TRANSDUCTION OF CA2+ SIGNALS UPON FERTILIZATION OF EGGS - IDENTIFICATION OF AN S-100 PROTEIN AS A MAJOR CA2+-BINDING PROTEIN

A transient increase in the level of free cytosolic Ca2+ is observed upon fertilization of the eggs of many species and is thought to represent a key event in the initiation of development. To identify components in the egg which could be involved in mediating such Ca2+ signals we searched for Ca2+-binding proteins in eggs of the fresh-water fish Misgurnus fossilis (loach). We show that loach eggs contain two major Ca2+-binding proteins which can be purified through their Ca2+-dependent interaction with a hydrophobic matrix. Protein sequencing revealed that the larger 18 kDa protein is calmodulin, while the smaller polypeptide of 10 kDa is a member of the S-100 protein family. This is the first report of the presence of an S-100 protein in vertebrate eggs and shows that this protein is found in two fold higher concentration than calmodulin. Since the 10 kDa protein shares 68% sequence identity with S-100alpha from bovine brain, it can be considered as the loach homologue of mammalian S-100alpha. During early embryonic development, de novo protein synthesis of calmodulin is observed at the earliest stages analyzed (mid-blastula), while de novo protein synthesis of the S-100alpha homologue begins with the mid-gastrula stage. Although both proteins are likely to serve as mediators of Ca2+ signals and/or as Ca2+-buffers in the egg, the tighter regulation of the synthesis of the S-100 protein as compared to calmodulin argues for an additional and probably more specialized function of the S-100alpha homologue in later embryogenesis, which could be restricted to certain cell types.