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J. Biol. Chem., Vol. 279, Issue 11, 9978-9986, March 12, 2004

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Dimerization of TonB Is Not Essential for Its Binding to the Outer Membrane Siderophore Receptor FhuA of Escherichia coli^*

Jiri Koedding, Peter Howard, Lindsay Kaufmann, Patrick Polzer, Ariel Lustig¶, and Wolfram Welte||

From the Fakultaet fuer Biologie, Universitaet Konstanz, Universitaetsstrasse 10, 78457 Konstanz, Germany, the Department of Microbiology and Immunology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E5, Canada, and ^¶Biozentrum Basel, 4056 Basel, Switzerland

FhuA belongs to a family of specific siderophore transport systems located in the outer membrane of Escherichia coli. The energy required for the transport process is provided by the proton motive force of the cytoplasmic membrane and is transmitted to FhuA by the protein TonB. Although the structure of full-length TonB is not known, the structure of the last 77 residues of a fragment composed of the 86 C-terminal amino acids was recently solved and shows an intertwined dimer (Chang, C., Mooser, A., Pluckthun, A., and Wlodawer, A. (2001) J. Biol. Chem. 276, 27535–27540). We analyzed the ability of truncated C-terminal TonB fragments of different lengths (77, 86, 96, 106, 116, and 126 amino acid residues, respectively) to bind to the receptor FhuA. Only the shortest TonB fragment, TonB-77, could not effectively interact with FhuA. We have also observed that the fragments TonB-77 and TonB-86 form homodimers in solution, whereas the longer fragments remain monomeric. TonB fragments that bind to FhuA in vitro also inhibit ferrichrome uptake via FhuA in vivo and protect cells against attack by bacteriophage 80.

Received for publication, October 24, 2003 , and in revised form, December 8, 2003.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The atomic coordinates and structure factors (code 1QXX) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^|| To whom correspondence should be addressed. Tel.: 49-07-531-882-206; Fax: 49-07-531-883-183; E-mail: wolfram.welte{at}uni-konstanz.de.

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