Determination of ion permeability through the channels made of porins from the outer membrane of Salmonella typhimurium in lipid bilayer membranes

Control of subunit stoichiometry in single-chain MspA nanopores

Transmembrane protein channels enable fast and highly sensitive detection of single molecules. Nanopore sequencing of DNA was achieved using an engineered Mycobacterium smegmatis porin A (MspA) in combination with a motor enzyme. Due to its favorable channel geometry, the octameric MspA pore exhibits the highest current level compared with other pore proteins. To date, MspA is the only protein nanopore with a published record of DNA sequencing. While widely used in commercial devices, nanopore sequencing of DNA suffers from significant base-calling errors due to stochastic events of the complex DNA-motor-pore combination and the contribution of up to five nucleotides to the signal at each position. Different mutations in specific subunits of a pore protein offer an enormous potential to improve nucleotide resolution and sequencing accuracy. However, individual subunits of MspA and other oligomeric protein pores are randomly assembled in vivo and in vitro, preventing the efficient production of designed pores with different subunit mutations. In this study, we converted octameric MspA into a single-chain pore by connecting eight subunits using peptide linkers. Lipid bilayer experiments demonstrated that single-chain MspA formed membrane-spanning channels and discriminated all four nucleotides identical to MspA produced from monomers in DNA hairpin experiments. Single-chain constructs comprising three, five, six, and seven connected subunits assembled to functional channels, demonstrating a remarkable plasticity of MspA to different subunit stoichiometries. Thus, single-chain MspA constitutes a new milestone in the optimization of MspA as a biosensor for DNA sequencing and many other applications by enabling the production of pores with distinct subunit mutations and pore diameters.

Magnesium therapy improves outcome in Streptococcus pneumoniae meningitis by altering pneumolysin pore formation

BACKGROUND AND PURPOSE

Streptococcus pneumoniae is the most common cause of bacterial meningitis in adults and is characterized by high lethality and substantial cognitive disabilities in survivors. Here, we have studied the capacity of an established therapeutic agent, magnesium, to improve survival in pneumococcal meningitis by modulating the neurological effects of the major pneumococcal pathogenic factor, pneumolysin.

EXPERIMENTAL APPROACH

We used mixed primary glial and acute brain slice cultures, pneumolysin injection in infant rats, a mouse meningitis model and complementary approaches such as Western blot, a black lipid bilayer conductance assay and live imaging of primary glial cells.

KEY RESULTS

Treatment with therapeutic concentrations of magnesium chloride (500 mg·kg^-1 in animals and 2 mM in cultures) prevented pneumolysin-induced brain swelling and tissue remodelling both in brain slices and in animal models. In contrast to other divalent ions, which diminish the membrane binding of pneumolysin in non-therapeutic concentrations, magnesium delayed toxin-driven pore formation without affecting its membrane binding or the conductance profile of its pores. Finally, magnesium prolonged the survival and improved clinical condition of mice with pneumococcal meningitis, in the absence of antibiotic treatment.

CONCLUSIONS AND IMPLICATIONS

Magnesium is a well-established and safe therapeutic agent that has demonstrated capacity for attenuating pneumolysin-triggered pathogenic effects on the brain. The improved animal survival and clinical condition in the meningitis model identifies magnesium as a promising candidate for adjunctive treatment of pneumococcal meningitis, together with antibiotic therapy.

A porin-like protein from oral secretions of Spodoptera littoralis larvae induces defense-related early events in plant leaves

Insect herbivory on plants is a complex incident consisting of at least two different aspects, namely mechanical damage and chemical challenge, as feeding insects introduce oral secretions (OS) into the wounded tissue of the attacked plant. Mechanical wounding alone is sufficient to induce a set of defense-related reactions in host plants, but some early events such as membrane potential (Vm) changes and cytosolic Ca²⁺-elevations can be triggered only by herbivores suggesting that OS-derived molecules are involved in those processes. Following an assay-guided purification based on planar lipid bilayer membrane technique in combination with proteomic analysis, a porin-like protein (PLP) of most likely bacterial origin was determined from collected OS of Spodoptera littoralis larvae. PLP exhibited channel-forming activity. Further, early defense-related events in plant-insect interaction were evaluated by using a purified fraction and α-hemolysin (α-HL) as a commercial pore-forming compound. Both up-regulated the calmodulin-like CML42 in Arabidopsis thaliana, which only responds to oral secretion and not to wounding. An elevation of in vivo [Ca²⁺](cyt) was not observed. Because membrane channel formation is a widespread phenomenon in plant-insect interactions, this PLP might represent an example for microbial compounds from the insect gut which are initially involved in plant-insect interactions.

The role of transport mechanisms in mycobacterium tuberculosis drug resistance and tolerance

In the fight against tuberculosis, cell wall permeation of chemotherapeutic agents remains a critical but largely unsolved question. Here we review the major mechanisms of small molecule penetration into and efflux from Mycobacterium tuberculosis and other mycobacteria, and outline how these mechanisms may contribute to the development of phenotypic drug tolerance and induction of drug resistance. M. tuberculosis is intrinsically recalcitrant to small molecule permeation thanks to its thick lipid-rich cell wall. Passive diffusion appears to account for only a fraction of total drug permeation. As in other bacterial species, influx of hydrophilic compounds is facilitated by water-filled open channels, or porins, spanning the cell wall. However, the diversity and density of M. tuberculosis porins appears lower than in enterobacteria. Besides, physiological adaptations brought about by unfavorable conditions are thought to reduce the efficacy of porins. While intracellular accumulation of selected drug classes supports the existence of hypothesized active drug influx transporters, efflux pumps contribute to the drug resistant phenotype through their natural abundance and diversity, as well as their highly inducible expression. Modulation of efflux transporter expression has been observed in phagocytosed, non-replicating persistent and multi-drug resistant bacilli. Altogether, M. tuberculosis has evolved both intrinsic properties and acquired mechanisms to increase its level of tolerance towards xenobiotic substances, by preventing or minimizing their entry. Understanding these adaptation mechanisms is critical to counteract the natural mechanisms of defense against toxic compounds and develop new classes of chemotherapeutic agents that positively exploit the influx and efflux pathways of mycobacteria.

