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 (PubMed:27303034). Partially contributes also to the assembly of acrosomal proteins to form an acrosomal granule (PubMed:27303034). Rodent specific isoform that participates in the formation of the acrosomal granule into the center of the acrosomal vesicle during early spermiogenesis (PubMed:27303034). In the fertilization process promotes ACR release from the acrosome during acrosomal exocytosis (PubMed:27303034). Binds pro-ACR. Does not bind the mature form of ACR. Binds pro-ACR. Does not bind the 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. Testis-specific (at protein level). Detected in pachytene spermatocytes and in haploid spermatids and in elongating spermatids. Testis-specific. Detected in pachytene spermatocytes and round spermatids but absent in elongating spermatids. Produced specifically in rodents. The N-terminus is blocked. Phosphorylated on Tyr residues in capacitated sperm. Synthesized as a 60-kDa precursor in pachytene spermatocytes, postmeiotic spermatocytes and haploid spermatids and is processed into the mature protein in elongating spermatids. Degraded in elongating spermatid. Deficient male and female are normal in behavior, body size, and health condition. Females exhibit normal fertility. However, males exhibit a severely reduced fertility. The acrosomal structure of spermatids is severely deformed at the later steps of spermiogenesis.