Molecular characterization of the Enterobacter aerogenes tonB gene: identification of a novel type of tonB box suppressor mutant

The Role of TonB Gene in Edwardsiella ictaluri Virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen that causes enteric septicemia in catfish (ESC). Stress factors including poor water quality, poor diet, rough handling, overcrowding, and water temperature fluctuations increase fish susceptibility to ESC. The TonB energy transducing system (TonB-ExbB-ExbD) and TonB-dependent transporters of Gram-negative bacteria support active transport of scarce resources including iron, an essential micronutrient for bacterial virulence. Deletion of the tonB gene attenuates virulence in several pathogenic bacteria. In the current study, the role of TonB (NT01EI_RS07425) in iron acquisition and E. ictaluri virulence were investigated. To accomplish this, the E. ictaluri tonB gene was in-frame deleted. Growth kinetics, iron utilization, and virulence of the EiΔtonB mutant were determined. Loss of TonB caused a significant reduction in bacterial growth in iron-depleted medium (p > 0.05). The EiΔtonB mutant grew similarly to wild-type E. ictaluri when ferric iron was added to the iron-depleted medium. The EiΔtonB mutant was significantly attenuated in catfish compared with the parent strain (21.69 vs. 46.91% mortality). Catfish surviving infection with EiΔtonB had significant protection against ESC compared with naïve fish (100 vs. 40.47% survival). These findings indicate that TonB participates in pathogenesis of ESC and is an important E. ictaluri virulence factor.

Molecular characterization of the TonB2 protein from the fish pathogen Vibrio anguillarum

In the fish pathogen Vibrio anguillarum the TonB2 protein is essential for the uptake of the indigenous siderophore anguibactin. Here we describe deletion mutants and alanine replacements affecting the final six amino acids of TonB2. Deletions of more than two amino acids of the TonB2 C-terminus abolished ferric-anguibactin transport, whereas replacement of the last three residues resulted in a protein with wild-type transport properties. We have solved the high-resolution solution structure of the TonB2 C-terminal domain by NMR spectroscopy. The core of this domain (residues 121-206) has an alphabetabetaalphabeta structure, whereas residues 76-120 are flexible and extended. This overall folding topology is similar to the Escherichia coli TonB C-terminal domain, albeit with two differences: the beta4 strand found at the C-terminus of TonB is absent in TonB2, and loop 3 is extended by 9 A (0.9 nm) in TonB2. By examining several mutants, we determined that a complete loop 3 is not essential for TonB2 activity. Our results indicate that the beta4 strand of E. coli TonB is not required for activity of the TonB system across Gram-negative bacterial species. We have also determined, through NMR chemical-shift-perturbation experiments, that the E. coli TonB binds in vitro to the TonB box from the TonB2-dependent outer membrane transporter FatA; moreover, it can substitute in vivo for TonB2 during ferric-anguibactin transport in V. anguillarum. Unexpectedly, TonB2 did not bind in vitro to the FatA TonB-box region, suggesting that additional factors may be required to promote this interaction. Overall our results indicate that TonB2 is a representative of a different class of TonB proteins.

Studies on colicin B translocation: FepA is gated by TonB

Colicin B is a 55 kDa dumbbell-shaped protein toxin that uses the TonB system (outer membrane transporter, FepA, and three cytoplasmic membrane proteins TonB/ExbB/ExbD) to enter and kill Escherichia coli. FepA is a 22-stranded beta-barrel with its lumen filled by an amino-terminal globular domain containing an N-terminal semiconserved region, known as the TonB box, to which TonB binds. To investigate the mechanism of colicin B translocation across the outer membrane, we engineered cysteine (Cys) substitutions in the globular domain of FepA. Colicin B caused increased exposure to biotin maleimide labelling of all Cys substitutions, but to different degrees, with TonB as well as the FepA TonB box required for all increases. Because of the large increases in exposure for Cys residues from T13 to T51, we conclude that colicin B is translocated through the lumen of FepA, rather than along the lipid-barrel interface or through another protein. Part of the FepA globular domain (residues V91-V142) proved relatively refractory to labelling, indicating either that the relevant Cys residues were sequestered by an unknown protein or that a significant portion of the FepA globular domain remained inside the barrel, requiring concomitant conformational rearrangement of colicin B during its translocation. Unexpectedly, TonB was also required for colicin-induced exposure of the FepA TonB box, suggesting that TonB binds FepA at a different site prior to interaction with the TonB box.

