3-DIMENSIONAL RECONSTRUCTION OF THE HEMOCYANIN OF THE PROTOBRANCH BIVALVE MOLLUSK NUCULA-HANLEYI FROM FROZEN-HYDRATED SPECIMENS

The didecameric hemocyanin of the bivalve mollusc Nucula hanleyi, obse rved in the electron microscope in vitreous ice, was subjected to a th ree-dimensional (3D) reconstruction by the random conical tilt series method. The 3D volume was analyzed by computing solid-body surface rep resentations and slices through the volume and by eroding the structur e progressively. The molecule was reconstructed from 288 and 63 images in the side- and end-on view orientations, respectively. It comprises three parts: one cylindrical wall composed of 10 oblique wall units a nd two collar complexes located at both ends of the cylinder and made up each of five arches and one annular collar. Erosion of the volume r eveals that the wall looks like a segment of a five-stranded right-han ded helix and that each oblique wall unit resembles a figure 8 incline d to the right. The 3D volumes reconstructed from Helix pomatia and N. hanleyi hemocyanins look perfectly similar, except that when the thre shold is expressed as a function of the molecular volume left apparent , the various connections between the wall, the arches, and the collar disappear at a lower threshold in N. hanleyi than in H. pomatia. Conv ersely, with thresholds expressed as a fraction of the dynamic range o f the 3D volume, the two structures are quite similar. This behavior m eans either that the molecular weights of Helix and Nucula hemocyanins (and therefore their molecular volumes) are not so different as indic ated in the literature or that all the portions of Nucula hemocyanin h ave molecular weights lower than those of Helix hemocyanins. (C) 1995 Academic Press, Inc.