Accessory subunit of the proton-transporting vacuolar (V)-ATPase protein pump, which is required for luminal acidification of secretory vesicles (PubMed:33065002). 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 (PubMed:27231034). Involved in membrane trafficking and Ca(2+)-dependent membrane fusion (PubMed:27231034). May play a role in the assembly of the V-type ATPase complex (Probable). 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 (PubMed:28296633). In islets of Langerhans cells, may regulate the acidification of dense-core secretory granules (By similarity). Accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump (PubMed:33065002). Interacts (via N-terminus) with ATP6AP2 (via N-terminus) (PubMed:33065002, PubMed:29127204). Interacts with RNASEK (PubMed:26212330). Not detected in trans-Golgi network. widely expressed, with highest levels in brain and lowest in liver and duodenum. N-glycosylated. The disease is caused by variants affecting the gene represented in this entry. Belongs to the vacuolar ATPase subunit S1 family. Truncated N-terminus. Truncated N-terminus.