Extracellular calcium reduction strongly increases the lytic capacity of pneumolysin from streptococcus pneumoniae in brain tissue

BACKGROUND

Streptococcus pneumoniae causes serious diseases such as pneumonia and meningitis. Its major pathogenic factor is the cholesterol-dependent cytolysin pneumolysin, which produces lytic pores at high concentrations. At low concentrations, it has other effects, including induction of apoptosis. Many cellular effects of pneumolysin appear to be calcium dependent.

METHODS

Live imaging of primary mouse astroglia exposed to sublytic amounts of pneumolysin at various concentrations of extracellular calcium was used to measure changes in cellular permeability (as judged by lactate dehydrogenase release and propidium iodide chromatin staining). Individual pore properties were analyzed by conductance across artificial lipid bilayer. Tissue toxicity was studied in continuously oxygenated acute brain slices.

RESULTS

The reduction of extracellular calcium increased the lytic capacity of the toxin due to increased membrane binding. Reduction of calcium did not influence the conductance properties of individual toxin pores. In acute cortical brain slices, the reduction of extracellular calcium from 2 to 1 mM conferred lytic activity to pathophysiologically relevant nonlytic concentrations of pneumolysin.

CONCLUSIONS

Reduction of extracellular calcium strongly enhanced the lytic capacity of pneumolysin due to increased membrane binding. Thus, extracellular calcium concentration should be considered as a factor of primary importance for the course of pneumococcal meningitis.

A protein important for antimicrobial peptide resistance, YdeI/OmdA, is in the periplasm and interacts with OmpD/NmpC

Antimicrobial peptides (AMPs) kill or prevent the growth of microbes. AMPs are made by virtually all single and multicellular organisms and are encountered by bacteria in diverse environments, including within a host. Bacteria use sensor-kinase systems to respond to AMPs or damage caused by AMPs. Salmonella enterica deploys at least three different sensor-kinase systems to modify gene expression in the presence of AMPs: PhoP-PhoQ, PmrA-PmrB, and RcsB-RcsC-RcsD. The ydeI gene is regulated by the RcsB-RcsC-RcsD pathway and encodes a 14-kDa predicted oligosaccharide/oligonucleotide binding-fold (OB-fold) protein important for polymyxin B resistance in broth and also for virulence in mice. We report here that ydeI is additionally regulated by the PhoP-PhoQ and PmrA-PmrB sensor-kinase systems, which confer resistance to cationic AMPs by modifying lipopolysaccharide (LPS). ydeI, however, is not important for known LPS modifications. Two independent biochemical methods found that YdeI copurifies with OmpD/NmpC, a member of the trimeric beta-barrel outer membrane general porin family. Genetic analysis indicates that ompD contributes to polymyxin B resistance, and both ydeI and ompD are important for resistance to cathelicidin antimicrobial peptide, a mouse AMP produced by multiple cell types and expressed in the gut. YdeI localizes to the periplasm, where it could interact with OmpD. A second predicted periplasmic OB-fold protein, YgiW, and OmpF, another general porin, also contribute to polymyxin B resistance. Collectively, the data suggest that periplasmic OB-fold proteins can interact with porins to increase bacterial resistance to AMPs.

The porin OmpD from nontyphoidal Salmonella is a key target for a protective B1b cell antibody response

Invasive nontyphoidal Salmonella (NTS), including Salmonella typhimurium (STm), are major yet poorly-recognized killers of infants in sub-Saharan Africa. Death in these children is usually associated with bacteremia, commonly in the absence of gastrointestinal symptoms. Evidence from humans and animal studies suggest that severe infection and bacteremia occur when specific Ab is lacking. Understanding how Ab responses to Salmonella are regulated will help develop vaccines against these devastating infections. STm induces atypical Ab responses characterized by prominent, accelerated, extrafollicular T-independent (TI) Ab against a range of surface antigens. These responses develop without concomitant germinal centers, which only appear as infection resolves. Here, we show STm rapidly induces a population of TI B220(+)CD5(-) B1b cells during infection and TI Ab from B1b cells targets the outer membrane protein (Omp) porins OmpC, OmpD and OmpF but not flagellin. When porins are used as immunogens they can ablate bacteremia and provide equivalent protection against STm as killed bacterial vaccine and this is wholly B cell-dependent. Furthermore Ab from porin-immunized chimeras, that have B1b cells, is sufficient to impair infection. Infecting with porin-deficient bacteria identifies OmpD, a protein absent from Salmonella Typhi, as a key target of Ab in these infections. This work broadens the recognized repertoire of TI protein antigens and highlights the importance of Ab from different B cell subsets in controlling STm infection. OmpD is a strong candidate vaccine target and may, in part, explain the lack of cross-protection between Salmonella Typhi and STm infections.

Cholesterol-dependent pore formation of Clostridium difficile toxin A

The large clostridial cytotoxins toxin A and toxin B from Clostridium difficile are major virulence factors known to cause antibiotic-associated diarrhea and pseudomembranous colitis. Both toxins mono-glucosylate and thereby inactivate small GTPases of the Rho family. Recently, it was reported that toxin B, but not toxin A, induces pore formation in membranes of target cells under acidic conditions. Here, we reassessed data on pore formation of toxin A in cells derived from human colon carcinoma. Treatment of 86Rb+-loaded cells with native or recombinant toxin A resulted in an increased efflux of radioactive cations induced by an acidic pulse. The efficacy of pore formation was dependent on membrane cholesterol, since cholesterol depletion of membranes with methyl-beta-cyclodextrin inhibited 86Rb+ efflux, and cholesterol repletion reconstituted pore-forming activity of toxin A. Similar results were obtained with toxin B. Consistently, methyl-beta-cyclodextrin treatment delayed intoxication of cells in a concentration-dependent manner. In black lipid membranes, toxin A induced ion-permeable pores only in cholesterol containing bilayers and at low pH. In contrast, release of glycosylphosphatidylinositol-anchored structures by phosphatidylinositol specific phospholipase C treatment did not reduce cell sensitivity toward toxins A and B. These data indicate that in colonic cells toxin A induces pore formation in an acidic environment (e.g. endosomes) similar to that reported for toxin B and suggest that pore formation by clostridial glucosylating toxins depends on the presence of cholesterol.

