Accessory subunit of the proton-transporting vacuolar (V)-ATPase protein pump, which is required for luminal acidification of secretory vesicles (PubMed:18713856). Guides the V-type ATPase into specialized subcellular compartments, such as neuroendocrine regulated secretory vesicles or the ruffled border of the osteoclast, thereby regulating its activity. Involved in membrane trafficking and Ca(2+)-dependent membrane fusion. May play a role in the assembly of the V-type ATPase complex. In aerobic conditions, involved in intracellular iron homeostasis, thus triggering the activity of Fe(2+) prolyl hydroxylase (PHD) enzymes, and leading to HIF1A hydroxylation and subsequent proteasomal degradation (By similarity). In islets of Langerhans cells, may regulate the acidification of dense-core secretory granules (PubMed:18713856). Accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump. Interacts (via N-terminus) with ATP6AP2 (via N-terminus). Interacts with RNASEK (By similarity). Not detected in trans-Golgi network. Expressed in brain cortex (at protein level) (PubMed:27231034). Highly expressed in islets of Langerhans (PubMed:18713856). Expressed in pancreatic acini, pituitary gland, adrenal gland, lung, brain and bone marrow (PubMed:18713856). N-glycosylated. Belongs to the vacuolar ATPase subunit S1 family.