Gene duplication and fusion have occurred frequently in the evolution of phosphagen kinases--a two-domain arginine kinase from the clam Pseudocardium sachalinensis.

In contrast to the 40 kDa arginine kinases from Molllusca and Arthropoda, the adductor muscle of the marine clam Pseudocardium sachalinensis contains an unusual arginine kinase consisting of an 86 kDa subunit. The cDNA encoding the 86 kDa arginine kinase was amplified by PCR and the cDNA-derived amino acid sequence of 724 residues was determined. The exact molecular mass for the protein was calculated to be 80941 Da. The amino acid sequence clearly indicates that Pseudocardium arginine kinase has a two-domain structure: the first domain residues 1-363 and the second domain 364-724. The two domains, which are separated by an intron of 176 bp in the gene, show 62% amino acid sequence identity. This two-domain arginine kinase from a mollusc represents yet another multiple-domain enzyme observed in the phosphagen kinase enzyme family. Two-domain and three-domain enzymes have been observed in three other diverse invertebrate groups. Thus, it is clear that gene duplication and subsequent fusion have occurred frequently, and likely independently, during the course of the evolution of this enzyme family. Comparison of the amino acid sequence in the GS region (a possible candidate for the guanidine substrate recognition site in the phosphagen kinase family) suggests that the first domain of Pseudocardium arginine kinase might not retain a complete enzyme activity, because the Asp-7 in the GS region, which is assumed to be involved in the recognition of the positive charge of arginine, was replaced by a Gly residue in the first domain.