Auxillary subunit of N-terminal acetyltransferase complexes which display alpha (N-terminal) acetyltransferase (NAT) activity. The NAT activity may be important for vascular, hematopoietic and neuronal growth and development. Required to control retinal neovascularization in adult ocular endothelial cells. In complex with XRCC6 and XRCC5 (Ku80), up-regulates transcription from the osteocalcin promoter. Component of the N-terminal acetyltransferase A complex (also called the NatA complex) composed of NAA10 and NAA15 (PubMed:12888564). Within the complex interacts with NAA10 (PubMed:12888564). Component of the N-terminal acetyltransferase A (NatA)/HYPK complex at least composed of NAA10, NAA15 and HYPK, which has N-terminal acetyltransferase activity (By similarity). In complex with NAA10, interacts with HYPK (By similarity). Component of the N-terminal acetyltransferase E (NatE) complex at least composed of NAA10, NAA15 and NAA50 (By similarity). Within the complex interacts with NAA10; the interaction is required for binding to NAA50 (By similarity). Interacts with NAAT50 (By similarity). The interaction of the NatA complex with NAA50 reduces the acetylation activity of the NatA complex (By similarity). Component of the N-terminal acetyltransferase E (NatE)/HYPK complex at least composed of NAA10, NAA15, NAA50 and HYPK (By similarity). In complex with NAA10 interacts with HYPK; the interaction with HYPK reduces the capacity of the NatA complex to interact with NAA50 (By similarity). Interacts with NAA11 (By similarity). Interacts with XRCC6 and XRCC5 (PubMed:12145306). Mainly cytoplasmic, nuclear in some cases. Present in the free cytosolic and cytoskeleton-bound polysomes, but not in the membrane-bound polysomes. Endothelial cells, osteoblasts and myeloid cells of the hematopoietic tissue. Present in adult ovary, bone marrow, brain, heart, kidney, testis and osteoblasts. Highly expressed in endothelial cells during embryonic vasculogenesis, and then down-regulated and restricted to specific endothelial cells. In the brain, expression is highest in regions that contain dividing and proliferating cells. As brain development progresses, expression restricts to the hippocampus and cerebellar cortex. Regulated by NMDA receptor. Acetylated. Cleaved by caspases during apoptosis. Contaminating sequence.