Effects of Removal and Reconstitution of Myosin Regulatory Light Chain and Troponin C on the Ca²⁺-sensitive ATPase Activity of Myofibrils from Scallop Striated Muscle

Abstract

In order to examine the involvement of troponin-linked Ca²⁺-regulation, in addition to well-known myosin-linked Ca²⁺-regulation, in the contraction of molluscan striated muscle, myofibrils from Ezo-giant scallop striated muscle were desensitized to Ca²⁺ by removing both myosin regulatory light chain and troponin C by treatment with a strong divalent cation chelator, CDTA. The ATPase level in the desensitized myofibrils was about half the maximum level in intact myofibrils regardless of the Ca²⁺-concentration at 25 and 15 C. In the absence of Ca²⁺, the ATPase of the desensitized myofibrils was suppressed by myosin regulatory light chain but not affected by troponin C at either temperature. The ATPase was activated at higher Ca²⁺-concentrations by both myosin regulatory light chain and troponin C, but the activating effects of these two proteins were affected differently by temperature. The activation of ATPase by myosin regulatory light chain was much greater than that by troponin C at 25_??_C, whereas the activation by troponin C was much greater than that by myosin regulatory light chain at 15 C. The maximum activation was only obtained in the presence of both myosin regulatory light chain and troponin C at these temperatures. These findings strongly suggest that the contraction of scallop striated muscle is regulated through both myosin-linked and troponin-linked Ca²⁺-regulation, and that the troponinlinked Ca²⁺-regulation is more significant at lower temperature.