Cloning and characterization of the structural gene for the class 2 protein of Neisseria meningitidis

Innate immune function of the neisserial porins and the relationship to vaccine adjuvant activity

Neisseria meningitidis is a Gram-negative pathogenic bacteria responsible for bacterial meningitis and septicemia. Porins are the most represented outer membrane proteins in the pathogenic Neisseria species, functioning as pores for the exchange of ions, and are characterized by a trimeric beta-barrel structure. Neisserial porins have been shown to act as adjuvants in the immune response via activation of B cells and other antigen-presenting cells. Their effect on the immune response is mediated by upregulation of the costimulatory molecule B7-2 (CD86) on the surface of antigen-presenting cells, an effect that is dependent on Toll-like receptor (TLR)2 and MyD88, through a cascade of signal transduction events mediated by direct binding of the porin to the TLR2-TLR1 heterodimer. This article summarizes work carried out investigating the mechanisms of the immune stimulating capacity of the neisserial porins (specifically meningococcal PorB), emphasizing cellular events involved in antigen-presenting cell activation and induction of expression of cell surface molecules involved in the immune response.

Structural basis for solute transport, nucleotide regulation, and immunological recognition of Neisseria meningitidis PorB

PorB is the second most prevalent outer membrane protein in Neisseria meningitidis. PorB is required for neisserial pathogenesis and can elicit a Toll-like receptor mediated host immune response. Here, the x-ray crystal structure of PorB has been determined to 2.3 A resolution. Structural analysis and cocrystallization studies identify three putative solute translocation pathways through the channel pore: One pathway transports anions nonselectively, one transports cations nonselectively, and one facilitates the specific uptake of sugars. During infection, PorB likely binds host mitochondrial ATP, and cocrystallization with the ATP analog AMP-PNP suggests that binding of nucleotides regulates these translocation pathways both by partial occlusion of the pore and by restricting the motion of a putative voltage gating loop. PorB is located on the surface of N. meningitidis and can be recognized by receptors of the host innate immune system. Features of PorB suggest that Toll-like receptor mediated recognition outer membrane proteins may be initiated with a nonspecific electrostatic attraction.

Mannose-binding lectin binds to two major outer membrane proteins, opacity protein and porin, of Neisseria meningitidis

Human mannose-binding lectin (MBL) provides a first line of defense against microorganisms by complement activation and/or opsonization in the absence of specific Ab. This serum collectin has been shown to activate complement when bound to repeating sugar moieties on several microorganisms, including encapsulated serogroup B and C meningococci, which leads to increased bacterial killing. In the present study, we sought to identify the meningococcal cell surface components to which MBL bound and to characterize such binding. Outer membrane complex containing both lipooligosaccharide (LOS) and proteins and LOS from Neisseria meningitidis were examined for MBL binding by dot blot and ELISA. MBL bound outer membrane complex but not LOS. The binding to bacteria by whole-cell ELISA did not require calcium and was not inhibited by N-acetyl-glucosamine or mannose. With the use of SDS-PAGE, immunoblot analysis, and mAbs specific for meningococcal opacity (Opa) proteins and porin proteins, we determined that MBL bound to Opa and porin protein B (porB). The N-terminal amino acid sequences of the two MBL binding proteins confirmed Opa and PorB. Purified PorB inhibited the binding of MBL to meningococci. Escherichia coli with surface-expressed gonococcal Opa bound significantly more MBL than did the control strain. The binding of human factor H to purified PorB was markedly inhibited by MBL in a dose-dependent manner. Meningococci incubated with human serum bound MBL as detected by ELISA. We conclude that MBL binds to meningococci by a novel target recognition of two nonglycosylated outer membrane proteins, Opa and PorB.

