Transduction of Ca²⁺ signals upon fertilization of eggs; identification of an S-100 protein as a major Ca²⁺-binding protein

Abstract

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-100α from bovine brain, it can be considered as the loach homologue of mammalian S-100α. During early embryonic development, de novo protein synthesis of calmodulin is observed at the earliest stages analyzed (mid-bastula), while de novo protein synthesis of the S-100α 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-100α homologue in later embryogenesis, which could be restricted to certain cell types.