Outer membrane porin proteins F, P, and D1 of Pseudomonas aeruginosa and PhoE of Escherichia coli: chemical cross-linking to reveal native oligomers

Directed Evolution of a Surface-Displayed Artificial Allylic Deallylase Relying on a GFP Reporter Protein

Artificial metalloenzymes (ArMs) combine characteristics of both homogeneous catalysts and enzymes. Merging abiotic and biotic features allows for the implementation of new-to-nature reactions in living organisms. Here, we present the directed evolution of an artificial metalloenzyme based on Escherichia coli surface-displayed streptavidin (SavSD hereafter). Through the binding of a ruthenium-pianostool cofactor to SavSD, an artificial allylic deallylase (ADAse hereafter) is assembled, which displays catalytic activity toward the deprotection of alloc-protected 3-hydroxyaniline. The uncaged aminophenol acts as a gene switch and triggers the overexpression of a fluorescent green fluorescent protein (GFP) reporter protein. This straightforward readout of ADAse activity allowed the simultaneous saturation mutagenesis of two amino acid residues in Sav near the ruthenium cofactor, expediting the screening of 2762 individual clones. A 1.7-fold increase of in vivo activity was observed for SavSD S112T-K121G compared to the wild-type SavSD (wt-SavSD). Finally, the best performing Sav isoforms were purified and tested in vitro (SavPP hereafter). For SavPP S112M-K121A, a total turnover number of 372 was achieved, corresponding to a 5.9-fold increase vs wt-SavPP. To analyze the marked difference in activity observed between the surface-displayed and purified ArMs, the oligomeric state of SavSD was determined. For this purpose, crosslinking experiments of E. coli cells overexpressing SavSD were carried out, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot. The data suggest that SavSD is most likely displayed as a monomer on the surface of E. coli. We hypothesize that the difference between the in vivo and in vitro screening results may reflect the difference in the oligomeric state of SavSD vs soluble SavPP (monomeric vs tetrameric). Accordingly, care should be applied when evolving oligomeric proteins using E. coli surface display.

Porin from Marine Bacterium Marinomonas primoryensis KMM 3633T: Isolation, Physico-Chemical Properties, and Functional Activity

Marinomonas primoryensis KMM 3633^T, extreme living marine bacterium was isolated from a sample of coastal sea ice in the Amursky Bay near Vladivostok, Russia. The goal of our investigation is to study outer membrane channels determining cell permeability. Porin from M. primoryensis KMM 3633^T (MpOmp) has been isolated and characterized. Amino acid analysis and whole genome sequencing were the sources of amino acid data of porin, identified as Porin_4 according to the conservative domain searching. The amino acid composition of MpOmp distinguished by high content of acidic amino acids and low content of sulfur-containing amino acids, but there are no tryptophan residues in its molecule. The native MpOmp existed as a trimer. The reconstitution of MpOmp into black lipid membranes demonstrated its ability to form ion channels whose conductivity depends on the electrolyte concentration. The spatial structure of MpOmp had features typical for the classical gram-negative porins. However, the oligomeric structure of isolated MpOmp was distinguished by very low stability: heat-modified monomer was already observed at 30 °C. The data obtained suggest the stabilizing role of lipids in the natural membrane of marine bacteria in the formation of the oligomeric structure of porin.

In Silico Structure and Sequence Analysis of Bacterial Porins and Specific Diffusion Channels for Hydrophilic Molecules: Conservation, Multimericity and Multifunctionality

Diffusion channels are involved in the selective uptake of nutrients and form the largest outer membrane protein (OMP) family in Gram-negative bacteria. Differences in pore size and amino acid composition contribute to the specificity. Structure-based multiple sequence alignments shed light on the structure-function relations for all eight subclasses. Entropy-variability analysis results are correlated to known structural and functional aspects, such as structural integrity, multimericity, specificity and biological niche adaptation. The high mutation rate in their surface-exposed loops is likely an important mechanism for host immune system evasion. Multiple sequence alignments for each subclass revealed conserved residue positions that are involved in substrate recognition and specificity. An analysis of monomeric protein channels revealed particular sequence patterns of amino acids that were observed in other classes at multimeric interfaces. This adds to the emerging evidence that all members of the family exist in a multimeric state. Our findings are important for understanding the role of members of this family in a wide range of bacterial processes, including bacterial food uptake, survival and adaptation mechanisms.

