Accessory component of the proton-transporting vacuolar (V)-ATPase protein pump involved in intracellular iron homeostasis. In aerobic conditions, required for intracellular iron homeostasis, thus triggering the activity of Fe(2+) prolyl hydroxylase (PHD) enzymes, and leading to HIF1A hydroxylation and subsequent proteasomal degradation. Necessary for endolysosomal acidification and lysosomal degradation (By similarity). May be involved in Golgi homeostasis (By similarity). Accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump. Seems not to be associated with recycling endosomes. Predominantly expressed in the heart, liver, kidney and testis and at lower levels in the brain and lung. Undetectable in the spleen and muscles. Strongly expressed at 13.5 dpc in a ventro-lateral area of striatum and piriform cortex. At 14.5 dpc, some of the positive cells shift toward the cortical plate. At this stage, strongly observed at the superficial layer of dorsal cortex, whereas that of ventral cortex decreases in its intensity. Expression extended further dorsally at 16.5 dpc. In the dorsal cortex, expressed in the ventricular zone at 13.5 dpc. At later stages, expression shifts to the cortical plate. At 15.5 dpc, localizes to both ventricular zone and cortical plate. At 16.5 dpc, almost all expression is in the deeper cortical plate, although some expressing cells are still detectable in the ventricular zone. In the lateral cortex, weak expression in the lateral ganglionic eminence at 13.5 dpc. At 15.5 dpc, strongly detected in the progenitor zones of the lateral ganglionic eminence and medial ganglionic eminence, as well as in tangentially migrating cells in the superficial area of lateral cortex. In the striatum and ventro-lateral cortex, expressed at both 13.5 dpc and 15.5 dpc. Modestly up-regulated by FGF2.