The regulatory role of myosin light chain kinase as an actin-binding protein

Myosin light chain kinase (MLCK) is present in muscle cells including those of smooth muscle as an actin-binding protein. By avoiding complication introduced as a result of kinase activity of MLCK, we have demonstrated regulatory role of MLCK through its actin-binding activity [Kohama et al. (1992) Biochem. Biophys. Res. Commun. 184, 1204-1211]. To analyze such a regulatory role of MLCK, we compared the effects of MLCK on the velocity of the movement of actin filaments on a surface coated with smooth muscle myosin with those of another actin-binding proteins in smooth muscle, namely, caldesmon (CaD) and calponin (CaP). Both CaD and CaP stimulated movement when their concentrations were low, but they inhibited movement as their concentrations were increased. Calmodulin (CaM) in the presence of Ca2+ (Ca-CaM) antagonized the inhibition but hardly affected the stimulation. The effect of MLCK, by contrast, was simply inhibitory when Ca-CaM was not present. No stimulation was observed until Ca-CaM was added. The inhibitory ability of these actin-binding proteins increased in the following order: CaD < CaP < MLCK. The effect of MLCK and CaD on movement was further examined on surfaces coated with skeletal muscle myosin. The basic effect was similar to that observed with smooth muscle myosin. However, 10-fold greater concentrations of MLCK and CaD were required for a comparable effect. Such an increase in the required concentration was also observed when the velocity of movement was increased by elevation of the temperature during the assay with smooth muscle myosin. Thus, it is the velocity of movement itself that determines the required concentrations of MLCK and CaD.