Transport-defective mutations alter the conformation of the energy-coupling motif of an outer membrane transporter

A disulfide chaperone knockout facilitates spin labeling and pulse EPR spectroscopy of outer membrane transporters

Pulse EPR measurements provide information on distances and distance distributions in proteins but require the incorporation of pairs of spin labels that are usually attached to engineered cysteine residues. In previous work, we demonstrated that efficient in vivo labeling of the Escherichia coli outer membrane vitamin B12 transporter, BtuB, could only be achieved using strains defective in the periplasmic disulfide bond formation (Dsb) system. Here, we extend these in vivo measurements to FecA, the E. coli ferric citrate transporter. As seen for BtuB, pairs of cysteines cannot be labeled when the protein is present in a standard expression strain. However, incorporating plasmids that permit an arabinose induced expression of FecA into a strain defective in the thiol disulfide oxidoreductase, DsbA, enables efficient spin-labeling and pulse EPR of FecA in cells. A comparison of the measurements made on FecA in cells with measurements made in reconstituted phospholipid bilayers suggests that the cellular environment alters the behavior of the extracellular loops of FecA. In addition to these in situ EPR measurements, the use of a DsbA minus strain for the expression of BtuB improves the EPR signals and pulse EPR data obtained in vitro from BtuB that is labeled, purified, and reconstituted into phospholipid bilayers. The in vitro data also indicate the presence of intermolecular BtuB-BtuB interactions, which had not previously been observed in a reconstituted bilayer system. This result suggests that in vitro EPR measurements on other outer membrane proteins would benefit from protein expression in a DsbA minus strain.

Extracellular loops of BtuB facilitate transport of vitamin B12 through the outer membrane of E. coli

Vitamin B12 (or cobalamin) is an enzymatic cofactor essential both for mammals and bacteria. However, cobalamin can be synthesized only by few microorganisms so most bacteria need to take it up from the environment through the TonB-dependent transport system. The first stage of cobalamin import to E. coli cells occurs through the outer-membrane receptor called BtuB. Vitamin B12 binds with high affinity to the extracellular side of the BtuB protein. BtuB forms a β-barrel with inner luminal domain and extracellular loops. To mechanically allow for cobalamin passage, the luminal domain needs to partially unfold with the help of the inner-membrane TonB protein. However, the mechanism of cobalamin permeation is unknown. Using all-atom molecular dynamics, we simulated the transport of cobalamin through the BtuB receptor embedded in an asymmetric and heterogeneous E. coli outer-membrane. To enhance conformational sampling of the BtuB loops, we developed the Gaussian force-simulated annealing method (GF-SA) and coupled it with umbrella sampling. We found that cobalamin needs to rotate in order to permeate through BtuB. We showed that the mobility of BtuB extracellular loops is crucial for cobalamin binding and transport and resembles an induced-fit mechanism. Loop mobility depends not only on the position of cobalamin but also on the extension of luminal domain. We provided atomistic details of cobalamin transport through the BtuB receptor showing the essential role of the mobility of BtuB extracellular loops. A similar TonB-dependent transport system is used also by many other compounds, such as haem and siderophores, and importantly, can be hijacked by natural antibiotics. Our work could have implications for future delivery of antibiotics to bacteria using this transport system.

Allosteric Signaling Is Bidirectional in an Outer-Membrane Transport Protein

In BtuB, the Escherichia coli TonB-dependent transporter for vitamin B₁₂, substrate binding to the extracellular surface unfolds a conserved energy coupling motif termed the Ton box into the periplasm. This transmembrane signaling event facilitates an interaction between BtuB and the inner-membrane protein TonB. In this study, continuous-wave and pulse electron paramagnetic resonance in a native outer-membrane preparation demonstrate that signaling also occurs from the periplasmic to the extracellular surface in BtuB. The binding of a TonB fragment to the periplasmic interface alters the configuration of the second extracellular loop and partially dissociates a spin-labeled substrate analog. Moreover, mutants in the periplasmic Ton box that are transport-defective alter the binding site for vitamin B₁₂ in BtuB. This work demonstrates that the Ton box and the extracellular substrate binding site are allosterically coupled in BtuB, and that TonB binding may initiate a partial round of transport.

