Inhibition of Actin Stimulation of Skeletal Muscle (A1)S-1 ATPase Activity by Caldesmon

We have previously shown that caldesmon inhibits the actin-activated ATPase activity of myosin subfragments in parallel with inhibition of myosin subfragment · ATP binding to actin (M. E. Hemric, and J. M. Chalovich, 1988, J. Biol. Chem. 263, 1878-1885; L. Velaz, R. H. Ingraham, and J. M. Chalovich, 1990, J. Biol. Chem. 265, 2929-2934). From these data, we suggested that caldesmon is a competitive inhibitor of binding of myosin subfragment-1 to actin. To confirm this result, we now show the effect of caldesmon on the steady-state parameters of ATP hydrolysis by (A1)S-1 at increasing actin concentrations. Low ionic strength conditions were used to maximize the interaction between (AL)S-1 and actin. In both the presence and absence of smooth muscle tropomyosin, caldesmon caused a twofold decrease in the kcat and more than a 12-fold change in the KATPase. Therefore, competition of binding of myosin to actin by caldesmon contributes to the reduction in ATPase activity in both the presence and the absence of tropomyosin.