The 1.0 Å crystal structure of Ca2+-bound calmodulin: an analysis of disorder and implications for functionally relevant plasticity

Calmodulin (CaM) is a highly conserved 17 kDa eukaryotic protein that can bind specifically to over 100 protein targets in response to a Ca2+ signal. Ca2+-CaM requires a considerable degree of structural plasticity to accomplish this physiological role; however, the nature and extent of this plasticity remain poorly characterized. Here, we present the 1.0 Å crystal structure of Paramecium tetraurelia Ca2+-CaM, including 36 discretely disordered residues and a fifth Ca2+ that mediates a crystal contact. The 36 discretely disordered residues are located primarily in the central helix and the two hydrophobic binding pockets, and reveal correlated side-chain disorder that may assist target-specific deformation of the binding pockets. Evidence of domain displacements and discrete backbone disorder is provided by translation-libration-screw (TLS) analysis and multiconformer models of protein disorder, respectively. In total, the evidence for disorder at every accessible length-scale in Ca2+-CaM suggests that the protein occupies a large number of hierarchically arranged conformational substates in the crystalline environment and may sample a quasi-continuous spectrum of conformations in solution. Therefore, we propose that the functionally distinct forms of CaM are less structurally distinct than previously believed, and that the different activities of CaM in response to Ca2+ may result primarily from Ca2+-mediated alterations in the dynamics of the protein.