The function of TMEM9 (transmembrane protein 9, Ensembl gene identifier ENSG00000116857) is as follows. Transmembrane protein that binds to and facilitates the assembly of lysosomal proton-transporting V-type ATPase (v-ATPase), resulting in enhanced lysosomal acidification and trafficking (PubMed:30374053). By bringing the v-ATPase accessory protein ATP6AP2 and the v-ATPase subunit ATP6V0D1 together, allows v-ATPase complex formation and activation (PubMed:30374053). TMEM9-controlled vesicular acidification induces hyperactivation of Wnt/beta-catenin signaling, involved in development, tissue homeostasis and tissue regeneration, through lysosomal degradation of adenomatous polyposis coli/APC (PubMed:30374053, PubMed:32380568). In the liver, involved in hepatic regeneration (PubMed:32380568). {ECO:0000269|PubMed:30374053, ECO:0000269|PubMed:32380568}.