ExbB protein in the cytoplasmic membrane of Escherichia coli forms a stable oligomer.

In Gram-negative bacteria like Escherichia coli the ExbB-ExbD-TonB protein complex is anchored to the cytoplasmic membrane and is involved in energization of outer membrane transport. Outer membrane proteins catalyze energy-coupled transport of scarce nutrients. Energy is derived from the protonmotive force of the cytoplasmic membrane which is transferred through ExbB-ExbD-TonB to the outer membrane transporters. Earlier studies showed that ExbB is the most abundant protein of the ExbB-ExbD-TonB complex and stabilizes TonB and ExbD. To advance understanding of the role of ExbB in the membrane organization of the ExbB-ExbD-TonB complex, His-tagged ExbB was solubilized with decyl maltoside and purified to electrophoretic homogeneity. Its size and shape were determined by blue native gel electrophoresis, size exclusion chromatography, transmission electron microscopy, and small-angle X-ray scattering. Decyl maltoside bound to ExbB was quantified by the anthrone method that determines the sugar moiety of decyl maltoside. The results obtained with the four methods consistently indicated that isolated ExbB adopts a stable homooligomer with four to six monomers. We propose that the ExbB homooligomer forms a platform on which ExbD and TonB are assembled to form the energy-transducing complex in the cytoplasmic membrane.