Sepia officinalis hemocyanin:: A refined 3D structure from field emission gun cryoelectron microscopy

The extracellular respiratory pigment of the cuttlefish Sepia officinalis was observed by cryoelectron microscopy with conventional LaB6 and field emission gun electron sources at 100 and 200 kV, respectively. Each image series was used to compute one 3D reconstruction volume with correction of the contrast transfer function by Wiener filtering. A strong boosting of the contrast was corrected by band-pass filtering of the final volumes, and a qualitative gain in resolution was observed when using the field emission gun electron microscope. In this volume, a strong signal is present down to 1/18 Angstrom(-1) and some meaningful information is obtained down to 1/12.5 Angstrom(-1). The complex is composed of five pairs of polypeptide chains and resembles a hollow cylinder with five wall oblique units and five inner arches. Three types of wall-wall connections termed pillar P1 to P3 are visible in this volume and the four functional units present in the arches are each linked to the wall by two arch-wall connections. The dispositions of the functional units in the arches of Sepia and Octopus hemocyanins share no common feature. (C) 2000 Academic Press.