Isolation and cDNA-derived amino acid sequences of hemoglobin and myoglobin from the deep-sea clam Calyptogena kaikoi

摘要

The heterodont clam Calyptogena kaikoi, living in the cold-seep area at a depth of 3761 m of the Nankai Trough, Japan, has abundant hemoglobins and myoglobins in erythrocytes and adductor muscle, respectively. Two types of hemoglobins (Hb I and Hb II) were isolated, and the complete amino acid sequences of Hb I (145 residues) and Hb II (137 residues) were obtained with combination of cDNA and protein sequencing. The amino acid sequences of C. kaikoi Hbs I and II differed from homologous chains of the congeneric clam Calyptogena soyoae in eight and five positions, respectively. The distal (E7) His, one of the functionally important residues in hemoglobin and myoglobin, was replaced by Gln in hemoglobins of C. kaikoi, A phylogenetic analysis of clam hemoglobins indicates that the evolutionary rate of Calyptogena hemoglobins is rather faster than those of other clams, suggesting that the mutation rate might be accelerated in the deep-sea animals around the areas of cold seeps or hydrothermal vents. On the other hand, it was found unexpectedly that two myoglobins Mbs I and II, isolated from the red adductor muscle, are identical in amino acid sequence Hbs I and II, respectively. Thus it was assumed that genes for Hbs I and II are also expressed in the muscle of C. kaikoi in substitution for myoglobin gene. This suggests that the major physiological role of globins in C. kaikoi is storage of oxygen under the low oxygen conditions, rather than circulating of oxygen.