NMR Studies of the E140Q Mutant of the Carboxy-Terminal Domain of Calmodulin Reveal Global Conformational Exchange in the Ca2+-Saturated State

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Abstract
In the present investigation, the Ca2+ activation of the C-terminal domain of bovine calmodulin and the effects of replacing the bidentate Ca2+-coordinating glutamic acid residue in the 12th and last position of loop IV with a glutamine are studied by NMR spectroscopy. The mutation E140Q results in sequential Ca2+ binding in this domain and has far-reaching effects on the structure of (Ca2+)2 TR2C, thereby providing further evidence for the critical role of this glutamic acid residue for the Ca2+-induced conformational change of regulatory EF-hand proteins. Analyses of the NOESY spectra of the mutant under Ca2+-saturated conditions, such that 97% of the protein is in the (Ca2+)2 form, revealed two sets of mutually exclusive NOEs. One set of NOEs is found to be consistent with the closed structure observed in the apo state of the C-terminal domain of the wild-type protein, while the other set supports the open structure observed in the Ca2+-saturated state. In addition, several residues in the hydrophobic core exhibit broadened resonances. We conclude that the (Ca2+)2 form of the mutant experiences a global conformational exchange between states similar to the closed and open conformations of the C-terminal domain of wild-type calmodulin. A population of 65 ± 15% of the open conformation and an exchange rate of (1−7) × 104 s-1 were estimated from the NMR data and the chemical shifts of the wild-type protein. From a Ca2+ titration of the 15N-labeled mutant, the macroscopic binding constants [log(K1) = 4.9 ± 0.3 and log(K2) = 3.15 ± 0.10] and the inherent chemical shifts of the intermediate (Ca2+)1 form of the mutant were determined using NMR. Valuable information was also provided on the mechanism of the Ca2+ activation and the roles of the structural elements in the two Ca2+-binding events. Comparison with the wild-type protein indicates that the (Ca2+)1 conformation of the mutant is essentially closed but that some rearrangement of the empty loop IV toward the Ca2+-bound form has occurred.