Conformational transition in the myosin hinge upon activation of muscle

We have determined the rates of chymotryptic proteolysis of the myosin hinge region in glycerinated rabbit psoas fibers and myofibrils in in rigor-inducing, activating, and relaxing buffers. The time course of formation of light meromyosin (LMM) provides a specific probe for cleavage within the hinge domain. In rigor-inducing and relaxing buffers proteolysis within the hinge is depressed, but on activation LMM is formed at a markedly increased rate, which is dependent on the concentration of MgATP. Peptide bond cleavage occurs at four widely separated sites spanning the length of the hinge domain. Only a trivial amount of proteolysis occurs at the head--rod swivel or within the heavy chain of the head itself (S-1 subunit) in rigor-inducing and relaxing solvents, and we find no significant change on activation. The rate of formation of LMM in rigor-inducing buffer is unchanged by addition of MgADP, Pi, or magnesium adenosine 5'-[beta, gamma-imido]triphosphate or in activating solvent at zero overlap between thick and thin filaments. These results provide evidence for a conformational (helix--coil) transition within the myosin hinge upon activation of skeletal muscle.