Structural comparison of cephalopod hemocyanins: Phylogenetic significance

Hemocyanins, the respiratory molecules of cephalopod mollusks, are hollow c ylinders with five internal arches. Three hemocyanins representative of thr ee orders of cephalopods (Benthoctopus species, Octopoda; Vampyroteuthis in fernalis, Vampyromorpha; Sepia officinalis, Sepioidea) were subjected to cr yoelectron microscopy and three-dimensional (3D) reconstruction. The struct ure of Benthoctopus hemocyanin, solved at 26.4-Angstrom resolution, possess es arches comprising two identical functional units. The similarity between these functional units and the structure recently observed in X-ray crysta llography for Octopus by Cuff ct al. (J. Mol. Biol., 1998, 232, 522-529) al lows the identification of their N- and C-terminal domains in this 3D recon struction volume. Conversely, arches present in the 3D reconstruction volum e of Sepia hemocyanin (21.8 Angstrom resolution) contain four functional un its that are disposed differently. The strong resemblance between the recon struction volumes of Vampyroteuthis (21.4-Angstrom resolution) and Benthoct opus hemocyanins suggests that Sepioidea diverged from a group containing O ctopoda and Vampyromorpha. (C) 1999 Academic Press.