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

Background: Calmodulin is a ubiquitous Ca2+-activated regulator of cellular processes in eukaryotes. The structures of the Ca2+-free (apo) and Ca2+-loaded states of calmodulin have revealed that Ca2+ 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. Results: The dynamics of the transition between closed and open conformations in the Ca2+-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 mus, with a mean of 21 +/- 3 mus. 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 Ca2+-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) = (2.7 +/- 1.0) x 10(4) s(-1). Conclusions: 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 Ca2+ off rate, suggesting that Ca2+ release may be gated by the conformational dynamics. Structural interpretation of estimated chemical shifts suggests a mechanism for ion release.