Acts as a GTPase-activating protein for Rab family protein(s) (PubMed:28067790, PubMed:19077034). 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). Interacts with ACTB (By similarity). Interacts with ARMC12, TOMM20, DNAH7 and RAP1A (By similarity). Interacts with RAB10 (PubMed:28067790). 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 round and elongated spermatids (at protein level). Expressed specifically in adult testis and very weakly in fetal brain. The arginine and glutamine fingers are critical for the GTPase-activating mechanism, they pull out Rab's 'switch 2' glutamine and insert in Rab's active site.