Membrane Thickness Varies Around the Circumference of the Transmembrane Protein BtuB

BtuB is a large outer-membrane beta-barrel protein that belongs to a class of active transport proteins that are TonB-dependent. These TonB-dependent transporters are based upon a 22-stranded antiparallel beta-barrel, which is notably asymmetric in its length. Here, site-directed spin labeling and simulated annealing were used to locate the membrane lipid interface surrounding BtuB when reconstituted into phosphatidylcholine bilayers. Positions on the outer facing surface of the beta-barrel and the periplasmic turns were spin-labeled and distances from the label to the membrane interface estimated by progressive power saturation of the electron paramagnetic resonance spectra. These distances were then used as atom-to-plane distance restraints in a simulated annealing routine, to dock the protein to two independent planes and produce a model representing the average position of the lipid phosphorus atoms at each interface. The model is in good agreement with the experimental data; however, BtuB is mismatched to the bilayer thickness and the resulting planes representing the bilayer interface are not parallel. In the model, the membrane thickness varies by 11 angstrom around the circumference of the protein, indicating that BtuB distorts the bilayer interface so that it is thinnest on the short side of the protein beta-barrel.