Three-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, observed in the electron microscope in vitreous ice, was subjected to a three-dimensional (3D) reconstruction by the random conical tilt series method. The 3D volume was analyzed by computing solid-body surface representations and slices through the volume and by eroding the structure 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 and 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 reveals that the wall looks like a segment of a five-stranded right-handed helix and that each oblique wall unit resembles a figure 8 inclined to the right. The 3D volumes reconstructed from Helix pomatia and N. hanleyi hemocyanins look perfectly similar, except that when the threshold 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. Conversely, with thresholds expressed as a fraction of the dynamic range of the 3D volume, the two structures are quite similar. This behavior means either that the molecular weights of Helix and Nucula hemocyanins (and therefore their molecular volumes) are not so different as indicated in the literature or that all the portions of Nucula hemocyanin have molecular weights lower than those of Helix hemocyanins.