Abstract
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 k cat and more than a 12-fold change in the K ATPase. 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.
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