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.

Porin OmpP2 of Haemophilus influenzae shows specificity for nicotinamide-derived nucleotide substrates

Haemophilus influenzae has an absolute requirement for NAD (factor V) because it lacks all biosynthetic enzymes necessary for de novo synthesis of that cofactor. Therefore, growth in vitro requires the presence of NAD itself, NMN, or nicotinamide riboside (NR). To address uptake abilities of these compounds, we investigated outer membrane proteins. By analyzing ompP2 knockout mutants, we found that NAD and NMN uptake was prevented, whereas NR uptake was not. Through investigation of the properties of purified OmpP2 in artificial lipid membrane systems, the substrate specificity of OmpP2 for NAD and NMN was determined, with KS values of approximately 8 and 4mm, respectively, in 0.1 m KCl, whereas no interaction was detected for the nucleoside NR and other purine or pyrimidine nucleotide or nucleoside species. Based on our analysis, we assume that an intrinsic binding site within OmpP2 exists that facilitates diffusion of these compounds across the outer membrane, recognizing carbonyl and exposed phosphate groups. Because OmpP2 was formerly described as a general diffusion porin, an additional property of acting as a facilitator for nicotinamide-based nucleotide transport may have evolved to support and optimize utilization of the essential cofactor sources NAD and NMN in H. influenzae.

Photodynamic activation of ion transport through lipid membranes and its correlation with an increased dielectric constant of the membrane

Illumination of biological membranes with visible light in the presence of membrane-active sensitizers (e.g. rose bengal) is known to inactivate transport proteins such as ion channels and ion pumps. In some cases, however, illumination gives rise to an activation of transport. This is shown here for ion channels formed by alamethicin in lipid membranes, and for porin channels, which were isolated from the outer membrane of E. coli (OmpC) and from the outer membrane of mitochondria (VDAC) and were reconstituted in lipid membranes. An activation (in the form of an increased conductance) was also observed in the presence of the cation carriers valinomycin and nonactin. The activation phenomena were only present, if the membranes were made from lipids containing unsaturated double bonds. Activation was reduced in the presence of the antioxidant vitamin E. We suggest that the activation of the different transport systems has a common physical basis, namely an increase of the dielectric constant, epsilon(m), of the membrane interior by the presence of polar oxidation products of photodynamically induced lipid peroxidation. Experimental evidence for an enhanced dielectric constant was obtained from the finding of a light-induced increase of the membrane capacitance in the presence of rose bengal.

The majorThiobacillus ferrooxidansouter membrane protein forms low conductance ion channels in planar lipid bilayers

A protein isolated and purified from the outer membrane of the acidophilic, chemolithotrophic bacterium, Thiobacillus ferrooxidans with an oligomeric molecular weight of 90,000 Da (p90) was incorporated into phosphatidylethanolamine planar lipid bilayers. The protein formed slightly anionic channels in KCl solutions, with a conductance of 25 pS in 100 mM KCl. The current-voltage relationship was linear between +/- 60 mV, and the conductance was a saturating function of the salt concentration. These channels fluctuated from a single open to closed state at low potentials, but present flickering activity at higher potentials.

The C-terminal domain of the Bordetella pertussis autotransporter BrkA forms a pore in lipid bilayer membranes

BrkA is a 103-kDa outer membrane protein of Bordetella pertussis that mediates resistance to antibody-dependent killing by complement. It is proteolytically processed into a 73-kDa N-terminal domain and a 30-kDa C-terminal domain as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. BrkA is also a member of the autotransporter family of proteins. Translocation of the N-terminal domain of the protein across the outer membrane is hypothesized to occur through a pore formed by the C-terminal domain. To test this hypothesis, we performed black lipid bilayer experiments with purified recombinant protein. The BrkA C-terminal protein showed an average single-channel conductance of 3.0 nS in 1 M KCl. This result strongly suggests that the C-terminal autotransporter domain of BrkA is indeed capable of forming a pore.

The gene bglH present in the bgl operon of Escherichia coli, responsible for uptake and fermentation of beta-glucosides encodes for a carbohydrate-specific outer membrane porin

The cryptic gene bglH from the Escherichia coli chromosome was cloned into a tacOP-driven expression vector. The resulting plasmid was transferred into the porin-deficient E. coli strain KS26 and the protein was expressed by addition of IPTG. The BglH protein was localized in the outer membrane. It was purified to homogeneity using standard methods. Reconstitution experiments with lipid bilayer membranes defined BglH as a channel-forming component, i.e. it is an outer membrane porin. The single-channel conductance of BglH (560 pS in 1 M KCl) was only one-third of that of the general diffusion porins of E. coli outer membrane. The presence of carbohydrates in the aqueous phase led to a dose-dependent block of ion transport through the channel, similar to that found for LamB (maltoporin) of E. coli and Salmonella typhimurium, which means that BglH is a porin specific for the uptake of carbohydrates. The binding constants of a variety of different carbohydrates were calculated from titration experiments of the BglH-induced membrane conductance. The tightest binding was observed with the aromatic beta-D-glucosides arbutin and salicin, and with gentibiose and cellobiose. Binding of maltooligosaccharides to BglH was in contrast to their binding to LamB in that it was much weaker, indicating that the binding site of BglH for carbohydrates is different from that of LamB (maltoporin). The kinetics of cellopentaose binding to BglH was investigated using the carbohydrate-induced current noise and was compared with that of cellopentaose binding to LamB (maltoporin) and ScrY (sucroseporin).

