Phospholipid scramblase involved in autophagy by mediating autophagosomal membrane expansion. 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 (By similarity). Required for lipopolysaccharide (LPS)-enhanced bacterial clearance through the autophagic pathway (PubMed:12626495, PubMed:22575510). 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. Expression is up-regulated during development and Legionella infection. 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. Displays severe developmental defects as well as a strong phagocytosis defect. Belongs to the ATG9 family.