Deletion and substitution analysis of the Escherichia coli TonB Q160 region

The active transport of iron siderophores and vitamin B(12) across the outer membrane (OM) of Escherichia coli requires OM transporters and the potential energy of the cytoplasmic membrane (CM) proton gradient and CM proteins TonB, ExbB, and ExbD. A region at the amino terminus of the transporter, called the TonB box, directly interacts with TonB Q160 region residues. R158 and R166 in the TonB Q160 region were proposed to play important roles in cocrystal structures of the TonB carboxy terminus with OM transporters BtuB and FhuA. In contrast to predictions based on the crystal structures, none of the single, double, or triple alanyl substitutions at arginyl residues significantly decreased TonB activity. Even the quadruple R154A R158A R166A R171A mutant TonB still retained 30% of wild-type activity. Up to five residues centered on TonB Q160 could be deleted without inactivating TonB or preventing its association with the OM. TonB mutant proteins with nested deletions of 7, 9, or 11 residues centered on TonB Q160 were inactive and appeared never to have associated with the OM. Because the 7-residue-deletion mutant protein (TonBDelta7, lacking residues S157 to Y163) could still form disulfide-linked dimers when combined with W213C or F202C in the TonB carboxy terminus, the TonBDelta7 deletion did not prevent necessary energy-dependent conformational changes that occur in the CM. Thus, it appeared that initial contact with the OM is made through TonB residues S157 to Y163. It is hypothesized that the TonB Q160 region may be part of a large disordered region required to span the periplasm and contact an OM transporter.

The Solution Structure of the C-terminal Domain of TonB and Interaction Studies with TonB Box Peptides

The TonB protein transduces energy from the proton gradient across the cytoplasmic membrane of Gram-negative bacteria to TonB-dependent outer membrane receptors. It is a critically important protein in iron uptake, and deletion of this protein is known to decrease virulence of bacteria in animal models. This system has been used for Trojan horse antibiotic delivery. Here, we describe the high-resolution solution structure of Escherichia coli TonB residues 103-239 (TonB-CTD). TonB-CTD is monomeric with an unstructured N terminus (103-151) and a well structured C terminus (152-239). The structure contains a four-stranded antiparallel beta-sheet packed against two alpha-helices and an extended strand in a configuration homologous to the C-terminal domain of the TolA protein. Chemical shift perturbations to the TonB-CTD (1)H-(15)N HSCQ spectrum titrated with TonB box peptides modeled from the E.coli FhuA, FepA and BtuB proteins were all equivalent, indicating that all three peptides bind to the same region of TonB. Isothermal titration calorimetry measurements demonstrate that TonB-CTD interacts with the FhuA-derived peptide with a K(D)=36(+/-7) microM. On the basis of chemical shift data, the position of Gln160, and comparison to the TolA gp3 N1 complex crystal structure, we propose that the TonB box binds to TonB-CTD along the beta3-strand.

