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 (ATG2A or ATG2B) 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). 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. Under amino acid starvation or rapamycin treatment, redistributes from a juxtanuclear clustered pool to a dispersed peripheral cytosolic pool (PubMed:18936157). The starvation-induced redistribution depends on ULK1 and ATG13 (PubMed:18936157). Highly expressed in placenta (trophoblast cells) and pituitary gland. Not expressed in vascular endothelial. By hypoxia, leading to inhibit NOS3 expression. 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 ATG9B-containing vesicles, thereby enabling growth into autophagosomes. The tyrosine-based sorting signal motif, also named YXX-psi motif, promotes interaction with the AP-4 complex. ATG9B gene is located on the opposite DNA strand of the NOS3 gene at chromosome 7q36. The genes are oriented in a tail-to-tail configuration and the mRNAs encoding ATG9B and NOS3 are complementary for 662 nucleotides. ATG9B transcription may a role in NOS3 transcription regulation. Belongs to the ATG9 family.