Membrane Thickness Varies Around the Circumference of the Transmembrane Protein BtuB

BtuB is a large outer-membrane β-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 β-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 β-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 Å 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 β-barrel.