Mutations in the Escherichia coli receptor FepA reveal residues involved in ligand binding and transport

FepA is the Escherichia coli outer membrane receptor for ferric enterobactin, colicin D and colicin B. The transport processes through FepA are energy-dependent, relying on the periplasmic protein TonB to interact with FepA. Through this interaction, TonB tranduces energy derived from the cytoplasmic membrane across the periplasmic space to FepA. In this study, random mutagenesis strategies were used to define residues of FepA important for its function. Both polymerase chain reaction (PCR)-generated random mutations in the N-terminal 180 amino acids of FepA and spontaneous chromosomal fepA mutations were selected by resistance to colicin B. The PCR mutagenesis strategy targeted the N-terminus because it forms a plug inside the FepA barrel that is expected to be involved in ligand binding, ligand transport, and interaction with TonB. We report the characterization of 15 fepA missense mutations that were localized to three regions of the FepA receptor. The first region was a stretch of eight amino acids referred to as the TonB box. The second region included extracellular loops of both the barrel and the plug. A third region formed a cluster near the barrel wall around positions 75 and 126 of the plug. These mutations provide initial insight into the mechanisms of ligand binding and transport through the FepA receptor.

Bordetella pertussis TonB, a Bvg-independent virulence determinant

In gram-negative bacteria, high-affinity iron uptake requires the TonB/ExbB/ExbD envelope complex to release iron chelates from their specific outer membrane receptors into the periplasm. Based on sequence similarities, the Bordetella pertussis tonB exbB exbD locus was identified on a cloned DNA fragment. The tight organization of the three genes suggests that they are cotranscribed. A putative Fur-binding sequence located upstream from tonB was detected in a Fur titration assay, indicating that the tonB exbB exbD operon may be Fur-repressed in high-iron growth conditions. Putative structural genes of the beta-subunit of the histone-like protein HU and of a new two-component regulatory system were identified upstream from tonB and downstream from exbD, respectively. A B. pertussis DeltatonB exbB::Km(r) mutant was constructed by allelic exchange and characterized. The mutant was impaired for growth in low-iron medium in vitro and could not use ferrichrome, desferal, or hemin as iron sources. Levels of production of the major bacterial toxins and adhesins were similar in the TonB(+)/TonB(-) pair. The DeltatonB exbB mutant was still responsive to chemical modulators of virulence; thus, the BvgA/BvgS two-component system is not TonB dependent. Nevertheless, in vivo in the mouse respiratory infection model, the colonization ability of the mutant was reduced compared to the parental strain.

Vibrio cholerae iron transport: haem transport genes are linked to one of two sets of tonB, exbB, exbD genes

Vibrio cholerae was found to have two sets of genes encoding TonB, ExbB and ExbD proteins. The first set (tonB1, exbB1, exbD1) was obtained by complementation of a V. cholerae tonB mutant. In the mutant, a plasmid containing these genes permitted transport via the known V. cholerae high-affinity iron transport systems, including uptake of haem, vibriobactin and ferrichrome. When chromosomal mutations in exbB1 or exbD1 were introduced into a wild-type V. cholerae background, no defect in iron transport was noted, indicating the existence of additional genes that can complement the defect in the wild-type background. Another region of the V. cholerae chromosome was cloned that encoded a second functional TonB/Exb system (tonB2, exbB2, exbD2). A chromosomal mutation in exbB2 also failed to exhibit a defect in iron transport, but a V. cholerae strain that had chromosomal mutations in both the exbB1 and exbB2 genes displayed a mutant phenotype similar to that of an Escherichia coli tonB mutant. The genes encoding TonB1, ExbB1, ExbD1 were part of an operon that included three haem transport genes (hutBCD), and all six genes appeared to be expressed from a single Fur-regulated promoter upstream of tonB1. A plasmid containing all six genes permitted utilization of haem by an E. coli strain expressing the V. cholerae haem receptor, HutA. Analysis of the hut genes indicated that hutBCD, which are predicted to encode a periplasmic binding protein (HutB) and cytoplasmic membrane permease (HutC and HutD), were required to reconstitute the V. cholerae haem transport system in E. coli. In V. cholerae, the presence of hutBCD stimulated growth when haemin was the iron source, but these genes were not essential for haemin utilization in V. cholerae.

