May be involved in the fusion of the spermatozoa with the oocyte during fertilization. Interacts with C3b. Interacts with C4b. Interacts with moesin/MSN. Inner acrosomal membrane of spermatozoa. Specifically expressed in testis. Within testis, present only in elongated spermatids and spermatozoa (at protein level). Not expressed in embryonic and immature rats. Expressed in parallel with synthesis of spermatids. O-glycosylated. N-glycosylated.