In muscle cells, the myosin-containing thick filaments and the actin-containing thin filaments form a double hexagonal lattice. Myosin subfragment-1 binds to the thin filament forming actomyosin crossbridges under a wide range of conditions, such as in the presence of ATP with Ca++ (contracting state), without Ca++ (relaxed state), and in the absence of ATP (rigor state). These attached states are part of the actomyosin ATPase cycle, where the generation of active contraction is driven by the process of ATP hydrolysis. Our goal is to characterize the structural and, simultaneously, mechanical properties of the attached crossbridges formed under these conditions by obtaining equatorial X-ray diffraction patterns and mechanical measurements from single, skinned psoas fiber preparations. The hexagonal lattice formed by the contractile filaments is better ordered in a single fiber preparation, whereby several equatorial X-ray reflections not reported previously were obtained. Such improved diffraction patterns led to the conclusion that attached crossbridges in relaxed fibers probably move more freely around the thin filament than those in rigor fibers, and the part of myosin head immobilized by attachment is smaller in the relaxed state. By combining X-ray diffraction techniques with mechanical measurements, it is found that generation of active axial force in muscle fiber is accompanied by a lateral (radial) component that can be of the same order of magnitude as the axial component. Meanwhile, the radial stiffness is about twice the axial stiffness. It was proposed earlier by H. E. Huxley (Science, 1969) that there are two free hinge regions in the myosin molecule to facilitate attachment and force generation. Our finding suggests that the hinges should be elastic rather than free.