Structural and functional analyses of the major outer membrane protein of Chlamydia trachomatis

Chlamydia trachomatis is a major pathogen throughout the world, and preventive measures have focused on the production of a vaccine using the major outer membrane protein (MOMP). Here, in elementary bodies and in preparations of the outer membrane, we identified native trimers of the MOMP. The trimers were stable under reducing conditions, although disulfide bonds appear to be present between the monomers of a trimer and between trimers. Cross-linking of the outer membrane complex demonstrated that the MOMP is most likely not in a close spatial relationship with the 60- and 12-kDa cysteine-rich proteins. Extraction of the MOMP from Chlamydia isolates under nondenaturing conditions yielded the trimeric conformation of this protein as shown by cross-linking and analysis by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis with different concentrations of acrylamide. Using circular dichroism spectroscopy, we determined that the trimers were formed mainly of beta-pleated sheet structures in detergent micelles. Using a liposomal swelling assay, the MOMP was found to have porin activity, and the size of the pore was estimated to be approximately 2 nm in diameter. The trimers were found to be stable in SDS at temperatures ranging from 4 to 37 degrees C and over a pH range of 5.0 to 8.0. In addition, the trimers of MOMP were found to be resistant to digestion with trypsin. In conclusion, these results show that the native conformation of the MOMP of C. trachomatis is a trimer with predominantly a beta-sheet structure and porin function.

[Interactions of the Pseudomonas aeruginosa outer membrane proteins with plasma fibronectins. Bacterial adhesin investigation]

Pseudomonas aeruginosa adherence is a complex phenomenon largely mediated by pili involving specific receptor-ligand interactions. Anti-fibronectin antibodies as well as plasmatic fibronectin are able to inhibit P. aeruginosa adherence onto A549 cells showing that matricial fibronectin is an actual receptor for this bacterium. Experiments performed in vitro with human plasmatic fibronectin used as receptor and outer membrane proteins of P. aeruginosa as ligands show the presence of four fibronectin-binding proteins. These proteins with molecular mass of 70 +/- 2, 60 +/- 2, 48 +/- 2 and 36 +/- 1 kDa should be adhesins of P. aeruginosa on epithelial cell matrix in a non-pilus mediated adherence.

Renaturation of recombinant Treponema pallidum rare outer membrane protein 1 into a trimeric, hydrophobic, and porin-active conformation

We have previously observed that while native Treponema pallidum rare outer membrane protein 1 (Tromp1) is hydrophobic and has porin activity, recombinant forms of Tromp1 do not possess these properties. In this study we show that these properties are determined by conformation and can be replicated by proper renaturation of recombinant Tromp1. Native Tromp1, but not the 47-kDa lipoprotein, extracted from whole organisms by using Triton X-114, was found to lose hydrophobicity after treatment in 8 M urea, indicating that Tromp1's hydrophobicity is conformation dependent. Native Tromp1 was purified from 0.1% Triton X-100 extracts of whole organisms by fast-performance liquid chromatography (FPLC) and shown to have porin activity in planar lipid bilayers. Cross-linking studies of purified native Tromp1 with an 11 A cross-linking agent showed oligomeric forms consistent with dimers and trimers. For renaturation studies of recombinant Tromp1 (rTromp1), a 31,109-Da signal-less construct was expressed in Escherichia coli and purified by FPLC. FPLC-purified rTromp1 was denatured in 8 M urea and then renatured in the presence of 0.5% Zwittergent 3,14 during dialysis to remove the urea. Renatured rTromp1 was passed through a Sephacryl S-300 gel exclusion column previously calibrated with known molecular weight standards. While all nonrenatured rTromp1 eluted from the column at approximately the position of the carbonic anhydrase protein standard (29 kDa), all renatured rTromp1 eluted at the position of the phosphorylase b protein standard (97 kDa), suggesting a trimeric conformation. Trimerization was confirmed by using an 11 A cross-linking agent which showed both dimers and trimers similar to that of native Tromp1. Triton X-114 phase separations showed that all of renatured rTromp1, but none of nonrenatured rTromp1, phase separated exclusively into the hydrophobic detergent phase, similar to native Tromp1. Circular dichroism of nonrenatured and renatured rTromp1 showed a marked loss in alpha-helical secondary structure of renatured rTromp1 compared to the nonrenatured form. Finally, renatured rTromp1, but not the nonrenatured form, showed porin activity in planar liquid bilayers. These results demonstrate that proper folding of rTromp1 results in a trimeric, hydrophobic, and porin-active conformation similar to that of the native protein.

