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 Ca2+, 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 (similar to 40 degrees C) of the CaM C-terminal domain, close to physiological temperatures.