Accessory subunit of the proton-transporting vacuolar (V)-ATPase protein pump, which is required for luminal acidification of secretory vesicles (PubMed:32764564). 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 (By similarity). Accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump (PubMed:32764564). Interacts (via N-terminus) with ATP6AP2 (via N-terminus) (PubMed:32764564). Interacts with RNASEK (By similarity). Not detected in trans-Golgi network. N-glycosylated. Belongs to the vacuolar ATPase subunit S1 family.