Structural comparison of cephalopod hemocyanins: phylogenetic significance.

Hemocyanins, the respiratory molecules of cephalopod mollusks, are hollow cylinders with five internal arches. Three hemocyanins representative of three orders of cephalopods (Benthoctopus species, Octopoda; Vampyroteuthis infernalis, Vampyromorpha; Sepia officinalis, Sepioidea) were subjected to cryoelectron microscopy and three-dimensional (3D) reconstruction. The structure of Benthoctopus hemocyanin, solved at 26.4-A resolution, possesses arches comprising two identical functional units. The similarity between these functional units and the structure recently observed in X-ray crystallography for Octopus by Cuff et al. (J. Mol. Biol., 1998, 232, 522-529) allows the identification of their N- and C-terminal domains in the 3D reconstruction volume. Conversely, arches present in the 3D reconstruction volume of Sepia hemocyanin (21.8 A resolution) contain four functional units that are disposed differently. The strong resemblance between the reconstruction volumes of Vampyroteuthis (21.4-A resolution) and Benthoctopus hemocyanins suggests that Sepioidea diverged from a group containing Octopoda and Vampyromorpha.