Crystal structure of the dimeric C-terminal domain of TonB reveals a novel fold

The TonB-dependent complex of Gram-negative bacteria couples the inner membrane proton motive force to the active transport of iron siderophore and vitamin B-12 across the outer membrane. The structural basis of that process has not been described so far in full detail. The crystal structure of the C-terminal domain of TonB from Escherichia coli has now been solved by multi-wavelength anomalous diffraction and refined at 1.55-Angstrom resolution, providing the first evidence that this region of TonB (residues 164-239) dimerizes. Moreover, the structure shows a novel architecture that has no structural homologs among any known proteins. The dimer of the C-terminal domain of TonB is cylinder-shaped with a length of 65 Angstrom and a diameter of 25 Angstrom. Each monomer contains three beta strands and a single alpha helix. The two monomers are intertwined with each other, and all six beta -strands of the dimer make a large antiparallel beta -sheet. We propose a plausible model of binding of TonB to FhuA and FepA, two TonB-dependent outer-membrane receptors.