Monomeric TonB and the Ton box are required for the formation of a high-affinity transporter-TonB complex

The energy-dependent uptake of trace nutrients by Gram-negative bacteria involves the coupling of an outer membrane transport protein to the transperiplasmic protein TonB. In this study, a soluble construct of Escherichia coli TonB (residues 33-239) was used to determine the affinity of TonB for outer membrane transporters BtuB, FecA, and FhuA. Using fluorescence anisotropy, TonB(33-239) was found to bind with high affinity (tens of nanomolar) to both BtuB and FhuA; however, no high-affinity binding to FecA was observed. In BtuB, the high-affinity binding of TonB(33-239) was eliminated by mutations in the Ton box, which yield transport-defective protein, or by the addition of a Colicin E3 fragment, which stabilizes the Ton box in a folded state. These results indicate that transport requires a high-affinity transporter-TonB interaction that is mediated by the Ton box. Characterization of TonB(33-239) using double electron-electron resonance (DEER) demonstrates that a significant population of TonB(33-239) exists as a dimer; moreover, interspin distances are in approximate agreement with interlocked dimers observed previously by crystallography for shorter TonB fragments. When the TonB(33-239) dimer is bound to the outer membrane transporter, DEER shows that the TonB(33-239) dimer is converted to a monomeric form, suggesting that a dimer-monomer conversion takes place at the outer membrane during the TonB-dependent transport cycle.

The N-terminal domain of a TonB-dependent transporter undergoes a reversible stepwise denaturation

Gram-negative bacteria contain a family of outer membrane transport proteins that function in the uptake of rare nutrients, such as iron and vitamin B(12). These proteins are termed TonB-dependent because transport requires an interaction with the inner-membrane protein TonB. Using a combination of site-directed spin labeling and chemical denaturation, we examined the site-specific unfolding of regions of the Escherichia coli vitamin B(12) transporter, BtuB. The data indicate that a portion of the N-terminal region of the protein, which occupies the lumen of the BtuB barrel, denatures prior to the unfolding of the barrel and that the free energy of folding for the N-terminus is smaller than that typically seen for globular proteins. Moreover, the data indicate that the N-terminal domain does not unfold in a single event but unfolds in a series of independent steps. The unfolding of the N-terminus is reversible, and removal of denaturant restores the native fold of the protein. These data are consistent with proposed transport mechanisms that involve a transient rearrangement or unfolding of the N-terminus of the protein, and they provide evidence of a specific protein conformation that might be an intermediate accessed during transport.

Osmolytes modulate conformational exchange in solvent-exposed regions of membrane proteins

Site-directed spin labeling (SDSL) was used to investigate local structure and conformational exchange in two bacterial outer-membrane TonB-dependent transporters, BtuB and FecA. Protecting osmolytes, such as polyethylene glycols (PEGs) are known to modulate a substrate-dependent conformational equilibrium in the energy coupling motif (Ton box) of BtuB. Here, we demonstrate that a segment that is N-terminal to the Ton box in BtuB, is in conformational exchange between ordered and disordered states with or without substrate. Protecting osmolytes shift this equilibrium to favor the more ordered, folded state. However, a segment of BtuB that is C-terminal to the Ton box that is not solvent exposed is insensitive to PEGs. Protecting osmolytes also modulate a conformational equilibrium in the Ton box of FecA, with larger molecular weight PEGs producing the largest shifts in the conformational free energy. These data indicate that solvent-exposed regions of these transporters undergo conformational exchange and that regions of these transporters that are involved in protein-protein interactions sample multiple conformational substates. The sensitivity to solute provides an explanation for differences seen between two high-resolution structures of BtuB, which each likely represent one conformation from a subset of states that are normally sampled by the protein. This work also illustrates how SDSL and osmolytes may be used to characterize and quantitate conformational equilibria in membrane proteins.

