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 (PubMed:28296633). May be involved in Golgi homeostasis (PubMed:26833330). Binds 20(S)-hydroxycholesterol (20(S)-OHC) (By similarity). {ECO:0000250|UniProtKB:Q5SYH2, ECO:0000269|PubMed:26833330, ECO:0000269|PubMed:28296633}. This is the function of Ensembl gene identifier ENSG00000244045 (VMA12, vacuolar ATPase assembly factor VMA12).