Regions of Escherichia coli TonB and FepA proteins essential for in vivo physical interactions

The transport of Fe(III)-siderophore complexes and vitamin B12 across the outer membrane of Escherichia coli is an active transport process requiring a cognate outer membrane receptor, cytoplasmic membrane-derived proton motive force, and an energy-transducing protein anchored in the cytoplasmic membrane, TonB. This process requires direct physical contact between the outer membrane receptor and TonB. Previous studies have identified an amino-terminally located region (termed the TonB box) conserved in all known TonB-dependent outer membrane receptors as being essential for productive energy transduction. In the present study, a mutation in the TonB box of the ferric enterochelin receptor FepA resulted in the loss of detectable in vivo chemical cross-linking between FepA and TonB. Protease susceptibility studies indicated this effect was due to an alteration of conformation rather than the direct disruption of a specific site of physical contact. This suggested that TonB residue 160, implicated in previous studies as a site of allele-specific suppression of TonB box mutants, also made a conformational rather than a direct contribution to the physical interaction between TonB and the outer membrane receptors. This possibility was supported by the finding that TonB carboxyl-terminal truncations that retained Gln-160 were unable to participate in TonB-FepA complex formation, indicating that this site alone was not sufficient to support the physical interactions involved in energy transduction. These studies indicated that the final 48 residues of TonB were essential to this physical interaction. This region contains a putative amphipathic helix which could facilitate TonB-outer membrane interaction. Amino acid replacements at one site in this region were found to affect energy transduction but did not appear to greatly alter TonB conformation or the formation of a TonB-FepA complex. The effects of amino acid substitutions at several other TonB sites were also examined.

Cloning and characterization of the exbB-exbD-tonB locus of Pasteurella haemolytica A1

A recombinant plasmid (pMG1) carrying Pasteurella haemolytica A1 DNA which complements a tonB mutation of Escherichia coli has been isolated. E. coli tonB metE which carries pMG1 exhibits growth kinetics in the presence of vitamin B12 similar to that of the wild-type host. In addition, the complemented E. coli is susceptible to killing by bacteriophage phi 80 and colicin B. Analysis of the nucleotide sequence in the complementing DNA showed that it codes for three genes in the order of exbB-exbD-tonB. This genetic organization has been reported in Haemophilus influenzae, H. ducreyi, Pseudomonas putida and Vibrio cholerae, and may represent a separate lineage of evolution from that of the Enterobacteriaceae in which tonB is unlinked with the accessory genes exbB and exbD. A comparison of the DNA flanking the exbB-exbD-tonB locus in P. haemolytica A1 and H. influenzae showed that the flanking regions are completely different between the two organisms.

Neisseria meningitidis tonB, exbB, and exbD genes: Ton-dependent utilization of protein-bound iron in Neisseriae

We have recently cloned and characterized the hemoglobin (Hb) receptor gene, hmbR, from Neisseria meningitidis. To identify additional proteins that are involved in Hb utilization, the N. meningitidis Hb utilization system was reconstituted in Escherichia coli. Five cosmids from N. meningitidis DNA library enabled a heme-requiring (hemA), HmbR-expressing mutant of E. coli to use Hb as both porphyrin and iron source. Nucleotide sequence analysis of DNA fragments subcloned from the Hb-complementing cosmids identified four open reading frames, three of them homologous to Pseudomonas putida, E. coli, and Haemophilus influenzae exbB, exbD, and tonB genes. The N. meningitidis TonB protein is 28.8 to 33.6% identical to other gram-negative TonB proteins, while the N. meningitidis ExbD protein shares between 23.3 and 34.3% identical amino acids with other ExbD and TolR proteins. The N. meningitidis ExbB protein was 24.7 to 36.1% homologous with other gram-negative ExbB and TolQ proteins. Complementation studies indicated that the neisserial Ton system cannot interact with the E. coli FhuA TonB-dependent outer membrane receptor. The N. meningitidis tonB mutant was unable to use Hb, Hb-haptoglobin complexes, transferrin, and lactoferrin as iron sources. Insertion of an antibiotic cassette in the 3' end of the exbD gene produced a leaky phenotype. Efficient usage of heme by N. meningitidis tonB and exbD mutants suggests the existence of a Ton-independent heme utilization mechanism. E. coli complementation studies and the analysis of N. meningitidis hmbR and hpu mutants suggested the existence of another Hb utilization mechanism in this organism.

