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). Interacts with the v-ATPase accessory protein ATP6AP2 and with the v-ATPase complex subunit ATP6V0D1; these interactions lead to the assembly of the v-ATPase complex. Highly expressed in adrenal gland, thyroid gland, testis, ovary and prostate (PubMed:12359240). Moderate expression in trachea, spinal cord, stomach, colon, small intestine and spleen (PubMed:12359240, PubMed:30374053). Low expression in bone marrow, lymph node, thymus and peripheral blood lymphocytes (PubMed:12359240). Expression is detected in hematopoietic cell lines including those of myeloid, erythroid, B- and T-cell origin (PubMed:12359240). The transmembrane domain (TMD) is essential for the interaction with ATP6AP2. N-glycosylated. Belongs to the TMEM9 family.