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Two types of Ca2+-regulations for the contraction in scallop striated myofibrils, 2'. e., myosinlinked 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 Ca”, 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 Ca”-concentrations was activated by these proteins. The ATPase activation by calmodulin at high Ca“ 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. The contraction of scallop striated muscle is regulated by two types of Ca2‘"—regulations, 1'. e., myosin—linked and troponin-linl<ed Ca2+-regulations (9). Both types of Ca2+—regulations in scallop striated myofibrils have been demonstrated to be desensitized by removing regulatory light chain (RLC) from myosin and Ca2+-binding Abbreviations; A, actin; TM, tropomyosin; TN-T, I and C, troponin T, I and C; ELC, myosin essential light chain; RLC, myosin regulatory light chain; CaM, calmodulin. Correspondence to: Dr Fumie Shiraishi at the above address. Tel: (092)642-6082 Fax: (092)642-6084 Email: shirai@clipharm.med.kyushu-u.ac.jp subunit (troponin C; TN-C) from troponin by treatment with a strong divalent cation chelator, CDTA. The ATPase of the CDTA—treated myofibrils from scallop striated muscle was activated regardless of Ca2+—concentrations and the Ca”-sensitivity was fully recovered after being reconstituted with both RLC and TN-C (9)Calmodulin (CaM) is a Ca2+—receptive protein for many biological functions. This protein has been shown to replace the Ca2+-—-regulating action of TN-C on the ATPase of troponin C—depleted myofibrils from vertebrate striated muscle (5). The present study was undertaken to investigate the effect of replacement of TN-C with calmodulin on the Ca2+—regulation of the contraction of invertebrate scallop striated muscle. Examination of the effects of bovine brain CaM,

biomedical research

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