Function of the membrane fusion protein, MexA, of the MexA, B-OprM efflux pump in Pseudomonas aeruginosa without an anchoring membrane

Kinetic study of membrane protein interactions: from three to two dimensions

Molecular interactions are contingent upon the system's dimensionality. Notably, comprehending the impact of dimensionality on protein-protein interactions holds paramount importance in foreseeing protein behaviour across diverse scenarios, encompassing both solution and membrane environments. Here, we unravel interactions among membrane proteins across various dimensionalities by quantifying their binding rates through fluorescence recovery experiments. Our findings are presented through the examination of two protein systems: streptavidin-biotin and a protein complex constituting a bacterial efflux pump. We present here an original approach for gauging a two-dimensional binding constant between membrane proteins embedded in two opposite membranes. The quotient of protein binding rates in solution and on the membrane represents a metric denoting the exploration distance of the interacting sites-a novel interpretation.

Efflux MexAB-Mediated Resistance in P. aeruginosa Isolated from Patients with Healthcare Associated Infections

Today, one of the most important challenges for physicians is the adequate treatment of infections due to multidrug resistant organism (MDR). Pseudomonas aeruginosa is considered an opportunistic organism causing different types of healthcare associated infections (HAIs). We aimed to investigate the MDR and pandrug resistance (PDR) rate in P. aeruginosa in our region and detect efflux-pump mexAB genes and the proposed binding interactions of five different categories of antimicrobial agents with the mexB pump. A total of 180 non-duplicated P. aeruginosa strains were isolated from patients with HAIs in the Suez Canal University Hospital. Phenotypically, minimum inhibitory concentration (MIC) was done for all MDR and PDR strains before and after addition of efflux pump inhibitor carbonyl cyanide m-chlorophenyl hydrazone (CCCP). Molecular detection of mexA and mexB genes was done by using polymerase chain reaction (PCR). Most of the isolated strains (126 strains) were MDR (70%); only 10 samples (5.5%) were PDR. MexA and mexB genes were detected in 88.2% (120 strains) and 70.5% (96 strains) of stains, respectively. All PDR strains (10 stains) carried both mexA and mexB genes. Efflux mexAB genes were detected in all MDR and PDR strains (136 strains). Molecular modeling studies were performed to investigate the modes of intermolecular binding interactions between the antimicrobial agents and mexB key amino acids that resulted in MDR and PDR. The current study reported high prevalence of MDR and PDR P. aeruginosa in patients with HAIs in the Suez Canal University Hospitals.

MexAB-OprM Efflux Pump of Pseudomonas aeruginosa Offers Resistance to Carvacrol: A Herbal Antimicrobial Agent

Carvacrol is a herbal antimicrobial agent with in vitro activity against several bacterial pathogens. However, multidrug resistant strains of Pseudomonas aeruginosa are resistant to herbal antimicrobial compounds including carvacrol. Resistance of P. aeruginosa to carvacrol is not well studied. This study was aimed to identify the gene(s) associated with carvacrol resistance, thus to understand its mechanisms in P. aeruginosa. A herbal drug resistant strain was isolated from a hospital environment. Carvacrol sensitive mutant was generated using transposon mutagenesis. The inactivated gene in the mutant was identified as mexA, which is part of the mexAB-oprM operon. Inactivation of the mexA gene resulted in a >31-fold reduction in MIC of carvacrol, whereas a >80-fold reduction was observed in the presence of drug efflux inhibitor phenylalanine-arginine β-naphthylamide (PAβN). The parental herbal-resistant strain was completely killed within 3 h of incubation in the presence of carvacrol and PAβN. The mexA inactivation did not affect the resistance to other herbal compounds used. The results demonstrate that resistance to carvacrol in P. aeruginosa is mediated by the MexAB-OprM efflux pump.

AcrB: a mean, keen, drug efflux machine

Gram-negative bacteria are intrinsically resistant against cytotoxic substances by means of their outer membrane and a network of multidrug efflux systems, acting in synergy. Efflux pumps from various superfamilies with broad substrate preferences sequester and pump drugs across the inner membrane to supply the highly polyspecific and powerful tripartite resistance-nodulation-cell division (RND) efflux pumps with compounds to be extruded across the outer membrane barrier. In Escherichia coli, the tripartite efflux system AcrAB-TolC is the archetype RND multiple drug efflux pump complex. The homotrimeric inner membrane component acriflavine resistance B (AcrB) is the drug specificity and energy transduction center for the drug/proton antiport process. Drugs are bound and expelled via a cycle of mainly three consecutive states in every protomer, constituting a flexible alternating access channel system. This review recapitulates the molecular basis of drug and inhibitor binding, including mechanistic insights into drug efflux by AcrB. It also summarizes 17 years of mutational analysis of the gene acrB, reporting the effect of every substitution on the ability of E. coli to confer resistance toward antibiotics (http://goethe.link/AcrBsubstitutions). We emphasize the functional robustness of AcrB toward single-site substitutions and highlight regions that are more sensitive to perturbation.

Focus on the Outer Membrane Factor OprM, the Forgotten Player from Efflux Pumps Assemblies

Antibiotics have been used extensively during several decades and we are now facing the emergence of multidrug resistant strains. It has become a major public concern, urging the need to discover new strategies to combat them. Among the different ways used by bacteria to resist antibiotics, the active efflux is one of the main mechanisms. In Gram-negative bacteria the efflux pumps are comprised of three components forming a long edifice crossing the complete cell wall from the inside to the outside of the cell. Blocking these pumps would permit the restoration of the effectiveness of the current antibiotherapy which is why it is important to increase our knowledge on the different proteins involved in these complexes. A tremendous number of experiments have been performed on the inner membrane protein AcrB from Escherichia coli and, to a lesser extent, the protein partners forming the AcrAB-TolC pump, but less information is available concerning the efflux pumps from other virulent Gram-negative bacteria. The present review will focus on the OprM outer membrane protein from the MexAB-OprM pump of Pseudomonas aeruginosa, highlighting similarities and differences compare to the archetypal AcrAB-TolC in terms of structure, function, and assembly properties.

A novel packing arrangement of AcrB in the lipid bilayer membrane

The central component AcrB of the Escherichia coli drug efflux complex AcrA-AcrB-TolC has been extensively investigated by X-ray crystallography of detergent-protein 3-D crystals. In these crystals, AcrB packs as trimers - the functional unit. We visualized the AcrB-AcrB interaction in its native environment by examining E. coli lipid reconstituted 2-D crystals, which were overwhelmingly formed by asymmetric trimers stabilized by strongly-interacting monomers from adjacent trimers. Most interestingly, we observed lattices formed by an arrangement of AcrB monomers distinct from that in traditional trimers. This hitherto unobserved packing, might play a role in the biogenesis of trimeric AcrB.

Tat pathway-mediated translocation of the Sec pathway substrate OprM, an outer membrane subunit of the resistance nodulation division xenobiotic extrusion pumps, in Pseudomonas Aeruginosa

BACKGROUND

Pseudomonas aeruginosa produces the Sec and Tat protein secretion machineries. The latter appears to be involved in the secretion of virulence factors, including phospholipase C (PlcH), and hence is a potential target of chemotherapeutic agents.

METHODS

The signal sequence of OprM, the outer membrane subunit of the xenobiotic extrusion pumps, was substituted with that of PlcH. The antibiotic susceptibility of oprM-deficient cells expressing the hybrid protein PlcH-OprM was evaluated using the agar dilution method.

