Temperature dependence of domain motions of calmodulin probed by NMR relaxation at multiple fields

Interdomain motions of Ca2+-ligated calmodulin were characterized by analyzing the nuclear magnetic resonance N-15 longitudinal relaxation rate R-1, transverse relaxation rate R-2, and steady-state {H-1}-N-15 NOE of the backbone amide group at three different magnetic field strengths (18.8, 14.1, and 8.5 T) and four different temperatures (21, 27, 35, and 43 degreesC). Between 35 and 43 degreesC, a larger than expected change in the amplitude and the time scale of the interdomain motion for both N- and C-domains was observed. We attribute this to the shift in population of four residues (74-77) in the central linker from predominantly helical to random coil in this temperature range. This is consistent with the conformation of these residues in the calmodulin-peptide complex, where they are nonhelical. The doubling of the disordered region of the central helix (residues 78-81 at room temperature) when temperature is raised from 35 to 43 degreesC results in larger amplitude interdomain motion. Our analysis of the NMR relaxation data quantifies subtle changes in the interdomain dynamics and provides an additional tool to monitor conformational changes in multidomain proteins.