His(20) provides the sole functionally significant side chain in the essential TonB transmembrane domain

Primary role of the Tol-Pal complex in bacterial outer membrane lipid homeostasis

Gram-negative bacteria are defined by an outer membrane (OM) that contributes to envelope integrity and barrier function. Building this bilayer require proper assembly of lipopolysaccharides, proteins, and phospholipids, yet how the balance of these components is achieved is unclear. One system long known for ensuring OM stability is the Tol-Pal complex, which has been implicated in maintaining OM lipid homeostasis. However, assignment of Tol-Pal function has been challenging, owing to its septal localization and associated role(s) during division. Here, we uncouple the function of Tol-Pal in OM lipid homeostasis from its impact on cell division in Escherichia coli, by engineering a chimeric complex that loses septal enrichment. We demonstrate that this peripherally-localized Tol-Pal complex is fully capable of maintaining lipid balance in the OM, thus restoring OM integrity and barrier. Our work establishes the primary function of the Tol-Pal complex in OM lipid homeostasis, independent of its role during division.

Study in the iron uptake mechanism of Pasteurella multocida

Pasteurella multocida infects a wide range of animals, causing hemorrhagic septicemia or infectious pneumonia. Iron is an essential nutrient for growth, colonization, and proliferation of P. multocida during infection of the host, and competition for iron ions in the host is a critical link in the pathogenesis of this pathogen. In recent years, there has been significant progress in the study of the iron uptake system of P. multocida, including its occurrence and regulatory mechanisms. In order to provide a systematic theoretical basis for the study of the molecular pathogenesis of the P. multocida iron uptake system, and generate new ideas for the investigation and development of molecular-targeted drugs and subunit vaccines against P. multocida, the mechanisms of iron uptake by transferrin receptors, heme receptors, and siderophores, and the mechanism of expression and regulation of the P. multocida iron uptake system are all described.

In vivo tests of the E. coli TonB system working model-interaction of ExbB with unknown proteins, identification of TonB-ExbD transmembrane heterodimers and PMF-dependent ExbD structures

The TonB system of Escherichia coli resolves the dilemma posed by its outer membrane that protects it from a variety of external threats, but also constitutes a diffusion barrier to nutrient uptake. Our working model involves interactions among a set of cytoplasmic membrane-bound proteins: tetrameric ExbB that serves as a scaffold for a dimeric TonB complex (ExbB 4 -TonB 2 ), and also engages dimeric ExbD (ExbB 4 -ExbD 2 ). Through a set of synchronized conformational changes and movements these complexes are proposed to cyclically transduce cytoplasmic membrane protonmotive force to energize active transport of nutrients through TonB-dependent transporters in the outer membrane (described in Gresock et al. , J. Bacteriol. 197:3433). In this work, we provide experimental validation of three important aspects of the model. The majority of ExbB is exposed to the cytoplasm, with an ∼90-residue cytoplasmic loop and an ∼50 residue carboxy terminal tail. Here we found for the first time, that the cytoplasmic regions of ExbB served as in vivo contacts for three heretofore undiscovered proteins, candidates to move ExbB complexes within the membrane. Support for the model also came from visualization of in vivo PMF-dependent conformational transitions in ExbD. Finally, we also show that TonB forms homodimers and heterodimers with ExbD through its transmembrane domain in vivo . This trio of in vivo observations suggest how and why solved in vitro structures of ExbB and ExbD differ significantly from the in vivo results and submit that future inclusion of the unknown ExbB-binding proteins may bring solved structures into congruence with proposed in vivo energy transduction cycle intermediates.

Force-Generation by the Trans-Envelope Tol-Pal System

The Tol-Pal system spans the cell envelope of Gram-negative bacteria, transducing the potential energy of the proton motive force (PMF) into dissociation of the TolB-Pal complex at the outer membrane (OM), freeing the lipoprotein Pal to bind the cell wall. The primary physiological role of Tol-Pal is to maintain OM integrity during cell division through accumulation of Pal molecules at division septa. How the protein complex couples the PMF at the inner membrane into work at the OM is unknown. The effectiveness of this trans-envelope energy transduction system is underscored by the fact that bacteriocins and bacteriophages co-opt Tol-Pal as part of their import/infection mechanisms. Mechanistic understanding of this process has been hindered by a lack of structural data for the inner membrane TolQ-TolR stator, of its complexes with peptidoglycan (PG) and TolA, and of how these elements combined power events at the OM. Recent studies on the homologous stators of Ton and Mot provide a starting point for understanding how Tol-Pal works. Here, we combine ab initio protein modeling with previous structural data on sub-complexes of Tol-Pal as well as mutagenesis, crosslinking, co-conservation analysis and functional data. Through this composite pooling of in silico, in vitro, and in vivo data, we propose a mechanism for force generation in which PMF-driven rotary motion within the stator drives conformational transitions within a long TolA helical hairpin domain, enabling it to reach the TolB-Pal complex at the OM.