NMR and EPR studies of membrane transporters

In order to fulfill their function, membrane transport proteins have to cycle through a number of conformational and/or energetic states. Thus, understanding the role of conformational dynamics seems to be the key for elucidation of the functional mechanism of these proteins. However, membrane proteins in general are often difficult to express heterologously and in sufficient amounts for structural studies. It is especially challenging to trap a stable energy minimum, e.g., for crystallographic analysis. Furthermore, crystallization is often only possible by subjecting the protein to conditions that do not resemble its native environment and crystals can only be snapshots of selected conformational states. Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy are complementary methods that offer unique possibilities for studying membrane proteins in their natural membrane environment and for investigating functional conformational changes, lipid interactions, substrate-lipid and substrate-protein interactions, oligomerization states and overall dynamics of membrane transporters. Here, we review recent progress in the field including studies from primary and secondary active transporters.

TonB induces conformational changes in surface-exposed loops of FhuA, outer membrane receptor of Escherichia coli

FhuA, outer membrane receptor of Escherichia coli, transports hydroxamate-type siderophores into the periplasm. Cytoplasmic membrane-anchored TonB transduces energy to FhuA to facilitate siderophore transport. Because the N-terminal cork domain of FhuA occludes the C-terminal beta-barrel lumen, conformational changes must occur to enable siderophore passage. To localize conformational changes at an early stage of the siderophore transport cycle, four anti-FhuA monoclonal antibodies (mAbs) were purified to homogeneity, and the epitopes that they recognize were determined by phage display. We mapped continuous and discontinuous epitopes to outer surface-exposed loops 3, 4, and 5 and to beta-barrel strand 14. To probe for conformational changes of FhuA, surface plasmon resonance measured mAb binding to FhuA in its apo- and siderophore-bound states. Changes in binding kinetics were observed for mAbs whose epitopes were mapped to outer surface-exposed loops. Further, we measured mAb binding in the absence and presence of TonB. After forming immobilized FhuA-TonB complexes, changes in kinetics of mAb binding to FhuA were even more pronounced compared with kinetics of binding in the absence of TonB. Measurement of extrinsic fluorescence of the dye MDCC conjugated to residue 336 in outer surface-exposed loop 4 revealed 33% fluorescence quenching upon ferricrocin binding and up to 56% quenching upon TonB binding. Binding of mAbs to apo- and ferricrocin-bound FhuA complemented by fluorescence spectroscopy studies showed that their cognate epitopes on loops 3, 4, and 5 undergo conformational changes upon siderophore binding. Further, our data demonstrate that TonB binding promotes conformational changes in outer surface-exposed loops of FhuA.

Membrane hydrocarbon thickness modulates the dynamics of a membrane transport protein

Nitroxide spin labels were incorporated into selected sites within the beta-barrel of the bacterial outer-membrane transport protein BtuB by site-directed mutagenesis, followed by chemical modification with a methanethiosufonate spin label. The electron paramagnetic resonance lineshapes of the spin-labeled side chain (R1) from these sites are highly variable, and have spectral parameters that reflect secondary structure and local steric constraints. In addition, these lineshape parameters correlate with crystallographic structure factors for Calpha carbons, suggesting that the motion of the spin label is modulated by both the local modes of motion of the spin label and the local dynamics of the protein backbone. Experiments performed as a function of lipid composition and sample temperature indicate that nitroxide spin labels on the exterior surface of BtuB, which face the membrane hydrocarbon, are not strongly influenced by the phase state of the bulk lipids. However, these spectra are modulated by membrane hydrocarbon thickness. Specifically, the values of the scaled mobility parameter for the R1 lineshapes are inversely proportional to the hydrocarbon thickness. These data suggest that protein dynamics and structure in BtuB are directly coupled to membrane hydrophobic thickness.

Substrate-dependent transmembrane signaling in TonB-dependent transporters is not conserved

Site-directed spin labeling (SDSL) was used to examine and compare transmembrane signaling events in the bacterial outer-membrane transport proteins BtuB, FecA, and FhuA. These proteins extract energy for transport by coupling to the transperiplasmic protein TonB, an interaction that is thought to be mediated by the Ton box, a highly conserved energy-coupling motif in these transporters. In the ferric citrate transporter, FecA, SDSL indicates that the Ton box undergoes a substrate-induced disorder transition similar to that seen for BtuB, the vitamin B(12) transporter. This conformational change produces an aqueous exposed, highly disordered protein fragment, which likely regulates transporter-TonB interactions. However, in the ferrichrome transporter, FhuA, SDSL does not reveal a substrate-induced unfolding transition. In this protein, with or without substrate, the Ton box conformation is found to be highly dynamic and constitutively unfolded. In addition, SDSL indicates that structural features seen in high-resolution models are not found in membrane-associated FhuA. Taken together, these data indicate that the Ton box of FhuA may always be available for interactions with TonB, implying that transporter-TonB interactions in FhuA are either constitutive or not regulated by the Ton box configuration.

