Acts as a GTPase-activating protein for Rab family protein(s) (PubMed:19077034, PubMed:28067790). Essential for the establishment of male fertility, and is required for both the production of normal sperm number and sperm function (By similarity). Plays an important role in the formation of intact mitochondria, outer dense fibers and axoneme within the sperm tail (By similarity). Essential for sperm mitochondrial sheath formation and for the interactions of ARMC12 with VDAC2 and VDAC3 (By similarity). May be involved in acrosome formation and cytoskeletal reorganization during spermiogenesis, possibly by regulating RAB3A activity (PubMed:21128978). {ECO:0000250|UniProtKB:Q9D9D3, ECO:0000269|PubMed:28067790, ECO:0000305|PubMed:19077034, ECO:0000305|PubMed:21128978}. This is the function of ENSG00000167139 (TBC1D21, TBC1 domain family member 21).