Phylogenetic Evidence for Frequent Positive Selection and Recombination in the Meningococcal Surface Antigen PorB

Previous estimates of rates of synonymous (d(S)) and nonsynonymous (d(N)) substitution among Neisseria meningitidis gene sequences suggested that the surface loops of the variable outer membrane protein PorB were under only weak selection pressure from the host immune response. These findings were consistent with studies indicating that PorB variants were not always protective in immunological and microbiological assays and questioned the suitability of this protein as a vaccine component. PorB, which is expressed at high levels on the surface of the meningococcus, has been implicated in mechanisms of pathogenesis and has also been used as a typing target in epidemiological investigations. In this work, using more precise estimates of selection pressures and recombination rates, we have shown that some residues in the surface loops of PorB are under very strong positive selection, as great as that observed in human immunodeficiency virus-1 surface glycoproteins, whereas amino acids within the loops and the membrane-spanning regions of the protein are under purifying selection, presumably because of structural constraints. Congruence tests showed that recombination occurred at a rate that was not sufficient to erase all phylogenetic similarity and did not greatly bias selection analysis. Homology models of PorB structure indicated that many strongly selected sites encoded residues that were predicted to be exposed to host immune responses, implying that this protein is under strong immune selection and requires further examination as a potential vaccine candidate. These data show that phylogenetic inference can be used to complement immunological and biochemical data in the choice of vaccine candidates.

Interactions of pathogenic neisseriae with epithelial cell membranes

The closely related bacterial pathogens Neisseria gonorrhoeae (gonococci, GC) and N. meningitidis (meningococci, MC) initiate infection at human mucosal epithelia. Colonization begins at apical epithelial surfaces with a multistep adhesion cascade, followed by invasion of the host cell, intracellular persistence, transcytosis, and exit. These activities are modulated by the interaction of a panoply of virulence factors with their cognate host cell receptors, and signals are sent from pathogen to host and host to pathogen at multiple stages of the adhesion cascade. Recent advances place us on the verge of understanding the colonization process at a molecular level of detail. In this review we describe the Neisseria virulence factors in the context of epithelial cell biology, placing special emphasis on the signaling functions of type IV pili, pilus-based twitching motility, and the Opa and Opc outermembrane adhesin/invasin proteins. We also summarize what is known about bacterial intracellular trafficking and growth. With the accelerated integration of tools from cell biology, biochemistry, biophysics, and genomics, experimentation in the next few years should bring unprecedented insights into the interactions of Neisseriae with their host.

The opcA and (psi)opcB regions in Neisseria: genes, pseudogenes, deletions, insertion elements and DNA islands

Previous data have indicated that the opc gene encoding an immunogenic invasin is specific to Neisseria meningitidis (Nm) and is lacking in Neisseria gonorrhoeae (Ng). The data presented here show that Nm and Ng both contain two paralogous opc-like genes, opcA, corresponding to the former opc gene, and (psi)opcB, a pseudogene. The predicted OpcA and OpcB proteins possess transmembrane regions with conserved non-polar faces but differ extensively in four of the five surface-exposed loops. Gonococcal OpcA was expressed weakly under in vitro conditions, and it is unknown whether these bacteria can express this protein at high levels. Analysis of the sequences flanking opcA and (psi)opcB revealed a framework of conserved housekeeping genes interspersed with DNA islands. These regions also contained several pseudogenes, deletions and IS elements, attesting to considerable genome plasticity. Both opcA and (psi)opcB are located on DNA islands that have probably been imported from unrelated bacteria. A third island encodes the dcmD/dcrD R/M genes in Ng versus a small open reading frame in most strains of Nm. Rare strains of Nm were identified in which the R/M island has been imported. DNA islands in Nm and Ng seem to have been acquired by recombination via conserved flanking housekeeping genes rather than by insertion of mobile genetic elements.

Characterization of the structure, function, and conformational stability of PorB class 3 protein from Neisseria meningitidis. A porin with unusual physicochemical properties

