The effects of caldesmon on the ATPase activities of rabbit skeletal-muscle myosin.

We studied the effects of caldesmon, a major actin- and calmodulin-binding protein found in a variety of muscle and non-muscle tissues, on the various ATPase activities of skeletal-muscle myosin. Caldesmon inhibited the actin-activated myosin Mg2+-ATPase, and this inhibition was enhanced by tropomyosin. In the presence of the troponin complex and tropomyosin, caldesmon inhibited the Ca2+-dependent actomyosin Mg2+-ATPase; this inhibition could be partly overcome by Ca2+/calmodulin. Caldesmon, phosphorylated to the extent of approximately 4 mol of Pi/mol of caldesmon, inhibited the actin-activated myosin Mg2+-ATPase to the same extent as did non-phosphorylated caldesmon. Both inhibitions could be overcome by Ca2+/calmodulin. Caldesmon also inhibited the Mg2+-ATPase activity of skeletal-muscle myosin in the absence of actin; this inhibition also could be overcome by Ca2+/calmodulin. Caldesmon inhibited the Ca2+-ATPase activity of skeletal-muscle myosin in the presence or absence of actin, at both low (0.1 M-KCl) and high (0.3 M-KCl) ionic strength. Finally, caldesmon inhibited the skeletal-muscle myosin K+/EDTA-ATPase at 0.1 M-KCl, but not at 0.3 M-KCl. Addition of actin resulted in no inhibition of this ATPase by caldesmon at either 0.1 M- or 0.3 M-KCl. These observations suggest that caldesmon may function in the regulation of actin-myosin interactions in striated muscle and thereby modulate the contractile state of the muscle. The demonstration that caldesmon inhibits a variety of myosin ATPase activities in the absence of actin indicates a direct effect of caldesmon on myosin. The inhibition of the actin-activated Mg2+-ATPase activity of myosin (the physiological activity) may not be due therefore simply to the binding of caldesmon to the actin filament causing blockage of myosin-cross-bridge-actin interaction.