The membrane of leaf peroxisomes contains a porin-like channel

Spinach leaf peroxisomes were purified by Percoll density gradient centrifugation. After several freeze-thaw cycles, the peroxisomal membranes were separated from the matrix enzymes by sucrose density gradient centrifugation. The purity of the peroxisomal membranes was checked by measuring the activities of marker enzymes and by using antibodies. Lipid bilayer membrane experiments with the purified peroxisomal membranes, solubilized with a detergent, demonstrated that the membranes contain a channel-forming component, which may represent the major permeability pathway of these membranes. Control experiments with membranes of other cell organelles showed that the peroxisomal channel was not caused by the contamination of the peroxisomes with mitochondria or chloroplasts. The peroxisomal channel had a comparatively small single channel conductance of 350 pS in 1 M KCl as compared with channels from other cell organelles. The channel is slightly anion selective, which is in accordance with its physiological function. The single channel conductance was found to be only moderately dependent on the salt concentration in the aqueous phase. This may be explained by the presence of positive point net charges in or near the channel or by the presence of a saturable binding site inside the channel. The possible role of the channel in peroxisomal metabolism is discussed.

Noise analysis of ion current through the open and the sugar-induced closed state of the LamB channel of Escherichia coli outer membrane: evaluation of the sugar binding kinetics to the channel interior

LamB, a sugar-specific channel of Escherichia coli outer membrane was reconstituted into lipid bilayer membranes and the current noise was investigated using fast Fourier transformation. The current noise through the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. The spectral density of the noise of the open LamB channels was a quadratic function of the applied voltage. Its magnitude was not correlated to the number of channels in the lipid bilayer membrane. Upon addition of sugars to the aqueous phase the current decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically, which indicated interaction of the sugars with the binding site inside the channel. The frequency dependence of the spectral density was of Lorentzian type, although the power of its frequency dependence was not identical to -2. Analysis of the power density spectra using a previously proposed simple model (Benz, R., A. Schmid, and G. H. Vos-Scheperkeuter. 1987. J. Membr. Biol. 100: 12-29), allowed the evaluation of the on- and the off-rate constants for the maltopentaose binding to the binding site inside the LamB channels. This means also that the maltopentaose flux through the LamB channel could be estimated by assuming a simple one-site, two-barrier model for the sugar transport from the results of the noise analysis.

Topology of PhoE porin: the 'eyelet' region

A model for the topology of the PhoE porin has been proposed according to which the polypeptide traverses the outer membrane sixteen times mostly as amphipathic beta-sheets, thereby exposing eight loops at the cell surface. Until now, no evidence has been obtained for the surface exposure of the third loop. Recently, the structure of porin of Rhodobacter capsulatus has been determined. The proposed model of PhoE is very similar to the structure of the R. capsulatus porin, which has an 'eyelet' region, extending into the interior of the pore. The proposed third external loop of PhoE might form a similar 'eyelet' region. To determine the location of the predicted third external loop of PhoE, multiple copies of an oligonucleotide linker encoding an antigenic determinant of VP1 protein of foot-and-mouth disease virus (FMDV) were inserted. All hybrid proteins were properly inserted in the outer membrane. The monoclonal antibody MA11, directed against the linear FMDV epitope, was able to bind only to intact cells expressing a hybrid PhoE protein with at least three copies of the FMDV epitope present. Antibiotic sensitivity tests and single-channel conductance measurements revealed that the insertions influenced the channel size. These results are consistent with a location of the third loop of PhoE within the pore channel.

The sugar-specific outer membrane channel ScrY contains functional characteristics of general diffusion pores and substrate-specific porins

Escherichia coli K-12 strain PS1-28-37 carries the multicopy plasmid pPSO28-37 containing a DNA fragment coding for two of the proteins that enable bacteria to utilize sucrose as sole carbon source. One of the different gene products of the plasmid is the outer membrane protein, ScrY. This protein was isolated and purified by chromatography across a gel filtration column. Reconstitution experiments with lipid bilayer membrane demonstrated that ScrY formed ion-permeable channels with properties very similar to those of general diffusion pores of enteric bacteria. The presence of sugars in the aqueous phase led to a dose-dependent block of ion transport through the channel, like the situation found with LamB (maltoporin) of Escherichia coli and Salmonella typhimurium. The binding constants of a variety of different sugars were determined. The stability constant for malto-oligosaccharide binding increased with increasing numbers of glucose residues. Disaccharides generally had a larger binding constant than monosaccharides. The binding of different sugars to ScrY and LamB of E. coli is discussed with respect to the kinetics of sugar movement through the channel.

Channel fluctuations induced by membrane attack complex C5B-9

The assembly of complement (C) components C5b-9 in membranes results in the formation of transmembrane lesions. The C9 component has been shown to be mainly responsible for formation of the ultrastructurally visible tubules associated with C5b-9 complexes. Several studies have disputed the role of C9 polymerization in C-mediated cytolysis on the grounds that C5b-9 lyses cells in the absence of tubular formation. Here, C5b-9 complexes were reconstituted into high-impedance planar lipid bilayers and shown to form channels which are heterogenous in size. The smallest channels had unitary conductances of 15 picoSiemens (pS) in 0.1 M NaCl. The closing of these channels showed voltage-dependence at membrane potentials exceeding 40 mV. These channels were more cation-selective, with K+ ions being favored over Na+. The 15-pS channels described here are much smaller than the channels attributed previously to either C5b-9 or polymerized C9 complexes but resemble channels formed by the C9b fragment, which does not polymerize into tubules. These results indicate that C5b-9 complexes are capable of damaging membranes by forming initially small ion channels which then aggregate in the membrane to form tubular lesions with much larger conductances. Like C5b-9, C5b-8 also increased membrane permeability. However, this increase in membrane conductance could not be resolved into single channels, suggesting that C5b-8 may induce membrane leakiness by perturbing the packing of membrane lipids, whereas addition of C9 results in authentic production of ion channels.

