Solid-state NMR investigation of the dynamics of the soluble and membrane-bound colicin Ia channel-forming domain

Solid-state NMR spectroscopy was employed to study the molecular dynamics of the colicin Ia channel domain in the soluble and membrane-bound states. In the soluble state, the protein executes small-amplitude librations (with root-mean-square angular fluctuations of 0-10 degrees) in the backbone and larger-amplitude motions (16-17 degrees) in the side chains. Upon membrane binding, the motional amplitudes increase significantly for both the backbone (12-16 degrees) and side chains (23-29 degrees), as manifested by the reduction in the C-H:and H-H dipolar couplings and N-15 chemical shift anisotropy. These motions occur not only on the pico- to nanosecond time scales, but also on the microsecond time scale, as revealed by the H-1 rotating frame-spin-lattice relaxation times. Average motional correlation times of 0.8 and 1.2 ps were extracted for the soluble and membrane-bound states, respectively. In comparison, both forms of the colicin Ia channel-domain are completely immobile on the millisecond scale. These results indicate that the colicin Ia channel domain has enhanced conformational mobility in the lipid bilayer compared to the soluble state. This membrane-induced mobility increase is consistent with the loss of tertiary structure of the protein in the membrane, which was previously suggested by the extended helical array model [Zakharov et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4282-4287]. An extended structure would also facilitate protein interactions with the mobile lipids and thus increase the protein internal motions. We speculate that the large mobility of the membrane-bound colicin Ia channel domain is a prerequisite for channel opening in the presence of a voltage gradient.