BACKBONE DYNAMICS OF CALMODULIN STUDIED BY N-15 RELAXATION USING INVERSE DETECTED 2-DIMENSIONAL NMR-SPECTROSCOPY - THE CENTRAL HELIX IS FLEXIBLE

The backbone dynamics of Ca2+-saturated recombinant Drosophila calmodulin has been studied by N-15 longitudinal and transverse relaxation experiments, combined with N-15{H-1} NOE measurements. Results indicate a high degree of mobility near the middle of the central helix of calmodulin, from residue K77 through S81, with order parameters (S2) in the 0.5-0.6 range. The anisotropy observed in the motion of the two globular calmodulin domains is much smaller than expected on the basis of hydrodynamic calculations for a rigid dumbbell type structure. This indicates that, for the purposes of N-15 relaxation, the tumbling of the N-terminal (L4-K77) and C-terminal (E82-S147) lobes of calmodulin is effectively independent. A slightly shorter motional correlation time (tau(c) almost-equal-to 6.3 ns) is obtained for the C-terminal domain compared to the N-terminal domain (tau(c) almost-equal-to 7.1 ns), in agreement with the smaller size of the C-terminal domain. A high degree of mobility, with order parameters of approximately 0.5, is also observed in the loop that connects the first with the second EF-hand type calcium binding domain and in the loop connecting the third and fourth calcium binding domain.