An endoplasmic reticulum protein, calreticulin, is transported into the acrosome of rat sperm

Recently, we purified a Ca²⁺-binding protein from rat spermatogenic cells [Biochem. Biophys. Res. Commun. 176, 1358-1364, 1991]. In the present study, this protein was identified as calreticulin, which is a resident protein of the endoplasmic reticulum (ER). Immunohistochemical studies revealed that calreticulin was present in the acrosome of both round spermatids and mature sperm. However, under immunoelectron microscopy, gold-particles were seen over other subcellular structures of spermatocytes, spermatids, and Sertoli cells. When the labeling density in subcellular structures of spermatids was analyzed, the acrosome was found to be most heavily labeled and the Golgi apparatus was second. The complete amino acid sequence of calreticulin, deduced from the cDNA sequence, shares a high degree of identity with that of the analogous mouse protein. The cDNA encoded a protein of 416 amino acids, including a 17-residue NH₂-terminal signal sequence. The mature protein contains a KDEL sequence as an ER signal at the COOH terminus. Sperm calreticulin contained no glycosyl moiety. Northern blot analysis of RNAs from purified populations of rat spermatogenic cells indicated that the calreticulin mRNA was present in both pre- and postmeiotic cells. Immunoblot analysis of calreticulin during developmental stages showed that calreticulin was detected in the testis between the ages of 5 and 50 days. Furthermore, purified rat calreticulin contained two Ca²⁺-binding sites, a low affinity/high capacity site and a high affinity/low capacity site. These results suggest that calreticulin, which is not specific to testis, is closely associated with spermatogenesis of rats. This ER protein may be incorporated into the acrosomal vesicle via the Golgi apparatus, without glycosylation, during spermiogenesis, and may play an important role in the regulation of cell functions such as sperm motility and acrosome reaction.