The Intrinsically Disordered Region of ExbD Is Required for Signal Transduction

The TonB system actively transports vital nutrients across the unenergized outer membranes of the majority of Gram-negative bacteria. In this system, integral membrane proteins ExbB, ExbD, and TonB work together to transduce the proton motive force (PMF) of the inner membrane to customized active transporters in the outer membrane by direct and cyclic binding of TonB to the transporters. A PMF-dependent TonB-ExbD interaction is prevented by 10-residue deletions within a periplasmic disordered domain of ExbD adjacent to the cytoplasmic membrane. Here, we explored the function of the ExbD disordered domain in more detail. In vivo photo-cross-linking through sequential pBpa substitutions in the ExbD disordered domain captured five different ExbD complexes, some of which had been previously detected using in vivo formaldehyde cross-linking, a technique that lacks the residue-specific information that can be achieved through photo-cross-linking: two ExbB-ExbD heterodimers (one of which had not been detected previously), previously detected ExbD homodimers, previously detected PMF-dependent ExbD-TonB heterodimers, and for the first time, a predicted, ExbD-TonB PMF-independent interaction. The fact that multiple complexes were captured by the same pBpa substitution indicated the dynamic nature of ExbD interactions as the energy transduction cycle proceeded in vivo In this study, we also discovered that a conserved motif-V45, V47, L49, and P50-within the disordered domain was required for signal transduction to TonB and to the C-terminal domain of ExbD and was the source of motif essentiality.IMPORTANCE The TonB system is a virulence factor for Gram-negative pathogens. The mechanism by which cytoplasmic membrane proteins of the TonB system transduce an electrochemical gradient into mechanical energy is a long-standing mystery. TonB, ExbB, and ExbD primary amino acid sequences are characterized by regions of predicted intrinsic disorder, consistent with a proposed multiplicity of protein-protein contacts as TonB proceeds through an energy transduction cycle, a complex process that has yet to be recapitulated in vitro This study validates a region of intrinsic disorder near the ExbD transmembrane domain and identifies an essential conserved motif embedded within it that transduces signals to distal regions of ExbD suggested to configure TonB for productive interaction with outer membrane transporters.

Two novel proteins, TtpB2 and TtpD2, are essential for iron transport in the TonB2 system of Vibrio vulnificus

In gram-negative bacteria, energy-dependent active transport of iron-bound substrates across the outer membrane is achieved through the TonB systems of proteins. Three TonB systems have been identified in the human pathogen Vibrio vulnificus. The TonB1 system contains three proteins: TonB1, ExbB1, and ExbD1. Both the TonB2 and TonB3 systems have been shown to also contain a fourth protein, TtpC2 and TtpC3, respectively. Here, we report and begin to characterize two additional proteins in the TonB2 and TonB3 systems: TtpB and TtpD. Both TtpB2 and TtpD2 are absolutely required for the function of the TonB2 system in V. vulnificus. However, although both TtpB3 and TtpD3 in the TonB3 system are related to the proteins in the TonB2 system, neither are active in iron transport. All six protein components of the TonB2 system-TonB2, ExbB2, ExbD2, TtpB2, TtpC2, and TtpD2-are essential for the uptake of both endogenously produced iron-bound siderophores and exogenous siderophores produced from other organisms. Through complementation, we have shown that V. vulnificus is capable of using different TtpD2 proteins from other Vibrio species to bring in multiple siderophores. In contrast, we also demonstrate that TtpB2 must come from V. vulnificus, and not other species within the genus, to complement mutations in the TonB2 system.

Tools and Approaches for Dissecting Protein Bacteriocin Import in Gram-Negative Bacteria

Bacteriocins of Gram-negative bacteria are typically multi-domain proteins that target and kill bacteria of the same or closely related species. There is increasing interest in protein bacteriocin import; from a fundamental perspective to understand how folded proteins are imported into bacteria and from an applications perspective as species-specific antibiotics to combat multidrug resistant bacteria. In order to translocate across the cell envelope and cause cell death, protein bacteriocins hijack nutrient uptake pathways. Their import is energized by parasitizing intermembrane protein complexes coupled to the proton motive force, which delivers a toxic domain into the cell. A plethora of genetic, structural, biochemical, and biophysical methods have been applied to find cell envelope components involved in bacteriocin import since their discovery almost a century ago. Here, we review the various approaches that now exist for investigating how protein bacteriocins translocate into Gram-negative bacteria and highlight areas of research that will need methodological innovations to fully understand this process. We also highlight recent studies demonstrating how bacteriocins can be used to probe organization and architecture of the Gram-negative cell envelope itself.

Assessing Energy-Dependent Protein Conformational Changes in the TonB System

Changes in conformation can alter a protein's vulnerability to proteolysis. Thus, in vivo differential proteinase sensitivity provides a means for identifying conformational changes that mark discrete states in the activity cycle of a protein. The ability to detect a specific conformational state allows for experiments to address specific protein-protein interactions and other physiological components that potentially contribute to the function of the protein. This chapter presents the application of this technique to the TonB-dependent energy transduction system of Gram-negative bacteria, a strategy that has refined our understanding of how the TonB protein is coupled to the ion electrochemical gradient of the cytoplasmic membrane.

Going Outside the TonB Box: Identification of Novel FepA-TonB Interactions In Vivo

In Gram-negative bacteria, the cytoplasmic membrane protein TonB transmits energy derived from proton motive force to energize transport of important nutrients through TonB-dependent transporters in the outer membrane. Each transporter consists of a beta barrel domain and a lumen-occluding cork domain containing an essential sequence called the TonB box. To date, the only identified site of transporter-TonB interaction is between the TonB box and residues ∼158 to 162 of TonB. While the mechanism of ligand transport is a mystery, a current model based on site-directed spin labeling and molecular dynamics simulations is that, following ligand binding, the otherwise-sequestered TonB box extends into the periplasm for recognition by TonB, which mediates transport by pulling or twisting the cork. In this study, we tested that hypothesis with the outer membrane transporter FepA using in vivo photo-cross-linking to explore interactions of its TonB box and determine whether additional FepA-TonB interaction sites exist. We found numerous specific sites of FepA interaction with TonB on the periplasmic face of the FepA cork in addition to the TonB box. Two residues, T32 and A33, might constitute a ligand-sensitive conformational switch. The facts that some interactions were enhanced in the absence of ligand and that other interactions did not require the TonB box argued against the current model and suggested that the transport process is more complex than originally conceived, with subtleties that might provide a mechanism for discrimination among ligand-loaded transporters. These results constitute the first study on the dynamics of TonB-gated transporter interaction with TonB in vivoIMPORTANCE The TonB system of Gram-negative bacteria has a noncanonical active transport mechanism involving signal transduction and proteins integral to both membranes. To achieve transport, the cytoplasmic membrane protein TonB physically contacts outer membrane transporters such as FepA. Only one contact between TonB and outer membrane transporters has been identified to date: the TonB box at the transporter amino terminus. The TonB box has low information content, raising the question of how TonB can discriminate among multiple different TonB-dependent transporters present in the bacterium if it is the only means of contact. Here we identified several additional sites through which FepA contacts TonB in vivo, including two neighboring residues that may explain how FepA signals to TonB that ligand has bound.