The Pseudomonas aeruginosa tonB gene encodes a novel TonB protein

The Pseudomonas aeruginosa tonB gene was cloned by complementation of the tonB mutation of Pseudomonas putida strain TE516 (W. Bitter, J. Tommassen & P.J. Weisbeek, 1993, Mol Microbiol 7, 117-130). The gene was 1025 bp in length, capable of encoding a protein of 36860 Da. As with previously described TonB proteins, the P. aeruginosa TonB (TonBp.a.) was rich in Pro residues (18.1%) and contained Glu-Pro/Lys-Pro repeats. Unlike previously described TonB proteins, however, TonBp.a. lacked an N-terminal membrane anchor (signal) sequence and contained, instead, a predicted internal signal/anchor sequence, expected to yield an atypical N-terminal cytoplasmic domain in this protein. TonB proteins are essential components in iron-siderophore uptake in bacteria, apparently functioning as energy transducers in coupling the energized state of the cytoplasmic membrane to outer-membrane receptor function. As expected, tonB derivatives of P. aeruginosa were defective in siderophore-mediated iron acquisition. tonB gene expression was inducible by iron-limitation, consistent with the identification of a Fur consensus binding sequence upstream of the gene. TonBp.a. showed substantially greater similarity to the Escherichia coli TonB protein than the Pseudomonas putida protein (31% identity vs. 20% identity) and tonBp.a. was able to complement deficiencies in the acquisition of ferric enterobactin and vitamin B12, and sensitivity to phage phi 80 of an E. coli tonB strain. The larger size of TonBp.a. and its ability to function in both E. coli and P. putida make it a unique TonB protein whose characterization should enhance our understanding of TonB function in bacteria.

Identification of TonB homologs in the family Enterobacteriaceae and evidence for conservation of TonB-dependent energy transduction complexes

The transport of Fe(III)-siderophore complexes and vitamin B12 across the outer membrane of Escherichia coli requires the TonB-dependent energy transduction system. A set of murine monoclonal antibodies (MAbs) was generated against an E. coli TrpC-TonB fusion protein to facilitate structure and function studies. In the present study, the epitopes recognized by these MAbs were mapped, and their distribution in gram-negative organisms was examined. Cross-species reactivity patterns obtained against TonB homologs of known sequence were used to refine epitope mapping, with some epitopes ultimately confirmed by inhibition experiments using synthetic polypeptides. Epitopes recognized by this set of MAbs were conserved in TonB homologs for 9 of 12 species in the family Enterobacteriaceae (including E. coli), including previously unidentified TonB homologs in Shigella, Citrobacter, Proteus, and Kluyvera species. These homologs were also detected by a polyclonal alpha-TrpC-TonB serum that additionally recognized the known Yersinia enterocolitica TonB homolog and a putative TonB homolog in Edwardsiella tarda. These antibody preparations failed to detect the known TonB homologs of either Pseudomonas putida or Haemophilus influenzae but did identify potential TonB homologs in several other nonenteric gram-negative species. In vivo chemical cross-linking experiments demonstrated that in addition to TonB, auxiliary components of the TonB-dependent energy transduction system are broadly conserved in members of the family Enterobacteriaceae, suggesting that the TonB system represents a common system for high-affinity active transport across the gram-negative outer membrane.