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.

Molecular basis for substrate-dependent transmembrane signaling in an outer-membrane transporter

Transmembrane signaling events that propagate through receptors and transporters have critical roles in cellular function and regulation. In the Escherichia coli vitamin B(12) transporter, BtuB, substrate binding to the extracellular surface of the protein triggers the unfolding of an energy coupling motif at the periplasmic surface. Here, the molecular interactions mediating this substrate-dependent transmembrane signaling event were investigated in a novel way by combining a two mutant cycle analysis with site-directed spin labeling (SDSL). SDSL was used to monitor the unfolding and conformational equilibrium of the energy-coupling motif, and a thermodynamic two-mutant cycle analysis was used to estimate pair-wise interaction free energies for a pair of charged residues (D316 and R14) within the protein interior. The data indicate that D316 and R14 are critical to this structural transition. Substrate binding is shown to reduce the interaction free energy between these residues, thereby triggering the unfolding of the energy coupling motif of this membrane transporter. The result indicates that SDSL when used in combination with a mutant cycle analysis provides an approach to examine the molecular interactions mediating signaling events in membrane proteins.

Energy-coupled outer membrane transport proteins and regulatory proteins

FhuA and FecA are two examples of energy-coupled outer membrane import proteins of gram-negative bacteria. FhuA transports iron complexed by the siderophore ferrichrome and serves as a receptor for phages, a toxic bacterial peptide, and a toxic protein. FecA transports diferric dicitrate and regulates transcription of an operon encoding five ferric citrate (Fec) transport genes. Properties of FhuA mutants selected according to the FhuA crystal structure are described. FhuA mutants in the TonB box, the hatch, and the beta-barrel are rather robust. TonB box mutants in FhuA FecA, FepA, Cir, and BtuB are compared; some mutations are suppressed by mutations in TonB. Mutant studies have not revealed a ferrichrome diffusion pathway, and tolerance to mutations in the region linking the TonB box to the hatch does not disclose a mechanism for how energy transfer from the cytoplasmic membrane to FhuA changes the conformation of FhuA such that bound substrates are released, the pore is opened, and substrates enter the periplasm, or how surface loops change their conformation such that TonB-dependent phages bind irreversibly and release their DNA into the cells. The FhuA and FecA crystal structures do not disclose the mechanism of these proteins, but they provide important information for specific functional studies. FecA is also a regulatory protein that transduces a signal from the cell surface into the cytoplasm. The interacting subdomains of the proteins in the FecA --> FecR --> FecI --> RNA polymerase signal transduction pathway resulting in fecABCDE transcription have been determined. Energy-coupled transporters transport not only iron and vitamin B12, but also other substrates of very low abundance such as sugars across the outer membrane; transcription regulation of the transport genes may occur similarly to that of the Fec transport genes.

Substrate-dependent unfolding of the energy coupling motif of a membrane transport protein determined by double electron-electron resonance

BtuB is a TonB-dependent transport protein that binds and carries vitamin B(12) across the outer membrane of Gram negative bacteria such as Escherichia coli. Previous work has demonstrated that the Ton box, a highly conserved segment near the N-terminus of the protein, undergoes an order-to-disorder transition upon the binding of substrate. Here, we incorporate pairs of nitroxide spin labels into membrane reconstituted BtuB and utilize a four-pulse double electron-electron resonance (DEER) experiment to measure distances between the Ton box and the periplasmic surface of the transporter with and without substrate. During reconstitution, the labeled membrane protein was diluted with wild-type protein, which significantly reduced the intermolecular electron spin-spin relaxation rate and increased the DEER signal-to-noise ratio. In the absence of substrate, each spin pair gives rise to a single distribution of distances that is consistent with the crystal structure obtained for BtuB; however, distances that are much longer are found in the presence of substrate, and the data are consistent with the existence of an equilibrium between folded and unfolded states of the Ton box. From these distances, a model for the position of the Ton box was constructed, and it indicates that the N-terminal end of the Ton box extends approximately 20 to 30 A into the periplasm upon the addition of substrate. We propose that this substrate-induced extension provides the signal that initiates interactions between BtuB and the inner membrane protein TonB.