LamB (maltoporin) of Salmonella typhimurium: isolation, purification and comparison of sugar binding with LamB of Escherichia coli

LamB (maltoporin) of Salmonella typhimurium was found to be more strongly associated with the murein than OmpF. It was purified in one step using a hydroxyapatite (HTP) column. Reconstitution of the pure protein with lipid bilayer membrane showed that LamB of S. typhimurium formed small ion-permeable channels with a single channel conductance of about 90 pS in 1 M KCl and some preference for cations over anions. The conductance concentration curve was linear, which suggested that LamB of S. typhimurium does not contain any binding site for ions. Pore conductance was completely inhibited by the addition of 20 mM maltotriose. Titration of the LamB-induced membrane conductance with different sugars, including all members of the maltooligosaccharide series up to seven glucose residues, suggested that the channel contains, like LamB (maltoporin) of Escherichia coli, a binding site for sugars. The binding constant of sugars of the maltooligosaccharide series increased with increasing number of glucose residues up to five (saturated). Small sugars had a higher stability constant for sugar binding relative to LamB of E. coli. The advantage of a binding site inside a specific porin for the permeation of solutes is discussed with respect to the properties of a general diffusion porin.

Specific immunoglobulin response in mice immunized with porins and challenged with Salmonella typhi

Specific antiporin antibody (IgG, IgM, and IgA) response was studied in control, infected, immunized-infected, and immunized mice. The activity of specific IgG immunoglobulins was found to be the highest in immunized and immunized-infected groups in which 87.5% protection had been observed by laboratory potency test in mice; the next-highest activities were those of IgM and IgA immunoglobulins. However, in the infected group the activity of specific IgM and IgG immunoglobulins was almost similar.

How electrolyte shielding influences the electrical potential in transmembrane ion channels

The electrical potential due to fixed charge distributions is strongly altered in the vicinity of a membrane and notably dependent on aqueous electrolyte concentration. We present an efficient way to solve the nonlinear Poisson-Boltzmann equation applicable to general cylindrically symmetric dielectric geometries. It generalizes Gouy-Chapman theory to systems containing transmembrane channels. The method is applied to three channel systems: gramicidin, gap junction, and porin. We find that for a long, narrow channel such as gramicidin concentration variation has little influence on the electrical image barrier to ion permeation. However, electrolyte shielding reduces the image induced contribution to the energy required for multiple occupancy. In addition, the presence of electrolyte significantly affects the voltage profile due to an applied potential, substantially compressing the electric field to the immediate vicinity of the pore itself. In the large diameter channels, where bulk electrolyte may be assumed to enter the pore, the electrolyte greatly reduces the image barrier to ion permeation. At physiological ionic strengths this barrier is negligible and the channel may be readily multiply occupied. At all ionic strengths considered (l greater than 0.005 M) the image barrier saturates rapidly and is essentially constant more than one channel radius from the entrance to the pore. At lower ionic strengths (l less than 0.016 M) there are noticeable (greater than 20 mV) energy penalties associated with multiple occupancy.

Characterization of the nucleoside-binding site inside the Tsx channel of Escherichia coli outer membrane. Reconstitution experiments with lipid bilayer membranes

Reconstitution of purified Tsx protein from Escherichia coli into lipid bilayer membranes showed that Tsx formed small ion-permeable channels with a single-channel conductance of 10 pS in 1 M KCl. The dependence of conductance versus salt concentration was linear, suggesting that Tsx has no binding site for ions. Conductance was inhibited by the addition of 20 mM adenosine. Titration of the Tsx-mediated membrane conductance with different solutes including free bases, nucleosides, and deoxynucleosides suggested that the channel contains a binding site for nucleosides but not for sugars or amino acids, and binding increased in the following order: free base, nucleoside, and deoxynucleoside. Among the five nucleosides the stability constant for the binding increased in the order of cytidine, guanosine, uridine, adenosine, and thymidine. Control experiments revealed that the binding of the nucleosides is independent of ion concentration in the aqueous phase, i.e. there was no competition between nucleosides and ions for the binding site inside the channel. The binding of the solutes to the channel interior can be explained by a one-site two-barrier model for the Tsx channel. The advantage of a binding site inside a specific porin for the permeation of solutes is discussed with respect to the properties of a general diffusion pore.

Pore formation by pho-controlled outer-membrane proteins of various Enterobacteriaceae in lipid bilayers

The structural genes of the PhoE porins of Klebsiella pneumonia, Enterobacter cloacae and Escherichia coli C, cloned in multicopy plasmids, were transfered into a porin-deficient E. coli K-12 strain, which was constitutive for the pho regulon, and the PhoE porins were isolated and purified. PhoE of Salmonella typhimurium could not be cloned but was isolated from a pho-constitutive strain. Reconstitution experiments with artificial lipid bilayer membranes showed that the different PhoE proteins formed pores exhibiting a single-channel conductance of about 200 pS at 0.1 M KCl. All PhoE porins formed anion-selective channels in KCl at neutral pH. The degree of the selectivity was dependent on the PhoE species. The different PhoE porins formed general diffusion pores similar to the general porins but exhibited a considerable advantage for the permeation of phosphate through the outer membrane as compared to the constitutive OmpC and OmpF porins of E. coli K-12.

Insertion mutagenesis on a cell-surface-exposed region of outer membrane protein PhoE of Escherichia coli K-12

Amino acid residue arginine-158 of the outer membrane protein PhoE of Escherichia coli K-12 has been shown to be cell-surface-exposed [Korteland et al. (1985) Eur. J. Biochem. 152, 691-697]. To study the effects of small insertions in this region of the protein on its biogenesis and characteristics, a unique restriction site was created by site-directed mutagenesis in a plasmid carrying the phoE gene and oligonucleotides of 12-74 bp were inserted. The insertions did not interfere with incorporation into the outer membrane since (a) several monoclonal antibodies, directed against the cell-surface-exposed part of PhoE protein, bound to whole cells producing the altered proteins and (b) the proteins formed functional pores for the uptake of beta-lactam antibiotics. The binding of one monoclonal antibody and of the PhoE-specific phages TC45 and TC45hrN3 was disturbed by the insertions, showing that this region of the protein is immunogenic and is involved in the binding of both of these phages. The functioning of the mutant pores was characterized both in vivo by studying the uptake of beta-lactam antibiotics and in vitro after the reconstitution of the proteins in black lipid films. The pore characteristics changed depending on the nature of the inserted amino acids. Addition of a negatively charged amino acid resulted in decreased anion-selectivity, whereas insertion of a positive charge and deletion of a negative charge had only a small influence.

