Ray A. Larsen

Protonmotive force, ExbB and ligand‐bound FepA drive conformational changes in TonB

TonB couples the cytoplasmic membrane protonmotive force (pmf) to active transport across the outer membrane, potentially through a series of conformational changes. Previous studies of a TonB transmembrane domain mutant (TonB‐ΔV17) and its phenotypical suppressor (ExbB‐A39E) suggested that TonB is conformationally sensitive. Here, two new mutations of the conserved TonB transmembrane domain SHLS motif were isolated, TonB‐S16L and ‐H20Y, as were two new suppressors, ExbB‐V35E and ‐V36D. Each suppressor ExbB restored at least partial function to the TonB mutants, although TonB‐ΔV17, for which both the conserved motif and the register of the predicted transmembrane domain α‐helix are affected, was the most refractory. As demonstrated previously, TonB can undergo at least one conformational change, provided both ExbB and a functional TonB transmembrane domain are present. Here, we show that this conformational change reflects the ability of TonB to respond to the cytoplasmic membrane proton gradient, and occurs in proportion to the level of TonB activity attained by mutant–suppressor pairs. The phenotype of TonB‐ΔV17 was more complex than the ‐S16L and ‐H20Y mutations, in that, beyond the inability to be energized efficiently, it was also conditionally unstable. This second defect was evident only after suppression by the ExbB mutants, which allow transmembrane domain mutants to be energized, and presented as the rapid turnover of TonB‐ΔV17. Importantly, this degradation was dependent upon the presence of a TonB‐dependent ligand, suggesting that TonB conformation also changes following the energy transduction event. Together, these observations support a dynamic model of energy transduction in which TonB cycles through a set of conformations that differ in potential energy, with a transition to a higher energy state driven by pmf and a transition to a lower energy state accompanying release of stored potential energy to an outer membrane receptor.

TonB-dependent energy transduction between outer and cytoplasmic membranes.

The TonB system of Escherichia coli (and most other Gram-negative bacteria) is distinguished by its importance to iron acquisition, its contribution to bacterial pathogenesis, and a unique and mysterious mechanism of action. This system somehow gathers the potential energy of the cytoplasmic membrane (CM) proton gradient and delivers it to active transporters in the outer membrane (OM). Our understanding of this system is confounded by the challenge of reconciling often contradictory in vivo and in vitro studies that are presented in this review.

Quantification of known components of the Escherichia coli TonB energy transduction system: TonB, ExbB, ExbD and FepA

The TonB‐dependent energy transduction system couples cytoplasmic membrane proton motive force to active transport of iron–siderophore complexes across the outer membrane in Gram‐negative bacteria. In Escherichia coli, the primary players known in this process to date are: FepA, the TonB‐gated transporter for the siderophore enterochelin; TonB, the energy‐transducing protein; and two cytoplasmic membrane proteins with less defined roles, ExbB and ExbD. In this study, we report the per cell numbers of TonB, ExbB, ExbD and FepA for cells grown under iron‐replete and iron‐limited conditions. Under iron‐replete conditions, TonB and FepA were present at 335 ± 78 and 504 ± 165 copies per cell respectively. ExbB and ExbD, despite being encoded from the same operon, were not equimolar, being present at 2463 ± 522 and 741 ± 105 copies respectively. The ratio of these proteins was calculated at one TonB:two ExbD:seven ExbB under all four growth conditions tested. In contrast, the TonB:FepA ratio varied with iron status and according to the method used for iron limitation. Differences in the method of iron limitation also resulted in significant differences in cell size, skewing the per cell copy numbers for all proteins.

The conserved proline‐rich Motif is not essential for energy transduction by Escherichia coliTonB 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 (TonBΔ(66–100)). TonBΔ(66–100) is fully capable of irreversible 80 adsorption, except under physiological circumstances where the periplasmic space is expanded. Based on the ability of TonBΔ(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.

Partial suppression of an Escherichia coli TonB transmembrane domain mutation (ΔV17) 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 amino‐terminal 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 ΔV17 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 TonBΔV17. Molecular modeling suggested that the ΔV17 mutation caused a significant change in the predicted conserved face of the TonB amino‐terminal membrane anchor. TonBΔV17 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 ΔV17 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 TonBΔV17 was not observed in the presence of ExbBA39E. Neither the ExbBA39E allele nor the absence of ExbB affected TonB or TonBΔV17 export. Unlike the tonBΔV17 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 TonBδV17, ExbBA39E pair resulted in a TonB with a greatly reduced half‐life (≅ 10 min). These results together with protease susceptibility studies suggest that ExbB functions by modulating the conformation of TonB.

