Calmodulin substitutes the activating action of troponin C and myosin regulatory light chain on the Ca²⁺-sensitive ATPase activity of myofibrils from scallop striated muscle

Two types of Ca²⁺-regulations for the contraction in scallop striated myofibrils, i. e., myosin-linked and troponin-linked Ca²⁺-regulations, were desensitized by removing both myosin regulatory light chain and troponin C from myofibrils by treatment with CDTA, and the effect of reconstitution with myosin regulatory light chain, troponin C, and bovine brain calmodulin was then examined on the Ca²⁺-sensitive ATPase activity of the desensitized myofibrils. The ATPase of the desensitized myofibrils was about half the maximum activity of the intact myofibrils regardless of Ca²⁺-concentrations. At low Ca²⁺, the ATPase of the desensitized myofibrils was inhibited by myosin regulatory light chain but was not affected by troponin C and calmodulin, whereas the ATPase at higher Ca²⁺-concentrations was activated by these proteins. The ATPase activation by calmodulin at high Ca²⁺ was larger than that of myosin regulatory light chain or troponin C and reached almost the same level as the ATPase activity of intact myofibrils. This activating effect of calmodulin was not affected by myosin regulatory light chain and troponin C. These results suggest that, at higher Ca²⁺-concentrations, calmodulin activate the ATPase activity of the desensitized myofibrils by substituting both troponin C and myosin regulatory light chain.