Accessory subunit of the proton-transporting vacuolar (V)-ATPase protein pump, which is required for luminal acidification of secretory vesicles (By similarity). In the germline, required for the trafficking of the receptor RME-2 to the oocyte cell membrane where it regulates the uptake of yolk proteins (PubMed:22768351). Also, plays an essential role in osmoregulation in the embryo, probably by regulating the proper formation of the eggshell (PubMed:22768351). Accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump. Expressed in pharynx, hypodermis, intestine, vulval hypodermis and the H-shape excretory cell. Expressed in larvae and adults. RNAi-mediated knockdown at the L1 larval stage causes arrest followed by death at the L3/L4 larval stages (PubMed:22768351). RNAi-mediated knockdown at the L4 larval stage does not affect development into adults (PubMed:22768351). However, only 26% of the progeny hatched into larvae which die at the L3 larval stage. RNAi-mediated knockdown at the L4 larval stage or in the germline causes egg compression in their uterus (PubMed:22768351). Fewer oocytes are produced in the oviduct and are endomitotic (PubMed:22768351). This results in a reduced rate of ovulation over time (PubMed:22768351). Oocytes fail to endocytose vitellogenin due to a failure to traffic receptor rme-2 to the cell membrane (PubMed:22768351). Also, embryos display osmotic sensitivity and defect in osmoregulation and cytokinesis probably due to an abnormal eggshell (PubMed:22768351). Belongs to the vacuolar ATPase subunit S1 family.