Solution X-ray Scattering Data Show Structural Differences between Yeast and Vertebrate Calmodulin: Implications for Structure/Function

We present here the first evidence, obtained by the use of solution X-ray scattering, of the solution structure of yeast calmodulin, a poor activator of vertebrate enzymes. The radius of gyration of yeast calmodulin decreased from 21.1 to 19.9 Å when excess Ca2+ ions were added. The profiles of the pair-distribution function suggested that yeast calmodulin without Ca2+ has a dumbbell-like shape which changes toward a rather asymmetric globular shape, from its dumbbell shape, by the binding of Ca2+. In the presence of a calmodulin binding peptide such as MLCK-22 (a synthetic peptide corresponding to residues 577−598 of skeletal myosin light chain kinase), the radius of gyration of yeast calmodulin decreased by 1.6 Å, and the molecular shape of it estimated from the profile of the pair-distribution function was globular but less compact than that of vertebrate calmodulin. These results for the structure of yeast calmodulin complexed with Ca2+ and with Ca2+−peptides are quite different from those of vertebrate calmodulin. Thus, the functional differences between yeast and vertebrate calmodulin which we reported previously [Matsuura, I., et al. (1993) J. Biol. Chem. 268, 13267−13273] have been interpreted on the basis of the structural differences between them. Moreover, the structural studies on chimeric proteins of chicken and yeast calmodulin suggest that Ca2+ binding at site IV is essential to form the full active dumbbell structure, which is characteristic of vertebrate-type calmodulin.