Effects of removal and reconstitution of myosin regulatory light chain andtroponin C on the Ca2+-sensitive ATPase activity of myofibrils from scallop striated muscle

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 musc le 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 1 5 degrees C. In the absence of Ca2+, the ATPase of the desensitized myofibr ils was suppressed by myosin regulatory light chain but not affected by tro ponin C at either temperature, The ATPase was activated at higher Ca2+-conc entrations by both myosin regulatory light chain and troponin C, but the ac tivating effects of these two proteins were affected differently by tempera ture. The activation of ATPase by myosin regulatory light chain was much gr eater than that by troponin C at 25 degrees C, whereas the activation by tr oponin C was much greater than that by myosin regulatory light chain at 15 degrees 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 i s regulated through both myosin-linked and troponin-linked Ca2+-regulation, and that the troponin-linked Ca2+-regulation is more significant at lower temperature.