Fluorescence High-Throughput Screening for Inhibitors of TonB Action

Gram-negative bacteria acquire ferric siderophores through TonB-dependent outer membrane transporters (TBDT). By fluorescence spectroscopic hgh-throughput screening (FLHTS), we identified inhibitors of TonB-dependent ferric enterobactin (FeEnt) uptake through Escherichia coli FepA (EcoFepA). Among 165 inhibitors found in a primary screen of 17,441 compounds, we evaluated 20 in secondary tests: TonB-dependent ferric siderophore uptake and colicin killing and proton motive force-dependent lactose transport. Six of 20 primary hits inhibited TonB-dependent activity in all tests. Comparison of their effects on [⁵⁹Fe]Ent and [¹⁴C]lactose accumulation suggested several as proton ionophores, but two chemicals, ebselen and ST0082990, are likely not proton ionophores and may inhibit TonB-ExbBD. The facility of FLHTS against E. coli led us to adapt it to Acinetobacter baumannii We identified its FepA ortholog (AbaFepA), deleted and cloned its structural gene, genetically engineered 8 Cys substitutions in its surface loops, labeled them with fluorescein, and made fluorescence spectroscopic observations of FeEnt uptake in A. baumannii Several Cys substitutions in AbaFepA (S279C, T562C, and S665C) were readily fluoresceinated and then suitable as sensors of FeEnt transport. As in E. coli, the test monitored TonB-dependent FeEnt uptake by AbaFepA. In microtiter format with A. baumannii, FLHTS produced Z' factors 0.6 to 0.8. These data validated the FLHTS strategy against even distantly related Gram-negative bacterial pathogens. Overall, it discovered agents that block TonB-dependent transport and showed the potential to find compounds that act against Gram-negative CRE (carbapenem-resistant Enterobacteriaceae)/ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens. Our results suggest that hundreds of such chemicals may exist in larger compound libraries.IMPORTANCE Antibiotic resistance in Gram-negative bacteria has spurred efforts to find novel compounds against new targets. The CRE/ESKAPE pathogens are resistant bacteria that include Acinetobacter baumannii, a common cause of ventilator-associated pneumonia and sepsis. We performed fluorescence high-throughput screening (FLHTS) against Escherichia coli to find inhibitors of TonB-dependent iron transport, tested them against A. baumannii, and then adapted the FLHTS technology to allow direct screening against A. baumannii This methodology is expandable to other drug-resistant Gram-negative pathogens. Compounds that block TonB action may interfere with iron acquisition from eukaryotic hosts and thereby constitute bacteriostatic antibiotics that prevent microbial colonization of human and animals. The FLHTS method may identify both species-specific and broad-spectrum agents against Gram-negative bacteria.

From Homodimer to Heterodimer and Back: Elucidating the TonB Energy Transduction Cycle

The TonB system actively transports large, scarce, and important nutrients through outer membrane (OM) transporters of Gram-negative bacteria using the proton gradient of the cytoplasmic membrane (CM). In Escherichia coli, the CM proteins ExbB and ExbD harness and transfer proton motive force energy to the CM protein TonB, which spans the periplasmic space and cyclically binds OM transporters. TonB has two activity domains: the amino-terminal transmembrane domain with residue H20 and the periplasmic carboxy terminus, through which it binds to OM transporters. TonB is inactivated by all substitutions at residue H20 except H20N. Here, we show that while TonB trapped as a homodimer through its amino-terminal domain retained full activity, trapping TonB through its carboxy terminus inactivated it by preventing conformational changes needed for interaction with OM transporters. Surprisingly, inactive TonB H20A had little effect on homodimerization through the amino terminus and instead decreased TonB carboxy-terminal homodimer formation prior to reinitiation of an energy transduction cycle. That result suggested that the TonB carboxy terminus ultimately interacts with OM transporters as a monomer. Our findings also suggested the existence of a separate equimolar pool of ExbD homodimers that are not in contact with TonB. A model is proposed where interaction of TonB homodimers with ExbD homodimers initiates the energy transduction cycle, and, ultimately, the ExbD carboxy terminus modulates interactions of a monomeric TonB carboxy terminus with OM transporters. After TonB exchanges its interaction with ExbD for interaction with a transporter, ExbD homodimers undergo a separate cycle needed to re-energize them.

IMPORTANCE

Canonical mechanisms of active transport across cytoplasmic membranes employ ion gradients or hydrolysis of ATP for energy. Gram-negative bacterial outer membranes lack these resources. The TonB system embodies a novel means of active transport across the outer membrane for nutrients that are too large, too scarce, or too important for diffusion-limited transport. A proton gradient across the cytoplasmic membrane is converted by a multiprotein complex into mechanical energy that drives high-affinity active transport across the outer membrane. This system is also of interest since one of its uses in pathogenic bacteria is for competition with the host for the essential element iron. Understanding the mechanism of the TonB system will allow design of antibiotics targeting iron acquisition.

