No abstract

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 angstroms 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 angstroms, 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 Ca(2+)-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.

Two-stage melting of the dilute gels of poly(γ-benzyl l-glutamate) in benzene has been studied by differential scanning calorimetry, wide-angle X-ray diffraction, and small-angle X-ray scattering using synchrotron radiation. Differential scanning calorimetry measurements show that the traces on heating exhibit two endothermic peaks at about 25 °C and at about 31 °C, thereby indicating the melting of two types of aggregates, while those on cooling exhibit no exotherm even at 0 °C, thereby indicating a large thermal hysteresis. Wide-angle X-ray diffraction measurements show that the profiles yield little of interest other than diffuse halos from the solvent except a diffuse small-angle scattering, thereby indicating that the aggregates do not contain a solid crystalline phase but a crystal−solvate phase. Small-angle X-ray scattering measurements show that the two-stage melting of the aggregates can be attributed to a melting first of the bundles of three rods and then of the bundles of two rods and that, above the two-stage melting temperature, the gels transform into the dilute and isotropic solutions, in which the poly(γ-benzyl l-glutamate) stiff chains are randomly dispersed and do not interact with each other. We suggest that the intermolecular interactions causing the bundles to aggregate are induced by the solvent.