Reversal of caldesmon binding to myosin with calcium-calmodulin or by phosphorylating 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 the effects of Ca(2+)-calmodulin and phosphorylation on the binding of caldesmon to myosin. Ca(2+)-calmodulin reduces the binding of caldesmon to myosin with the same effectiveness as it does the binding of caldesmon to actin. However, Ca(2+)-calmodulin is ineffective in antagonizing the binding of the purified myosin-binding region of caldesmon to myosin. These and other results suggest that Ca(2+)-calmodulin binding to the COOH-terminal region of caldesmon is responsible for reversal of binding to myosin. Phosphorylation of the NH2-terminal region of caldesmon by the co-purifying kinase, calmodulin-dependent protein kinase II, weakens but does not eliminate the binding of caldesmon to smooth muscle myosin. Finally, phosphorylation of smooth muscle myosin by smooth muscle myosin light chain kinase has no effect on the binding of caldesmon to myosin. Since Ca(2+)-calmodulin and phosphorylation of caldesmon weaken the binding of caldesmon to both actin and myosin, these events may be coordinately regulated.