Cross-Bridge Movement and the Conformational State of the Myosin Hinge in Skeletal Muscle

Our recent experiments (Chiao and Harrington, 1979) on glycerinated skeletal-muscle fibers in rigor using the cross-linking method showed that the myosin heads could be cross-linked to the backbone of the thick filament while they are still attached to the thin filament. We further demonstrated that the heads could be made to move out from the thick filament by a small change in pH from 7.4 to 8.0. This behavior suggests that relatively small changes in the local ionic environment can release the cross-bridge from the thick-filament surface. Similar experiments (Sutoh et al., 1978a) with synthetic myosin filaments showed that cross-linking of the subfragment-1 (S-l) subunits was markedly depressed at high pH (8.0-8.3). The myosin heads in synthetic filaments showed pH-sensitive association with the filament backbone similar to those in muscle fibers, which suggests that release of the heads from the thick-filament surface in muscle fibers at high pH is due to the intrinsic property of myosin rather than to an ionic effect between filaments. The possibility that such a release could lead to force generation through an a-helix-random coil transition in the subfragment-2 (S-2) region of myosin during a cross-bridge cycle has been considered in earlier papers (Harrington, 1971, 1979; Tsong et al., 1979) and is supported by thermal melting experiments on the S-2 fragments isolated from rabbit skeletal myosin. Most of the melting in this structure appears to occur in the light meromyosin-heavy meromyosin (LMM-HMM) “hinge” region (Harrington et al., unpublished). Hence, if this segment were to be stabilized in the α-helical state when the head is close to the filament backbone, but became destabilized following attachment of the head to the thin filament during a cross-bridge cycle, the consequent partial melting of the S-2 segment would produce enough shortening force to account for the tension developed in active muscle (Harrington, 1979).