Two-domain hemoglobin from the blood clam, Barbatia lima. The cDNA-derived amino acid sequence

The blood clam, Barbatia lima, from Kochi, Japan, expresses a tetrameric (alpha 2 beta 2) and a polymeric hemoglobin in erythrocytes. The latter hemoglobin is composed of unusual 34-kDa hemoglobin with a two-domain structure, and its molecular mass (about 430 kDa) is exceptionally large for an intracellular hemoglobin. The 3' and 5' parts of the cDNA of B. lima two-domain globin have been amplified separately by polymerase chain reaction and the complete nucleotide sequence of 1147 bp was determined. The open reading frame is 930 nucleotides in length and encodes a protein with 309 amino acid residues, of which 73 amino acids were identified directly by protein sequencing. The mature protein begins with the acetylated Ser, and thus the N-terminus Met is cleaved. The molecular mass for the protein was calculated to the 35,244 Da. The cDNA-derived amino acid sequence of B. lima two-domain globin shows 89% homology with that of two-domain globin from B. reeveana, a North American species. The sequence homology between the two domains is 75%, suggesting that the two-domain globin resulted from the gene duplication of an ancestral 17-kDa globin.