PorB proteins constitute the vast majority of channels in neisserial outer membranes and can be subdivided within meningococcal strains into two distinct and mutually exclusive families that are designated as class 2 and class 3 proteins. We recently characterized the functional activity and conformational stability of a PorB class 2 protein from Neisseria meningitidis (Minetti, C. A. S. A., Tai, J. Y., Blake, M. S., Pullen, J. K., Liang, S. M., and Remeta, D. P. (1997) J. Biol. Chem. 272, 10710-10720). To evaluate the structure-function relatedness among the PorB proteins, we have employed a combination of electrophoretic and spectroscopic techniques to assess the conformational stability of zwittergent-solubilized class 3 trimers. The functional, physicochemical, and structural properties of the meningococcal class 2 and class 3 proteins are comparable with the notable exception that the latter exhibits a significantly higher susceptibility to SDS. The SDS-induced dissociation and partial unfolding of PorB class 3 is characterized by a single two-state transition with a midpoint at 0.35% SDS. The native trimeric assembly dissociates reversibly, forming partially folded monomers that retain the characteristic beta-sheet content of the transmembrane domain with a concomitant increase in random coil structure arising from unfolding the rigid surface loops. These results provide new insight into the elucidation of porin folding pathways and the factors that govern the overall structural stability of meningococcal proteins.

Successful recovery of the normal electrophysiological properties of PorB (class 3) porin from Neisseria meningitidis after expression in Escherichia coli and renaturation

Neisseria meningitidis PorB class 3 porins obtained either from native membranes (wild-type) or recovered from inclusion bodies following expression in Escherichia coli (recombinant), have been reconstituted into solvent-free planar phospholipid membranes. The wild-type and recombinant porins exhibited the same single-trimer conductance (1-1.3 nS in 200 mM NaCl), tri-level closure pattern, characteristic of functional channel trimers, and pattern of insertion into planar membranes. Both proteins were open at low voltages and displayed two voltage-dependent closure processes, one at positive and the other at negative potentials. Both showed asymmetric voltage dependence such that one gating process occurred at lower voltages (Vo=15 mV) than the other (Vo=25 mV). The sign of the potential that resulted in closure at low voltages varied from membrane to membrane indicating that they may have the property of auto-directed insertion (in analogy to the mitochondrial channel, VDAC). In the case of the recombinant porin, the steepness of the voltage dependence of one gating process was slightly less (n=1.3) than that observed for the other process or for the wild-type channel (n=1.5-1.7). Both channels have a high (40%) probability of closure even at 0 mV. While both channels show a slight selectivity for Cl- over Na+, the selectivity of the recombinant porin is a bit higher (permeability ratio of 2.8 vs. 1.6) as measured using a 2-fold salt gradient. Thus, the method employed to refold the recombinant porin was successful in not only restoring wild-type structure [H.L. Qi, J.Y. Tai, M.S. Blake, Expression of large amounts of Neisserial porin proteins in Escherichia coli and refolding of the proteins into native trimers, Infect. Immun. 62 (1994) 2432-2439; C.A.S.A. Minetti, J.Y. Tai, M.S. Blake, J.K. Pullen, S.M. Liang, D.P. Remeta, Structural and functional characterization of a recombinant PorB class 2 protein from Neisseria meningitidis. Conformational stability and porin activity, J. Biol. Chem. 272 (1997) 10710-10720] but also the overall electrophysiological function.

T-Cell epitope mapping the PorB protein of serogroup B Neisseria meningitidis in B10 congenic strains of mice

T-cell epitope mapping the meningococcal serotype 15 PorB protein performed in this study in three congenic strains of mice with B10 genetic background revealed at least three murine T-cell epitopes (55-72, 163-180, and 226-261), located in the highly conserved putative transmembrane regions of Neisserial porins. Proliferation assays with popliteal lymph node cells derived from mice immunized with the PorB protein or with synthetic 18-mer peptides showed that epitope 163-180 immunized only in the H-2d haplotype, epitope 55-72 could be presented by both H-2f and H-2s molecules, while the 226-261 region covered by three overlapping peptides could be efficiently recognized in context of all three MHC class II haplotypes studied. Inhibition experiments with blocking I-Aalpha- and I-Ealpha-specific mAb showed that peptide 163-180 was presented by I-Ad and peptide 244-261 was presented by both I-Af and I-As. In addition, evidence was obtained that peptide 226-243 was presented in context of H-2d or I-As haplotypes and peptide 55-72 was presented in context of I-Af and I-As loci. Finally, the Norwegian outer membrane vesicle vaccine, but not the purified PorB protein, could recall responses in mice immunized with synthetic peptides corresponding to the 226-261 region. Altogether, these results suggest that T-cell epitopes identified on the serotype 15 PorB protein, particularly those presented by several MHC class II molecules (e.g., 226-261), could have important implications for the development of meningococcal vaccines.

