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Conformational Selection and Functional Dynamics of Calmodulin: A 19F Nuclear Magnetic Resonance Study

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journal contribution
posted on 16.09.2014, 00:00 by Joshua Hoang, R. Scott Prosser
Calcium-bound calmodulin (CaM-4Ca2+) is innately promiscuous with regard to its protein interaction network within the cell. A key facet of the interaction process involves conformational selection. In the absence of a binding peptide, CaM-4Ca2+ adopts an equilibrium between a native state (N) and a weakly populated near-native peptide-bound-like state (I), whose lifetime is on the order of 1.5 ms at 37 °C, based on 19F nuclear magnetic resonance (NMR) Carr–Purcell–Meiboom–Gill (CPMG) relaxation dispersion measurements. This peptide-bound-like state of CaM-4Ca2+ is entropically stabilized (ΔS = 280 ± 35 J mol–1 K–1) relative to the native state, water-depleted, and likely parental to specific bound states. Solvent depletion, conformational selection, and flexibility of the peptide-bound-like state may be important in priming the protein for binding. At higher temperatures, the exchange rate, kex, appears to markedly slow, suggesting the onset of misfolded or off-pathway states, which retards interconversion between N and I. 19F NMR CPMG relaxation dispersion experiments with both CaM-4Ca2+ and the separate N-terminal and C-terminal domains reveal the cooperative role of the two domains in the binding process and the flexibility of the N-terminal domain in facilitating binding. Thus, when calcium binds, calmodulin establishes its interaction with a multitude of protein binding partners, through a combination of conformational selection to a state that is parental to the peptide-bound state and, finally, induced fit.