Membrane Protein Complex ExbB4-ExbD1-TonB1 from Escherichia coli Demonstrates Conformational Plasticity

Iron acquisition at the outer membrane (OM) of Gram-negative bacteria is powered by the proton motive force (PMF) of the cytoplasmic membrane (CM), harnessed by the CM-embedded complex of ExbB, ExbD, and TonB. Its stoichiometry, ensemble structural features, and mechanism of action are unknown. By panning combinatorial phage libraries, periplasmic regions of dimerization between ExbD and TonB were predicted. Using overexpression of full-length His6-tagged exbB-exbD and S-tagged tonB, we purified detergent-solubilized complexes of ExbB-ExbD-TonB from Escherichia coli. Protein-detergent complexes of ∼230 kDa with a hydrodynamic radius of ∼6.0 nm were similar to previously purified ExbB₄-ExbD₂ complexes. Significantly, they differed in electronegativity by native agarose gel electrophoresis. The stoichiometry was determined to be ExbB₄-ExbD₁-TonB₁. Single-particle electron microscopy agrees with this stoichiometry. Two-dimensional averaging supported the phage display predictions, showing two forms of ExbD-TonB periplasmic heterodimerization: extensive and distal. Three-dimensional (3D) particle classification showed three representative conformations of ExbB₄-ExbD₁-TonB₁. Based on our structural data, we propose a model in which ExbD shuttles a proton across the CM via an ExbB interprotein rearrangement. Proton translocation would be coupled to ExbD-mediated collapse of extended TonB in complex with ligand-loaded receptors in the OM, followed by repositioning of TonB through extensive dimerization with ExbD. Here we present the first report for purification of the ExbB-ExbD-TonB complex, molar ratios within the complex (4:1:1), and structural biology that provides insights into 3D organization.

IMPORTANCE

Receptors in the OM of Gram-negative bacteria allow entry of iron-bound siderophores that are necessary for pathogenicity. Numerous iron-acquisition strategies rely upon a ubiquitous and unique protein for energization: TonB. Complexed with ExbB and ExbD, the Ton system links the PMF to OM transport. Blocking iron uptake by targeting a vital nanomachine holds promise in therapeutics. Despite much research, the stoichiometry, structural arrangement, and molecular mechanism of the CM-embedded ExbB-ExbD-TonB complex remain unreported. Here we demonstrate in vitro evidence of ExbB₄-ExbD₁-TonB₁ complexes. Using 3D EM, we reconstructed the complex in three conformational states that show variable ExbD-TonB heterodimerization. Our structural observations form the basis of a model for TonB-mediated iron acquisition.

The role of ExbD in periplasmic pH homeostasis in Helicobacter pylori

BACKGROUND

Helicobacter pylori, a neutralophile, colonizes the acidic environment of the human stomach by employing acid acclimation mechanisms that regulate periplasmic and cytoplasmic pH. The regulation of urease activity is central to acid acclimation. Inactive urease apoenzyme, UreA/B, requires nickel for activation. Accessory proteins UreE, F, G, and H are required for nickel insertion into apoenzyme. The ExbB/ExbD/TonB complex transfers energy from the inner to outer membrane, providing the driving force for nickel uptake. Therefore, the aim of this study was to determine the contribution of ExbD to pH homeostasis.

MATERIALS AND METHODS

A nonpolar exbD knockout was constructed and survival, growth, urease activity, and membrane potential were determined in comparison with wildtype.

RESULTS

Survival of the ΔexbD strain was significantly reduced at pH 3.0. Urease activity as a function of pH and UreI activation was similar to the wildtype strain, showing normal function of the proton-gated urea channel, UreI. The increase in total urease activity over time in acid seen in the wildtype strain was abolished in the ΔexbD strain, but recovered in the presence of supraphysiologic nickel concentrations, demonstrating that the effect of the ΔexbD mutant is due to loss of a necessary constant supply of nickel. In acid, ΔexbD also decreased its ability to maintain membrane potential and periplasmic buffering in the presence of urea.

CONCLUSIONS

ExbD is essential for maintenance of periplasmic buffering and membrane potential by transferring energy required for nickel uptake, making it a potential nonantibiotic target for H. pylori eradication.

ExbB cytoplasmic loop deletions cause immediate, proton motive force-independent growth arrest

The Escherichia coli TonB system consists of the cytoplasmic membrane proteins TonB, ExbB, and ExbD and multiple outer membrane active transporters for diverse iron siderophores and vitamin B12. The cytoplasmic membrane proteins harvest and transmit the proton motive force (PMF) to outer membrane transporters. This system, which spans the cell envelope, has only one component with a significant cytoplasmic presence, ExbB. Characterization of sequential 10-residue deletions in the ExbB cytoplasmic loop (residues 40 to 129; referred to as Δ10 proteins) revealed that it was required for all TonB-dependent activities, including interaction between the periplasmic domains of TonB and ExbD. Expression of eight out of nine of the Δ10 proteins at chromosomal levels led to immediate, but reversible, growth arrest. Arrest was not due to collapse of the PMF and did not require the presence of ExbD or TonB. All Δ10 proteins that caused growth arrest were dominant for that phenotype. However, several were not dominant for iron transport, indicating that growth arrest was an intrinsic property of the Δ10 variants, whether or not they could associate with wild-type ExbB proteins. The lack of dominance in iron transport also ruled out trivial explanations for growth arrest, such as high-level induction. Taken together, the data suggest that growth arrest reflected a changed interaction between the ExbB cytoplasmic loop and one or more unknown growth-regulatory proteins. Consistent with that, a large proportion of the ExbB cytoplasmic loop between transmembrane domain 1 (TMD1) and TMD2 is predicted to be disordered, suggesting the need for interaction with one or more cytoplasmic proteins to induce a final structure.

