title: Effects of thin and thick filament proteins on calcium binding and exchange with cardiac troponin C.
abstract: Understanding the effects of thin and thick filament proteins on the kinetics of Ca(2+) exchange with cardiac troponin C is essential to elucidating the Ca(2+)-dependent mechanisms controlling cardiac muscle contraction and relaxation. Unlike labeling of the endogenous Cys-84, labeling of cardiac troponin C at a novel engineered Cys-53 with 2-(4'-iodoacetamidoanilo)napthalene-6-sulfonic acid allowed us to accurately measure the rate of calcium dissociation from the regulatory domain of troponin C upon incorporation into the troponin complex. Neither tropomyosin nor actin alone affected the Ca(2+) binding properties of the troponin complex. However, addition of actin-tropomyosin to the troponin complex decreased the Ca(2+) sensitivity ( approximately 7.4-fold) and accelerated the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 2.5-fold). Subsequent addition of myosin S1 to the reconstituted thin filaments (actin-tropomyosin-troponin) increased the Ca(2+) sensitivity ( approximately 6.2-fold) and decreased the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 8.1-fold), which was completely reversed by ATP. Consistent with physiological data, replacement of cardiac troponin I with slow skeletal troponin I led to higher Ca(2+) sensitivities and slower Ca(2+) dissociation rates from troponin C in all the systems studied. Thus, both thin and thick filament proteins influence the ability of cardiac troponin C to sense and respond to Ca(2+). These results imply that both cross-bridge kinetics and Ca(2+) dissociation from troponin C work together to modulate the rate of cardiac muscle relaxation.
authors.affiliation: Department of Physiology and Cell Biology, The Ohio State University, Columbus, Ohio 43210, USA. davis.812@osu.edu
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Davis
authors.forename: Catalina
authors.initials: C
authors.lastname: Norman
authors.forename: Tomoyoshi
authors.initials: T
authors.lastname: Kobayashi
authors.forename: R John
authors.initials: RJ
authors.lastname: Solaro
authors.forename: Darl R
authors.initials: DR
authors.lastname: Swartz
authors.forename: Svetlana B
authors.initials: SB
authors.lastname: Tikunova
mesh_terms: D000199:Actins / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D008297:Male; D009200:Myocardial Contraction / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D011485:Protein Binding; D051381:Rats; D012518:Sarcomeres / Q000502:physiology*; D019209:Troponin C / Q000378:metabolism*
chemical_list: D000199:Actins; D019209:Troponin C; D009218:Myosins; D002118:Calcium