Pseudomonas aeruginosa antigens as potential vaccines

Pseudomonas aeruginosa is one of the most important opportunistic bacterial pathogens in humans and animals. This organism is ubiquitous and has high intrinsic resistance to antibiotics due to the low permeability of the outer membrane and the presence of numerous multiple drug efflux pumps. Various cell-associated and secreted antigens of P. aeruginosa have been the subject of vaccine development. Among pseudomonas antigens, the mucoid substance, which is an extracellular slime consisting predominantly of alginate, was found to be heterogenous in terms of size and immunogenicity. High molecular mass alginate components (30-300 kDa) appear to contain conserved epitopes while lower molecular mass alginate components (10-30 kDa) possess conserved epitopes in addition to unique epitopes. Surface-exposed antigens including O-antigens (O-specific polysaccharide of LPS) or H-antigens (flagellar antigens) have been used for serotyping due to their highly immunogenic nature. Chemical structures of repeating units of O-specific polysaccharides have been elucidated and these data allowed the identification of 31 chemotypes of P. aeruginosa. Conserved epitopes among all serotypes of P. aeruginosa are located in the core oligosaccharide and the lipid A region of LPS and immunogens containing these epitopes induce cross-protective immunity in mice against different P. aeruginosa immunotypes. To examine the protective properties of OM proteins, a vaccine containing P. aeruginosa OM proteins of molecular masses ranging from 20 to 100 kDa has been used in pre-clinical and clinical trials. This vaccine was efficacious in animal models against P. aeruginosa challenge and induced high levels of specific antibodies in human volunteers. Plasma from human volunteers containing anti-P. aeruginosa antibodies provided passive protection and helped the recovery of 87% of patients with severe forms of P. aeruginosa infection. Vaccines prepared from P. aeruginosa ribosomes induced protective immunity in mice, but the efficacy of ribosomal vaccines in humans is not yet known. A number of recent studies indicated the potential of some P. aeruginosa antigens that deserve attention as new vaccine candidates. The outer core of LPS was implicated to be a ligand for binding of P. aeruginosa to airway and ocular epithelial cells of animals. However, heterogeneity exists in this outer core region among different serotypes. Epitopes in the inner core are highly conserved and it has been demonstrated to be surface-accessible, and not masked by O-specific polysaccharide. The use of an in vivo selection/expression technology (IVET) by a group of researchers identified a number of P. aeruginosa proteins that are expressed in vivo and essential for virulence. Two of these in vivo-expressed proteins are FptA (ferripyochelin receptor protein) and a homologue of an LPS biosynthetic enzyme. Our laboratory has identified a highly conserved protein, WbpM, and P. aeruginosa with a deficiency in this protein produces only rough LPS and became serum sensitive. Results from these studies have provided the foundation for a variety of vaccine formulations.

Influence of OprM expression on multiple antibiotic resistance in Pseudomonas aeruginosa

MexA-MexB-OprM is an efflux system in Pseudomonas aeruginosa. OprM overproduced from the cloned gene was able to complement OprM-deficient mutants but did not alter the resistance of a wild-type P. aeruginosa strain to the different antimicrobial agents tested. This suggests that OprM cannot function by itself to efflux antibiotics, including beta-lactams targeted to the periplasm.

The Fusobacterium nucleatum major outer-membrane protein (FomA) forms trimeric, water-filled channels in lipid bilayer membranes

The pore-forming activity of the major outer-membrane protein FomA of the anaerobic Fusobacterium nucleatum was studied in artificial lipid bilayer membranes. FomA was isolated from F. nucleatum strains Fev1, ATCC 10953, and ATCC 25586 by extraction with lithium dodecyl sulfate and lithium chloride and had an apparent molecular mass of about 40 kDa. When solubilized at low temperatures, the protein ran with an apparent molecular mass of about 62 kDa on SDS/PAGE. Cross-linking experiments and two-dimensional SDS/PAGE gave evidence that the 62-kDa protein band represented the trimeric form of FomA. The protein trimers were susceptible to SDS and temperature. The stability of the porin trimers varied among the strains. The properties of the FomA channels were studied in reconstitution experiments with black lipid bilayer membranes. The F. nucleatum porins formed channels with single-channel conductances in the range 0.66-1.30 nS in M KCl. The single-channel conductance was a function of the mobilities of the ions present in the aqueous solution bathing the bilayer membrane. This means that FomA forms general diffusion channels since (a) the conductance showed a linear dependence on the salt concentration, (b) the ion selectivity was small and varied for the three strains, and (c) the channels did not exhibit any binding site for maltotriose or triglycine. The water-filled channel was voltage dependent, and conductance decrements were observed at transmembrane potentials of +/- 50 mV. The conductance decrement steps were about one-third of the total conductance of a functional unit in its fully 'open' state. This strongly suggests that the trimer is the functional unit of the porin.