Mutations in Escherichia coli ExbB transmembrane domains identify scaffolding and signal transduction functions and exclude participation in a proton pathway

The TonB system couples cytoplasmic membrane proton motive force (pmf) to active transport of diverse nutrients across the outer membrane. Current data suggest that cytoplasmic membrane proteins ExbB and ExbD harness pmf energy. Transmembrane domain (TMD) interactions between TonB and ExbD allow the ExbD C terminus to modulate conformational rearrangements of the periplasmic TonB C terminus in vivo. These conformational changes somehow allow energization of high-affinity TonB-gated transporters by direct interaction with TonB. While ExbB is essential for energy transduction, its role is not well understood. ExbB has N-terminus-out, C-terminus-in topology with three TMDs. TMDs 1 and 2 are punctuated by a cytoplasmic loop, with the C-terminal tail also occupying the cytoplasm. We tested the hypothesis that ExbB TMD residues play roles in proton translocation. Reassessment of TMD boundaries based on hydrophobic character and residue conservation among distantly related ExbB proteins brought earlier widely divergent predictions into congruence. All TMD residues with potentially function-specific side chains (Lys, Cys, Ser, Thr, Tyr, Glu, and Asn) and residues with probable structure-specific side chains (Trp, Gly, and Pro) were substituted with Ala and evaluated in multiple assays. While all three TMDs were essential, they had different roles: TMD1 was a region through which ExbB interacted with the TonB TMD. TMD2 and TMD3, the most conserved among the ExbB/TolQ/MotA/PomA family, played roles in signal transduction between cytoplasm and periplasm and the transition from ExbB homodimers to homotetramers. Consideration of combined data excludes ExbB TMD residues from direct participation in a proton pathway.

The ttpC gene is contained in two of three TonB systems in the human pathogen Vibrio vulnificus, but only one is active in iron transport and virulence

The TonB system of proteins is required for the energy-dependent active transport of iron-bound substrates across the outer membrane of gram-negative bacteria. We have identified three TonB systems within the human pathogen Vibrio vulnificus. The TonB1 system contains the TonB1, ExbD1, and ExbB1 proteins, whereas both the TtpC2-TonB2 and TtpC3-TonB3 systems contain an additional fourth protein, TtpC. Here we report that TtpC3, although highly related to TtpC2, is inactive in iron transport, whereas TtpC2 is essential for the function of the TtpC2-TonB2 system in V. vulnificus. This protein, together with TonB2, is absolutely required for both the uptake of endogenously produced iron-bound siderophores as well as siderophores produced from other organisms. Through complementation we show that V. vulnificus is capable of using different TtpC2 proteins from other Vibrio species to drive the uptake of multiple siderophores. We have also determined that aerobactin, a common bacterial siderophore involved in virulence of enteric bacteria, can only be brought into the cell using the TtpC2-TonB2 system, indicating an important evolutionary adaptation of TtpC2 and TonB2. Furthermore, in the absence of TonB1, TtpC2 is essential for a fully virulent phenotype as demonstrated using 50% lethal dose (LD(50)) experiments in mice.

Identification of functionally important TonB-ExbD periplasmic domain interactions in vivo

In gram-negative bacteria, the cytoplasmic membrane proton-motive force energizes the active transport of TonB-dependent ligands through outer membrane TonB-gated transporters. In Escherichia coli, cytoplasmic membrane proteins ExbB and ExbD couple the proton-motive force to conformational changes in TonB, which are hypothesized to form the basis of energy transduction through direct contact with the transporters. While the role of ExbB is not well understood, contact between periplasmic domains of TonB and ExbD is required, with the conformational response of TonB to presence or absence of proton motive force being modulated through ExbD. A region (residues 92 to 121) within the ExbD periplasmic domain was previously identified as being important for TonB interaction. Here, the specific sites of periplasmic domain interactions between that region and the TonB carboxy terminus were identified by examining 270 combinations of 45 TonB and 6 ExbD individual cysteine substitutions for disulfide-linked heterodimer formation. ExbD residues A92C, K97C, and T109C interacted with multiple TonB substitutions in four regions of the TonB carboxy terminus. Two regions were on each side of the TonB residues known to interact with the TonB box of TonB-gated transporters, suggesting that ExbD positions TonB for correct interaction at that site. A third region contained a functionally important glycine residue, and the fourth region involved a highly conserved predicted amphipathic helix. Three ExbD substitutions, F103C, L115C, and T121C, were nonreactive with any TonB cysteine substitutions. ExbD D25, a candidate to be on a proton translocation pathway, was important to support efficient TonB-ExbD heterodimerization at these specific regions.

The ExbD periplasmic domain contains distinct functional regions for two stages in TonB energization

The TonB system of gram-negative bacteria energizes the active transport of diverse nutrients through high-affinity TonB-gated outer membrane transporters using energy derived from the cytoplasmic membrane proton motive force. Cytoplasmic membrane proteins ExbB and ExbD harness the proton gradient to energize TonB, which directly contacts and transmits this energy to ligand-loaded transporters. In Escherichia coli, the periplasmic domain of ExbD appears to transition from proton motive force-independent to proton motive force-dependent interactions with TonB, catalyzing the conformational changes of TonB. A 10-residue deletion scanning analysis showed that while all regions except the extreme amino terminus of ExbD were indispensable for function, distinct roles for the amino- and carboxy-terminal regions of the ExbD periplasmic domain were evident. Like residue D25 in the ExbD transmembrane domain, periplasmic residues 42 to 61 facilitated the conformational response of ExbD to proton motive force. This region appears to be important for transmitting signals between the ExbD transmembrane domain and carboxy terminus. The carboxy terminus, encompassing periplasmic residues 62 to 141, was required for initial assembly with the periplasmic domain of TonB, a stage of interaction required for ExbD to transmit its conformational response to proton motive force to TonB. Residues 92 to 121 were important for all three interactions previously observed for formaldehyde-cross-linked ExbD: ExbD homodimers, TonB-ExbD heterodimers, and ExbD-ExbB heterodimers. The distinct requirement of this ExbD region for interaction with ExbB raised the possibility of direct interaction with the few residues of ExbB known to occupy the periplasm.