Mechanism of ion transport through the anion-selective channel of the Pseudomonas aeruginosa outer membrane

Protein P trimers isolated and purified from Pseudomonas aeruginosa outer membrane were reconstituted in planar lipid bilayer membranes from diphytanoyl phosphatidylcholine. The protein trimers formed highly anion-specific channels with an average single channel conductance of 160 pS in 0.1 M Cl solution. A variety of different nonvalent anions were found to be permeable through the channel, which suggests a channel diameter between 0.5 and 0.7 nm. The selectivity for the halides followed the Eisenman sequence AVI (without At-). The ion transport through the protein P channel could be explained reasonably well by a one-site, two-barrier model. The stability constant of the binding of Cl- to the site was 20 M-1 at neutral pH. The binding of anions to the site was pH dependent, which suggested that several charges are involved in the closely spaced selectivity filter. Permeability ratios for different anions as calculated from bi-ionic potentials showed agreement with corresponding ratios of single channel conductances. The protein P channels were not voltage-gated and had lifetimes of the order of several minutes. The current-voltage curves were linear for membrane potentials up to 150 mV, which suggested that Nernst-Planck-type barriers rather than Eyring barriers were involved in the movement of anions through the protein P channel.

Large cation-selective pores from rat liver peroxisomal membranes incorporated to planar lipid bilayers

Fusion of a highly purified fraction of rat liver peroxisomal membranes to planar lipid bilayers incorporates large, cation-selective voltage-dependent pores. The PK/PCl ratio of these pores, estimated in KCl gradients, is close to 4. The pores display several conductance states and spend most of the time open at voltages near 0 mV, closing at more positive and negative voltages. At voltages near 0 mV the most frequent open state has a conductance of 2.4 nS in 0.3 M KCl. At voltages more positive and more negative than 10 mV the most frequent open state displays a conductance of 1.2 nS in 0.3 M KCl. With these results pore diameters of 3 and 1.5 nm, respectively, can be estimated. We suggest that these pores might account for the unusually high permeability of peroxisomes to low molecular weight solutes. Fusion also incorporates a perfectly anion-selective, two-open states channel with conductances of 50 and 100 pS in 0.1 M KCl.

Pore formation by the mitochondrial porin of rat brain in lipid bilayer membranes

The porin of the outer membrane of rat-brain mitochondria was isolated and purified. The protein showed a single band of apparent Mr 35,500 on dodecyl sulfate-containing polyacrylamide gels. The incorporation of rat-brain porin into artificial lipid bilayer membranes showed that it is able to form pores with an average single-channel conductance of 400 pS in 0.1 M KCI. The pores were found to be voltage-dependent and switched to substrates at higher transmembrane potentials. The voltage-dependence of the rat brain pore was considerably smaller than that of the other known eukaryotic porins. The possible role of the rat-brain porin in the regulation of transport process across the outer mitochondrial membrane is discussed.

Ionic channels formed by Staphylococcus aureus alpha-toxin: voltage-dependent inhibition by divalent and trivalent cations

The interaction of Staphylococcus aureus alpha-toxin with planar lipid membranes results in the formation of ionic channels whose conductance can be directly measured in voltage-clamp experiments. Single-channel conductance depends linearly on the solution conductivity suggesting that the pores are filled with aqueous solution; a rough diameter of 11.4 +/- 0.4 A can be estimated for the pore. The conductance depends asymmetrically on voltage and it is slightly anion selective at pH 7.0, which implies that the channels are asymmetrically oriented into the bilayer and that ion motion is restricted at least in a region of the pore. The pores are usually open in a KCl solution but undergo a dose- and voltage-dependent inactivation in the presence of di- and trivalent cations, which is mediated by open-closed fluctuations at the single-channel level. Hill plots indicate that each channel can bind two to three inactivating cations. The inhibiting efficiency follows the sequence Zn2+ greater than Tb3+ greater than Ca2+ greater than Mg2+ greater than Ba2+, suggesting that carboxyl groups of the protein may be involved in the binding step. A voltage-gated inactivation mechanism is proposed which involves the binding of two polyvalent cations to the channel, one in the open and one in the closed configuration, and which can explain voltage, dose and time dependence of the inactivation.

Antibodies to porin antigens of Salmonella typhi induced during typhoid infection in humans

Immunoglobulin G (IgG)- and IgM-specific antibody titers against Salmonella typhi Ty2 porins have been measured in 30 paired typhoid sera by enzyme-linked immunosorbent assay. These studies have found that IgG serum titers of acute and convalescent sera were 625 and 5,000 times higher, respectively than the control serum titers. The same typhoid sera were titrated with S. typhi Ty2 flagellin and S. typhi lipopolysaccharide. The titers against these antigens were considerably lower than those against the porins. The highest IgM-specific titer has also been found against porins in convalescent-phase sera. However, the largest increase in IgM-specific titer compared with the control group titer was obtained against flagellin during the acute phase of typhoid. The lowest increases in antibody titer were obtained with the IgM-specific anti-lipopolysaccharide in both types of sera. This may be because many normal individuals in endemic areas already have IgM titers against lipopolysaccharide. This study has provided good evidence that porins are excellent antigens and that IgG-specific antiporin titers may be of diagnostic value in typhoid infections in endemic areas.

Native and chemically modified porin channels from Salmonella typhi Ty2 in planar lipid bilayers

Native porins, from Salmonella typhi Ty2 outer membrane, and porins alkylated with pyridoxal phosphate (Plp) were studied in planar lipid bilayers. The conductance of bilayers exposed to native or chemically modified porins increases in discrete jumps. Conductance histograms for native porins displayed two major peaks at 1.7 and 6.7 nS (in 0.5 M KCl). On the other hand, Plp-treated porins exhibited a single major peak at 1 nS. The relation between bilayer conductance and native porin concentration was linear. However, this relation became logarithmic in the presence of modified porins. The results support the notion that alkaline reduction of S. typhi Ty2 porins with Plp dissociates porin channel trimers in a reversible fashion.

