Sperm surface membrane protein that may be involved in spermatogenesis and fertilization. Binds 1 zinc ion per subunit. Expressed specifically in testis. Adult levels are reached by day 25 after birth. The conserved cysteine present in the cysteine-switch motif binds the catalytic zinc ion, thus inhibiting the enzyme. The dissociation of the cysteine from the zinc ion upon the activation-peptide release activates the enzyme.