ExbD mutants define initial stages in TonB energization

Cytoplasmic membrane proteins ExbB and ExbD of the Escherichia coli TonB system couple cytoplasmic membrane protonmotive force (pmf) to TonB. TonB transmits this energy to high-affinity outer membrane active transporters. ExbD is proposed to catalyze TonB conformational changes during energy transduction. Here, the effect of ExbD mutants and changes in pmf on TonB proteinase K sensitivity in spheroplasts was examined. Spheroplasts supported the pmf-dependent formaldehyde cross-link between periplasmic domains of TonB and ExbD, indicating that they constituted a biologically relevant in vivo system to study changes in TonB proteinase K sensitivity. Three stages in TonB energization were identified. In Stage I, ExbD L123Q or TonB H20A prevented proper interaction between TonB and ExbD, rendering TonB sensitive to proteinase K. In Stage II, ExbD D25N supported conversion of TonB to a proteinase-K-resistant form, but not energization of TonB or formation of the pmf-dependent formaldehyde cross-link. Addition of protonophores had the same effect as ExbD D25N. This suggested the existence of a pmf-independent association between TonB and ExbD. TonB proceeded to Stage III when pmf was present, again becoming proteinase K sensitive, but now able to form the pmf-dependent cross-link to ExbD. Absence or presence of pmf toggled TonB between Stage II and Stage III conformations, which were also detected in wild-type cells. ExbD also underwent pmf-dependent conformational changes that were interdependent with TonB. These observations supported the hypothesis that ExbD couples TonB to the pmf, with concomitant transitions of ExbD and TonB periplasmic domains from unenergized to energized heterodimers.

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.

The same periplasmic ExbD residues mediate in vivo interactions between ExbD homodimers and ExbD-TonB heterodimers

The TonB system couples cytoplasmic membrane proton motive force to TonB-gated outer membrane transporters for active transport of nutrients into the periplasm. In Escherichia coli, cytoplasmic membrane proteins ExbB and ExbD promote conformational changes in TonB, which transmits this energy to the transporters. The only known energy-dependent interaction occurs between the periplasmic domains of TonB and ExbD. This study identified sites of in vivo homodimeric interactions within ExbD periplasmic domain residues 92 to 121. ExbD was active as a homodimer (ExbD(2)) but not through all Cys substitution sites, suggesting the existence of conformationally dynamic regions in the ExbD periplasmic domain. A subset of homodimeric interactions could not be modeled on the nuclear magnetic resonance (NMR) structure without significant distortion. Most importantly, the majority of ExbD Cys substitutions that mediated homodimer formation also mediated ExbD-TonB heterodimer formation with TonB A150C. Consistent with the implied competition, ExbD homodimer formation increased in the absence of TonB. Although ExbD D25 was not required for their formation, ExbD dimers interacted in vivo with ExbB. ExbD-TonB interactions required ExbD transmembrane domain residue D25. These results suggested a model where ExbD(2) assembled with ExbB undergoes a transmembrane domain-dependent transition and exchanges partners in localized homodimeric interfaces to form an ExbD(2)-TonB heterotrimer. The findings here were also consistent with our previous hypothesis that ExbD guides the conformation of the TonB periplasmic domain, which itself is conformationally dynamic.

Mutations in the ExbB cytoplasmic carboxy terminus prevent energy-dependent interaction between the TonB and ExbD periplasmic domains

The TonB system of Gram-negative bacteria provides passage across the outer membrane (OM) diffusion barrier that otherwise limits access to large, scarce, or important nutrients. In Escherichia coli, the integral cytoplasmic membrane (CM) proteins TonB, ExbB, and ExbD couple the CM proton motive force (PMF) to active transport of iron-siderophore complexes and vitamin B(12) across the OM through high-affinity transporters. ExbB is an integral CM protein with three transmembrane domains. The majority of ExbB occupies the cytoplasm. Here, the importance of the cytoplasmic ExbB carboxy terminus (residues 195 to 244) was evaluated by cysteine scanning mutagenesis. D211C and some of the substitutions nearest the carboxy terminus spontaneously formed disulfide cross-links, even though the cytoplasm is a reducing environment. ExbB N196C and D211C substitutions were converted to Ala substitutions to stabilize them. Only N196A, D211A, A228C, and G244C substitutions significantly decreased ExbB activity. With the exception of ExbB(G244C), all of the substituted forms were dominant. Like wild-type ExbB, they all formed a formaldehyde cross-linked tetramer, as well as a tetramer cross-linked to an unidentified protein(s). In addition, they could be formaldehyde cross-linked to ExbD and TonB. Taken together, the data suggested that they assembled normally. Three of four ExbB mutants were defective in supporting both the PMF-dependent formaldehyde cross-link between the periplasmic domains of TonB and ExbD and the proteinase K-resistant conformation of TonB. Thus, mutations in a cytoplasmic region of ExbB prevented a periplasmic event and constituted evidence for signal transduction from cytoplasm to periplasm in the TonB system.

Taking the Escherichia coli TonB transmembrane domain "offline"? Nonprotonatable Asn substitutes fully for TonB His20

The TonB system of Gram-negative bacteria uses the proton motive force (PMF) of the cytoplasmic membrane to energize active transport of nutrients across the outer membrane. The single transmembrane domain (TMD) anchor of TonB, the energy transducer, is essential. Within that TMD, His20 is the only TMD residue that is unable to withstand alanine replacement without a loss of activity. H20 is required for a PMF-dependent conformational change, suggesting that the importance of H20 lies in its ability to be reversibly protonated and deprotonated. Here all possible residues were substituted at position 20 (H20X substitutions). The His residue was also relocated throughout the TonB TMD. Surprisingly, Asn, a structurally similar but nonprotonatable residue, supported full activity at position 20; H20S was very weakly active. All the remaining substitutions, including H20K, H20R, H20E, and H20D, the obvious candidates to mimic a protonated state or support proton translocation, were inactive. A second-site suppressor, ExbB(A39E), indiscriminately reactivated the majority of H20 substitutions and relocations, including H20V, which cannot be made protonatable. These results suggested that the TonB TMD was not on a proton conductance pathway and thus only indirectly responds to PMF, probably via ExbD.