RESULTS

The PlcH-OprM-expressing cells showed resistance to various MexAB-OprM substrate antibiotics. To evaluate the translocation route of PlcH-OprM, tatC encoding an indispensable component of the Tat machinery was knocked out in oprM-deficient cells. The tatC-oprM double mutant expressing PlcH-OprM exhibited antibiotic hypersusceptibility like the oprM-deficient cells, indicating that PlcH-OprM was translocated across the inner membrane exclusively through the Tat system.

CONCLUSIONS

This system can be used for the screening of Tat system inhibitors and will be an excellent model for the study of secretion and biogenesis of the β-barrel outer membrane proteins.

Modeling the Tripartite Drug Efflux Pump Archetype: Structural and Functional Studies of the Macromolecular Constituents Reveal More Than Their Names Imply

It is a remarkable age in molecular biology when one can argue that our current understanding of a process is influenced as much by structural studies as it is by genetic and physiological manipulations. This statement is particularly poignant with membrane proteins for which structural knowledge has been long impeded by the inability to easily obtain crystal structures in a lipid matrix. Thus, several high-resolution structures of the components comprising tripartite multidrug efflux pumps from Escherichia coli and Pseudomonas aeruginosa are now available and were received with much acclaim over ever-evolving crystal structures of soluble, aqueous proteins. These structures, in conjunction with functional mutagenesis studies, have provided insight into substrate capture and binding domains and redefined the potential interactions between individual pump constituents. However, correct assembly of the components is still a matter of debate as is the functional contribution of each to the translocation of drug substrates over long distances spanning the Gram-negative cell envelope.

Stoichiometry of the MexA-OprM binding, as investigated by blue native gel electrophoresis

Multidrug resistance has become a serious concern in the treatment of bacterial infections. A prominent role is ascribed to the active efflux of xenobiotics out of the bacteria by a tripartite protein machinery. The mechanism of drug extrusion is rather well understood, thanks to the X-ray structures obtained for the Escherichia coli TolC/AcrA/AcrB model system and the related Pseudomonas aeruginosa OprM/MexA/MexB. However, many questions remain unresolved, in particular the stoichiometry of the efflux pump assembly. On the basis of blue native polyacrylamide gel electrophoresis (BN-PAGE) (Wittig et al., Nat. Protoc. 2006, 1, 418-428), we analyzed the binding stoichiometry of both palmitylated and non-palmitylated MexA with the cognate partner OprM trimer at different ratios and detergent conditions. We found that β-octyl glucopyranoside (β-OG) detergent was not suitable for this technique. Then we proved that MexA has to be palmitylated in order to stabilized the complex formation with OprM. Finally, we provided evidence for a two by two (2, 4, 6, or upper) binding of palmitylated MexA per trimer of OprM.

Genome-wide analysis and literature-based survey of lipoproteins in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen able to cause acute or chronic infections. Like all other Pseudomonas species, P. aeruginosa has a large genome, >6 Mb, encoding more than 5000 proteins. Many proteins are localized in membranes, among them lipoproteins, which can be found tethered to the inner or the outer membrane. Lipoproteins are translocated from the cytoplasm and their N-terminal signal peptide is cleaved by the signal peptidase II, which recognizes a specific sequence called the lipobox just before the first cysteine of the mature lipoprotein. A majority of lipoproteins are transported to the outer membrane via the LolCDEAB system, while those having an avoidance signal remain in the inner membrane. In Escherichia coli, the presence of an aspartate residue after the cysteine is sufficient to cause the lipoprotein to remain in the inner membrane, while in P. aeruginosa the situation is more complex and involves amino acids at position +3 and +4 after the cysteine. Previous studies indicated that there are 185 lipoproteins in P. aeruginosa, with a minority in the inner membrane. A reanalysis led to a reduction of this number to 175, while new retention signals could be predicted, increasing the percentage of inner-membrane lipoproteins to 20 %. About one-third (62 out of 175) of the lipoprotein genes are present in the 17 Pseudomonas genomes sequenced, meaning that these genes are part of the core genome of the genus. Lipoproteins can be classified into families, including those outer-membrane proteins having a structural role or involved in efflux of antibiotics. Comparison of various microarray data indicates that exposure to epithelial cells or some antibiotics, or conversion to mucoidy, has a major influence on the expression of lipoprotein genes in P. aeruginosa.

Tat pathway-mediated translocation of the sec pathway substrate protein MexA, an inner membrane component of the MexAB-OprM xenobiotic extrusion pump in Pseudomonas aeruginosa

Pseudomonas aeruginosa is equipped with the Sec and Tat protein secretion systems, which translocate the xenobiotic transporter MexAB-OprM and the pathogenic factor phospholipase C (PlcH), respectively. When the signal sequence of MexA was replaced with that of PlcH, the hybrid protein was successfully expressed and recovered from the periplasmic fraction, suggesting that the hybrid protein had been translocated across the inner membrane. MexA-deficient cells harboring the plasmid carrying the plcH-mexA fusion gene showed antibiotic resistance comparable to that of the wild-type cells. This result suggested that MexA secreted via the Tat machinery was properly assembled and functioned as a subunit of the MexAB-OprM efflux pump. A mutation was introduced into the chromosomal tatC gene encoding an inner membrane component of the Tat protein secretion machinery in mexA-deficient cells, and they were transformed with the plasmid carrying the plcH-mexA fusion gene. The transformants showed antibiotic susceptibility comparable to that of mexA-deficient cells, indicating that the hybrid protein was not transported to the periplasm. Whole-cell lysate of the mexA-tatC double mutant harboring the plcH-mexA plasmid produced mainly unprocessed PlcH-MexA. The periplasmic fraction showed no detectable anti-MexA antibody-reactive material. On the basis of these results, we concluded that MexA could be translocated across the inner membrane through the Tat pathway and assembled with its cognate partners, MexB and OprM, and that this complex machinery was fully functional. This hybrid protein translocation system has the potential to be a powerful screening tool for antimicrobial agents targeting the Tat system, which is not present in mammalian cells.

The assembled structure of a complete tripartite bacterial multidrug efflux pump

Bacteria like Escherichia coli and Pseudomonas aeruginosa expel drugs via tripartite multidrug efflux pumps spanning both inner and outer membranes and the intervening periplasm. In these pumps a periplasmic adaptor protein connects a substrate-binding inner membrane transporter to an outer membrane-anchored TolC-type exit duct. High-resolution structures of all 3 components are available, but a pump model has been precluded by the incomplete adaptor structure, because of the apparent disorder of its N and C termini. We reveal that the adaptor termini assemble a beta-roll structure forming the final domain adjacent to the inner membrane. The completed structure enabled in vivo cross-linking to map intermolecular contacts between the adaptor AcrA and the transporter AcrB, defining a periplasmic interface between several transporter subdomains and the contiguous beta-roll, beta-barrel, and lipoyl domains of the adaptor. With short and long cross-links expressed as distance restraints, the flexible linear topology of the adaptor allowed a multidomain docking approach to model the transporter-adaptor complex, revealing that the adaptor docks to a transporter region of comparative stability distinct from those key to the proposed rotatory pump mechanism, putative drug-binding pockets, and the binding site of inhibitory DARPins. Finally, we combined this docking with our previous resolution of the AcrA hairpin-TolC interaction to develop a model of the assembled tripartite complex, satisfying all of the experimentally-derived distance constraints. This AcrA(3)-AcrB(3)-TolC(3) model presents a 610,000-Da, 270-A-long efflux pump crossing the entire bacterial cell envelope.

