Direct determination of myosin filament symmetry in scallop striated adductor muscle by rapid freezing and freeze substitution.

Chemically skinned, relaxed bundles of fibers from the striated adductor muscle of the scallop Placopecten magellanicus were rapidly frozen and freeze-substituted. In the electron microscope, ultrathin transverse sections of embedded specimens showed, in many cases, clear regularly organized projections (crossbridges) protruding from the backbones of the myosin filaments. In the majority of cases the number of projections was directly observed to be seven: this was confirmed by alignment and averaging of the images using correlation methods. The rotational power spectrum of the average image showed a strong peak at N = 7. Tilting of sections in the electron microscope showed that the long-pitch crossbridge helices were right-handed. These and other observations confirm directly the essential features of the low-resolution three-dimensional helical reconstruction of negatively stained scallop filaments calculated previously.