The TonB dimeric crystal structures do not exist in vivo

The TonB system energizes transport of nutrients across the outer membrane of Escherichia coli using cytoplasmic membrane proton motive force (PMF) for energy. Integral cytoplasmic membrane proteins ExbB and ExbD appear to harvest PMF and transduce it to TonB. The carboxy terminus of TonB then physically interacts with outer membrane transporters to allow translocation of ligands into the periplasmic space. The structure of the TonB carboxy terminus (residues ~150 to 239) has been solved several times with similar results. Our previous results hinted that in vitro structures might not mimic the dimeric conformations that characterize TonB in vivo. To test structural predictions and to identify irreplaceable residues, the entire carboxy terminus of TonB was scanned with Cys substitutions. TonB I232C and N233C, predicted to efficiently form disulfide-linked dimers in the crystal structures, did not do so. In contrast, Cys substitutions positioned at large distances from one another in the crystal structures efficiently formed dimers. Cys scanning identified seven functionally important residues. However, no single residue was irreplaceable. The phenotypes conferred by changes of the seven residues depended on both the specific assay used and the residue substituted. All seven residues were synergistic with one another. The buried nature of the residues in the structures was also inconsistent with these properties. Taken together, these results indicate that the solved dimeric crystal structures of TonB do not exist. The most likely explanation for the aberrant structures is that they were obtained in the absence of the TonB transmembrane domain, ExbB, ExbD, and/or the PMF.

The TonB system of Gram-negative bacteria is an attractive target for development of novel antibiotics because of its importance in iron acquisition and virulence. Logically, therefore, the structure of TonB must be accurately understood. TonB functions as a dimer in vivo, and two different but similar crystal structures of the dimeric carboxy-terminal ~90 amino acids gave rise to mechanistic models. Here we demonstrate that the crystal structures, and therefore the models based on them, are not biologically relevant. The idiosyncratic phenotypes conferred by substitutions at the only seven functionally important residues in the carboxy terminus suggest that similar to interaction of cytochromes P450 with numerous substrates, these residues allow TonB to differentially interact with different outer membrane transporters. Taken together, data suggest that TonB is maintained poised between order and disorder by ExbB, ExbD, and the proton motive force (PMF) before energy transduction to the outer membrane transporters.

Cytoplasmic membrane protonmotive force energizes periplasmic interactions between ExbD and TonB

The TonB system of Escherichia coli (TonB/ExbB/ExbD) transduces the protonmotive force (pmf) of the cytoplasmic membrane to drive active transport by high-affinity outer membrane transporters. In this study, chromosomally encoded ExbD formed formaldehyde-linked complexes with TonB, ExbB and itself (homodimers) in vivo. Pmf was required for detectable cross-linking between TonB-ExbD periplasmic domains. Consistent with that observation, the presence of inactivating transmembrane domain mutations ExbD(D25N) or TonB(H20A) also prevented efficient formaldehyde cross-linking between ExbD and TonB. A specific site of periplasmic interaction occurred between ExbD(A92C) and TonB(A150C) and required functional transmembrane domains in both proteins. Conversely, neither TonB, ExbB nor pmf were required for ExbD dimer formation. These data suggest two possible models where either dynamic complex formation occurred through transmembrane domains or the transmembrane domains of ExbD and TonB configure their respective periplasmic domains. Analysis of T7-tagged ExbD with anti-ExbD antibodies revealed that a T7 tag was responsible both for our previous failure to detect T7-ExbD-ExbB and T7-ExbD-TonB formaldehyde-linked complexes and for the concomitant artefactual appearance of T7-ExbD trimers.

Energy-dependent immunity protein release during tol-dependent nuclease colicin translocation

Nuclease colicins bind their target receptor in the outer membrane of sensitive cells in the form of a high affinity complex with their cognate immunity proteins. Upon cell entry the immunity protein is lost from the complex by means that are poorly understood. We have developed a sensitive fluorescence assay that has enabled us to study the molecular requirements for immunity protein release. Nuclease colicins use members of the tol operon for their translocation across the outer membrane. We have demonstrated that the amino-terminal 80 residues of the colicin E9 molecule, which is the region that interacts with TolB, are essential for immunity protein release. Using tol deletion strains we analyzed the cellular components necessary for immunity protein release and found that in addition to a requirement for tolB, the tolA deletion strain was most affected. Complementation studies showed that the mutation H22A, within the transmembrane segment of TolA, abolishes immunity protein release. Investigation of the energy requirements demonstrated that the proton motive force of the cytoplasmic membrane is critical. Taken together these results demonstrate for the first time a clear energy requirement for the uptake of a nuclease colicin complex and suggest that energy transduced from the cytoplasmic membrane to the outer membrane by TolA could be the driving force for immunity protein release and concomitant translocation of the nuclease domain.

TonB-dependent maltose transport by Caulobacter crescentus

We have shown previously that Caulobacter crescentus grows on maltodextrins which are actively transported across the outer membrane by the MalA protein. Evidence for energy-coupled transport was obtained by deletion of the exbB exbD genes which abolished transport. However, removal of the TonB protein, which together with the ExbB ExbD proteins is predicted to form an energy-coupling device between the cytoplasmic membrane and the outer membrane, left transport unaffected. Here we identify an additional tonB gene encoded by the cc2334a ORF, which when deleted abolished maltose transport. MalA contains a TonB box that reads EEVVIT and is predicted to interact with TonB. Replacement of valine number 15 in the TonB box by proline abolished maltose transport. Maltose was transported across the cytoplasmic membrane by the MalY protein (CC2283). Maltose transport was induced by maltose and repressed by the MalI protein (CC2284). In addition to MalA, MalY and MalI, the mal locus encodes two predicted cytoplasmic alpha-amylases (CC2285 and CC2286) and a periplasmic glucoamylase (CC2282). The TonB dependence together with the previously described ExbB ExbD dependence demonstrates energy-coupled maltose transport across the outer membrane. MalY is involved in maltose transport across the cytoplasmic membrane by a presumably ion-coupled mechanism.

