Acrosomal protein that maintains proacrosin (pro-ACR) as an enzymatically inactive zymogen in the acrosome. Involved also in the acrosome formation. Binds proacrosin (pro-ACR). Does not bind the mature form of ACR. Expression restricted to testis in normal tissue. Expressed in a wide spectrum of cancers, including bladder, breast, liver, lung and colon cancers. Phosphorylated on Tyr residues in capacitated sperm. The N-terminus is blocked. Synthesized as a 60-kDa precursor, the 32-kDa mature form is post-translationally produced by the removal of the N-terminal half of the precursor during sperm maturation in the testis and/or epididymis.