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 calmo dulin decreased from 21.1 to 19.9 Angstrom when excess Ca2+ ions were added. The profiles of the pair-distribution function suggested that y east calmodulin without Ca2+ has a dumbbell-like shape which changes t oward a rather asymmetric globular shape, from its dumbbell shape, by the binding of Ca2+. In the presence of a calmodulin binding peptide s uch as MLCK-22 (a synthetic peptide corresponding to residues 577-598 of skeletal myosin light chain kinase), the radius of gyration of yeas t calmodulin decreased by 1.6 Angstrom, and the molecular shape of it estimated from the profile of the pair-distribution function was globu lar 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. T hus, the functional differences between yeast and vertebrate calmoduli n which we reported previously [Matsuura, I., et al. (1993) J. Biol. C hem. 268, 13267-13273] have been interpreted on the basis of the struc tural differences between them. Moreover, the structural studies on ch imeric proteins of chicken and yeast calmodulin suggest that Ca2+ bind ing at site IV is essential to form the full active dumbbell structure , which is characteristic of vertebrate-type calmodulin.