Two-domain haemoglobin of the blood clam Barbatia lima resulted from the recent gene duplication of the single-domain δ chain Available to Purchase

The blood clam Barbatia lima subsp. from Amami-Oshima, Japan, expresses three types of haemoglobins in erythrocytes: a tetramer (α₂β₂), a homodimer (∆₂) and a polymer consisting of two kinds of chains, a 34 kDa two-domain (2D) globin and a ∆ chain. This is in sharp contrast to the congeneric clams B. reeveana (a North American species) and B. lima from Kochi, Japan, each containing a tetramer and a polymer consisting of the 2D globin, but not the ∆ chain. We have determined the cDNA-derived amino acid sequences of all four chains, α (163 residues), β (155 residues), ∆ (152 residues) and 2D (308 residues) of B. lima (Amami-Oshima). The α chain has an extremely long N-terminal extension of 20 residues that may form a ‘pre-A helix’, and this makes the α chain the longest known globin. B. lima α and β chains show about 50% sequence identity with the α and β chains, respectively, of tetrameric haemoglobin from a related clam, Anadara trapezia. The B. lima homodimeric ∆ chain shows 71-74% identity with each of the two domains of the 2D chain, but only 39% identity with the homodimeric γ chain of Anadara. In addition, the alignment of amino acid sequences of the ∆ chain and the two domains of the 2D chain revealed that the ∆ chain lacks one amino acid (Lys) at the C-terminus, suggesting that the C-terminal Lys (codon AAA or AAG) of the two domains of 2D chain could result from the stop codon TAA in the ∆ chain by nucleotide substitutions. These results, together with the fact that the ∆ and 2D chains form a polymeric haemoglobin, indicates that the ∆ chain is the ancestral single-domain globin for the 2D globin. The ∆ chain is expressed only in B. lima (Amami-Oshima), and appears to be a relic of molecular evolution.