Tracking membrane protein association in model membranes

Membrane proteins are essential in the exchange processes of cells. In spite of great breakthrough in soluble proteins studies, membrane proteins structures, functions and interactions are still a challenge because of the difficulties related to their hydrophobic properties. Most of the experiments are performed with detergent-solubilized membrane proteins. However widely used micellar systems are far from the biological two-dimensions membrane. The development of new biomimetic membrane systems is fundamental to tackle this issue.We present an original approach that combines the Fluorescence Recovery After fringe Pattern Photobleaching technique and the use of a versatile sponge phase that makes it possible to extract crucial informations about interactions between membrane proteins embedded in the bilayers of a sponge phase. The clear advantage lies in the ability to adjust at will the spacing between two adjacent bilayers. When the membranes are far apart, the only possible interactions occur laterally between proteins embedded within the same bilayer, whereas when membranes get closer to each other, interactions between proteins embedded in facing membranes may occur as well.After validating our approach on the streptavidin-biotinylated peptide complex, we study the interactions between two membrane proteins, MexA and OprM, from a Pseudomonas aeruginosa efflux pump. The mode of interaction, the size of the protein complex and its potential stoichiometry are determined. In particular, we demonstrate that: MexA is effectively embedded in the bilayer; MexA and OprM do not interact laterally but can form a complex if they are embedded in opposite bilayers; the population of bound proteins is at its maximum for bilayers separated by a distance of about 200 A, which is the periplasmic thickness of Pseudomonas aeruginosa. We also show that the MexA-OprM association is enhanced when the position and orientation of the protein is restricted by the bilayers. We extract a stoichiometry for the complex that exhibits a strong pH dependance: from 2 to 6 MexA per OprM trimer when the pH decreases from 7.5 to 5.5.Our technique allows to study membrane protein associations in a membrane environment. It provides some challenging information about complexes such as geometry and stoichiometry.

Structural and functional diversity of bacterial membrane fusion proteins

Membrane Fusion Proteins (MFPs) are functional subunits of multi-component transporters that perform diverse physiological functions in both Gram-positive and Gram-negative bacteria. MFPs associate with transporters belonging to Resistance-Nodulation-cell Division (RND), ATP-Binding Cassette (ABC) and Major Facilitator (MF) superfamilies of proteins. Recent studies suggested that MFPs interact with substrates and play an active role in transport reactions. In addition, the MFP-dependent transporters from Gram-negative bacteria recruit the outer membrane channels to expel various substrates across the outer membrane into external medium. This review is focused on the diversity, structure and molecular mechanism of MFPs that function in multidrug efflux. Using phylogenetic approaches we analyzed diversity and representation of multidrug MFPs in sequenced bacterial genomes. In addition to previously characterized MFPs from Gram-negative bacteria, we identified MFPs that associate with RND-, MF- and ABC-type transporters in Gram-positive bacteria. Sequence analyses showed that MFPs vary significantly in size (200-650 amino acid residues) with some of them lacking the signature alpha-helical domain of multidrug MFPs. Furthermore, many transport operons contain two- or three genes encoding distinct MFPs. We further discuss the diversity of MFPs in the context of current views on the mechanism and structure of MFP-dependent transporters.

The Enterobacter aerogenes outer membrane efflux proteins TolC and EefC have different channel properties

The outer membrane proteins TolC and EefC from Enterobacter aerogenes are involved in multidrug resistance as part of two resistance-nodulation-division efflux systems. To gain more understanding in the molecular mechanism underlying drug efflux, we have undertaken an electrophysiological characterization of the channel properties of these two proteins. TolC and EefC were purified in their native trimeric form and then reconstituted in proteoliposomes for patch-clamp experiments and in planar lipid bilayers. Both proteins generated a small single channel conductance of about 80 pS in 0.5 M KCl, indicating a common gated structure. The resultant pores were stable, and no voltage-dependent openings or closures were observed. EefC has a low ionic selectivity (P(K)/P(Cl)= approximately 3), whereas TolC is more selective to cations (P(K)/P(Cl)= approximately 30). This may provide a possible explanation for the difference in drug selectivity between the AcrAB-TolC and EefABC efflux systems observed in vivo. The pore-forming activity of both TolC and EefC was severely inhibited by divalent cations entering from the extracellular side. Another characteristic of the TolC and EefC channels was the systematic closure induced by acidic pH. These results are discussed in respect to the physiological functions and structural models of TolC and EefC.

Organization of reconstituted lipoprotein MexA onto supported lipid membrane

MexA, a periplasmic component of OprM-MexA-MexB tripartite multidrug efflux pump from Pseudomonas aeruginosa, is natively anchored via its fatty acid in the bacteria inner membrane protruding into the periplasm. We used supported lipid bilayer (SLB) to attach the protein to a single leaflet mimicking its perisplamic orientation. For that purpose, we studied the solubilization of DOPC lipid bilayer supported on silica surface with beta-octyl glucoside (betaOG). First we showed that SLBs resist to betaOG concentrations that usually solubilize liposomes. Native form of MexA was directly inserted in the outer leaflet at (betaOG concentrations in a range of 20-25 mM). Second, observations by cryo-electron microscopy (cryoEM) revealed a dense protein layer attached to the surface corresponding to a 13-nm layer of MexA proteins. Analysis of protein densities allows proposing a schematic organization of native MexA inserted in lipid membrane. This structural organization provides further insights with respect to the partially solved structure of the soluble form.

PelC is a Pseudomonas aeruginosa outer membrane lipoprotein of the OMA family of proteins involved in exopolysaccharide transport

Pseudomonas aeruginosa is a gram-negative bacterium, opportunistic pathogen, which causes severe acute or chronic infections, as is the case with cystic fibrosis patients. Chronic infections are frequently accompanied by the development of the bacterial population into a specialized community called biofilm. The pelA-G gene cluster of P. aeruginosa has been shown to be involved in pellicle production and biofilm formation. The pel genes have been proposed to contribute to the formation of the exopolysaccharide-containing pellicle. However, the function and the subcellular localization of the seven different Pel proteins are poorly understood. Based on bioinformatics analysis, we have previously considered that PelF is a putative glycosyltransferase (GT4 family), whereas PelG is a Wzx-like polysaccharide transporter from the PST family. In this study we have further characterized the PelC protein. We have shown that PelC is an outer membrane lipoprotein. The N-terminal signal peptide of the PelC lipoprotein is sufficient to target the protein into the membranes. However, by constructing various PelC hybrid proteins we also proposed that efficient and functional outer membrane insertion of PelC requires not only the signal peptide and the lipid modification, but also requires the C-terminal domain of PelC. Because the gene encoding the outer membrane lipoprotein PelC is part of a putative gene cluster involved in exopolysaccharide biogenesis, we suggest that PelC is a new member of the outer membrane auxiliary (OMA) family of lipoprotein whose Wza, involved in Escherichia coli capsular polysaccharide transport, is an archetype.

