Acrosomal protein that maintains proacrosin (pro-ACR) as an enzymatically inactive zymogen in the acrosome. Involved also in the acrosome formation. Binds proacrosin (ACR). Does not bind the mature form of ACR. Specifically expressed in testis. The N-terminus is blocked. Phosphorylated on Tyr residues in capacitated sperm. 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.