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. 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. 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. Atg1 phosphorylation is required for preautophagosome elongation. Belongs to the ATG9 family.