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 o f the colicin Ia channel domain in the soluble and membrane-bound states. I n 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 backbo ne (12-16 degrees) and side chains (23-29 degrees), as manifested by the re duction in the C-H:and H-H dipolar couplings and N-15 chemical shift anisot ropy. 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 fra me-spin-lattice relaxation times. Average motional correlation times of 0.8 and 1.2 ps were extracted for the soluble and membrane-bound states, respe ctively. In comparison, both forms of the colicin Ia channel-domain are com pletely 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 incr ease is consistent with the loss of tertiary structure of the protein in th e membrane, which was previously suggested by the extended helical array mo del [Zakharov et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4282-4287]. A n extended structure would also facilitate protein interactions with the mo bile lipids and thus increase the protein internal motions. We speculate th at the large mobility of the membrane-bound colicin Ia channel domain is a prerequisite for channel opening in the presence of a voltage gradient.