Slow Conformational Dynamics and Unfolding of the Calmodulin C-Terminal Domain

Calmodulin (CaM) is a calcium-sensing protein whose physiological function requires large-scale conformational changes. In the absence of Ca 2+, the C-terminal domain of CaM exhibits conformational dynamics on a microseconds time scale. This dynamics has been interpreted as transitions between closed (apo) and open (hololike) conformers based on NMR experiments and, alternatively, as a two-state unfolding process based on temperature-jump fluorescence absorption spectroscopy. We develop a coarse-grained model to study the conformational dynamics of the CaM C-terminal domain. By construction, the coarse Hamiltonian reproduces the experimentally estimated conformer populations, melting temperature, and the microseconds time scale of the structural relaxation process. Simulations of this model suggest a three-state equilibrium among the closed apo form, an open hololike form, and a (partially) unfolded form. At lower temperatures, the well-folded apo conformation exchanges with hololike conformers; at elevated temperatures, the dynamics is dominated by exchange between the apo and unfolded states. The resulting dynamics of the C-terminal domain appears to be effectively two-state-like, with the respective third state being only scarcely populated at most temperatures. The transition between the two dominant exchange mechanisms occurs near the melting temperature (∼40 °C) of the CaM C-terminal domain, close to physiological temperatures.