Effect of skeletal muscle myosin light chain 2 on the calcium-sensitive interaction of myosin and heavy meromyosin with regulated actin

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Abstract
A low-speed centrifugation assay was used to examine the binding of myosin filaments to F-actin and to regulated actin in the presence of MgATP. While the cross-linking of F-actin by myosin was Ca2+ insensitive, much less regulated actin was cross-linked by myosin in the absence of Ca2+ than in its presence. Removal of the 19,000-dalton, phosphorylatable light chain from myosin resulted in the loss of this Ca2+ sensitivity. Readdition of this light chain partially restored the Ca2+-sensitive cross-linking of regulated actin by myosin. Urea gel electrophoresis was used to distinguish that fraction of heavy meromyosin which conatains intact phosphorylatable light chain from that which contains a 17,000-dalton fragment of this light chain. In the absence of Ca2+, heavy meromyosin which contained digested light chain bound to regulate actin in MgATP about 10-fold more tightly than did heavy meromyosin which contained intact light chain. The regulated actin-activated ATPases of heavy meromyosin also showed that cleavage of this light chain causes a substantial increase in the affinity of heavy meromyosin for regulated actin in the absence of Ca2+. Thus, the binding of both myosin and heavy meromyosin to regulated actin is Ca2+ sensitive, and this sensitivity is dependent on the phosphorylatable light chain.