In vivo evidence of TonB shuttling between the cytoplasmic and outer membrane in Escherichia coli

Gram‐negative bacteria are able to convert potential energy inherent in the proton gradient of the cytoplasmic membrane into active nutrient transport across the outer membrane. The transduction of energy is mediated by TonB protein. Previous studies suggest a model in which TonB makes sequential and cyclic contact with proteins in each membrane, a process called shuttling. A key feature of shuttling is that the amino‐terminal signal anchor must quit its association with the cytoplasmic membrane, and TonB becomes associated solely with the outer membrane. However, the initial studies did not exclude the possibility that TonB was artifactually pulled from the cytoplasmic membrane by the fractionation process. To resolve this ambiguity, we devised a method to test whether the extreme TonB amino‐terminus, located in the cytoplasm, ever became accessible to the cys‐specific, cytoplasmic membrane‐impermeant molecule, Oregon Green® 488 maleimide (OGM) in vivo. A full‐length TonB and a truncated TonB were modified to carry a sole cysteine at position 3. Both full‐length TonB and truncated TonB (consisting of the amino‐terminal two‐thirds) achieved identical conformations in the cytoplasmic membrane, as determined by their abilities to cross‐link to the cytoplasmic membrane protein ExbB and their abilities to respond conformationally to the presence or absence of proton motive force. Full‐length TonB could be amino‐terminally labelled in vivo, suggesting that it was periplasmically exposed. In contrast, truncated TonB, which did not associate with the outer membrane, was not specifically labelled in vivo. The truncated TonB also acted as a control for leakage of OGM across the cytoplasmic membrane. Further, the extent of labelling for full‐length TonB correlated roughly with the proportion of TonB found at the outer membrane. These findings suggest that TonB does indeed disengage from the cytoplasmic membrane during energy transduction and shuttle to the outer membrane.

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

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 α-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.

Performance of Standard Phenotypic Assays for TonB Activity, as Evaluated by Varying the Level of Functional, Wild-Type TonB

The ability of gram-negative bacterial cells to transport cobalamin and iron-siderophore complexes and their susceptibility to killing by some bacteriophages and colicins are characteristics routinely used to assay mutations of proteins in the TonB-dependent energy transduction system. These assays vary greatly in sensitivity and are subject to perturbation by overexpression of TonB and, perhaps, other proteins that contribute to the process. Thus, the choice of assay and the means by which a potential mutant is expressed can greatly influence the interpretation and recognition of a given mutant. In the present study, we expressed TonB at several different quantified levels in cells that were then subjected to a panel of assays. Our results suggest that it is reasonable to regard the assays as having windows of sensitivity. Thus, while no single assay satisfactorily spans the potential range of TonB activity, it is evident that certain assays are better suited for resolving small deviations from wild-type levels of activity, with others most useful when activity levels are very low. It is apparent from the results that the application of all possible assays to the characterization of new mutants will yield the most meaningful results.

Interactions of the Energy Transducer TonB with Noncognate Energy-Harvesting Complexes

The TonB and TolA proteins are energy transducers that couple the ion electrochemical potential of the cytoplasmic membrane to support energy-dependent processes at the outer membrane of the gram-negative envelope. The transfer of energy to these transducers is facilitated by energy-harvesting complexes, which are heteromultimers of cytoplasmic membrane proteins with homologies to proton pump proteins of the flagellar motor. Although the cognate energy-harvesting complex best services each transducer, components of the complexes (for TonB, ExbB and ExbD; for TolA, TolQ and TolR) are sufficiently similar that each complex can imperfectly replace the other. Previous investigations of this molecular cross talk considered energy-harvesting complex components expressed from multicopy plasmids in strains in which the corresponding genes were interrupted by insertions, partially absent due to polarity, or missing due to a larger deletion. These questions were reexamined here using strains in which individual genes were removed by precise deletions and, where possible, components were expressed from single-copy genes with native promoters. By more closely approximating natural stoichiometries between components, this study provided insight into the roles of energy-harvesting complexes in both the energization and the stabilization of TonB. Further, the data suggest a distinct role for ExbD in the TonB energy transduction cycle.

Death of the TonB Shuttle Hypothesis

A complex of ExbB, ExbD, and TonB couples cytoplasmic membrane (CM) proton motive force (pmf) to the active transport of large, scarce, or important nutrients across the outer membrane (OM). TonB interacts with OM transporters to enable ligand transport. Several mechanical models and a shuttle model explain how TonB might work. In the mechanical models, TonB remains attached to the CM during energy transduction, while in the shuttle model the TonB N terminus leaves the CM to deliver conformationally stored potential energy to OM transporters. Previous studies suggested that TonB did not shuttle based on the activity of a GFP–TonB fusion that was anchored in the CM by the GFP moiety. When we recreated the GFP–TonB fusion to extend those studies, in our hands it was proteolytically unstable, giving rise to potentially shuttleable degradation products. Recently, we discovered that a fusion of the Vibrio cholerae ToxR cytoplasmic domain to the N terminus of TonB was proteolytically stable. ToxR–TonB was able to be completely converted into a proteinase K-resistant conformation in response to loss of pmf in spheroplasts and exhibited an ability to form a pmf-dependent formaldehyde crosslink to ExbD, both indicators of its location in the CM. Most importantly, ToxR–TonB had the same relative specific activity as wild-type TonB. Taken together, these results provide conclusive evidence that TonB does not shuttle during energy transduction. We had previously concluded that TonB shuttles based on the use of an Oregon Green® 488 maleimide probe to assess periplasmic accessibility of N-terminal TonB. Here we show that the probe was permeant to the CM, thus permitting the labeling of the TonB N-terminus. These former results are reinterpreted in the context that TonB does not shuttle, and suggest the existence of a signal transduction pathway from OM to cytoplasm.