title: Rotational dynamics of calcium-free calmodulin studied by 15N-NMR relaxation measurements.
abstract: The backbone motions of calcium-free Xenopus calmodulin have been characterized by measurements of the 15N longitudinal relaxation times (T1) at 51 and 61 MHz, and by conducting transverse relaxation (T2), spin-locked transverse relaxation (T1 rho), and 15N-[1H] heteronuclear NOE measurements at 61 MHz 15N frequency. Although backbone amide hydrogen exchange experiments indicate that the N-terminal domain is more stable than calmodulin's C-terminal half, slowly exchanging backbone amide protons are found in all eight alpha-helices and in three of the four short beta-strands. This confirms that the calcium-free form consists of stable secondary structure and does not adopt a 'molten globule' type of structure. However, the C-terminal domain of calmodulin is subject to conformational exchange on a time scale of about 350 microseconds, which affects many of the C-terminal domain residues. This results in significant shortening of the 15N T2 values relative to T1 rho, whereas the T1 rho and T2 values are of similar magnitude in the N-terminal half of the protein. A model in which the motion of the protein is assumed to be isotropic suggests a rotational correlation time for the protein of about 8 ns but quantitatively does not agree with the magnetic field dependence of the T1 values and does not explain the different T2 values found for different alpha-helices in the N-terminal domain. These latter parameters are compatible with a flexible dumb-bell model in which each of calmodulin's two domains freely diffuse in a cone with a semi-angle of about 30 degrees and a time constant of about 3 ns, whereas the overall rotation of the protein occurs on a much slower time scale of about 12 ns. The difference in the transverse relaxation rates observed between the amides in helices C and D suggests that the change in interhelical angle upon calcium binding is less than predicted by Herzberg et al. Strynadka and James [Strynadka, N. C. J. & James, M. N. G. (1988) Proteins Struct. Funct. Genet. 3, 1-17].
authors.affiliation: Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520, USA.
authors.forename: N
authors.initials: N
authors.lastname: Tjandra
authors.forename: H
authors.initials: H
authors.lastname: Kuboniwa
authors.forename: H
authors.initials: H
authors.lastname: Ren
authors.forename: A
authors.initials: A
authors.lastname: Bax
mesh_terms: D000818:Animals; D016880:Anisotropy; D002118:Calcium / Q000032:analysis; D002147:Calmodulin / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry; D012399:Rotation; D014981:Xenopus
chemical_list: D002147:Calmodulin; D011994:Recombinant Proteins; D002118:Calcium