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.