Solutes modify a conformational transition in a membrane transport protein

The bacterial outer-membrane vitamin B(12) transporter, BtuB, undergoes a dramatic order-to-disorder transition in its N-terminal energy-coupling motif (Ton box) upon substrate binding. Here, site-directed spin labeling (SDSL) is used to show that a range of solutes prevents this conformational change when ligand is bound to BtuB, resulting in a more ordered Ton box structure. For each solute examined, the data indicate that solutes effectively block this conformational transition through an osmotic mechanism. The molecular weight dependence of this solute effect has been examined for a series of polyethylene glycols, and a sharp molecular weight cutoff is observed. This cutoff indicates that solutes are preferentially excluded from a cavity within the protein as well as the protein surface. Furthermore, the sensitivity of the conformational change to solution osmolality is consistent with a structural model predicted by SDSL. When the Ton box is unfolded by detergents or mutations (rather than by ligand binding), solutes, such as polyethylene glycols and salts, also induce a more structured compacted conformation. These results suggest that conformational changes in this class of outer membrane transporters, which involve modest energy differences and changes in hydration, may be modulated by a range of solutes, including solutes typically used in protein crystallization.

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.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Competing ligands stabilize alternate conformations of the energy coupling motif of a TonB-dependent outer membrane transporter

BtuB is a TonB-dependent outer-membrane transporter for vitamin B12 (or cyanocobalamin, CN-Cbl) in Escherichia coli. The binding of CN-Cbl is believed to promote an unfolding or undocking of the Ton box, the conserved N-terminal energy coupling motif at the periplasmic surface of the transporter. This structural change may facilitate the interaction of BtuB with the inner membrane protein TonB. In this work, the effect of the receptor binding fragment of colicin E3 (E3R) on the conformation of the Ton box was examined with site-directed spin labeling. Addition of E3R reverses the undocking of the Ton box that is promoted by CN-Cbl, consistent with a competitive binding between the substrate and the colicin fragment. EPR spectroscopy indicates that the Ton box is in a two-state equilibrium between docked and undocked conformations. In the absence of substrate, the docked conformation is the predominant state; however, the equilibrium can be shifted to the undocked state by the addition of detergents or site-specific proline substitutions. Even when the undocking is induced by detergents or by certain proline mutations, E3R binding shifts the equilibrium to the docked conformation. Thus, two competitive extracellular ligands, CN-Cbl and ER3, transduce opposite conformations of the N-terminal Ton box. Substrate binding stabilizes an undocked conformation, whereas E3R binding stabilizes a docked conformation of the Ton box.

Differential substrate-induced signaling through the TonB-dependent transporter BtuB

The BtuB transporter mediates high-affinity binding and TonB-dependent active transport of vitamin B12 [cyanocobalamin (CNCbl)] across the outer membrane of Escherichia coli. A characteristic feature of TonB-dependent transporters is the Ton box, a conserved sequence near the N terminus and exposed to the periplasm. Crosslinking to TonB and site-directed spin labeling indicated that the Ton box of BtuB undergoes a substantial conformational transition in response to CNCbl binding, but only slight movement was seen in crystal structures. An in vivo method of detecting substrate-induced changes in the Ton box environment measured reaction of a biotin maleimide derivative with cysteine substitutions through the N-terminal region of BtuB between positions 1 and 31. The degree of maleimide labeling of different residues correlated with their accessibility in the crystal structure. Labeling of many positions was increased strongly when CNCbl was present, consistent with the undocking of this region proposed from spin-labeling analyses. The receptor-binding domain of colicin E3, which binds to BtuB competitively with CNCbl, resulted in decreased labeling. Both substrate-induced transitions occur in and beyond the Ton box and were affected by transport-uncoupling substitutions. Thus, two transport substrates that bind competitively to the extracellular face of BtuB stabilize opposite transitions of the Ton box.