The rare outer membrane protein, OmpL1, of pathogenic Leptospira species is a heat-modifiable porin

The outer membranes of invasive spirochetes contain unusually small amounts of transmembrane proteins. Pathogenic Leptospira species produce a rare 31-kDa surface protein, OmpL1, which has a deduced amino acid sequence predictive of multiple transmembrane beta-strands. Studies were conducted to characterize the structure and function of this protein. Alkali, high-salt, and urea fractionation of leptospiral membranes demonstrated that OmpL1 is an integral membrane protein. The electrophoretic mobility of monomeric OmpL1 was modifiable by heat and reduction; complete denaturation of OmpL1 required prolonged boiling in sodium dodecyl sulfate (SDS), 8 M urea, and 2-mercaptoethanol. When solubilized in SDS at low temperature, a small proportion of OmpL1 exhibited an apparent molecular mass of approximately 90 kDa, indicating the existence of an SDS-unstable oligomer. OmpL1 dimers and trimers were demonstrated by nearest neighbor chemical cross-linking. In order to generate purified protein for functional studies, the ompL1 gene was ligated into the pMMB66 expression plasmid under control of the tac promoter. Although expression in Escherichia coli was toxic, most of the OmpL1 produced was found in the outer membrane, as determined by subcellular fractionation. Purified recombinant OmpL1 was reconstituted into planar lipid bilayers, demonstrating an average single channel conductance of 1.1 nS, similar to the major porin activity of native leptospiral membranes. These findings indicate that OmpL1 spans the leptospiral outer membrane and functions as a porin.

Development of multiple-antibiotic-resistant (Mar) mutants of Pseudomonas aeruginosa after serial exposure to fluoroquinolones

Laboratory-derived fluoroquinolone-resistant mutants were created by serially passaging wild-type Pseudomonas aeruginosa on fluoroquinolone-containing agar to obtain high-level fluoroquinolone resistance (e.g., ciprofloxacin MIC of 1,024 micrograms/ml). With increases of 4- to 32-fold in MICs of fluoroquinolones, these organisms demonstrated (relative to wild-type) normal morphology, resistance to fluoroquinolones only, no change in fluoroquinolone uptake, and no change in lipopolysaccharide profiles or outer membrane protein profiles. Complementation with wild-type Escherichia coli gyrA restored fluoroquinolone susceptibility, suggesting that these were gyrA mutants. After 4- to 32-fold increases in fluoroquinolone MICs (with continued passage on fluoroquinolone-containing agar) isolates demonstrated altered morphology, a multiple-antibiotic-resistant (Mar) phenotype (including cross-resistance to beta-lactams, chloramphenicol, and tetracycline), reduced fluoroquinolone uptake and altered outer membrane proteins (reductions in the 25- and 38-kDa bands as well as several bands in the 43- to 66-kDa region). Complementation with wild-type E. coli gyrA partially reduced the level of fluoroquinolone resistance by approximately 8- to 32-fold, suggesting that these mutants displayed both gyrA and non-gyrA mutations.

Overexpression of algE in Escherichia coli: subcellular localization, purification, and ion channel properties

Alginate-producing (mucoid) strains of Pseudomonas aeruginosa possess a 54-kDa outer membrane (OM) protein (AlgE) which is missing in nonmucoid bacteria. The coding region of the algE gene from mucoid P. aeruginosa CF3/M1 was subcloned in the expression vector pT7-7 and expressed in Escherichia coli. The level of expression of recombinant AlgE was seven times higher than that of the native protein in P. aeruginosa. Recombinant AlgE was found mainly in the OM. A putative precursor protein (56 kDa) of AlgE could be immunologically detected in the cytoplasmic membrane (CM). Surface exposition of AlgE in the OM of E. coli was indicated by labeling lysine residues with N-hydroxysuccinimide-biotin. Secondary-structure analysis suggested that AlgE is anchored in the OM by 18 membrane-spanning beta-strands, probably forming a beta-barrel. Recombinant AlgE was purified, and isoelectric focusing revealed a pI of 4.4. Recombinant AlgE was spontaneously incorporated into planar lipid bilayers, forming ion channels with a single-channel conductance of 0.76 nS in 1 M KCl and a mean lifetime of 0.7 ms. Single-channel current measurements in the presence of other salts as well as reversal potential measurements in salt gradients revealed that the AlgE channel was strongly anion selective. For chloride ions, a weak binding constant (Km = 0.75 M) was calculated, suggesting that AlgE might constitute an ion channel specific for another particular anion, e.g., polymannuronic acid, which is a precursor of alginate. Consistent with this idea, the open-state probability of the channel decreased when GDP-mannuronic acid was added. The AlgE channel was inactivated when membrane voltages higher than +85 mV were applied. The electrophysiological characteristics of AlgE, including its rectifying properties, are quite different from those of typical porins.

