This application is part of an effort to understand the macromolecular mechanics of force production by muscle. Specifically, the role of myosin crossbrige bending in force production and energy storage will be investigated. Three recently discovered structural properties of the Sl portion of the muscle myosin crossbridge will be examined: its permanent electric dipole moment, its segmental flexibility, and its elastic structural distortion under the influence of an applied electric field. The myosin crossbridge is a protein fragment, isolated from muscle, that is capable of combining with actin to transduce MgATP chemical energy into mechanical energy by an unknown structural mechanism. The proposed experiments will test the hypotheses that (1) the part of the myosin crossbridge called S1 bends while bound to actin in order to generate force, that (2) S1 can be reversibly and elastically distorted in order to temporarily store the mechanical energy as elastic energy and that (3) enzymatic removal of a 2000 dalton segment of S1 is the cause of the trypsin-induced loss of its actin-activated MgATPase activity. Segmental motion and elastic distortion will be measured primarily by the method of transient electrical birefringence, using greatly improved modern instrumentation. By adjusting the strength and duration of an applied electric field pulse, different aspects of the isolated S1 structure can be investigated quantitatively in solution. Weak short pulses will selectively align flexity attached smaller segments. Weak long pulses will rotate the entire molecule. Strong short pulses will transiently distort it. The electric field produces a torque on the large permanent electric dipole moment of S1 and the energetics of this interaction can be measured quantitatively. The hydrodynamic behavior of S1 during and after the applied field will be analyzed to obtain dynamic structural information. The structural parameters obtained for crossbridges from muscles from different sources, and for chemically and enzymically modified crossbridges, will be correlated to the known muscle properties. The fate of a trypsin-produced 2000 dalton S1 heavy chain fragment, and its effects on S1 structural dynamics and ATPase activity, will be determined.
