Regulatory light chains of myosin
Scallop myosin is unusual in that the regulatory light chains (RLC) can be reversibly dissociated on removal of divalent metal ions. In this thesis the mechanism of the RLC dissociation was studied using several approaches. The dissociation of divalent metal ions from the non-specific divalent metal ion binding site of the scallop RLC was followed, either by a pH indicator method, or by a Mn2+ displacement method. The binding and release of the RLC itself, from the heavy chain/essential light chain complex was followed using the fluorophare 8-anilino-1-naphthalene sulphonate, which was found to be specific for the RLC binding site. The evaluation of the mechanism was greatly aided by the serendipitous discovery, that Mercenaria RLC bound to the scallop heavy chain/essential light chain complex in the absence of divalent metal ions. This allowed RLC exchange experiments to be performed which suggested the nature of the proposed mechanism. The dissociation of the RLC from the scallop heavy chain/ essential light chain complex, is largely explained by a refractory state mechanism. In this mechanism, the heavy chain/essential light chain complex is envisaged to exist in two forms, a nascent state which occurs immediately after RLC dissociation and a refractory state which is favoured on a long term basis. The formation of this refractory state, is the driving force for the net dissociation of the RLC from scallop myosin and subfragments. Differences in the degree of RLC dissociation (HMM cf S1 +rd) on addition of EDTA are explained by a change in the equilibrium constant for the refractory state transition. The mechanism is discussed with reference to existing structural information on RLC denuded myosin.