Amino acids at positions 3 and 4 determine the membrane specificity of Pseudomonas aeruginosa lipoproteins

Escherichia coli lipoproteins with Asp at position 2 remain in the inner membrane, whereas those having other amino acids are targeted to the outer membrane by the Lol system. However, inner membrane lipoproteins without Asp at position 2 are found in other Gram-negative bacteria. MexA of Pseudomonas aeruginosa, an inner membrane-specific lipoprotein involved in multidrug efflux, has Gly at position 2. To identify the residue or region of MexA that functions as an inner membrane retention signal, we constructed chimeric lipoproteins comprising various regions of MexA and an outer membrane lipoprotein, OprM, and analyzed their membrane localization. Lys and Ser at positions 3 and 4, respectively, were found to be critical for the inner membrane localization of MexA in P. aeruginosa. Substitution of these residues with Leu and Ile, which are present in OprM, was sufficient to target the chimeric lipoprotein to the outer membrane and to abolish the ability of MexA to confer drug resistance. The membrane specificity of a model lipoprotein, lipoMalE, a lipidated variant of the periplasmic maltose-binding protein of E. coli, was also determined by the residues at positions 3 and 4 in P. aeruginosa. In contrast to the widely accepted "+2 rule" for E. coli lipoproteins, these results suggest a new "+3, +4 rule" for lipoprotein sorting in P. aeruginosa, namely, the final destination of lipoproteins is determined by the residues at positions 3 and 4.

Membrane protein selectively oriented on solid support and reconstituted into a lipid membrane

Mimetic functional membranes on solid support are now emerging for the development of membrane biosensor or for the study of membrane-mediated processes and should have an important impact on biodiagnostics. We established a method to reconstitute a membrane protein into a lipid membrane in a selective orientation on a solid support. Membrane protein OprM, a component of OprM-MexA-MexB multidrug efflux pump, solubilized in detergent was immobilized via its extracellular domain on aminosilane-modified silica surface. The oriented protein was reconstituted into a lipid membrane by detergent removal. The membrane protein reconstitution process carried out on silica nanoparticles and on planar silica surfaces was followed by cryo-electron microscopy (cryo-EM) and quartz crystal microbalance with dissipation monitoring (QCM-D) respectively. The selective protein orientation on aminosilane-modified silica surface was assessed by cryo-EM and was compared to the nonspecific protein deposition on silica surface. Finally, the binding of MexA, a periplasmic component of the tripartite efflux complex, was monitored with QCM-D on the oriented OprM protein monolayer. The large adsorbed mass gave a direct evidence of the high affinity of MexA with the periplasmic helical part of OprM.

Conformational flexibility in the multidrug efflux system protein AcrA

Intrinsic resistance to multiple drugs in many gram-negative bacterial pathogens is conferred by resistance nodulation cell division efflux pumps, which are composed of three essential components as typified by the extensively characterized Escherichia coli AcrA-AcrB-TolC system. The inner membrane drug:proton antiporter AcrB and the outer membrane channel TolC export chemically diverse compounds out of the bacterial cell, and require the activity of the third component, the periplasmic protein AcrA. The crystal structures of AcrB and TolC have previously been determined, and we complete the molecular picture of the efflux system by presenting the structure of a stable fragment of AcrA. The AcrA fragment resembles the elongated sickle shape of its homolog Pseudomonas aeruginosa MexA, being composed of three domains: beta-barrel, lipoyl, and alpha-helical hairpin. Notably, unsuspected conformational flexibility in the alpha-helical hairpin domain of AcrA is observed, which has potential mechanistic significance in coupling between AcrA conformations and TolC channel opening.

Purification of outer membrane vesicles from Pseudomonas aeruginosa and their activation of an IL-8 response

Considerable lung injury results from the inflammatory response to Pseudomonas aeruginosa infections in patients with cystic fibrosis (CF). The P. aeruginosa laboratory strain PAO1, an environmental isolate, and isolates from CF patients were cultured in vitro and outer membrane vesicles from those cultures were quantitated, purified, and characterized. Vesicles were produced throughout the growth phases of the culture and vesicle yield was strain-independent. Strain-dependent differences in the protein composition of vesicles were quantitated and identified. The aminopeptidase PaAP (PA2939) was highly enriched in vesicles from CF isolates. Vesicles from all strains elicited IL-8 secretion by lung epithelial cells. These results suggest that P. aeruginosa colonizing the CF lung may produce vesicles with a particular composition and that the vesicles could contribute to inflammation.

Modeling the Tripartite Drug Efflux Pump Archetype: Structural and Functional Studies of the Macromolecular Constituents Reveal More Than Their Names Imply

It is a remarkable age in molecular biology when one can argue that our current understanding of a process is influenced as much by structural studies as it is by genetic and physiological manipulations. This statement is particularly poignant with membrane proteins for which structural knowledge has been long impeded by the inability to easily obtain crystal structures in a lipid matrix. Thus, several highresolution structures of the components comprising tripartite multidrug efflux pumps from Escherichia coli and Pseudomonas aeruginosa are now available and were received with much acclaim over ever-evolving crystal structures of soluble, aqueous proteins. These structures, in conjunction with functional mutagenesis studies, have provided insight into substrate capture and binding domains and redefined the potential interactions between individual pump constituents. However, correct assembly of the components is still a matter of debate as is the functional contribution of each to the translocation of drug substrates over long distances spanning the Gram-negative cell envelope.

The eefABC multidrug efflux pump operon is repressed by H-NS in Enterobacter aerogenes

The Enterobacter aerogenes eefABC locus, which encodes a tripartite efflux pump, was cloned by complementation of an Escherichia coli tolC mutant. E. aerogenes deltaacrA expressing EefABC became less susceptible to a wide range of antibiotics. Data from eef::lacZ fusions showed that eefABC was not transcribed in the various laboratory conditions tested. However, increased transcription from Peef was observed in an E. coli hns mutant. In addition, EefA was detected in E. aerogenes expressing a dominant negative E. coli hns allele.

Crystal Structure of the Drug Discharge Outer Membrane Protein, OprM, of Pseudomonas aeruginosa

The OprM lipoprotein of Pseudomonas aeruginosa is a member of the MexAB-OprM xenobiotic-antibiotic transporter subunits that is assumed to serve as the drug discharge duct across the outer membrane. The channel structure must differ from that of the porin-type open pore because the protein facilitates the exit of antibiotics but not the entry. For better understanding of the structure-function linkage of this important pump subunit, we studied the x-ray crystallographic structure of OprM at the 2.56-angstroms resolution. The overall structure exhibited trimeric assembly of the OprM monomer that consisted mainly of two domains: the membrane-anchoring beta-barrel and the cavity-forming alpha-barrel. OprM anchors the outer membrane by two modes of membrane insertions. One is via the covalently attached NH(2)-terminal fatty acids and the other is the beta-barrel structure consensus on the outer membrane-spanning proteins. The beta-barrel had a pore opening with a diameter of about 6-8 angstroms, which is not large enough to accommodate the exit of any antibiotics. The periplasmic alpha-barrel was about 100 angstroms long formed mainly by a bundle of alpha-helices that formed a solvent-filled cavity of about 25,000 angstroms(3). The proximal end of the cavity was tightly sealed, thereby not permitting the entry of any molecule. The result of this structure was that the resting state of OprM had a small outer membrane pore and a tightly closed periplasmic end, which sounds plausible because the protein should not allow free access of antibiotics. However, these observations raised another unsolved problem about the mechanism of opening of the OprM cavity ends. The crystal structure offers possible mechanisms of pore opening and pump assembly.

