title: The Ca2+/Mg2+ sites of troponin C modulate crossbridge-mediated thin filament activation in cardiac myofibrils.
abstract: The Ca(2+)/Mg(2+) sites (III and IV) located in the C-terminal domain of cardiac troponin C (cTnC) have been generally considered to play a purely structural role in keeping the cTnC bound to the thin filament. However, several lines of evidence, including the discovery of cardiomyopathy-associated mutations in the C-domain, have raised the possibility that these sites may have a more complex role in contractile regulation. To explore this possibility, the ATPase activity of rat cardiac myofibrils was assayed under conditions in which no Ca(2+) was bound to the N-terminal regulatory Ca(2+)-binding site (site II). Myosin-S1 was treated with N-ethylmaleimide to create strong-binding myosin heads (NEM-S1), which could activate the cardiac thin filament in the absence of Ca(2+). NEM-S1 activation was assayed at pCa 8.0 to 6.5 and in the presence of either 1mM or 30 μM free Mg(2+). ATPase activity was maximal when sites III and IV were occupied by Mg(2+) and it steadily declined as Ca(2+) displaced Mg(2+). The data suggest that in the absence of Ca(2+) at site II strong-binding myosin crossbridges cause the opening of more active sites on the thin filament if the C-domain is occupied by Mg(2+) rather than Ca(2+). This finding could be relevant to the contraction-relaxation kinetics of cardiac muscle. As Ca(2+) dissociates from site II of cTnC during the early relaxing phase of the cardiac cycle, residual Ca(2+) bound at sites III and IV might facilitate the switching off of the thin filament and the detachment of crossbridges from actin.
authors.affiliation: Boston Biomedical Research Institute, 64 Grove Street, Watertown, MA 02472, USA.
authors.forename: Franklin
authors.initials: F
authors.lastname: Fuchs
authors.forename: Zenon
authors.initials: Z
authors.lastname: Grabarek
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D009210:Myofibrils / Q000378:metabolism*; D051381:Rats; D019209:Troponin C / Q000378:metabolism*
chemical_list: D019209:Troponin C; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002118:Calcium