Interactions between the outer membrane ferric citrate transporter FecA and TonB: studies of the FecA TonB box

Both induction of transcription of the ferric citrate transport genes and transport of ferric citrate by the Escherichia coli outer membrane receptor FecA require energy derived from the proton motive force (PMF) of the inner membrane. The energy is transduced to FecA by the inner membrane complex, TonB, ExbB, and ExbD. Region 160 of TonB and the conserved TonB box of other TonB-dependent receptors are implicated as sites of interaction. In the present study, the postulated TonB box (D(80)A(81)L(82)T(83)V(84)) of FecA was deleted in frame, with a subsequent loss of both FecA functions. DALTV of FecA could be functionally replaced with the core TonB boxes of FhuA (DTITV) and FepA (DTIVV). Each residue of the TonB box of FecA was sequentially replaced with cysteine residues, and only the D80C replacement showed a loss (reduction) of both FecA functions. A physical interaction between TonB and FecA was demonstrated using both in vivo site-specific disulfide bond cross-linking and nonspecific formaldehyde (FA) cross-linking. Pairwise combinations of FecA (DALTV)/Cys substitutions were cross-linked via disulfide bond formation with TonBQ160C, TonBQ162C, and TonBY163C. Unexpectedly, this cross-linking was not enhanced by substrate (ferric citrate). In contrast, the TonB-FecA interaction was enhanced by ferric citrate in the FA-cross-linking assay. Energy derived from the PMF was not required for the TonB-FecA interaction in either the disulfide- or FA-cross-linking assay. TonB/CysExbB/ExbD(D25N) was still able to cross-link with the FecA (DALTV)/Cys derivatives in a tonB tolQ background, even though ExbD25N renders the TonB/ExbBD complex nonfunctional (V. Braun, S. Gaisser, C. Herrmann, K. Kampfenkel, H. Killmann, and I. Traub, J. Bacteriol. 178:2836-2845, 1996). TonB cross-linked to FecA via FA was not inhibited by either carbonylcyanide-m-chlorophenylhydrazone or 1 mM 2,4-dinitrophenol, which dissipate the electrochemical potential of the cytoplasmic membrane and disrupt both FecA functions. The studies shown here demonstrate the significance of the TonB box for FecA functions and are consistent with the view that it is the structure and not the sequence of the TonB box that is important for activity. Demonstrated here for the first time is the physical interaction of TonB and FecA, which is enhanced by ferric citrate.