Immunochemical and biological characterization of outer membrane proteins of Porphyromonas endodontalis

Outer membrane proteins (OMP) of Porphyromonas endodontalis HG 370 (ATCC 35406) were prepared from the cell envelope fraction of the organisms. The cell envelope that had been obtained by sonication of the whole cells was extracted in 2% lithium dodecyl sulfate and then successively chromatographed with Sephacryl S-200 HR and DEAE-Sepharose Fast Flow. Two OMP fractions, OMP-I and OMP-II, were obtained, and their immunochemical properties and induction of specific antibodies were examined. The OMP-I preparation consisted of a major protein with an apparent molecular mass of 31 kDa and other moderate to minor proteins of 40.3, 51.4, 67, and 71.6 kDa, while the OMP-II preparation contained 14-, 15.5-, 27-, and 44-kDa proteins as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis. OMP-I was found to form hydrophilic diffusion pores by incorporation into artificial liposomes composed of egg yolk phosphatidylcholine and dicetylphosphate, indicating that OMP-I exhibited significant porin activity. However, the liposomes containing heat-denatured OMP-I were scarcely active. Spontaneous and antigen-specific immunoglobulin M (IgM)-, IgG-, and IgA-secreting spot-forming cells (SFC) enzymatically dissociated into single-cell suspensions from chronically inflamed periapical tissues and were enumerated by enzyme-linked immunospot assay. In patients with radicular cysts or dental granulomas, the major isotype of spontaneous SFC was IgG. In radicular cysts, the OMP-II-specific IgG SFC represented 0.13% of the total IgG SFC, while the antigen-specific IgA or IgM SFC was not observed. It was also found that none of these mononuclear cells produced antibodies specific for OMP-I or lipopolysaccharide of P. endodontalis.

Polyphosphate-selective porin OprO of Pseudomonas aeruginosa: expression, purification and sequence

The oprO gene of Pseudomonas aeruginosa codes for a polyphosphate-specific porin and terminates 458 bp upstream of the start codon for the phosphate-specific porin OprP. OprO was found to be expressed only under phosphate-starvation conditions in both wild-type and oprP::Tn501 mutant P. aeruginosa strains. However, unlike the rest of the genes of the Pho regulon, including oprP, expression of oprO required cells to be in the stationary growth phase in addition to phosphate starvation. Wild-type P. aeruginosa cells were grown in fermentor culture under these conditions and fractionated by selective solubilization in octylpolyoxyethylene detergent solution. Solubilized OprO was separated from OprP by application to a Mono Q FPLC column and elution with a salt gradient and shown to be functionally identical to cloned OprO produced in Escherichia coli. DNA sequencing of oprO showed the gene product to be highly homologous to OprP, with 76% identity and 16% conserved substitutions. Most genes of the Pho regulon possess a modified -35 region called the Pho box. Two such elements, separated by 4 bp were found in oprO. DNA sequencing also revealed a second Pho box in the oprP gene with the same spacing.

Decreased outer membrane permeability in imipenem-resistant mutants of Pseudomonas aeruginosa

The outer membrane of imipenem-resistant mutants of Pseudomonas aeruginosa was shown to have decreased permeability to imipenem but not to cephaloridine. These experiments were performed with intact cells and liposomes containing imipenem-hydrolyzing beta-lactamase derived from Pseudomonas maltophilia, in both cases utilizing an imipenem concentration of 50 microM. In contrast, liposome swelling assays using imipenem at 8 mM detected no significant difference between the imipenem-resistant mutants and their parents. It was found that the outer membrane of P. aeruginosa has a saturable specific channel through which imipenem travels mainly at low concentrations, whereas at high concentrations this pathway is obscured by diffusion through nonspecific porin channels.

