Structure of thick filaments from insect flight muscle.

The supramolecular organization of thick (myosin) filaments isolated from insect flight muscle was studied using negative staining and shadowing techniques. The electron microscopical findings favour the two-stranded arrangement of double cross-bridges rather than a four- or six-stranded structures of single cross-bridges. The thick filament backbone consists of 12-subfilaments of myosin rods with a diameter of about 4 nm in agreement with the X-ray data. Furthermore, about 4 nm stripping was observed on the filament shaft, resembling to the structure of the light meromyosin paracrystals with a periodicity of 4.8 nm. The existence of a hinge region at the origin of the projections and between the tail and myosin heads are confirmed. According to the morphological observations the stalks of the projections can be characterized with flexible properties, as well. At high level of the supramolecular organization, the myosin filaments, the connecting filaments and the Z-filaments are systematically interconnected after a remarkable myosin filament branching (trifurcation) at the Z line level, which supports the view that a continuous longitudinal filament network constitutes the structure of the myofibrils.