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

Two types of Ca2+-regulations for the contraction in scallop striated myofi brils, i. e., myosin-linked and troponin-linked Ca2+-regulations, were dese nsitized by removing both myosin regulatory light chain and troponin C from myofibrils by treatment with CDTA, and the effect of reconstitution with m yosin regulatory light chain, troponin C, and bovine brain calmodulin was t hen examined on the Ca2+-sensitive ATPase activity of the desensitized myof ibrils. The ATPase of the desensitized myofibrils was about half the maximu m activity of the intact myofibrils regardless of Ca2+-concentrations. At l ow Ca2+, the ATPase of the desensitized myofibrils was inhibited by myosin regulatory light chain but was not affected by troponin C and calmodulin, w hereas the ATPase at higher Ca2+-concentrations was activated by these prot eins. 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 troponi n 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.