Molecular dynamics simulations of the bacterial outer membrane protein FhuA: a comparative study of the ferrichrome-free and bound states

FhuA is one of the more complex members of the superfamily of bacterial outer membrane proteins. Its primary function is to provide a binding site on the outer membrane surface for siderophores, such as ferrichrome, and subsequently to facilitate their energy-dependent transport across the membrane, presumably powered by the TonB-ExbBD protein complex that resides in the cytoplasmic membrane. Crystal structures of FhuA with and without a bound ferrichrome molecule have provided some clues as to the initial stages of the siderophore transport mechanism. In the current study, we have employed 10-ns duration molecular dynamics simulations of FhuA and of the FhuA-ferrichrome complex, both embedded in a phospholipid bilayer, to probe the short timescale dynamics of this integral membrane protein, and to explore possible mechanistic implications of this dynamic behavior. Analysis of the dynamics of the protein suggests that the extracellular loops move as relatively rigid entities relative to the transmembrane beta-barrel. Comparison of the two simulations (with and without bound ferrichrome) revealed some ligand-induced changes in loop mobility. Specifically, loop L8 appears to be involved in a mechanism whereby the binding site is gated closed upon ligand binding. Analysis of the dynamics of water molecules within the core of the FhuA protein provided no evidence for a water-permeable protopore through which the ferrichrome might pass without a major perturbation of the FhuA protein. Overall, these simulations support the proposal that binding of ferrichrome initiates a signaling mechanism that ultimately leads to the TonB-mediated partial or total removal of the core domain from the beta-barrel, thus opening up a permeable pore. These simulations are among the longest that have been performed on a complex membrane protein. However, a simple analysis of sampling reveals that the description of protein motions is far from complete.

Touch and go: tying TonB to transport

The TonB system of Gram-negative bacteria appears to exist for the purpose of transducing the protonmotive force energy from the cytoplasmic membrane, where it is generated, to the outer membrane, where it is needed for active transport of iron siderophores, vitamin B12 and, in pathogens, iron from host-binding proteins. In this review, we bring the reader up to date on the developments in the field since the authors each wrote reviews in this journal in 1990.

Acquisition of siderophores in gram-negative bacteria

The outer membrane of Gram-negative bacteria constitutes a permeability barrier that protects the cell from exterior hazards, but also complicates the uptake of nutrients. In the case of iron, the challenge is even greater, because of the scarcity of this indispensable element in the cell's surroundings. To solve this dilemma, bacteria have evolved sophisticated mechanisms whereby the concerted actions of receptor, transporter and energy-transducing proteins ensure that there is a sufficient supply of iron-containing compounds, such as siderophores.

Demonstration and characterization of a specific interaction between gonococcal transferrin binding protein A and TonB

Iron scavenging by Neisseria gonorrhoeae is accomplished by the expression of receptors that are specific for host iron-binding proteins, such as transferrin and lactoferrin. Efficient transferrin-iron acquisition is dependent on the combined action of two proteins, designated TbpA and TbpB. TbpA is a TonB-dependent outer membrane receptor, whereas TbpB is lipid modified and serves to increase the efficiency of transferrin-iron uptake. Both proteins, together or separately, can be isolated from the gonococcal outer membrane by using affinity chromatography techniques. In the present study, we identified an additional protein in transferrin-affinity preparations, which had an apparent molecular mass of 45 kDa. The ability to copurify this protein by transferrin affinity was dependent upon the presence of TbpA and not TbpB. The amino-terminal sequence of the 45-kDa protein was identical to the amino terminus of gonococcal TonB, indicating that TbpA stably interacted with TonB, without the addition of chemical cross-linkers. Using immunoprecipitation, we could recover TbpA-TonB complexes without the addition of transferrin, suggesting that ligand binding was not a necessary prerequisite for TonB interaction. In contrast, a characterized TonB box mutant of TbpA did not facilitate interaction between these two proteins such that complexes could be isolated. We generated an in-frame deletion of gonococcal TonB, which removed 35 amino acids, including a Neisseria-specific, glycine-rich domain. This mutant protein, like the parental TonB, energized TbpA to enable growth on transferrin. Consistent with the functionality of this deletion derivative, TbpA-TonB complexes could be recovered from this strain. The results of the present study thus begin to define the requirements for a functional interaction between gonococcal TbpA and TonB.