Acts as a GTPase-activating protein for Rab family protein (s) (PubMed:30360518). Essential for the establishment of male fertility, and is required for both the production of normal sperm number and sperm function (PubMed:32976492). Plays an important role in the formation of intact mitochondria, outer dense fibers and axoneme within the sperm tail (PubMed:32976492). Essential for sperm mitochondrial sheath formation and for the interactions of ARMC12 with VDAC2 and VDAC3 (PubMed:33536340). May be involved in acrosome formation and cytoskeletal reorganization during spermiogenesis, possibly by regulating RAB3A activity (PubMed:21128978). Interacts with ACTB (PubMed:21128978). Interacts with ARMC12 (PubMed:33536340). Interacts with TOMM20 and DNAH7 (PubMed:32976492). Interacts with RAP1A (PubMed:30360518). Interacts with RAB10 (By similarity). Located at the edge of the acrosomal region, neck and annulus during spermiogenesis. Colocalizes with RAB3A at the acrosome-acroplaxome and neck regions of spermatids. Colocalizes with ACTB at the neck region in elongated spermatids. Expressed in testis, specifically in elongating and elongated spermatids (at protein level) (PubMed:21128978, PubMed:30360518, PubMed:33536340). Expressed in the sperm midpiece (at protein level) (PubMed:32976492, PubMed:33536340). Detected from postnatal day 35 onward. Male mice are sterile, characterized by defects in sperm tail structure and diminished sperm motility (PubMed:32976492, PubMed:33536340). The mitochondria of the sperm-tail has an abnormal irregular arrangement, abnormal diameter, and structural defects and the axoneme structure of sperm tails is severely disturbed (PubMed:32976492). Sperm mitochondria cannot form a proper mitochondrial sheath at the subsequent mitochondrial compaction step, although they can coil around the flagellum (PubMed:33536340).