG4A; the interaction is direct and promotes ATG9A trafficking (PubMed:33773106). Mainly localizes to the trans-Golgi network (TGN) and the endosomal system; cycles between them though vesicle trafficking (PubMed:27316455, PubMed:27663665). Export from the TGN to promote formation of autophagosomes is mediated by the AP-4 complex (PubMed:29180427, PubMed:30262884). Under amino acid starvation or rapamycin treatment, redistributes to preautophagosomal structure/phagophore assembly site (PAS) (PubMed:16940348). The starvation-induced redistribution depends on ULK1, ATG13, as well as SH3GLB1 (PubMed:16940348). Upon autophagy induction, a small portion transiently localizes to the autophagic membranes (PubMed:22456507). Recruited to damaged mitochondria during mitophagy in a RIMOC1-dependent manner (PubMed:34432599). Forms a homotrimer with a solvated central pore, which is connected laterally to the cytosol through the cavity within each protomer (PubMed:32610138, PubMed:33106659). 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 ATG9A-containing vesicles, thereby enabling growth into autophagosomes (PubMed:33106659). The tyrosine-based sorting signal motif, also named YXX-psi motif, promotes interaction with the AP-4 complex. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the ATG9 family. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.