Real-Time Probing of Membrane Transport in Living Microbial Cells Using Single Nanoparticle Optics and Living Cell Imaging†

Membrane transport plays a leading role in a wide spectrum of cellular and subcellular pathways, including multidrug resistance (MDR), cellular signaling, and cell-cell communication. Pseudomonas aeruginosa is renowned for its intriguing membrane transport mechanisms, such as the interplay of membrane permeability and extrusion machinery, leading to selective accumulation of specific intracellular substances and MDR. Despite extensive studies, the mechanisms of membrane transport in living microbial cells remain incompletely understood. In this study, we directly measure real-time change of membrane permeability and pore sizes of P. aeruginosa at the nanometer scale using the intrinsic color index (surface plasmon resonance spectra) of silver (Ag) nanoparticles as the nanometer size index probes. The results show that Ag nanoparticles with sizes ranging up to 80 nm are accumulated in living microbial cells, demonstrating that these Ag nanoparticles transport through the inner and outer membrane of the cells. In addition, a greater number of larger intracellular Ag nanoparticles are observed in the cells as chloramphenicol concentration increases, suggesting that chloramphenicol increases membrane permeability and porosity. Furthermore, studies of mutants (nalB-1 and DeltaABM) show that the accumulation rate of intracellular Ag nanoparticles depends on the expression level of the extrusion pump (MexAB-OprM), suggesting that the extrusion pump plays an important role in controlling the accumulation of Ag nanoparticles in living cells. Moreover, the accumulation kinetics measured by Ag nanoparticles are similar to those measured using a small fluorescent molecule (EtBr), eliminating the possibility of steric and size effects of Ag nanoparticle probes. Susceptibility measurements also suggest that a low concentration of Ag nanoparticles (1.3 pM) does not create significant toxicity for the cells, further validating that single Ag nanoparticles (1.3 pM) can be used as biocompatible nanoprobes for the study of membrane transport kinetics in living microbial cells.

Structure of the periplasmic component of a bacterial drug efflux pump

Multidrug resistance among Gram-negative bacteria is conferred by three-component membrane pumps that expel diverse antibiotics from the cell. These efflux pumps consist of an inner membrane transporter such as the AcrB proton antiporter, an outer membrane exit duct of the TolC family, and a periplasmic protein known as the adaptor. We present the x-ray structure of the MexA adaptor from the human pathogen Pseudomonas aeruginosa. The elongated molecule contains three linearly arranged subdomains; a 47-A-long alpha-helical hairpin, a lipoyl domain, and a six-stranded beta-barrel. In the crystal, hairpins of neighboring MexA monomers pack side-by-side to form twisted arcs. We discuss the implications of the packing of molecules within the crystal. On the basis of the structure and packing, we suggest a model for the key periplasmic interaction between the outer membrane channel and the adaptor protein in the assembled drug efflux pump.

Assembly of the MexAB-OprM multidrug efflux system of Pseudomonas aeruginosa: identification and characterization of mutations in mexA compromising MexA multimerization and interaction with MexB

The membrane fusion protein (MFP) component, MexA, of the MexAB-OprM multidrug efflux system of P. aeruginosa is proposed to link the inner (MexB) and outer (OprM) membrane components of this pump as a probable oligomer. A cross-linking approach confirmed the in vivo interaction of MexA and MexB, while a LexA-based assay for assessing protein-protein interaction similarly confirmed MexA multimerization. Mutations compromising the MexA contribution to antibiotic resistance but yielding wild-type levels of MexA were recovered and shown to map to two distinct regions within the N- and C-terminal halves of the protein. Most of the N-terminal mutations occurred at residues that are highly conserved in the MFP family (P68, G72, L91, A108, L110, and V129), consistent with these playing roles in a common feature of these proteins (e.g., oligomerization). In contrast, the majority of the C-terminal mutations occurred at residues poorly conserved in the MFP family (V264, N270, H279, V286, and G297), with many mapping to a region of MexA that corresponds to a region in the related MFP of Escherichia coli, AcrA, that is implicated in binding to its RND component, AcrB (C. A. Elkins and H. Nikaido, J. Bacteriol. 185:5349-5356, 2003). Given the noted specificity of MFP-RND interaction in this family of pumps, residues unique to MexA may well be important for and define the MexA interaction with its RND component, MexB. Still, all but one of the MexA mutations studied compromised MexA-MexB association, suggesting that native structure and/or proper assembly of the protein may be necessary for this.

Crystal Structure of the Membrane Fusion Protein, MexA, of the Multidrug Transporter in Pseudomonas aeruginosa

The MexAB-OprM efflux pump of Pseudomonas aeruginosa is central to multidrug resistance of this organism, which infects immunocompromised hospital patients. The MexA, MexB, and OprM subunits were assumed to function as the membrane fusion protein, the body of the transporter, and the outer membrane channel protein, respectively. For better understanding of this important xenobiotic transporter, we show the x-ray crystallographic structure of MexA at a resolution of 2.40 A. The global MexA structure showed unforeseen new features with a spiral assembly of six and seven protomers that were joined together at one end by a pseudo 2-fold image. The protomer showed a new protein structure with a tandem arrangement consisting of at least three domains and presumably one more. The rod domain had a long hairpin of twisted coiled-coil that extended to one end. The second domain adjacent to the rod alpha-helical domain was globular and constructed by a cluster of eight short beta-sheets. The third domain located distal to the alpha-helical rod was globular and composed of seven short beta-sheets and one short alpha-helix. The 13-mer was shaped like a woven rattan cylinder with a large internal tubular space and widely opened flared ends. The 6-mer and 7-mer had a funnel-like structure consisting of a tubular rod at one side and a widely opened flared funnel top at the other side. Based on these results, we constructed a model of the MexAB-OprM pump assembly. The three pairs of MexA dimers interacted with the periplasmic alpha-barrel domain of OprM via the alpha-helical hairpin, the second domain interacted with both MexB and OprM at their contact site, and the third and disordered domains probably interacted with the distal domain of MexB. In this fashion, the MexA subunit connected MexB and OprM, indicating that MexA is the membrane bridge protein.

Enhancement of the mexAB-oprM efflux pump expression by a quorum-sensing autoinducer and its cancellation by a regulator, MexT, of the mexEF-oprN efflux pump operon in Pseudomonas aeruginosa

nfxC-type cells of Pseudomonas aeruginosa that produce the MexEF-OprN efflux pump exhibit resistance to fluoroquinolones and chloramphenicol and hypersusceptibility to most classical beta-lactam antibiotics. We investigated the molecular mechanism of how the nfxC mutation causes beta-lactam hypersusceptibility. The MexAB-OprM extrusion pump transports and confers resistance to beta-lactam antibiotics. Interestingly, expression of the mexAB-oprM operon reached the highest level during the mid-stationary growth phase in both wild-type and nfxC-type mutant strains, suggesting that expression of the mexAB-oprM operon may be controlled by cell density-dependent regulation such as quorum sensing. This assumption was verified by demonstrating that exogenous addition of the quorum-sensing autoinducer N-butyryl-L-homoserine lactone (C4-HSL) enhanced the expression of MexAB-OprM, whereas N-(3-oxododecanoyl)-L-homoserine lactone had only a slight effect. Furthermore, this C4-HSL-mediated enhancement of mexAB-oprM expression was repressed by MexT, a positive regulator of the mexEF-oprN operon. It was concluded that beta-lactam hypersusceptibility in nfxC-type mutant cells is caused by MexT-mediated cancellation of C4-HSL-mediated enhancement of MexAB-OprM expression.

