Complete sequence of the 24-mer hemocyanin of the tarantula Eurypelma californicum - Structure and intramolecular evolution of the subunits

Hemocyanins are large oligomeric respiratory proteins found in many arthrop ods and molluscs. The hemocyanin of the tarantula Eurypelma californicum is a 24-mer protein complex with molecular mass of 1,726,459 Da that consists of seven different polypeptides (a-g); each occupying a distinct position within the native molecule, Here we report the complete molecular structure of the E. californicum hemocyanin as deduced from the corresponding cDNAs, This represents the first complex arthropod hemocyanin to be completely se quenced. The different subunits display 52-66% amino acid sequence identity . Within the subunits, the central domain, which bears the active center wi th the copper-binding sites A and B, displays the highest degree of identit y. Using a homology modeling approach, the putative three-dimensional struc ture of individual subunits was deduced and compared. Phylogenetic analyses suggest that differentiation of the individual subunits occurred 400-550 m illion years ago. The hemocyanin of the stemline Chelicerata was probably a hexamer built up of six distinct subunit types a, b/c, d, e, f, and g, whe reas that of the early Arachnida was originally a 24-mer that emerged after the differentiation of subunits b and c.