Reduced positive feedback regulation between myosin crossbridge and cardiac troponin C in fast skeletal myofibrils.
Several studies have shown that substitution of cardiac troponin C into fast skeletal muscle causes a marked reduction in cooperativity of Ca(2+)-activation of both myofibrillar ATPase and tension development. To clarify the underlying mechanisms, in the present study, Ca2+ binding to cardiac troponin C inserted into fast skeletal myofibrils was measured. Two classes of binding sites with different affinities (classes 1 and 2) were clearly identified, which were equivalent stoichiometrically to the two high-affinity sites (sites III and IV) and a single low-affinity site (site II) of troponin C, respectively. Ca2+ binding to class-2 sites and Ca(2+)-activation of myofibrillar ATPase occurred in roughly the same Ca2+ concentration range, indicating that site II is responsible for Ca2+ -regulation. Myosin crossbridge interactions with actin, both in the presence and absence of ATP, enhanced the Ca2+ binding affinity of only class-2 sites. These effects of myosin crossbridges, however, were much smaller than the effects on the Ca2+ binding to the low-affinity sites of fast skeletal troponin C, which are responsible for regulating fast skeletal myofibrillar ATPase. These findings provide strong evidence that the reduction in the cooperative response to Ca2+ upon substituting cardiac troponin C into fast skeletal myofibrils is due to a decrease in the positive feedback interaction between myosin crossbridge attachment and Ca2+ binding to the regulatory site of troponin C.
Morimoto
S
S
Department of Pharmacology, Faculty of Medicine, Kyushu University, Fukuoka.
Ohtsuki
I
I
D000199:Actins / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D004789:Enzyme Activation; D005246:Feedback / Q000502:physiology*; D008297:Male; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism; D009210:Myofibrils / Q000201:enzymology / Q000378:metabolism*; D018994:Myosin Light Chains / Q000032:analysis; D009218:Myosins / Q000378:metabolism*; D011817:Rabbits; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
D000199:Actins; D018994:Myosin Light Chains; D014336:Troponin; D019209:Troponin C; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D009218:Myosins; D002118:Calcium