Genomic structure of the sponge, Halichondria okadai calcyphosine gene

Calcyphosine is an EF-hand Ca(2+)-binding protein, which was first isolated from the canine thyroid. It is phosphorylated in a cyclic AMP (cAMP)-dependent manner; then it is thought to be implicated in the cross-signaling between the cAMP and calcium-phosphatidylinositol cascades. Here, we isolated the DNA complementary to RNA (cDNA) of an EF-hand Ca(2+)-binding protein from the sponge, Halichondria okadai and determined its genomic structure. The deduced sequence of the sponge Ca(2+)-binding protein showed significant similarity (about 40% identity) with those of mammal calcyphosines, and the intron positions were well conserved between the sponge and human calcyphosine genes. We considered that the isolated cDNA was that of sponge calcyphosine, and that sponge and mammalian calcyphosines evolved from a common ancestor gene. Recent cDNA projects have revealed that a calcyphosine cDNA is also expressed by human, mouse, and the ascidia. These cDNAs have more than 60% identity with sponge calcyphosine and each other, and all are composed of 208 amino acid residues. On the constructed phylogenetic trees, calcyphosines are essentially divided into two groups, types-I and -II calcyphosines. Type-I calcyphosine may be specific to mammals, and type-II is widely distributed among metazoan species. This suggests that type-II calcyphosine is a rather ancient gene with some essential function.