Outer-membrane permeability of bacteria

Gram-negative bacteria evolved to survive under the conditions in which a number of hazardous compounds are abundant. The outer membrane which protects the cell interior acts as a barrier against such hazardous agents, yet the cells must incorporate the chemicals that are essential for the cellular activity. The devices that Gram-negative bacteria developed to incorporate such essence are the transmembrane pores. These pores could be subdivided into three categories: (1) pore made of porins has a weak solute selectivity; (2) pore made of lamB protein and tsx proteins hold intermediate solute specificity. and (3) pores for the diffusion of vitamin B12 and ferric ion-chelator complexes have a tight solute specificity. Porins are identified from a number of Gram-negatives and from the outer membrane of mitochondria of various sources. Studies on the diffusion properties of these outer-membrane proteins provided essential information to understand membrane transports.

Properties of the leak permeability induced by a cytotoxic protein from Pseudomonas aeruginosa (PACT) in rat erythrocytes and black lipid membranes

A cytotoxic protein, isolated from Pseudomonas aeruginosa (PACT), was tested on red blood cells of rats and on black lipid membranes for changes of membrane permeability. In rat erythrocytes PACT induces lysis indicative of the formation of a leak permeable to monovalent ions. The dose response curve for the PACT-induced hemolysis demonstrates that the rate of lysis as well as the fraction of lytic cells increases with increasing toxin concentration. Furthermore, the leak pathway discriminates hydrophilic non-electrolytes according to their molecular weight. The findings indicate formation by PACT of a pore with an apparent radius of about 1.2 nm. In pure lipid membranes PACT forms hydrophilic pathways with moderate selectivity for small cations over small anions. The presence of cholesterol is a prerequisite for the occurrence of these PACT-induced permeability changes.

The mechanism of ion selectivity of OmpF-porin pores of Escherichia coli

The OmpF porin from the outer membrane of Escherichia coli acts as a lightly cation-selective pore, allowing the diffusion of small cations and cationic molecules, whose Mr are a little larger than the threshold exclusion limit. To ascertain the mechanism of this cation selectivity, we have examined a possible influence of cationic solutes on the fluorescence emission and the circular dichroic spectrum of tryptophan residues of the porin trimer, searching for conformational change(s). The diffusion of cationic solutes was determined with the native and the amidated porins in the presence or the absence of the effector cations. The following results were obtained. (a) Cations, e.g. spermidine, caused fluorescence quenching in the native trimer, with a half-maximum fluorescence quenching at 11-18 microM. A change in the circular dichroic spectrum was also recorded at around 280 nm. (b) The dissociation constant of spermidine to the native trimer was calculated to be 16 microM as determined by the method of equilibrium dialysis. (c) The cation-caused fluorescence quenching was reversed when the carboxyl groups of the trimer were modified by the amidation reaction, though amidation of the trimer resulted in no significant change in the fluorescence intensity. (d) The diffusion rate of N-benzyloxycarbonyl-glycyl-L-prolyl-L-arginine p-nitroanilide through the native and the amidated porins was lowered in the presence and the absence, respectively, of cations. Both the extent of fluorescence quenching in the presence of cation and the rate of cation diffusion were inversely proportional to the number of amidated carboxyl residues. The relative fluorescence quenching of the porin trimer (the amidated versus the native) in the presence of cations was linearly related to the relative solute diffusion via the porin (the amidated versus the native). These results suggested that cations caused a conformational change in the trimer, resulting in an easier diffusion of the solutes. The results suggested further that a limited number of carboxyl groups in the pore interior are involved in the cation selectivity of OmpF-porin pores.

Characterisation of channels induced in planar bilayer membranes by detergent solubilised Escherichia coli porins

Purified OmpF, OmpC, NmpC, PhoE and Lc (Protein 2) porins from the Escherichia coli outer membrane were incorporated into planar phospholipid bilayer membranes and the permeability properties of the pores studied. Triton X-100 solubilised porin samples showed large and reproducible increases in membrane conductivity composed of discreet single-channel events. The magnitude of the cation selectivity found for the porins was in the order OmpC greater than OmpF greater than NmpC = Lc; PhoE was anion selective. For the cation selective porins the cation/anion permeability ratios in a variety of solutes ranged from 6 to 35. Further information on the internal structure of the porins was obtained by examination of the single-channel conductance and this was used to interpret macroscopic observations and to estimate single-channel diameters. The same porins solubilised in SDS exhibited slight conductance increase with no observable single-channel activity. Use of on-line microcomputer techniques confirmed the ohmic current vs. voltage behaviour for all the single porin channels examined.

Ion selectivity of gram-negative bacterial porins

Twelve different porins from the gram-negative bacteria Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa, and Yersinia pestis were reconstituted into lipid bilayer membranes. Most of the porins, except outer membrane protein P, formed large, water-filled, ion-permeable channels with a single-channel conductance between 1.5 and 6 nS in 1 M KCl. The ions used for probing the pore structure had the same relative mobilities while moving through the porin pore as they did while moving in free solution. Thus the single-channel conductances of the individual porins could be used to estimate the effective channel diameters of these porins, yielding values ranging from 1.0 to 2.0 nm. Zero-current potential measurements in the presence of salt gradients across lipid bilayer membranes containing individual porins gave results that were consistent with the conclusions drawn from the single-channel experiments. For all porins except protein P, the channels exhibited a greater cation selectivity for less mobile anions and a greater anion selectivity for less mobile cations, which again indicated that the ions were moving inside the pores in a fashion similar to their movement in the aqueous phase. Three porins, PhoE and NmpC of E. coli and protein P of P. aeruginosa, formed anion-selective pores. PhoE and NmpC were only weakly anion selective, and their selectivity was dependent on the mobility of the ions. In contrast, cations were unable to enter the selectivity filter of the protein P channel. This resulted in a high anion selectivity for all salts tested in this study. The other porins examined, including all of the known constitutive porins of the four gram-negative bacteria studied, were cation selective with a 3- to 40-fold preference for K+ ions over Cl- ions.

