Configurations of myosin heads in the crab striated muscle as studied by X-ray diffraction.

The configurations of myosin projections in striated muscles from the marine crab, Portunus trituberculatus were described in the relaxed and rigor states at the full overlap length of the thin and thick filaments. The crystallographic period of the thick filament is 101.5 nm (14.5 nm X 7) and the thick filament has four-fold rotational symmetry. In the relaxed state, the myosin projections sit about 19 nm from the thick filament axis, lying just between the surface of the thick filament backbone and that of the thin filament. They have an elongated structure with the length of 10 nm approximately 12 nm and a maximum axial thickness of about 4 nm. They are tilted axially by 20 degrees approximately 30 degrees to the thick filament axis. The configuration of the resting projections sensitively depends on the ionic strength and pH of the solution. In the rigor state, myosin heads are bound periodically to the thin filaments ( Namba , Wakabayashi & Mitsui , 1980); four myosin heads attach in groups every 38.3 nm to successive actin molecules of each strand of F-actin. Most of the bound myosin head is incorporated in the thin filament with the centre of gravity 2.8 nm from the thin filament axis. They are inclined at about 30 degrees to and slewed round the thin filament axis.