Fluorometric Studies on the Light Chains of Skeletal Muscle Myosin

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Abstract
To elucidate the role of light chains in the actin-myosin interaction, we measured the changes in reactivities of the SH groups of light chains with a fluorescent thiol reagent, N-(7-dimethyl-amino-4-methylcoumarinyl)maleimide, under various conditions differing in the manner of actin-myosin interaction and in the activity of actin-activated Mg2+-ATPase at 20°C. The reactivity was not greatly influenced by changes in the manner of protein aggregation. An apparent difference observed between the states before and after superprecipitation was mainly attributed to the resulting ADP. The changes in reactivities of alkali light chains apparently depended on the actin-activated Mg2+-ATPase activity of myosin during the reaction with the thiol reagent. Since it can be considered that the population of actin-myosin complex in the reaction solution is larger at high ATPase activity of myosin than at low ATPase activity, alkali light chains are supposed to participate in the actin-myosin interaction. The change in alkali light chain-2 was more marked than that in alkali light chain-1. However, the finding that alkali light chains also showed large changes at 0°C, where the actin-activated Mg2+-ATPase activity of myosin was very low, suggests much more complicated mechanisms for such reactivity changes. Although the mechanisms of the reactivity changes in light chains and actin are not clear, these results agree well with our previously reported observations using glycerinated muscle.