REVERSAL OF CALDESMON BINDING TO MYOSIN WITH CALCIUM-CALMODULIN OR BYPHOSPHORYLATING CALDESMON

Caldesmon, an actin-binding protein from smooth muscle and non-muscle cells, has previously been shown to bind stoichiometrically to smooth muscle myosin in an ATP-dependent manner. We now show quantitatively t he effects of Ca2+-calmodulin and phosphorylation on the binding of ca ldesmon to myosin. Ca2+-calmodulin reduces the binding of caldesmon to myosin with the same effectiveness as it does the binding of caldesmo n to actin. However, Ca2+-calmodulin is ineffective in antagonizing th e binding of the purified myosin-binding region of caldesmon to myosin . These and other results suggest that Ca2+-calmodulin binding to the COOH-terminal region of caldesmon is responsible for reversal of bindi ng to myosin. Phosphorylation of the NH2-terminal region of caldesmon by the co-purifying kinase, calmodulin-dependent protein kinase II, we akens but does not eliminate the binding of caldesmon to smooth muscle myosin. Finally, phosphorylation of smooth muscle myosin by smooth mu scle myosin light chain kinase has no effect on the binding of caldesm on to myosin. Since Ca2+-calmodulin and phosphorylation of caldesmon w eaken the binding of caldesmon to both actin and myosin, these events may be coordinately regulated.