Efflux-mediated drug resistance in bacteria

Drug resistance in bacteria, and especially resistance to multiple antibacterials, has attracted much attention in recent years. In addition to the well known mechanisms, such as inactivation of drugs and alteration of targets, active efflux is now known to play a major role in the resistance of many species to antibacterials. Drug-specific efflux (e.g. that of tetracycline) has been recognised as the major mechanism of resistance to this drug in Gram-negative bacteria. In addition, we now recognise that multidrug efflux pumps are becoming increasingly important. Such pumps play major roles in the antiseptic resistance of Staphylococcus aureus, and fluoroquinolone resistance of S. aureus and Streptococcus pneumoniae. Multidrug pumps, often with very wide substrate specificity, are not only essential for the intrinsic resistance of many Gram-negative bacteria but also produce elevated levels of resistance when overexpressed. Paradoxically, 'advanced' agents for which resistance is unlikely to be caused by traditional mechanisms, such as fluoroquinolones and beta-lactams of the latest generations, are likely to select for overproduction mutants of these pumps and make the bacteria resistant in one step to practically all classes of antibacterial agents. Such overproduction mutants are also selected for by the use of antiseptics and biocides, increasingly incorporated into consumer products, and this is also of major concern. We can consider efflux pumps as potentially effective antibacterial targets. Inhibition of efflux pumps by an efflux pump inhibitor would restore the activity of an agent subject to efflux. An alternative approach is to develop antibacterials that would bypass the action of efflux pumps.

Using Nanoparticle Optics Assay for Direct Observation of the Function of Antimicrobial Agents in Single Live Bacterial Cells†

Multidrug resistance (MDR) has been reported in both prokaryotes and eukaryotes, underscoring the challenge of design and screening of more efficacious new drugs. For instance, the efflux pump of Pseudomonas aeruginosa (gram-negative bacteria) can extrude a variety of structurally and functionally diverse substrates, which leads to MDR. In this study, we present a new platform that studies modes of action of antibiotics in living bacterial cells (P. aeruginosa), in real-time, at nanometer scale and single-cell resolution using nanoparticle optics and single living cell imaging. The color index of silver (Ag) nanoparticles (violet, blue, green, and red) is used as the sized index (30 +/- 10, 50 +/- 10, 70 +/- 10, and 90 +/- 10 nm) for real-time measurement of sized transformation of the cell wall and membrane permeability at the nanometer scale. We have demonstrated that the number of Ag nanoparticles accumulated in cells increases as the aztreonam (AZT) concentration increases and as incubation time increases, showing that AZT induces the sized transformation of membrane permeability and the disruption of the cell wall. The results demonstrate that nanoparticle optics assay can be used as a new powerful tool for real-time characterization of modes of action of antimicrobial agents in living cells at the nanometer scale. Furthermore, studies of mutants of WT bacteria (nalB-1 and DeltaABM), suggest that an efflux pump (MexA-MexB-OprM) effectively extrudes substrates (nanoparticles) out of the cells, indicating that the MDR mechanism involves the induction of changes in membrane permeability and the intrinsic pump machinery.

Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria

For translocation across the cell envelope of Gram-negative bacteria, substances have to overcome two permeability barriers, the inner and outer membrane. Channel-tunnels are outer membrane proteins, which are central to two distinct export systems: the type I secretion system exporting proteins such as toxins or proteases, and efflux pumps discharging antibiotics, dyes, or heavy metals and thus mediating drug resistance. Protein secretion is driven by an inner membrane ATP-binding cassette (ABC) transporter while drug efflux occurs via an inner membrane proton antiporter. Both inner membrane transporters are associated with a periplasmic accessory protein that recruits an outer membrane channel-tunnel to form a functional export complex. Prototypes of these export systems are the hemolysin secretion system and the AcrAB/TolC drug efflux pump of Escherichia coli, which both employ TolC as an outer membrane component. Its remarkable conduit-like structure, protruding 100 A into the periplasmic space, reveals how both systems are capable of transporting substrates across both membranes directly from the cytosol into the external environment. Proteins of the channel-tunnel family are widespread within Gram-negative bacteria. Their involvement in drug resistance and in secretion of pathogenic factors makes them an interesting system for further studies. Understanding the mechanism of the different export apparatus could help to develop new drugs, which block the efflux pumps or the secretion system.

Linkage of the efflux-pump expression level with substrate extrusion rate in the MexAB-OprM efflux pump of Pseudomonas aeruginosa

The amount of the subunit proteins of the MexAB-OprM efflux pump in Pseudomonas aeruginosa was quantified by the immunoblotting method. A single cell of the wild-type strain contained about 2500, 1000, and 1200 copies of MexA, MexB, and OprM, respectively, and their stoichiometry therefore was 2:1:1. The mexR mutant produced an eightfold higher level of these proteins than did wild-type cells. Assuming that MexB and OprM exist as a trimer in a pump assembly, the total number of MexAB-OprM per wild-type cell was calculated to be about 400 assemblies. The substrate efflux rate of MexAB-OprM was calculated from the fluorescent intensity of ethidium in intact cells that a single cell extruded ethidium at a maximum of about 3 x 10(-19) mol s(-1) and, therefore, the turnover rate of a single pump unit was predicted to be about 500 s(-1).

Direct observation of substrate induction of resistance mechanism in Pseudomonas aeruginosa using single live cell imaging

The resistance mechanism in three strains of Pseudomonas aeruginosa, Delta ABM (devoid of MexAB-OprM), WT, and nalB-1 (overexpression of MexAB-OprM), was investigated using real-time single live cell imaging and fluorescence spectroscopy. Time courses of fluorescence intensity of these three strains in ethidium bromide (EtBr) showed that accumulation kinetics and extrusion machinery were highly dependent upon pump substrate (EtBr) concentration. At high substrate concentration (100 microM), the accumulation kinetic profiles in the cells at earlier incubation times were similar to those observed in low concentration. As EtBr accumulated in the cells reached a critical concentration, the fluorescence intensity of Delta ABM decreased below the fluorescence intensity of EtBr in buffer solution. This result suggested an inductive mechanism in the development of substrate resistance in P. aeruginosa. Substrates appeared to trigger the degradation of EtBr in Delta ABM. Unlike bulk measurements, single live cell imaging overcame the ensemble measurement of bulk analysis and showed that efflux machinery and resistance mechanism in individual cells were not synchronized.

Single-molecule detection of efflux pump machinery in Pseudomonas aeruginosa

Real-time single-molecule microscopy and spectroscopy were used to monitor single molecules moving in and out of live bacterial cells, Pseudomonas aeruginosa. Ethidium bromide (EtBr) was chosen as the fluorescence probe because it emitted a weak fluorescence in aqueous solution (outside of the cells) and became strongly fluorescent as it entered the cells and intercalated with DNA. Such changes in fluorescence intensity by individual EtBr molecules were measured to determine the influx and efflux rates of EtBr by the cells. The transport rates for EtBr through the energized extrusion pumps of these strains (WT, nalB-1, and DeltaABM) of P. aeruginosa were measured and showed stochastic behavior with the average being (2.86+/-0.12), (2.80+/-0.13), and (2.74+/-0.39) x s(-1), respectively. The transport rates of the three strains were independent of substrate concentration at the single-molecule level. In contrast to bulk (many molecules) measurements, single-molecule detection allowed the influx and efflux kinetics to be observed in low substrate concentrations at the molecular level.

