Journal of Molecular Biology
Volume 216, Issue 4, 20 December 1990, Pages 897-910
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TonB protein of Salmonella typhimurium: A model for signal transduction between membranes

https://doi.org/10.1016/S0022-2836(99)80009-6Get rights and content

The tonB gene product is required for several outer membrane transport processes in bacteria. The tonB gene from Salmonella typhimurium was sequenced and found to be similar to that of Escherichia coli. The TonB protein is highly proline-rich and includes an unusual segment consisting of multiple X-Pro dipeptide repeats. A synthetic peptide corresponding to this segment has been used to raise anti-TonB antibodies. TonB was shown to be associated with the cytoplasmic membrane, apparently anchored via a single hydrophobic N-terminal segment. Protease accessibility studies, and the use of a series of TonB-β-lactamase fusions, showed that the rest of the TonB protein is periplasmic. Unusually, export of TonB is not accompanied by cleavage of the N-terminal signal peptide. In the accompanying paper, we show that TonB interacts directly with the outer membrane FhuA (TonA) receptor. Thus, TonB must span the periplasm, providing a link between the cytoplasmic membrane and receptors in the outer membrane. On the basis of these data, and those published by other laboratories, we propose a model whereby TonB serves as a “mechanical” linkage that, by transmitting protein conformational changes from the cytoplasmic membrane across the periplasm, acts as a means of coupling energy to outer membrane transport processes. Such a mechanism has general implications for signal transduction within and between proteins.

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      Ligands are still able to bind their respective OM transporters in protonophore-treated cells.14,15 ExbD (141 amino acids) and TonB (239 amino acids) have identical membrane topologies of a single transmembrane domain (TMD), with the majority of each protein occupying the periplasmic space.16–18 ExbB (244 amino acids) has three TMDs, with the majority of its soluble domains localized to the cytoplasm.19,20

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    On sabbatical leave from Lawrence University, Appleton, WI 54912, U.S.A.

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