title: Structure of the Mg2+-loaded C-lobe of cardiac troponin C bound to the N-domain of cardiac troponin I: comparison with the Ca2+-loaded structure.
abstract: Cardiac troponin C (cTnC) is the Ca(2+)-binding component of the troponin complex and, as such, is the Ca(2+)-dependent switch in muscle contraction. This protein consists of two globular lobes, each containing a pair of EF-hand metal-binding sites, connected by a linker. In the N lobe, Ca(2+)-binding site I is inactive and Ca(2+)-binding site II is primarily responsible for initiation of muscle contraction. The C lobe contains Ca(2+)/Mg(2+)-binding sites III and IV, which bind Mg(2+) with lower affinity and play a structural as well as a secondary role in modulating the Ca(2+) signal. To understand the structural consequences of Ca(2+)/Mg(2+) exchange in the C lobe, we have determined the NMR solution structure of the Mg(2+)-loaded C lobe, cTnC(81-161), in a complex with the N domain of cardiac troponin I, cTnI(33-80), and compared it with a refined Ca(2+)-loaded structure. The overall tertiary structure of the Mg(2+)-loaded C lobe is very similar to that of the refined Ca(2+)-loaded structure as evidenced by the root-mean-square deviation of 0.94 A for all backbone atoms. While metal-dependent conformational changes are minimal, substitution of Mg(2+) for Ca(2+) is characterized by condensation of the C-terminal portion of the metal-binding loops with monodentate Mg(2+) ligation by the conserved Glu at position 12 and partial closure of the cTnI hydrophobic binding cleft around site IV. Thus, conformational plasticity in the Ca(2+)/Mg(2+)-dependent binding loops may represent a mechanism to modulate C-lobe cTnC interactions with the N domain of cTnI.
authors.affiliation: Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati, College of Medicine, 231 Albert Sabin Way, Medical Sciences Building, Cincinnati Ohio 45267-0524, USA.
authors.forename: Natosha L
authors.initials: NL
authors.lastname: Finley
authors.forename: Jack W
authors.initials: JW
authors.lastname: Howarth
authors.forename: Paul R
authors.initials: PR
authors.lastname: Rosevear
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D020013:Calcium Signaling; D002413:Cations, Divalent / Q000378:metabolism; D018360:Crystallography, X-Ray; D008274:Magnesium / Q000378:metabolism*; D051379:Mice; D009206:Myocardium / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism; 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: D002413:Cations, Divalent; D010446:Peptide Fragments; D020033:Protein Isoforms; D011994:Recombinant Proteins; D019209:Troponin C; D019210:Troponin I; D008274:Magnesium; D002118:Calcium