title: Identification of a region of troponin I important in signaling cross-bridge-dependent activation of cardiac myofilaments.
abstract: Force generating strong cross-bridges are required to fully activate cardiac thin filaments, but the molecular signaling mechanism remains unclear. Evidence demonstrating differential extents of cross-bridge-dependent activation of force, especially at acidic pH, in myofilaments in which slow skeletal troponin I (ssTnI) replaced cardiac TnI (cTnI) indicates the significance of a His in ssTnI that is an homologous Ala in cTnI. We compared cross-bridge-dependent activation in myofilaments regulated by cTnI, ssTnI, cTnI(A66H), or ssTnI(H34A). A drop from pH 7.0 to 6.5 induced enhanced cross-bridge-dependent activation in cTnI myofilaments, but depressed activation in cTnI(A66H) myofilaments. This same drop in pH depressed cross-bridge-dependent activation in both ssTnI myofilaments and ssTnI(H34A) myofilaments. Compared with controls, cTnI(A66H) myofilaments were desensitized to Ca(2+), whereas there was no difference in the Ca(2+)-force relationship between ssTnI and ssTnI(H34A) myofilaments. The mutations in cTnI and ssTnI did not affect Ca(2+) dissociation rates from cTnC at pH 7.0 or 6.5. However, at pH 6.5, cTnI(A66H) had lower affinity for cTnT than cTnI. We also probed cross-bridge-dependent activation in myofilaments regulated by cTnI(Q56A). Myofilaments containing cTnI(Q56A) demonstrated cross-bridge-dependent activation that was similar to controls containing cTnI at pH 7.0 and an enhanced cross-bridge-dependent activation at pH 6.5. We conclude that a localized N-terminal region of TnI comprised of amino acids 33-80, which interacts with C-terminal regions of cTnC and cTnT, is of particular significance in transducing signaling of thin filament activation by strong cross-bridges.
authors.affiliation: Center for Cardiovascular Research, Department of Physiology and Biophysics, University of Illinois College of Medicine, Chicago, Illinois 60612, USA.
authors.forename: Patti L
authors.initials: PL
authors.lastname: Engel
authors.forename: Tomoyoshi
authors.initials: T
authors.lastname: Kobayashi
authors.forename: Brandon
authors.initials: B
authors.lastname: Biesiadecki
authors.forename: Jonathan
authors.initials: J
authors.lastname: Davis
authors.forename: Svetlana
authors.initials: S
authors.lastname: Tikunova
authors.forename: Steven
authors.initials: S
authors.lastname: Wu
authors.forename: R John
authors.initials: RJ
authors.lastname: Solaro
mesh_terms: D000818:Animals; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D003001:Cloning, Molecular; D003432:Cross-Linking Reagents / Q000494:pharmacology; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D051379:Mice; D009154:Mutation; D009206:Myocardium / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D015398:Signal Transduction; D019210:Troponin I / Q000737:chemistry*
chemical_list: D003432:Cross-Linking Reagents; D019210:Troponin I; D005982:Glutathione Transferase; D002118:Calcium