title: Molecular dynamics simulations of the cardiac troponin complex performed with FRET distances as restraints.
abstract: Cardiac troponin (cTn) is the Ca(2+)-sensitive molecular switch that controls cardiac muscle activation and relaxation. However, the molecular detail of the switching mechanism and how the Ca(2+) signal received at cardiac troponin C (cTnC) is communicated to cardiac troponin I (cTnI) are still elusive. To unravel the structural details of troponin switching, we performed ensemble Förster resonance energy transfer (FRET) measurements and molecular dynamic (MD) simulations of the cardiac troponin core domain complex. The distance distributions of forty five inter-residue pairs were obtained under Ca(2+)-free and saturating Ca(2+) conditions from time-resolved FRET measurements. These distances were incorporated as restraints during the MD simulations of the cardiac troponin core domain. Compared to the Ca(2+)-saturated structure, the absence of regulatory Ca(2+) perturbed the cTnC N-domain hydrophobic pocket which assumed a closed conformation. This event partially unfolded the cTnI regulatory region/switch. The absence of Ca(2+), induced flexibility to the D/E linker and the cTnI inhibitory region, and rotated the cTnC N-domain with respect to rest of the troponin core domain. In the presence of saturating Ca(2+) the above said phenomenon were absent. We postulate that the secondary structure perturbations experienced by the cTnI regulatory region held within the cTnC N-domain hydrophobic pocket, coupled with the rotation of the cTnC N-domain would control the cTnI mobile domain interaction with actin. Concomitantly the rotation of the cTnC N-domain and perturbation of the D/E linker rigidity would control the cTnI inhibitory region interaction with actin to effect muscle relaxation.
authors.affiliation: Voiland School of Chemical Engineering and Bioengineering and The Department of Integrated Physiology and Neuroscience, Washington State University, Pullman, Washington, United States of America.
authors.forename: Jayant James
authors.initials: JJ
authors.lastname: Jayasundar
authors.affiliation: Voiland School of Chemical Engineering and Bioengineering and The Department of Integrated Physiology and Neuroscience, Washington State University, Pullman, Washington, United States of America.
authors.forename: Jun
authors.initials: J
authors.lastname: Xing
authors.affiliation: Department of Chemistry and Biochemistry, South Dakota State University, Brookings, South Dakota, United States of America.
authors.forename: John M
authors.initials: JM
authors.lastname: Robinson
authors.affiliation: The Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, Alabama, United States of America.
authors.forename: Herbert C
authors.initials: HC
authors.lastname: Cheung
authors.affiliation: Voiland School of Chemical Engineering and Bioengineering and The Department of Integrated Physiology and Neuroscience, Washington State University, Pullman, Washington, United States of America.
authors.forename: Wen-Ji
authors.initials: WJ
authors.lastname: Dong
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer / Q000379:methods*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008024:Ligands; D051379:Mice; D015233:Models, Statistical; D056004:Molecular Dynamics Simulation; D009206:Myocardium / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry; D055672:Static Electricity; D019209:Troponin C / Q000378:metabolism*
chemical_list: D000199:Actins; D008024:Ligands; D011994:Recombinant Proteins; D019209:Troponin C; D002118:Calcium