Accessory subunit of the proton-transporting vacuolar (V)-ATPase protein pump, which is required for luminal acidification of secretory vesicles. Accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump (By similarity). May interact with ATP6AP2 (PubMed:29995586). Belongs to the vacuolar ATPase subunit S1 family.