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

The heterodont clam Calyptogena kaikoi, living in the cold-seep area at a d epth of 3761 m of the Nankai Trough, Japan, has abundant hemoglobins and my oglobins in erythrocytes and adductor muscle, respectively. Two types of he moglobins (Hb I and Hb II) were isolated, and the complete amino acid seque nces of Hb 1(145 residues) and rib II (137 residues) were obtained with com bination of cDNA and protein sequencing. The amino acid sequences of C. kai koi Hbs I and II differed from homologous chains of the congeneric clam Cal yptogena soyoae in eight and five positions, respectively. The distal (E7) His, one of the functionally important residues in hemoglobin and myoglobin , was replaced by Gin in hemoglobins of C. kaikoi. A phylogenetic analysis of clam hemoglobins indicates that the evolutionary rate of Calyptogena hem oglobins is rather faster than those of other clams, suggesting that the mu tation rate might be accelerated in the deep-sea animals around the areas o f cold seeps or hydrothermal vents. On the other hand, it was found unexpec tedly that two myoglobins Mbs I and II, isolated from the red adductor musc le, are identical in amino acid sequence Hbs I and II, respectively. Thus i t 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 maj or physiological role of globins in C, kaikoi is storage of oxygen under th e low oxygen conditions, rather than circulating of oxygen. (C) 2000 Elsevi er Science B.V. All rights reserved.