A novel assembly process of the multicomponent xenobiotic efflux pump in Pseudomonas aeruginosa

The nfxC-type cells of Pseudomonas aeruginosa show resistance to a wide range of structurally and functionally diverse antibiotics, which is a phenomenon that is mainly attributable to the expression of the MexEF-OprN xenobiotic transporter. The MexF, MexE and OprN subunits of this transporter are located on the inner membrane, the periplasm and the outer membrane, respectively, and are assumed to function as an energy-dependent transporter, a bridge connecting the inner and outer membranes and outer membrane channel respectively. The nfxC-type cells showed a single protein band of MexF and OprN, whereas MexE appeared as three distinct bands in an SDS-polyacrylamide gel electrophoretogram. The mutant cells lacking MexF produced undetectable OprN and only a full-size of MexE even though the cells had unimpaired oprN and mexE. Expression of the plasmid-borne MexF in this mutant fully restored OprN and three MexE bands. Another class of mutants producing a full amount of MexF yielded undetectable OprN and two MexE bands lacking the smallest protein species suggesting that the presence of the smallest MexE subunit is required for stabilization of OprN. To identify which part of MexE was needed for stabilization and assembly of OprN, the carboxyl-terminal-truncated MexE tagged with polyhistidine was constructed and protein bands were visualized in the presence of MexF with an antibody raised against polyhistidine or MexE. The results revealed that the proteolytic processing of MexE would occur at carboxyl terminal amino acids between 11 and 16, thereby suggesting that the presence of the C-terminal truncated MexE is essential for stabilization and the proper assembly of OprN. Nucleotide sequencing of mutant mexFs, which produce a wild-type level of MexF but are unable to support the production of the smallest MexE, thereby destabilizing OprN, revealed that all the mutations were located within two large periplasmic domains of MexF between transmembrane segments 1-2 and 7-8. Taking these findings together, we concluded that two large periplasmic domains of MexF interact with MexE thereby promoting programmed processing of MexE, and this complex eventually assists the correct assembly and sorting of OprN.

Single live cell imaging for real-time monitoring of resistance mechanism in Pseudomonas aeruginosa

We have developed and applied single live cell imaging for real-time monitoring of resistance kinetics of Pseudomonas aeruginosa. Real-time images of live cells in the presence of a particular substrate (EtBr) provided the first direct insights of resistance mechanism with both spatial and temporal information and showed that the substrate appeared to be accumulated in cytoplasmic space, but not periplasmic space. Three mutants of P. aeruginosa, PAO4290 (a wild-type expression level of MexAB-OprM), TNP030#1 (nalB-1, MexAB-OprM over expression mutant), and TNP076 (DeltaABM, MexAB-OprM deficient mutant), were used to investigate the roles of these three membrane proteins (MexAB-OprM) in the resistance mechanism. Ethidium bromide (EtBr) was chosen as a fluorescence probe for spectroscopic measurement of bulk cell solution and single cell imaging of bulk cells. Bulk measurement indicated, among three mutants, that nalB-1 accumulated the least EtBr and showed the highest resistance to EtBr, whereas DeltaABM accumulated the most EtBr and showed the lowest resistance to EtBr. This result demonstrated the MexAB-OprM proteins played the roles in resistance mechanism by extruding EtBr out of cells. Unlike the bulk measurement, imaging and analysis of bulk cells at single cell resolution demonstrated individual cell had its distinguished resistance kinetics and offered the direct observation of the regulation of influx and efflux of EtBr with both spatial and temporal resolution. Unlike fluorescent staining assays, live cell imaging provided the real-time kinetic information of transformation of membrane permeability and efflux pump machinery of three mutants. This research constitutes the first direct imaging of resistance mechanism of live bacterial cells at single cell resolution and opens up the new possibility of advancing the understanding of bacteria resistance mechanism.

The MexJK efflux pump of Pseudomonas aeruginosa requires OprM for antibiotic efflux but not for efflux of triclosan

Using the biocide triclosan as a selective agent, several triclosan-resistant mutants of a susceptible Pseudomonas aeruginosa strain were isolated. Cloning and characterization of a DNA fragment conferring triclosan resistance from one of these mutants revealed a hitherto uncharacterized efflux system of the resistance nodulation cell division (RND) family, which was named MexJK and which is encoded by the mexJK operon. Expression of this operon is negatively regulated by the product of mexL, a gene located upstream of and transcribed divergently from mexJK. The triclosan-resistant mutant contained a single nucleotide change in mexL, which caused an amino acid change in the putative helix-turn-helix domain of MexL. The MexL protein belongs to the TetR family of repressor proteins. The MexJK system effluxed tetracycline and erythromycin but only in the presence of the outer membrane protein channel OprM; OprJ and OprN did not function with MexJK. Triclosan efflux required neither of the outer membrane protein channels tested but necessitated the MexJ membrane fusion protein and the MexK inner membrane RND transporter. The results presented in this study suggest that MexJK may function as a two-component RND pump for triclosan efflux but must associate with OprM to form a tripartite antibiotic efflux system. Furthermore, the results confirm that triclosan is an excellent tool for the study of RND multidrug efflux systems and that this popular biocide therefore readily selects mutants which are cross-resistant with antibiotics.

Secondary-site mutation restores the transport defect caused by the transmembrane domain mutation of the xenobiotic transporter MexB in Pseudomonas aeruginosa

It has been suggested that the MexB subunit of the MexAB-OprM efflux transporter of Pseudomonas aeruginosa exports xenobiotics in an energy-dependent manner. To investigate the role of the transmembrane segments (TMS) of MexB in the transporter activity, we isolated 24 spontaneous mutants showing hypersusceptibility to antibiotics. Among them, three mutations were located at TMS-3, TMS-4, and TMS-10 having amino acid substitution Leu376vPro, Gly397vVal, and Val928vGly, respectively. A secondary mutation, which suppressed the defect caused by the Val928vGly mutation in TMS-10, was found at the 403rd amino acid residue in TMS-4 with a change of glycine to serine, suggesting that TMS-4 and TMS-10 may be in close proximity. This result provided strong support for the recent notion that negatively charged residues in TMS-4 might form a salt-bridge with a positive charge in TMS-10 (Guan, L., and Nakae, T. (2001) J. Bacteriol. 183, 1734-1739). The transporter function impaired by the Gly397vVal mutation in TMS-4 was recovered by the secondary mutation, Gln998vHis, in the loop between TMS-11 and TMS-12, thereby suggesting that TMS-4 and TMS-11 or TMS-12 might also be in close proximity. Thus, it is most likely that TMS-4, TMS-10, and TMS-11 or TMS-12 are packed close three dimensionally.

