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J. Biol. Chem., Vol. 276, Issue 11, 8111-8117, March 16, 2001
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From the School of Molecular Biosciences, Washington State
University, Pullman, Washington 99164-4233
The cytoplasmic membrane protein TonB couples the
proton electrochemical potential of the cytoplasmic membrane to
transport events at the outer membrane of Gram-negative bacteria. The
amino-terminal signal anchor of TonB and its interaction with the
cytoplasmic membrane protein ExbB are essential to this process. The
TonB signal anchor is predicted to form an
Conserved Residues Ser16 and His20 and
Their Relative Positioning Are Essential for TonB Activity,
Cross-linking of TonB with ExbB, and the Ability of TonB to Respond to
Proton Motive Force*
-helix, with a conserved face comprised of residues Ser16, His20,
Leu27, and Ser31. Deletion of either
Ser16 or His20 or of individual intervening but
not flanking residues rendered TonB inactive and unable to assume a
proton motive force-dependent conformation. In
vivo formaldehyde cross-linking experiments revealed that the
ability of this subset of mutants to form a characteristic heterodimer
with ExbB was greatly diminished. Replacement of residues 17-19 by
three consecutive alanines produced a wild type TonB allele, indicating
that the intervening residues (Val, Cys, and Ile) contributed only to
spacing. These data indicated that the spatial relationship of
Ser16 to His20 was essential to function and
suggested that the motif HXXXS defines the minimal
requirement for the coupling of TonB to the cytoplasmic membrane
electrochemical gradient. Deletion of Trp11 resulted in a
TonB that remained active yet was unable to cross-link with ExbB.
Because Trp11 was demonstrably not involved in the actual
cross-linking, these results suggest that the TonB/ExbB interaction
detected by cross-linking occurred at a step in the energy transduction
cycle distinct from the coupling of TonB to the electrochemical gradient.
*
This work was supported by Grant GM42146 from the National
Institutes of General Medical Sciences (to K. P.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.:
509-335-5614; E-mail: postle@mail.wsu.edu.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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