Immunolocalization of CRES (Cystatin-related epididymal spermatogenic) protein in the acrosomes of mouse spermatozoa.

The CRES (cystatin-related epididymal spermatogenic) protein is a member of the cystatin superfamily of cysteine protease inhibitors and exhibits highly restricted expression in the reproductive tract. We have previously shown that CRES protein is present in elongating spermatids in the testis and is synthesized and secreted by the proximal caput epididymal epithelium. The presence of CRES protein in developing germ cells and in the luminal fluid surrounding maturing spermatozoa prompted us to examine whether CRES protein is associated with spermatozoa. In the studies presented, indirect immunofluorescence, immunogold electron microscopy, and Western blot analysis demonstrated that CRES protein is localized in sperm acrosomes and is released during the acrosome reaction. Interestingly, while the 19- and 14-kDa CRES proteins were present in testicular and proximal caput epididymal spermatozoa, the 14-kDa CRES protein was the predominant form present in mid-caput to cauda epididymal spermatozoa. Furthermore, following the ionophore-induced acrosome reaction, CRES protein localization was similar to that of proacrosin/acrosin in that it was detected in the soluble fraction as well as associated with the acrosome-reacted spermatozoa. The presence of CRES protein in the sperm acrosome, a site of high hydrolytic and proteolytic activity, suggests that CRES may play a role in the regulation of intraacrosomal protein processing or may be involved in fertilization.