Molecular cloning and evolution of lobster hemocyanin

In the American lobster, Homarus americanus, oxygen is transported by a hemocyanin that is composed 2 × 6 subunits. N-terminal sequencing show the presence of three distinct subunit types (α, β and γ). We cloned the cDNA of one of these subunits that belong to the α-type. It encodes a hemocyanin subunit of 654 amino acids with a molecular mass of 84.8 kDa, which is synthesized in the hepatopancreas. Phylogenetic analyses of the crustacean hemocyanin sequences show two well-separated clades, which correspond to the α and γ-type subunits. Sequences of β-type subunits are still unknown. The γ-sequences have evolved about 15% faster than the α-subunits, consistent with the proposed conservative function of the latter. Under the assumption of a molecular clock we calculated that α- and γ-subunits split about 214 ± 14 million years ago, suggesting their divergence only in the decapod Crustacea.