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Journal of Bacteriology, November 1998, p. 6031-6038, Vol. 180, No. 22
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Interactions in the TonB-Dependent Energy Transduction Complex: ExbB and ExbD Form Homomultimers

Penelope I. Higgs,¹ Paul S. Myers,^2, and Kathleen Postle^1,2,*

Departments of Microbiology¹ and Biochemistry and Biophysics,² Washington State University, Pullman, Washington 99164-4233

Received 20 July 1998/Accepted 9 September 1998

The cytoplasmic membrane proteins ExbB and ExbD support TonB-dependent active transport of iron siderophores and vitamin B₁₂ across the essentially unenergized outer membrane of Escherichia coli. In this study, in vivo formaldehyde cross-linking analysis was used to investigate the interactions of T7 epitope-tagged ExbB or ExbD proteins. ExbB and ExbD each formed two unique cross-linked complexes which were not dependent on the presence of TonB, the outer membrane receptor protein FepA, or the other Exb protein. Cross-linking analysis of ExbB- and ExbD-derived size variants demonstrated instead that these ExbB and ExbD complexes were homodimers and homotrimers and suggested that ExbB also interacted with an unidentified protein(s). Cross-linking analysis of epitope-tagged ExbB and ExbD proteins with TonB antisera afforded detection of a previously unrecognized TonB-ExbD cross-linked complex and confirmed the composition of the TonB-ExbB cross-linked complex. The implications of these findings for the mechanism of TonB-dependent energy transduction are discussed.

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^* Corresponding author. Mailing address: Department of Microbiology, Washington State University, P.O. Box 644233, Pullman, WA 99164-4233. Phone: (509) 335-5614. Fax: (509) 335-1907. E-mail: postle{at}mail.wsu.edu.

Present address: Millennium Biotherapeutics Inc., Boston, MA.

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Journal of Bacteriology, November 1998, p. 6031-6038, Vol. 180, No. 22
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

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