A new membrane-associated Ca(2+)-binding protein of rat spermatogenic cells: its purification and characterization

Strong expression of the calreticulin gene in the liver of Rana rugosa tadpoles, but not adult frogs

In the present paper we report the purification of calreticulin (CLT) from livers of the frog, Rana rugosa, the cloning and sequencing of its cDNA, and the CLT gene expression. CLT with M(r) = 52 kDa, estimated by SDS-PAGE, was purified from frog livers. Using rat CLT cDNA as a probe, a 2.4-kilobase frog cDNA clone was isolated from a frog liver cDNA library. The cDNA encoded 419 amino acids including an 18-residue NH2-terminal signal sequence that was 76% homologous to the rat CLT sequence and was 84% homologous to the partial sequence of Xenopus laevis CLT (Treves et al. [1992] Biochem. J. 287:579-581). Phylogenetic relationships estimated from the amino acid sequence of CLTs showed no pronounced variation between the two frog species, R. rugosa and X. laevis. Northern blot analysis indicated that the CLT mRNA level was very high in the liver of tadpoles, but extremely low in adult frogs. Expression levels were also very high in the premature ovary, while moderate expression was observed in the testis and brain of adult frogs. However, there was little histological change in the liver of tadpoles during development. Furthermore, CLT was recognized by Western blot analysis of total proteins in the liver of adult frogs. Immunostaining showed that CLT was distributed in the cytoplasm of liver cells. These results suggest that the expression of the CLT gene is tissue-dependent in the frog, R. rugosa, and that CLT probably functions biochemically in liver cells even when its gene expression is low.

Review of conformation-specific affinity purification methods for plasma vitamin K-dependent proteins

There are seven known vitamin K-dependent proteins in blood. These proteins require calcium ion for expressing their full biological activities. Calcium ion also induces conformational changes in this class of proteins. Taking advantage of the ligand induced conformational changes, a number of unique approaches of affinity chromatography have been developed. These methodologies have been very useful tools for both the purification and for understanding the structure-function relationships of this class of proteins. One method is the use of metal ion dependent immunoaffinity chromatography. The antigen can be dissociated from the antibodies with either the removal or addition of calcium ion under physiological conditions. The other method is pseudoaffinity chromatography. This method uses conventional ion-exchange or hydrophobic resin and manipulates the mobilities of the proteins on these resins by the presence or absence of calcium ions. Researchers working with other calcium binding proteins or other proteins that are known to undergo ligand induced conformational changes may benefit from the experience of these unique conformation-specific affinity chromatography approaches.

Antisera to calreticulin inhibits sperm motility in mice

Mouse sperm were rapidly immobilized when exposed to rabbit antisera against rat calreticulin. The inhibition of sperm motility was concentration dependent at dilutions 1:50-350. The velocity of sperm did not change significantly as long as they were motile. Neither motility nor velocity of sperm was affected by adding sheep antisera to bovine calmodulin. The antisera to calreticulin also inhibited in vitro fertilization of mouse eggs. Using the avidin-biotin-peroxidase complex method, the antigen was found to be localized in the acrosome of sperm. The results indicate that calreticulin is present in the acrosome of mouse sperm and may play an important role in sperm motility.

Calreticulin is present in the acrosome of spermatids of rat testis

We have already succeeded in purifying a calcium-binding protein (CalBP) from rat spermatogenic cells [Nakamura et al., Biochem. Biophys. Res. Commun., 176 (1991) 1358]. In this study, the location of this protein within rat testis was examined, using a rabbit antisera for this protein. The antigen was localized on the developing acrosomes during spermiogenesis. The NH2-terminal amino acid sequence obtained for rat CalBP was identical to that of calreticulin obtained for the skeletal muscle of mice and closely resembled that for rabbit calreticulin. On the immunoblot analysis, the purified rat CalBP reacted with an antibody raised against rabbit skeletal muscle calreticulin. The results indicate that calreticulin is present in the acrosome of spermatids of rat testes.