Structure and supramolecular architecture of membrane channel-forming peptides

Peptides gathering together to induce channels in lipid bilayers may be classified in several categories according to the spatial structures involved. For example, gramicidin A forms intramolecular tubes, alamethicin, bundles of helical rods with intermolecular pores, porins (being proteins, properly speaking) are rich in beta-sheets that may form barrels, whereas cyclic peptides might stack together resulting in the formation of pores. The chemical structure of these compounds is now well characterized. The transmembrane electrical signals that they transmit are also typical of the particular supramolecular configurations (or architecture). Investigations in this field are thus relevant to structure-function relationship studies due to the availability of natural or synthetic analogues allowing the measurement of the influence of physico-chemical parameters upon the energy profiles of the pores. Consequently, questions such as the existence and probabilities of conductance substrates, their voltage-dependence and their ion or molecular selectivity can be tackled. Today, the loosest aspect of these studies lies in the actual molecular conformations and architecture in the membranes of the peptide aggregates, the knowledge of which remains imprecise, even 'at rest' in the best-studied cases. This review attempts to point out still unresolved questions and to propose some plausible approaches concerning, for example: 1) the configurations of the molecular aggregates responsible for ion transfer; 2) the mechanisms for channel-opening and closing (gating); 3) the eventual cooperative phenomena between channels, via the bilayer or interfacial components. Possible applications of these structures will be tentatively outlined.

Nearest neighbor analysis of outer membrane proteins of nontypeable Haemophilus influenzae

The arrangement of outer membrane proteins on the surface of nontypeable Haemophilus influenzae was investigated with cleavable and noncleavable bis-imidate cross-linking agents. Whole organisms were subjected to cross-linking agents, and oligomers of proteins were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, two-dimensional gel electrophoresis, and immunoblot assay, using monoclonal antibodies to outer membrane proteins. The major outer membrane protein (P2) formed dimers and trimers detected by all three methods. Oligomers of other outer membrane proteins were not detected. These data indicate that P2 exists as a trimer on the outer membrane and suggest that other outer membrane proteins exist as monomers on the outer membrane.

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.

Large-scale purification and biochemical characterization of crystallization-grade porin protein P from Pseudomonas aeruginosa

A large-scale purification scheme was developed for lipopolysaccharide-free protein P, the phosphate-starvation-inducible outer-membrane porin from Pseudomonas aeruginosa. This highly purified protein P was used to successfully form hexagonal crystals in the presence of n-octyl-beta-glucopyranoside. Amino-acid analysis indicated that protein P had a similar composition to other bacterial outer membrane proteins, containing a high percentage (50%) of hydrophilic residues. The amino-terminal sequence of this protein, although not homologous to either outer membrane protein, PhoE or OmpF, of Escherichia coli, was found to have an analogous protein-folding pattern. Protein P in the native trimer form was capable of maintaining a stable functional trimer after proteinase cleavage. This suggested the existence of a strongly associated tertiary and quaternary structure. Circular dichroism studies confirmed these results in that a large proportion of the protein structure was determined to be beta-sheet and resistant to acid pH and heating in 0.1% sodium dodecyl sulphate.

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.

Isolation of the outer membrane and characterization of the major outer membrane protein from Spirochaeta aurantia

The outer membrane of Spirochaeta aurantia was isolated after cells were extracted with sodium lauryl sarcosinate and was subsequently purified by differential centrifugation and KBr isopycnic gradient centrifugation. The purified outer membrane was obtained in the form of carotenoid-containing vesicles. Four protein species with apparent molecular weights of 26,000 (26K), 36.5K, 41K, and 48.5K were readily observed as components of the vesicles. The 36.5K protein was the major polypeptide and constituted approximately 90% of the outer membrane protein observed on sodium dodecyl sulfate-polyacrylamide gels. Under mild denaturing conditions the 36.5K major protein exhibited an apparent molecular weight of approximately 90,000. This, together with the results of protein cross-linking studies, indicates that the 36.5K polypeptide has an oligomeric conformation in the native state. Reconstitution of solubilized S. aurantia outer membrane into lipid bilayer membranes revealed the presence of a porin, presumably the 36.5K protein, with an estimated channel diameter of 2.3 nm based on the measured single channel conductance of 7.7 nS in 1 M KCl.

Demonstration and chemical modification of a specific phosphate binding site in the phosphate-starvation-inducible outer membrane porin protein P of Pseudomonas aeruginosa

