The conformation of calmodulin: A substantial environmentally sensitive helical transition in Ca 4-calmodulin with potential mechanistic function

Abstract

The conformation of Ca 4-calmodulin in solution, as assessed by far-UV peptide circular dichroism, contains significantly less α-helix than the proposed X-ray crystal structure. We now show that Ca 4-calmodulin adopts significant additional helical structure in solution in the presence of a helicogenic solvent (50%, v/v, aqueous 2,2,2-trifluoroethanol or 50%, v/v, methylpentane-5,5-diol). We suggest that the long continuous helix (residues 66–92 of the crystal structure)is not necessarily a normal feature of the calmodulin structure in solution, and may be due in part to the conditions of crystallisation. This result is supported by time-resolved tyrosine fluorescence anisotropy studies indicating that Ca 4-calmodulin in solution is an essentially compact globular structure which undergoes isotropic rotational motion. We conclude that, under appropriate ionic and apolar environmental conditions, Ca 4-calmodulin undergoes a substantial helical transition, which may involve residues in the central region of the molecule. Such a transition could have an important function in determining specificity and affinity in interactions of calmodulin with different target sequences of Ca 2+-dependent regulatory enzymes.