Porin from bacterial and mitochondrial outer membranes

The outer membrane of gram-negative bacteria acts as a molecular filter with defined exclusion limit for hydrophilic substances. The exclusion limit is dependent on the type of bacteria and has for enteric bacteria like Escherichia coli and Salmonella typhimurium a value between 600 and 800 Daltons, whereas molecules with molecular weights up to 6000 can penetrate the outer membrane of Pseudomonas aeruginosa. The molecular sieving properties result from the presence of a class of major proteins called porins which form trimers of identical subunits in the outer membrane. The porin trimers most likely contain only one large but well-defined pore with a diameter between 1.2 and 2 nm. Mitochondria are presumably descendents of gram-negative bacteria. The outer membrane of mitochondria contains in agreement with this hypothesis large pores which are permeable for hydrophilic substances with molecular weights up to 6000. The mitochondrial porins are processed by the cell and have molecular weights around 30,000 Daltons. There exists some evidence that the pore is controlled by electric fields and metabolic processes.

Formation and properties of tetramers of band 3 protein from human erythrocyte membranes in planar lipid bilayers

Lipid bilayer experiments were performed in the presence of solubilized band 3 protein from human red cell membranes. Band 3 protein increased the conductance of the lipid membranes by several orders of magnitude. Membrane conductance was found to be dependent on the fourth power of protein concentration. This shows that four band 3 subunits form an ion permeable pathway in the lipid bilayer membranes. It also shows that, in the membranes, the protein molecules undergo an association equilibrium which involves at least the monomer and the tetramer of the protein, relaxation towards equilibrium being rapid on the time scale of the experiment. The increase in bilayer conductance induced by the band 3 tetramer could be inhibited by pretreatment of the protein with several SH-reagents (pCMB, pCMBS, DTNB) which also inhibit water transport across the human red cell membrane. Other SH-reagents which do not influence water transport (iodoacetamide, N-ethylmaleimide) did not show any influence on the band 3 induced conductance increase. A band 3-mediated exchange of anions comparable to that in the erythrocyte membrane did not occur in the system studied by us. Our results suggest that, in the human erythrocyte membrane, a pore formed by the band 3 tetramer could be the pathway responsible for the protein-mediated part of water transport.

Outer-membrane protein PhoE from Escherichia coli forms anion-selective pores in lipid-bilayer membranes

Porin PhoE of the outer membrane of Escherichia coli was isolated and purified. Reconstitution experiments with lipid bilayer membranes showed that this protein formed pores which had a single channel conductance of 210 pS at 0.1 M KCl. The PhoE pores were obviously not voltage-controlled or regulated. In contrast to pores formed by the OmpF porin from E. coli the PhoE channel was found to be anion-selective at neutral pH. Chloride is about three to ten times more permeable through the pore than alkali ions. On the basis of the observed pH dependence of the permeability ratio of anions and cations, this anionic selectivity is explained by the assumption that the PhoE pore contains an excess of fixed positive charges.

Properties of chemically modified porin from Escherichia coli in lipid bilayer membranes

Purified porin OmpF from Escherichia coli outer membrane was chemically modified by acetylation and succinylation of amino groups and by amidation of the carboxyl groups. Native and chemically modified porins were incorporated into lipid bilayer membranes and the permeability properties of the pores were studied. Acetylation and succinylation of the porin trimers had almost no influence on the single channel conductance in the presence of small cations and anions and the cation selectivity remained essentially unchanged as compared with the native porin. Amidation had also only little influence on the single channel conductance and changed the pore conductance at maximum by less than 50%, whereas the cation selectivity of the porin is completely lost after amidation. The results suggest that the structure of the porin pore remains essentially unchanged after chemical modification of the pores and that their cation selectivity is caused by an excess of negatively charged groups inside the pore and/or on the surface of the protein. Furthermore, it seems very unlikely that the pore contains any positively charged group at neutral pH.

Effects of growth temperature, 47-megadalton plasmid, and calcium deficiency on the outer membrane protein porin and lipopolysaccharide composition of Yersinia pestis EV76

The expression of several virulence determinants of Yersinia pestis is known to be dependent on the in vitro growth temperature. One of these, calcium dependence, is associated with the presence of a 47-megadalton plasmid. We have examined the effects of incubation temperature, calcium in the growth medium, the presence of the 47-megadalton plasmid on the outer membrane protein, and the lipopolysaccharide composition of Y. pestis EV76. When cells were grown at 37 degrees C as opposed to 26 degrees C, a change in lipopolysaccharide composition and a decrease in the amount of an outer membrane protein (protein E) were observed. The lipopolysaccharide obtained from cells incubated at 37 degrees C had a lower proportion of 2 keto-3-deoxyoctanate, a lower phosphate to 2-keto-3-deoxyoctanate ratio, and an increased gel mobility upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis when compared with lipopolysaccharide obtained from cells grown at 26 degrees C. Because of its growth temperature-related abundance, we investigated the nature of protein E. This protein had physical properties similar to those of other enterobacterial porins, including apparent formation of an oligomer on sodium dodecyl sulfate-polyacrylamide gels when solubilized at low temperature, acidic isoelectric point, and strong noncovalent association with the peptidoglycan. Protein E was purified and shown to form an aqueous channel in planar lipid membranes with a conductance of 1.1 nS in 1 M KCl. In addition to growth temperature-related alterations in the lipopolysaccharide and porin components of the outer membrane, the amount of three spots in two-dimensional polyacrylamide gels was shown to be related to the temperature or the presence of calcium during growth. One of these spots was shown to contain residual unmodified portions of two major heat-modifiable proteins which failed to shift to their heat-modified positions on gels, despite solubilization at 100 degrees C for 10 min before electrophoresis. The other two spots were the heat-modified and unmodified forms of another outer membrane protein (J) which did not appear in the isoelectric focusing gel of cells grown at 37 degrees C. It is proposed that the appearance of these spots in two-dimensional analyses is related to the lipopolysaccharide composition of the cells from which the outer membrane is derived and reflects lipopolysaccharide-protein interactions or calcium-protein interactions.