Energy-coupled transport and signal transduction through the gram-negative outer membrane via TonB-ExbB-ExbD-dependent receptor proteins

Iron in the form of ferric siderophore complexes and vitamin B12 are transported through the outer membrane of Gram-negative bacteria by a mechanism which consumes energy. There is no known energy source in the outer membrane or in the adjacent periplasmic space so that energy is provided by the electrochemical potential across the cytoplasmic membrane. Energy flows from the cytoplasmic into the outer membrane via a complex consisting of the TonB, ExbB and ExbD proteins which are anchored in the cytoplasmic membrane. It is proposed that the TonB--ExbB--ExbD complex opens--via an energized conformation of the TonB protein--channels in the outer membrane, formed by proteins which serves as highly specific binding sites for the various ferric siderophores and vitamin B12. In addition, outer membrane receptors together with the TonB--ExbB--ExbD complex are directly involved in induction of the transcription of ferric citrate and pseudobactin transport genes of Escherichia coli and Pseudomonas putida, respectively.

Partial suppression of an Escherichia coli TonB transmembrane domain mutation (delta V17) by a missense mutation in ExbB

Active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane of Escherichia coli appears to be dependent upon the ability of the TonB protein to couple cytoplasmic membrane-generated protonmotive force to outer membrane receptors. TonB is supported in this role by an auxiliary protein, ExbB, which, in addition to stabilizing TonB against the activities of endogenous envelope proteases, directly contributes to the energy transduction process. The topological partitioning of TonB and ExbB to either side of the cytoplasmic membrane restricts the sites of interaction between these proteins primarily to their transmembrane domains. In this study, deletion of valine 17 within the aminoterminal transmembrane anchor of TonB resulted in complete loss of TonB activity, as well as loss of detectable in vivo crosslinking into a 59 kDa complex believed to contain ExbB. The delta V17 mutation had no effect on TonB export. The loss of crosslinking appeared to reflect conformational changes in the TonB/ExbB pair rather than loss of interaction since ExbB was still required for some stabilization of TonB delta V17. Molecular modeling suggested that the delta V17 mutation caused a significant change in the predicted conserved face of the TonB amino-terminal membrane anchor. TonB delta V17 was unable to achieve the 23 kDa proteinase K-resistant form in lysed sphaeroplasts that is characteristic of active TonB. Wild-type TonB also failed to achieve the proteinase K-resistant configuration when ExbB was absent. Taken together these results suggested that the delta V17 mutation interrupted productive TonB-ExbB interactions. The apparent ability to crosslink to ExbB as well as a limited ability to transduce energy were restored by a second mutation (A39E) in or near the first predicted transmembrane domain of the ExbB protein. Consistent with the weak suppression, a 23 kDa proteinase K-resistant form of TonB delta V17 was not observed in the presence of ExbBA39E. Neither the ExbBA39E allele nor the absence of ExbB affected TonB or TonB delta V17 export. Unlike the tonB delta V17 mutation, the exbBA39E mutation did not greatly alter a modelled ExbB transmembrane domain structure. Furthermore, the suppressor ExbBA39E functioned normally with wild-type TonB, suggesting that the suppressor was not allele specific. Contrary to expectations, the TonB delta V17, ExbBA39E pair resulted in a TonB with a greatly reduced half-life (approximately 10 min). These results together with protease susceptibility studies suggest that ExbB functions by modulating the conformation of TonB.

A functional tonB gene is required for both utilization of heme and virulence expression by Haemophilus influenzae type b

Haemophilus influenzae is nearly unique among facultatively anaerobic bacteria in its absolute requirement for exogenously supplied heme for aerobic growth. In this study, a mutant analysis strategy was used to facilitate identification of H. influenzae cell envelope components involved in the uptake of heme. Chemical mutagenesis was employed to produce a mutant of a nontypeable H. influenzae strain unable to utilize either protein-bound forms of heme or low levels of free heme. This mutant was transformed with a plasmid shuttle vector-based genomic library constructed from the same wild-type nontypeable H. influenzae strain, and a growth selection technique was used to obtain a recombinant clone that could utilize heme. Analysis of the DNA insert in the recombinant plasmid revealed the presence of several open reading frames, one of which encoded a 28-kDa protein with significant similarity to the TonB protein of Escherichia coli. This H. influenzae gene product was able to complement a tonB mutation in E. coli, allowing the E. coli tonB mutant to form single colonies on minimal medium containing vitamin B12. When this H. influenzae gene was inactivated by insertional mutagenesis techniques and introduced into the chromosome of wild-type strains of H. influenzae type b, the resultant transformants lost their abilities to utilize heme and produce invasive disease in an animal model. Genetic restoration of the ability to express this TonB homolog resulted in the simultaneous acquisition of both heme utilization ability and virulence. These results indicate that the H. influenzae TonB protein is required not only for heme utilization by this pathogen in vitro, but also for virulence of H. influenzae type b in an animal model.

