Myosin-Linked Regulation: A Chemical Approach

Calcium is know to play a central role in controlling many cellular processes, for example, in secretion, in the visual cycle and in motility and has been implicated together with cyclic AMP in development, and in the control of many metabolic pathways; however, its role in regulating contraction in muscle is probably the best understood. Muscular contraction is controlled by specific calcium receptor proteins located on either the thick or thin filaments which are involved in switching actin-myosin interaction ON or OFF in response to changes in calcium concentration. In vertebrate striated and cardiac muscles, the regulatory proteins troponin and tropomyosin are associated with the thin filaments (Fig. 1). The extensive electron microscope, X-ray and biochemical evidence suggests that calcium induced changes in the troponin complex may be transmitted via a movement of the tropomyosin in such a way that it affects those sites in the actin molecules which are required for interaction with the myosin cross-bridges (see reviews Weber and Murray, 27; Huxley, 13). In molluscan muscles, the troponin complex is absent and instead calcium regulation of contractile activity requires the presence of specific regulatory light chains on the myosin (Fig. 1) (15, 24). However, little is known about how these regulatory light chains, under calcium, control, effect the transition of the myosin cross-bridge from the resting to the active state.