NagA-dependent uptake of N-acetyl-glucosamine and N-acetyl-chitin oligosaccharides across the outer membrane of Caulobacter crescentus

Among the 67 predicted TonB-dependent outer membrane transporters of Caulobacter crescentus, NagA was found to be essential for growth on N-acetyl-beta-D-glucosamine (GlcNAc) and larger chitin oligosaccharides. NagA (93 kDa) has a predicted typical domain structure of an outer membrane transport protein: a signal sequence, the TonB box EQVVIT, a hatch domain of 147 residues, and a beta-barrel composed of 22 antiparallel beta-strands linked by large surface loops and very short periplasmic turns. Mutations in tonB1 and exbBD, known to be required for maltose transport via MalA in C. crescentus, and in two additional predicted tonB genes (open reading frames cc2327 and cc3508) did not affect NagA-mediated GlcNAc uptake. nagA is located in a gene cluster that encodes a predicted PTS sugar transport system and two enzymes that convert GlcNAc-6-P to fructose-6-P. Since a nagA insertion mutant did not grow on and transport GlcNAc, diffusion of GlcNAc through unspecific porins in the outer membrane is excluded. Uptake of GlcNAc into tonB and exbBD mutants and reduction but not abolishment of GlcNAc transport by agents which dissipate the electrochemical potential of the cytoplasmic membrane (0.1 mM carbonyl cyanide 3-chlorophenylhydrazone and 1 mM 2,4-dinitrophenol) suggest diffusion of GlcNAc through a permanently open pore of NagA. Growth on (GlcNAc)(3) and (GlcNAc)(5) requires ExbB and ExbD, indicating energy-coupled transport by NagA. We propose that NagA forms a small pore through which GlcNAc specifically diffuses into the periplasm and functions as an energy-coupled transporter for the larger chitin oligosaccharides.

TonB system, in vivo assays and characterization

The multiprotein TonB system of Escherichia coli involves proteins in both the cytoplasmic membrane and the outer membrane. By a still unclear mechanism, the proton-motive force of the cytoplasmic membrane is used to catalyze active transport through high-affinity transporters in the outer membrane. TonB, ExbB, and ExbD are required to transduce the cytoplasmic membrane energy to these transporters. For E. coli, transport ligands consist of iron-siderophore complexes, vitamin B(12), group B colicins, and bacteriophages T1 and ø80. Our experimental philosophy is that data gathered in vivo, where all known and unknown components are present at balanced chromosomal levels in the whole cell, can be interpreted with less ambiguity than when a subset of components is overexpressed or analysed in vitro. This chapter describes in vivo assays for the TonB system and their application.

Modulation by substrates of the interaction between the HasR outer membrane receptor and its specific TonB-like protein, HasB

TonB is a cytoplasmic membrane protein required for active transport of various essential substrates such as heme and iron siderophores through the outer membrane receptors of Gram-negative bacteria. This protein spans the periplasm, contacts outer membrane transporters by its C-terminal domain, and transduces energy from the protonmotive force to the transporters. The TonB box, a relatively conserved sequence localized on the periplasmic side of the transporters, has been shown to directly contact TonB. While Serratia marcescens TonB functions with various transporters, HasB, a TonB-like protein, is dedicated to the HasR transporter. HasR acquires heme either freely or via an extracellular heme carrier, the hemophore HasA, that binds to HasR and delivers heme to the transporter. Here, we study the interaction of HasR with a HasB C-terminal domain and compare it with that obtained with a TonB C-terminal fragment. Analysis of the thermodynamic parameters reveals that the interaction mode of HasR with HasB differs from that with TonB, the difference explaining the functional specificity of HasB for HasR. We also demonstrate that the presence of the substrate on the extracellular face of the transporter modifies, via enthalpy-entropy compensation, the interaction with HasB on the periplasmic face. The transmitted signal depends on the nature of the substrate. While the presence of heme on the transporter modifies only slightly the nature of interactions involved between HasR and HasB, hemophore binding on the transporter dramatically changes the interactions and seems to locally stabilize some structural motifs. In both cases, the HasR TonB box is the target for those modifications.

Solute carrier 11 cation symport requires distinct residues in transmembrane helices 1 and 6

Ubiquitous solute carriers 11 (SLC11) contribute to metal-ion homeostasis by importing Me(2+) and H(+) into the cytoplasm. To identify residues mediating cation symport, Escherichia coli proton-dependent manganese transporter (MntH) was mutated at five SLC11-specific transmembrane (TM) sites; each mutant activity was compared with wild-type MntH, and the biochemical results were tested by homology threading. Cd(2+) and H(+) uptake kinetics were analyzed in whole cells as a function of pH and temperature, and right-side out membrane vesicles were used to detail energy requirements and to probe site accessibility by Cys replacement and thiol modification. This approach revealed that TM segment 1 (TMS1) residue Asp(34) couples H(+) and Me(2+) symport and contributes to MntH forward transport electrogenicity, whereas the TMS6 His(211) residue mediates pH-dependent Me(2+) uptake; MntH Asn(37), Asn(250), and Asn(401) in TMS1, TMS7, and TMS11 participate in transporter cycling and/or helix packing interactions. These biochemical results fit the LeuT/SLC6 structural fold, which suggests that conserved peptide motifs Asp(34)-Pro-Gly (TMS1) and Met-Pro-His(211) (TMS6) form antiparallel "TM helix/extended peptide" boundaries, lining a "pore" cavity and enabling H(+)-dependent Me(2+) import.

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.

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.