Arginine kinase evolved twice: evidence that echinoderm arginine kinase originated from creatine kinase

Arginine kinase (AK) was isolated from the longitudinal muscle of the sea c ucumber Stichopus japonicus. Unlike the monomeric 40 kDa AKs from molluscs and arthropods, but like the cytoplasmic isoenzymes of vertebrate creatine kinase (CK), the Stichopus enzyme was dimeric. To explore the evolutionary origin of the dimeric AK, we determined its cDNA-derived amino acid sequenc e of 370 residues. A comparison of the sequence with those of other enzymes belonging to the phosphagen kinase family indicated that the entire amino acid sequence of Stichopus AK is apparently much more similar to vertebrate CKs than to all other AKs. A phylogenetic tree also strongly suggests that the Stichopus AK has evolved from CK. These results support the conclusion that AK evolved at least twice during the evolution of phosphagen kinases: first at an early stage of phosphagen kinase evolution (its descendants ar e molluscan and arthropod AKs) and secondly from CK later in metazoan evolu tion. A comparison of the amino acid sequence around the guanidino specific ity (GS) region (which is a possible candidate for the guanidine substrate recognition site in the phosphagen kinase family) of the Stichopus enzyme w ith those of other phosphagen kinases showed that the GS region of the Stic hopus enzyme was of the AK type: five amino acid deletions in the flexible loop region that might help to accommodate larger guanidine substrates in t he active site. The presence of the AK-type deletions in the Stichopus AK, even though it seems that the enzyme's most immediate ancestor was probably CK, strongly suggests that the GS region has a role in substrate specifici ty. Stichopus AK and presumably other echinoderm AKs seem to have evolved f rom the CK gene; the sequence of GS region might have been replaced by the AK type via exon shuffling. The presence of an intron near the GS region in the Stichopus AK gene supports this hypothesis.