MOTIONS OF CALMODULIN CHARACTERIZED USING BOTH BRAGG AND DIFFUSE-X-RAY SCATTERING

Background: Calmodulin is a calcium-activated regulatory protein which can bind to many different targets. The protein resembles a highly fl exible dumbbell, and bends in the middle as it binds, This and other m otions must be understood to formulate a realistic model of calmodulin function. Results: Using the Bragg reflections from X-ray crystallogr aphy, a multiple-conformer refinement of a calmodulin-peptide complex shows anisotropic displacements, with high variations of dihedral angl es in several nonhelical domains: the flexible linker; three of the fo ur calcium-binding sites (including both of the N-terminal sites); and a turn connecting the C-terminal EF-hand calcium-binding domains, Thr ee-dimensional maps of the large scale diffuse X-ray scattering data s how isotropic liquid-like motions with an unusually small correlation length. Three-dimensional maps of the small scale diffuse streaks show highly coupled, anisotropic motions along the head-to-tail molecular packing direction in the unit cell, There is also weak coupling perpen dicular to the head-to-tail packing direction, particularly across a c avity occupied by the disordered linker domain of the molecule. Conclu sions: Together, the Bragg and diffuse scattering present a self-consi stent description of the motions in the flexible linker of calmodulin, The other mobile regions of the protein are also of great interest, I n particular, the high variations in the calcium-binding sites are lik ely to influence how strongly they bind ions. This is especially impor tant in the N-terminal sites, which regulate the activity of the molec ule.