title: Magnesium-calcium exchange in cardiac troponin C bound to cardiac troponin I.
abstract: Understanding the process of Ca(2+)/Mg(2+)exchange during muscle excitation and relaxation is fundamental to elucidating the mechanism of Ca(2+)-regulated muscle contraction. During the resting phase, the C-domain of cardiac troponin C may be occupied by either Ca(2+)or Mg(2+). Here, complexes of recombinant cardiac troponin C(81-161) and the N terminus of cardiac troponin I, representing residues 33-80, were generated in the presence of saturating Mg(2+). Heteronuclear multi-dimensional nuclear magnetic resonance experiments were used to obtain backbone assignments of the Mg(2+)-loaded complex. In the presence of cardiac troponin I, the affinity of site IV for Mg(2+)is increased. Comparison of Mg(2+)and Ca(2+)-loaded complexes reveals that chemical shift differences are primarily localized to metal-binding sites III and IV, defining positions within these sites that have distinct Ca(2+)/Mg(2+)conformations. The observed transition from the Mg(2+)-loaded to Ca(2+)-loaded form demonstrates that sites III and IV fill simultaneously with Ca(2+)displacing Mg(2+). However, even in the absence of excess Ca(2+), Mg(2+)does not readily displace Ca(2+)in the isolated binary complex. Thus, the Mg(2+)-loaded conformer may only represent a small fraction of the total cardiac troponin C found in the sarcomere.
authors.affiliation: Department of Molecular Genetics, Biochemistry, and Microbiology, College of Medicine, University of Cincinnati, 231 Bethseda Avenue, Cincinnati, Ohio 45267, USA.
authors.forename: N
authors.initials: N
authors.lastname: Finley
authors.forename: A
authors.initials: A
authors.lastname: Dvoretsky
authors.forename: P R
authors.initials: PR
authors.lastname: Rosevear
mesh_terms: D017920:Antiporters / Q000378:metabolism*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D027682:Cation Transport Proteins; D004591:Electrophoresis, Polyacrylamide Gel; D008024:Ligands; D008274:Magnesium / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D009206:Myocardium / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D019209:Troponin C / Q000737:chemistry / Q000378:metabolism*; D019210:Troponin I / Q000737:chemistry / Q000378:metabolism*
chemical_list: D017920:Antiporters; D027682:Cation Transport Proteins; D008024:Ligands; D011994:Recombinant Proteins; D019209:Troponin C; D019210:Troponin I; C115377:calcium magnesium antiporter; D008274:Magnesium; D002118:Calcium