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

A transient increase in the level of free cytosolic Ca2+ is observed u pon fertilization of the eggs of many species and is thought to repres ent a key event in the initiation of development. To identify componen ts in the egg which could be involved in mediating such Ca2+ signals w e searched for Ca2+-binding proteins in eggs of the fresh-water fish M isgurnus fossilis (loach). We show that loach eggs contain two major C a2+-binding proteins which can be purified through their Ca2+-dependen t interaction with a hydrophobic matrix. Protein sequencing revealed t hat the larger 18 kDa protein is calmodulin, while the smaller polypep tide of 10 kDa is a member of the S-100 protein family. This is the fi rst 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 wit h S-100alpha from bovine brain, it can be considered as the loach homo logue of mammalian S-100alpha. During early embryonic development, de novo protein synthesis of calmodulin is observed at the earliest stage s analyzed (mid-blastula), while de novo protein synthesis of the S-10 0alpha homologue begins with the mid-gastrula stage. Although both pro teins 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-1 00 protein as compared to calmodulin argues for an additional and prob ably more specialized function of the S-100alpha homologue in later em bryogenesis, which could be restricted to certain cell types.