title: The troponin tail domain promotes a conformational state of the thin filament that suppresses myosin activity.
abstract: In cardiac and skeletal muscles tropomyosin binds to the actin outer domain in the absence of Ca(2+), and in this position tropomyosin inhibits muscle contraction by interfering sterically with myosin-actin binding. The globular domain of troponin is believed to produce this B-state of the thin filament (Lehman, W., Hatch, V., Korman, V. L., Rosol, M., Thomas, L. T., Maytum, R., Geeves, M. A., Van Eyk, J. E., Tobacman, L. S., and Craig, R. (2000) J. Mol. Biol. 302, 593-606) via troponin I-actin interactions that constrain the tropomyosin. The present study shows that the B-state can be promoted independently by the elongated tail region of troponin (the NH(2) terminus (TnT-(1-153)) of cardiac troponin T). In the absence of the troponin globular domain, TnT-(1-153) markedly inhibited both myosin S1-actin-tropomyosin MgATPase activity and (at low S1 concentrations) myosin S1-ADP binding to the thin filament. Similarly, TnT-(1-153) increased the concentration of heavy meromyosin required to support in vitro sliding of thin filaments. Electron microscopy and three-dimensional reconstruction of thin filaments containing TnT-(1-153) and either cardiac or skeletal muscle tropomyosin showed that tropomyosin was in the B-state in the complete absence of troponin I. All of these results indicate that portions of the troponin tail domain, and not only troponin I, contribute to the positioning of tropomyosin on the actin outer domain, thereby inhibiting muscle contraction in the absence of Ca(2+).
authors: Department of Internal Medicine, The University of Iowa, Iowa City, Iowa 52242, USA. larry-tobacman@uiowa.edu
authors: Larry S
authors: LS
authors: Tobacman
authors: Mahta
authors: M
authors: Nihli
authors: Carol
authors: C
authors: Butters
authors: Mark
authors: M
authors: Heller
authors: Victoria
authors: V
authors: Hatch
authors: Roger
authors: R
authors: Craig
authors: William
authors: W
authors: Lehman
authors: Earl
authors: E
authors: Homsher
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002417:Cattle; D004789:Enzyme Activation; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology; D015879:Myosin Subfragments / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011487:Protein Conformation; D011817:Rabbits; D014336:Troponin / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D000199:Actins; D015879:Myosin Subfragments; D014336:Troponin; D017301:Ca(2+) Mg(2+)-ATPase