Myosin-Linked Regulatory Systems: The Role of the Light Chains

Muscular contraction is regulated by the inter-action of calcium with the contractile lements. In vertebrate muscles the specific regulatory pro-teins, tropomyosin, and the troponin complex (Ebashi and Kodama, 1965, 1966) are associated with the thin filaments (Ohtsuki et al., 1967). Tropomyosin is required for relaxation, its role being to mediate the changes on troponin which affect the active sites of actin (Ebashi and Endo, 1968; Weber and Bremel, 1971). In molluscan muscles calcium regulation is associated with myosin; troponin and its components are absent, and tropomyosin, although present on the thin filaments, is not required for regulation. Purified molluscan myosin preparations bind calcium, and their ATPase activity depends on calcium when combined with pure actin (Kendrick-Jones et al., 1970). In this paper we define those components of the myosin preparations which are responsible for calcium regulation. The removal of a particular light chain from myosin with EDTA leads to a loss of calcium sensitivity, which can be quantita-tively regained when the light chain is recombined with the myosin in the presence of divalent cations. Therefore, the presence of this light chain on myosin is required for regulation by calcium, and it functions as a regulatory subunit. The light chain inhibits the actin-activated ATPase activity of scallop myosin in the absence of calcium by blocking those sites on myosin which combine with actin. In the presence of calcium the inhibitory effect of the light chain is removed. Possible mechanisms describing how the light chain may control interactions between actin and myosin will be discussed. A detailed escription of these observations will be published elsewhere (Szent GySrgyi et al., 1973). In these studies we focused our attention on the myosins isolated from the striated adductor muscles of two species of scallops, Aequipecten irradians and Placopecten magellanicus. These scallop myosins were prepared by the procedures previously outlined (Szent-Gy5rgyi et al., 1971) and were found to have identical behavior.