Acrosomal protein that maintains proacrosin (pro-ACR) as an enzymatically inactive zymogen in the acrosome (PubMed:8144514). Involved also in the acrosome formation (By similarity). Binds specifically to the 55- and 53-kDa proacrosins and the 49-kDa acrosin intermediate, but is not capable of binding 43-kDa acrosin intermediate and 32-kDa mature acrosin. Appears to be present only in the round spermatids and elongating spermatids. Colocalizes in the acrosome with proacrosin. Specifically expressed in testis. The N-terminus is blocked. Synthesized as a 60-kDa precursor, the 35-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. Phosphorylated on Tyr residues in capacitated sperm.