Plasma membrane protease present on mature sperm that may be involved in sperm function during epididymal maturation and/or fertilization. Binds 1 zinc ion per subunit. Monomer. Expressed exclusively in testis and more specifically on the surface of mature sperm. Localized to the equatorial region of the plasma membrane of cauda epididymal sperm. Adult levels are reached by day 20 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. The prodomain is removed during sperm passage through the caput epididymis after the protein has reached the cell surface. Not processed in the secretory pathway.