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

The extracellular respiratory pigment of the cuttlefish Sepia officinalis w as observed by cryoelectron microscopy with conventional LaB6 and field emi ssion gun electron sources at 100 and 200 kV, respectively. Each image seri es was used to compute one 3D reconstruction volume with correction of the contrast transfer function by Wiener filtering. A strong boosting of the co ntrast was corrected by band-pass filtering of the final volumes, and a qua litative gain in resolution was observed when using the field emission gun electron microscope. In this volume, a strong signal is present down to 1/1 8 Angstrom(-1) and some meaningful information is obtained down to 1/12.5 A ngstrom(-1). The complex is composed of five pairs of polypeptide chains an d resembles a hollow cylinder with five wall oblique units and five inner a rches. Three types of wall-wall connections termed pillar P1 to P3 are visi ble 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 t he functional units in the arches of Sepia and Octopus hemocyanins share no common feature. (C) 2000 Academic Press.