The interaction of phosphate ions with the Pseudomonas aeruginosa anion-specific protein P channel was probed. The single-channel conductance of protein P incorporated into planar lipid bilayer membranes in the presence of 0.3 M H2PO-4 was shown to be 6.0 pS, demonstrating that protein P channels allowed the permeation of phosphate. When large numbers of protein P channels were incorporated into lipid bilayer membranes in the presence of 40 mM Cl-, addition of small concentrations of phosphate resulted in reduction of macroscopic Cl- conductance in a dose- (and pH-) dependent fashion. This allowed calculation of an I50 value of e.g. 0.46 mM at pH 7.0, suggesting that the affinity of protein P for its normal substrate phosphate was at least 60-100-fold greater than the affinity of the channel for other ions such as chloride. Pyrophosphate and the phosphate analogue, arsenate, also inhibited macroscopic Cl- conductance through protein P with I50 values at pH 7.0 of 4.9 mM and 1.3 mM, respectively. To probe the nature of the phosphate binding site, the epsilon-amino groups of available lysine residues of protein P were chemically modified. Acetylation and carbamylation which produced uncharged, modified lysines destroyed both the anion (e.g. Cl-) binding site and the phosphate binding site as determined by single-channel experiments and macroscopic conductance inhibition experiments respectively. Nevertheless, the modified proteins still retained their trimeric configuration and their ability to reconstitute single channels in lipid bilayer membranes. Methylation, which allowed retention of the charge on the modified lysine residues, increased the Kd of the channel for Cl- 33-fold and the I50 for phosphate inhibition of macroscopic Cl- conductance 2.5-4-fold. A molecular model for the phosphate binding site of the protein P channel is presented.

Cross-linking analysis of yeast mitochondrial outer membrane

By enrichment of contact sites between the two mitochondrial boundary membranes it has been shown that this fraction contained a high activity of glutathione transferase and hexokinase which was bound to the outer membrane pore protein (Ohlendieck, K. et al. (1986) Biochim. Biophys. Acta 860, 672-689). Therefore, an interaction between the three proteins in the contact sites has been suggested. Cross-linking experiments with isolated outer membrane of yeast mitochondria show that glutathione transferase and the pore protein are already associated in the free outer membrane. Porin appeared to adopt four different oligomeric complexes in the membrane, including interactions with a 14 kDa polypeptide, which has glutathione transferase activity. The latter polypeptide could be phosphorylated by intrinsic or extrinsic protein kinases, while the porin itself was not phosphorylated. Yeast hexokinase, when bound to the outer membrane, was able to cross-link to the pore protein.

Monoclonal antibodies to Pseudomonas aeruginosa ferripyochelin-binding protein

Hybridomas secreting specific monoclonal antibodies against the Pseudomonas aeruginosa ferripyochelin-binding protein (FBP) were isolated. These monoclonal antibodies reacted with FBP in immunoblots of outer membrane preparations from all serotypes of P. aeruginosa. Two of the monoclonal antibodies also reacted with FBP in strains of P. putida, P. fluorescens, and P. stutzeri. These antibodies did not react with outer membranes of P. cepacia, "P. multivorans," P. maltophilia, or other gram-negative organisms. The monoclonal antibodies were opsonophagocytic and blocked the binding of [59Fe]ferripyochelin to isolated outer membranes of strain PAO. By indirect immunofluorescence techniques, the monoclonal antibodies were used to demonstrate that FBP is present on the cell surface of P. aeruginosa cells grown in low-iron but not high-iron medium. These observations were confirmed by using 125I in surface-labeling techniques.

Phosphate-starvation-induced outer membrane proteins of members of the families Enterobacteriaceae and Pseudomonodaceae: demonstration of immunological cross-reactivity with an antiserum specific for porin protein P of Pseudomonas aeruginosa

Bacteria from members of the families Enterobacteriaceae and Pseudomonadaceae were grown under phosphate-deficient (0.1 to 0.2 mM Pi) conditions and examined for the production of novel membrane proteins. Of the 17 strains examined, 12 expressed a phosphate-starvation-induced outer membrane protein which was heat modifiable in that after solubilization in sodium dodecyl sulfate at low temperature the protein ran on gels as a diffuse band of higher apparent molecular weight, presumably an oligomer form, which shifted to an apparent monomer form after solubilization at high temperature. These proteins fell into two classes based on their monomer molecular weights and the detergent conditions required to release the proteins from the peptidoglycan. The first class, expressed by species of the Pseudomonas fluorescens branch of the family Pseudomonadaceae, was similar to the phosphate-starvation-inducible, channel-forming protein P of Pseudomonas aeruginosa. The second class resembled the major enterobacterial porin proteins and the phosphate-regulated PhoE protein of Escherichia coli. Using a protein P-trimer-specific polyclonal antiserum, we were able to demonstrate cross-reactivity of the oligomeric forms of both classes of these proteins on Western blots. However, this antiserum did not react with the monomeric forms of any of these proteins, including protein P monomers. With a protein P-monomer-specific antiserum, no reactivity was seen with any of the phosphate-starvation-inducible membrane proteins (in either oligomeric or monomeric form), with the exception of protein P monomers. These results suggest the presence of conserved antigenic determinants only in the native, functional proteins.

