Fast-time scale dynamics of outer membrane protein A by extended model-free analysis of NMR relaxation data

In order to better understand the dynamics of an integral membrane protein, backbone amide 15N NMR dynamics measurements of the β-barrel membrane protein OmpA have been performed at three magnetic fields. A total of nine relaxation data sets were globally analyzed using an extended model-free formalism. The diffusion tensor was found to be prolate axially symmetric with an axial ratio of 5.75, indicating a possible rotation of the protein within the micelle. The generalized order parameters gradually decreased from the mid-plane towards the two ends of the barrel, counteracting the dynamic gradient of the lipids in a matching bilayer, and were dramatically reduced in the extracellular loops. Large-scale internal motions on the ns time scale indicate that entire loops most likely undergo concerted (“sea anemone”-like) motions emanating from their anchoring points on the barrel. The case of OmpA in DPC micelles also illustrates inherent limitations of analyzing the data with even the most sophisticated current models of the model-free formalism. It is likely that conformational exchange processes on the ms–μs also play a role in describing the motions of some residues, but their analysis did not produce unique results that could be independently verified.