Dynamics of the transition between open and closed conformations in a calmodulin C-terminal domain mutant

Calmodulin is a ubiquitous Ca(2+)-activated regulator of cellular processes in eukaryotes. The structures of the Ca(2+)-free (apo) and Ca(2+)-loaded states of calmodulin have revealed that Ca(2+) binding is associated with a transition in each of the two domains from a closed to an open conformation that is central to target recognition. However, little is known about the dynamics of this conformational switch.

The dynamics of the transition between closed and open conformations in the Ca(2+)-loaded state of the E140Q mutant of the calmodulin C-terminal domain were characterized under equilibrium conditions. The exchange time constants (tau(ex)) measured for 42 residues range from 13 to 46 micros, with a mean of 21 +/- 3 micros. The results suggest that tau(ex) varies significantly between different groups of residues and that residues with similar values exhibit spatial proximity in the structures of apo and/or Ca(2+)-saturated wild-type calmodulin. Using data for one of these groups, we obtained an open population of p(o) = 0.50 +/- 0.17 and a closed --> open rate constant of k(o) = x 10(4) s(-1).

The conformational exchange dynamics appear to involve locally collective processes that depend on the structural topology. Comparisons with previous results indicate that similar processes occur in the wild-type protein. The measured rates match the estimated Ca(2+) off rate, suggesting that Ca(2+) release may be gated by the conformational dynamics. Structural interpretation of estimated chemical shifts suggests a mechanism for ion release.