Energy-dependent receptor activities of Escherichia coli K-12: mutated TonB proteins alter FhuA receptor activities to phages T5, T1, phi 80 and to colicin M

The activity of the FhuA receptor in the outer membrane of Escherichia coli is dependent on the TonB, ExbB and ExbD proteins which are anchored to the cytoplasmic membrane. Only infection by phage T5 occurs independently of TonB, ExbB and ExbD. In this paper we describe mutated FhuA proteins which displayed either an increased or decreased FhuA activity to phage T5 when combined with mutated TonB proteins. These results suggest conformational changes in FhuA by TonB which are recognized by phage T5. Similar results were obtained with colicin M and the phages T1 and phi 80. It is proposed that the FhuA mutant proteins assume conformations which are either improved or impaired by the TonB derivatives. For the direct interaction of FhuA with TonB regions which are located outside the TonB box of FhuA and the region around residue 160 of TonB are important.

TonB protein and energy transduction between membranes

TonB protein couples cytoplasmic membrane electrochemical potential to active transport of iron-siderophore complexes and vitamin B12 through high-affinity outer membrane receptors of Gram-negative bacteria. The mechanism of energy transduction remains to be determined, but important concepts have already begun to emerge. Consistent with its function, TonB is anchored in the cytoplasmic membrane by its uncleaved amino terminus while largely occupying the periplasm. Both the connection to the cytoplasmic membrane and the amino acid sequences of the anchor are essential for activity. TonB directly associates with a number of envelope proteins, among them the outer membrane receptors and cytoplasmic membrane protein ExbB. ExbB and TonB interact through their respective transmembrane domains. ExbB is proposed to recycle TonB to an active conformation following energy transduction to the outer membrane. TonB most likely associates with the outer membrane receptors through its carboxy terminus, which is required for function. In contrast, the novel proline-rich region of TonB can be deleted without affecting function. A model that incorporates this information, as well as tempered speculation, is presented.

The conserved proline-rich motif is not essential for energy transduction by Escherichia coli TonB protein

TonB protein functions as an energy transducer, coupling cytoplasmic membrane electrochemical potential to the active transport of vitamin B12 and Fe(III)-siderophore complexes across the outer membrane of Escherichia coli and other Gram-negative bacteria. Accumulated evidence indicates that TonB is anchored in the cytoplasm, but spans the periplasmic space to interact physically with outer membrane receptors. It has been presumed that this ability is caused by a conserved (Glu-Pro)n-(Lys-Pro)m repeat motif, predicted to assume a rigid, linear conformation of sufficient length to reach the outer membrane. Based on in vitro studies with synthetic peptides and purified FhuA outer membrane receptor, it has been suggested that this region contains a site that directly binds outer membrane receptors and is essential for energy transduction. We have found a TonB lacking the (Glu-Pro)n-(Lys-Pro)m repeat motif (TonB delta(66-100)). TonB delta(66-100) is fully capable of irreversible phi 80 adsorption, except under physiological circumstances where the periplasmic space is expanded. Based on the ability of TonB delta(66-100) to interact with outer membrane receptors and components of the energy transduction apparatus under normal physiological conditions, it is evident that the TonB proline-rich region has no role in energy transduction other than to provide a physical extension sufficient to reach the outer membrane.