Phospholipid scramblase involved in autophagy by mediating autophagosomal membrane expansion (PubMed:12114572, PubMed:24805779). 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). In addition to autophagy, also plays a role in necrotic cell death (By similarity). Plays an essential role in plant nutrient recycling (PubMed:12114572). Homotrimer; forms a homotrimer with a central pore that forms a path between the two membrane leaflets. Expressed in roots, leaves, stems and flowers. 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. Mutant plants are hypersensitive to nitrogen or carbon starvation and show early bolting senescence. Belongs to the ATG9 family. Low resolution structures by electron microscopy suggested the presence of six transmembrane regions (PubMed:31276439). However, high resolution structures in human and S.pombe showed that it is composed of four transmembrane and two intramembrane regions (By similarity). Truncated N-terminus.