Isolation of a Tn501 insertion mutant lacking porin protein P of Pseudomonas aeruginosa

In order to demonstrate a role for anion-specific protein P channels in phosphate transport in Pseudomonas aeruginosa PAO, we wished to isolate a transposon insertion mutant deficient in protein P. A number of transposon delivery systems were tested which yielded, for the most part, whole plasmid inserts. Plasmid pMT1000 (Tsuda et al. 1984), a temperature-sensitive R68 plasmid carrying the transposon Tn501, was successfully employed in the isolation of a Tn501 insertion mutant lacking protein P under normally inducing conditions. To identify the mutant deficient in protein P, a protein P-specific polyclonal antiserum was used. This mutant, strain H576, was deficient in high-affinity phosphate transport exhibiting a Km for uptake (3.60 +/- 0.64 microM) almost ten times greater than that of the wild type strain (Km = 0.39 microM). There was, however, no change in the Vmax for high-affinity phosphate transport as a result of the loss of protein P in this mutant. The protein P-deficiency of the mutant correlated with a growth defect in a phosphate-limited medium, resulting in an 18%-35% decrease in growth when compared with the wild type.

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.

Immunotherapeutic potential of monoclonal antibodies against Pseudomonas aeruginosa protein F

To unambiguously demonstrate the immunotherapeutic potential of outer membrane porin protein F from Pseudomonas aeruginosa, a series of monoclonal antibodies have been isolated and demonstrated to be specific for protein F by Western blotting procedures. The antibodies recognize a surface-exposed antigenic site that is conserved on all Pseudomonas aeruginosa strains tested to date. One of these monoclonal antibodies named MA4-4 resulted in passive protection against subsequent infections by Pseudomonas aeruginosa in two different mouse infection models. In vitro studies using human polymorphonuclear leukocytes suggested that this antibody opsonized Pseudomonas aeruginosa for phagocytosis. The data suggest that immunotherapy based on porin protein F has definite potential for success.

Dimeric porin from Paracoccus denitrificans

Paracoccus denitrificans was shown to contain a 33,000-dalton porin, which produced pores of large (1.6 to 1.8 nm) diameter. Cross-linking studies showed that the porin existed as dimers in the outer membrane.

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.

Chemical modification of the anion selectivity of the PhoE porin from the Escherichia coli outer membrane

The PhoE porin of Escherichia coli is induced by phosphate deprivation and when purified, forms moderately anion-selective channels in lipid bilayer membranes. To further investigate the basis of anion selectivity, PhoE was chemically acetylated with acetic anhydride. Acetylation modified the mobility and staining characteristics of the PhoE porin on SDS-polyacrylamide gel electrophoresis but the acetylated protein was still found in its normal trimeric state after solubilization in SDS at low temperatures. Furthermore, the acetylated PhoE porin retained its ability to reconstitute into lipid bilayer membranes and the single channel conductance in 1 M KCl was unaltered. Zero-current potential measurements demonstrated that whereas the native PhoE porin was anion-selective, a 30-40-fold increase in preference for cations upon acetylation resulted in the acetylated PhoE porin being cation-selective. Increasing the pH of KCl solutions bathing lipid bilayer membranes from pH 3 to pH 6 caused symmetrical 4-fold increases in the selectivity of both the native and acetylated PhoE proteins for cations. In contrast, increasing the pH from 7 to 9 caused a 2.5-fold increase in selectivity only for the native PhoE porin. These results suggest that the basis of anion selectivity in the native PhoE porin is fixed protonated amino groups (possibly on lysines) in or near the channel, and furthermore indicate that deprotonated carboxyl groups have a strong influence on ion selectivity.

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.

Surface localization of Pseudomonas aeruginosa outer membrane porin protein F by using monoclonal antibodies

Hybridomas secreting highly specific monoclonal antibodies against porin protein F of Pseudomonas aeruginosa were isolated. These antibodies interacted with protein F in outer membranes isolated from strains representing the 17 serotypes of P. aeruginosa and from another 15 clinical isolates from patients with cystic fibrosis. The cell surface localization of antigenic sites on protein F was shown by indirect immunofluorescent techniques with these monoclonal antibodies. No fluorescence was observed on a protein F-deficient strain H283 of P. aeruginosa. Another monoclonal antibody specific for outer membrane lipoprotein H2 of P. aeruginosa showed no fluorescence on intact, wild-type bacterial cells, but was able to interact with a rough, LPS-deficient mutant.