Substrate-triggered recruitment of the TolC channel-tunnel during type I export of hemolysin by Escherichia coli

A defining event in type I export of hemolysin by Escherichia coli is the substrate-triggered recruitment of the TolC channel-tunnel by an inner membrane complex. This complex comprises a traffic ATPase (HlyB) and the 478 residue adaptor protein (HlyD), which contacts TolC during recruitment. HlyD has a large periplasmic domain (amino acid residues 81-478) linked by a single transmembrane helix to a small N-terminal cytosolic domain (1-59). Export was disabled by deletion of the ca 60 amino acid residue cytosolic domain of HlyD, even though the truncated HlyD (HlyDDelta45) was, like the wild-type, able to trimerise in the cytosolic membrane, and interact with the traffic ATPase. The mutant HlyB/HlyDDelta45 inner membrane complex engaged the hemolysin substrate, but this substrate-engaged complex failed to trigger recruitment of TolC. Further analyses showed that HlyDDelta45 was specifically unable to bind the substrate. The result suggests that substrate engagement by the traffic ATPase alone is insufficient to trigger TolC recruitment, and that substrate binding to the HlyD cytosolic domain is essential. Analysis of three further N-terminal deletion variants, HlyDDelta26, HlyDDelta26-45 and HlyDDelta34-38, indicated that an extreme N-terminal amphipathic helix and a cytosolic cluster of charged residues are central to the cytosolic domain function. The cytosolic amphipathic helix was not essential for substrate engagement or TolC recruitment, but export was impaired without it. In contrast, when the charged amino acid residues were deleted, the substrate was still engaged by HlyD but engagement was unproductive, i.e. TolC recruitment was not triggered. Our results are compatible with the HlyD cytosolic domain mediating transduction of the substrate binding signal directly, presumably to the HlyD periplasmic domain, to trigger recruitment of TolC and assemble the type I export complex.

Identification of essential charged residues in transmembrane segments of the multidrug transporter MexB of Pseudomonas aeruginosa

The MexABM efflux pump exports structurally diverse xenobiotics, utilizing the proton electrochemical gradient to confer drug resistance on Pseudomonas aeruginosa. The MexB subunit traverses the inner membrane 12 times and has two, two, and one charged residues in putative transmembrane segments 2 (TMS-2), TMS-4, and TMS-10, respectively. All five residues were mutated, and MexB function was evaluated by determining the MICs of antibiotics and fluorescent dye efflux. Replacement of Lys342 with Ala, Arg, or Glu and Glu346 with Ala, Gln, or Asp in TMS-2 did not have a discernible effect. Ala, Asn, or Lys substitution for Asp407 in TMS-4, which is well conserved, led to loss of activity. Moreover, a mutant with Glu in place of Asp407 exhibited only marginal function, suggesting that the length of the side chain at this position is important. The only replacements for Asp408 in TMS-4 or Lys939 in TMS-10 that exhibited significant function were Glu and Arg, respectively, suggesting that the native charge at these positions is required. In addition, double neutral mutants or mutants in which the charged residues Asp407 and Lys939 or Asp408 and Lys939 were interchanged completely lost function. An Asp408-->Glu/Lys939-->Arg mutant retained significant activity, while an Asp407-->Glu/Lys939-->Arg mutant exhibited only marginal function. An Asp407-->Glu/Asp408-->Glu double mutant also lost activity, but significant function was restored by replacing Lys939 with Arg (Asp407-->Glu/Asp408-->Glu/Lys939-->Arg). Taken as a whole, the findings indicate that Asp407, Asp408, and Lys939 are functionally important and raise the possibility that Asp407, Asp408, and Lys939 may form a charge network between TMS-4 and TMS-10 that is important for proton translocation and/or energy coupling.

Molecular mechanism of MexR-mediated regulation of MexAB-OprM efflux pump expression in Pseudomonas aeruginosa

To elucidate the molecular mechanism by which the MexR repressor regulates expression of the MexAB-OprM efflux pump, we investigated MexR and the mexR-mexA intergenic DNA (mexOP) interaction, and transcription of the mexA-lacZ reporter gene containing different lengths of mexOP. Homogeneously purified MexR bound specifically to mexOP proximal to mexR. The mexOP-lacZ fusion gene lacking the region immediately proximal to mexR showed minimum enzyme activity, thereby suggesting that a promoter element is located between mexR and the MexR-binding sites. These observations explain the mechanism of self-regulation of mexR expression as well as low and elevated expression of MexAB-OprM in the wild-type strain and nalB-type mutant, respectively, of Pseudomonas aeruginosa.

Mutational analysis of the OprM outer membrane component of the MexA-MexB-OprM multidrug efflux system of Pseudomonas aeruginosa

OprM is the outer membrane component of the MexA-MexB-OprM efflux system of Pseudomonas aeruginosa. Multiple-sequence alignment of this protein and its homologues identified several regions of high sequence conservation that were targeted for site-directed mutagenesis. Of several deletions which were stably expressed, two, spanning residues G199 to A209 and A278 to N286 of the mature protein, were unable to restore antibiotic resistance in OprM-deficient strains of P. aeruginosa. Still, mutation of several conserved residues within these regions did not adversely affect OprM function. Mutation of the highly conserved N-terminal cysteine residue, site of acylation of this presumed lipoprotein, also did not affect expression or activity of OprM. Similarly, substitution of the OprM lipoprotein signal, including consensus lipoprotein box, with the signal peptide of OprF, the major porin of this organism, failed to impact on expression or activity. Apparently, acylation is not essential for OprM function. A large deletion at the N terminus, from A12 to R98, compromised OprM expression to some extent, although the deletion derivative did retain some activity. Several deletions failed to yield an OprM protein, including one lacking an absolutely conserved LGGGW sequence near the C terminus of the protein. The pattern of permissive and nonpermissive deletions was used to test a topology model for OprM based on the recently published crystal structure of the OprM homologue, TolC (V. Koronakis, A. Sharff, E. Koronakis, B. Luisi, and C. Hughes, Nature 405:914-919, 2000). The data are consistent with OprM monomer existing as a substantially periplasmic protein with four outer membrane-spanning regions.

Localization of the outer membrane subunit OprM of resistance-nodulation-cell division family multicomponent efflux pump in Pseudomonas aeruginosa

The outer membrane subunit OprM of the multicomponent efflux pump of Pseudomonas aeruginosa has been assumed to form a transmembrane xenobiotic exit channel across the outer membrane. We challenged this hypothesis to clarify the underlying ambiguity by manipulating the amino-terminal signal sequence of the OprM protein of the MexAB-OprM efflux pump in P. aeruginosa. [(3)H]Palmitate uptake experiments revealed that OprM is a lipoprotein. The following lines of evidence unequivocally established that the OprM protein functioned at the periplasmic space. (i) The OprM protein, in which a signal sequence including Cys-18 was replaced with that of periplasmic azurin, appeared in the periplasmic space but not in the outer membrane fraction, and the protein fully functioned as the pump subunit. (ii) The hybrid OprM containing the N-terminal transmembrane segment of the inner membrane protein, MexF, appeared exclusively in the inner membrane fraction. The hybrid protein containing 186 or 331 amino acid residues of MexF was fully active for the antibiotic extrusion, but a 42-residue protein was totally inactive. (iii) The mutant OprM, in which the N-terminal cysteine residue was replaced with another amino acid, appeared unmodified with fatty acid and was fractionated in both the periplasmic space and the inner membrane fraction but not in the outer membrane fraction. The Cys-18-modified OprM functioned for the antibiotic extrusion indistinguishably from that in the wild-type strain. We concluded, based on these results, that the OprM protein was anchored in the outer membrane via fatty acid(s) attached to the N-terminal cysteine residue and that the entire polypeptide moiety was exposed to the periplasmic space.