Acrosomal protein that maintains proacrosin (pro-ACR) as an enzymatically inactive zymogen in the acrosome. Involved also in the acrosome formation. Maintains pro-ACR as an enzymatically inactive zymogen in the acrosome until acrosomal exocytosis. Partially contributes also to the assembly of acrosomal proteins to form an acrosomal granule. Rodent specific isoform that participates in the formation of the acrosomal granule into the center of the acrosomal vesicle during early spermiogenesis. In the fertilization process promotes ACR release from the acrosome during acrosomal exocytosis. Binds pro-ACR. Does not bind the mature form of ACR. Binds pro-ACR. Does not bind mature form of ACR. Colocalized with pro-ACR in the acrosomal granules of early round spermatids. Highly enriched in acrosomal granules of round spermatids, and spread over the apical end of the nucleus in elongating spermatids. Colocalized in the acrosome of epididymal sperm with pro-ACR. 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.