Proton NMR study of rabbit skeletal muscle troponin C: magnesium-induced conformational change

Authors:
  • KOKenji Ogura
    Kenji Ogura
  • YHYasushi Hasegawa
    Yasushi Hasegawa
  • KYKoichi Yagi
    Koichi Yagi
  • KHKunio Hikichi
    Kunio Hikichi
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Abstract
Binding of Mg2+ to rabbit skeletal muscle troponin C (TnC) is studied by means of two-dimensional (2D) 1H NMR spectroscopy. Using the sequence-specific resonance assignment method we assign several resonances of TnC in the Mg2+-saturated state. Assigned resonances are used as probes of the following titration experiments: (1) Mg2+ titration of apo-TnC, (2) Mg2+ titration of Ca2TnC, and (3) Mg2+ titration of Ca4TnC. In experiment 1, the slow-exchange behavior is observed for resonances of Phe99, Asp107, Gly108, Tyr109, Ile110, Asp111, His125, Gly144, Arg145, Ile146, Asp147, and Phe 148 located at the high-affinity Ca2+-binding sites in the C-terminal-half domain. In experiments 1 and 2, the fast-exchange behavior is observed for resonances of Gly32, Asp33, Ser35, Gly68, Thr69, and Asp71 located at the low-affinity Ca2+-binding sites in the N-terminal-half domain. These results suggest that Mg2+ ions bindi to the N domain as well as the C domain. In experiment 3, no spectral change is observed for all above-mentioned residues in the C domain and also for Gly32 and Gly68 in the N domain. It can be concluded that all Ca2+-binding sites in both the N and C domains can bind Mg2+ ions. No significant change is observed for resonances of Phe23, Ile34, Val68, and Phe72 in experiments 1 and 2. These results suggest that Mg2+ binding to the N domain does not induce conformational change in the hydrophobic region of the N domain. 2D-NMR spectra and Mg2+-titration data suggest that the antiparallel .beta.-sheet conformation is formed in both the N and C domains when Mg2+ ions bind to the two domains.

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