Positive selection adaptation of two-domain arginine kinase (AK) from cold seep Vesicomyidae clams.

Arginine kinase (AK) is an important member of Phosphagen kinases which engage in energy metabolism process, and AKs from cold seep clams may develop an effective mechanism to adapt a special habitat (e.g. low temperature). Three Vesicomyidae clams and seven Veneridae clams (belong to the same Order Veneroida) were chosen to analyze the evolution of two-domain AKs. In the present study, ten two-domain AKs were identified and Neighbor-joining tree showed that AKs were divided into two groups. Branch-site model indicated that two-domain AKs were subjected to strong positive selection (ω = 17.5058). 16 positively selective sites were detected and five of them showed posterior probabilities of 0.95 or more. Comparative analysis found that domain 2 might be suffered from more evolutionary selection pressure than domain 1, as most positively sites were located at domain 2. Residue Pro (positively selective site) (587P in ApAK) in domain 2 from all Vesicomyidae AKs might participate in change of the synergism and in the function of its cold-adapted characteristics. In conclusion, our studies provide evidence of positive Darwinian selection in the two-domain AKs family of Vesicomyidae clams, and may contribute to a better understanding of its adaptation mechanisms to cold seep habitats.