Structural and functional characterization of a recombinant PorB class 2 protein from Neisseria meningitidis. Conformational stability and porin activity

An outer membrane PorB class 2 protein from Neisseria meningitidis has been overexpressed in Escherichia coli, isolated from inclusion bodies, and refolded in the presence of zwitterionic detergent. The purified recombinant and native (strain M986) counterpart exhibit most of the typical functional and structural properties that are characteristic of bacterial porins. Channel forming activity has been monitored by incorporating class 2 into reconstituted liposomes and measuring the permeation rates of various oligosaccharides through the proteoliposomes to derive a pore diameter of approximately 1.6 nm. Structural studies employing a combination of spectroscopic and electrophoretic techniques reveal that recombinant and native class 2 are identical in terms of overall conformational stability. Both proteins form stable trimers in zwitterionic detergent and retain significant secondary and tertiary structure in the presence of SDS. The thermal unfolding of zwittergen-solubilized class 2 trimers (Tm = 88 degrees C) is reversible and characterized by solvent exposure of aromatic residues with concomitant disruption of tertiary and partial loss of secondary structures. SDS-induced destabilization and irreversible unfolding of the native trimeric assembly occurs at temperatures above 60 degrees C. Our physicochemical studies of PorB class 2 protein furnish significant insight regarding the structural and functional properties of this meningococcal outer membrane protein within the porin superfamily.

Cloning, sequencing and expression of an Eikenella corrodens gene encoding a component protein of the lectin-like adhesin complex

A lectin-like substance (LS), that was isolated from Eikenella corrodens (Ec) 1073, migrated as proteins of about 300 and 45 kDa upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. In this study, we cloned the gene encoding the 45-kDa protein and predicted its structure and function. Based on the N-terminal 23-amino acid (aa) sequence of this protein, we cloned the region for its N-terminus. We cloned the entire gene by means of gene walking using polymerase chain reaction and Southern hybridization. The nucleotide sequences of cloned fragments revealed an open reading frame encoding a polypeptide of 330 aa (M(r), 35748). This ORF displayed high homology to those of porins of Neisseria species. Using the T7-expression system, the 45-kDa protein was produced in E. coli. Our results suggested that the 45-kDa protein of Ec 1073 is a component of the EcLS complex, and that it is the major outer membrane protein.

Cloning of a Neisseria meningitidis gene for L-lactate dehydrogenase (L-LDH): evidence for a second meningococcal L-LDH with different regulation

We report the cloning of lldA, a Neisseria meningitidis gene for L-lactate dehydrogenase (L-LDH). Escherichia coli contains a single L-LDH gene (lldD) in the lld operon (previously lct). E. coli grown in complex media does not have L-LDH activity, but the activity is induced by growth in defined medium with L-lactate as the carbon source. In contrast, meningococci contain at least one L-LDH in addition to the lldA gene product. These enzymes are active in meningococci grown in complex media and are not dependent on growth in L-lactate. The predicted amino acid sequence of lldA is homologous to that of E. coli lldD and of other prokaryotic and eukaryotic flavin mononucleotide-containing enzymes that catalyze the oxidation of L-lactate and other small alpha-hydroxy acids. A mutant with a deletion in lldA was found to have reduced L-LDH activity. However, this mutant was able to grow on L-lactate, indicating that a second L-LDH must exist. Activity of the lldA enzyme was affected by growth conditions, being increased by growth on a defined medium with either L-lactate or pyruvate as the carbon source. For meningococci grown on a complex medium, activity of the lldA enzyme was increased by growth on plates or in well-aerated broth. A second L-lactate-oxidizing activity was seen in bacteria grown in poorly aerated broth. Neisseria gonorrhoeae contains a homolog of lldA. As for meningococci, mutation of the gonococcal lldA reduced L-LDH activity but did not affect growth on L-lactate.

Neisserial porins induce B lymphocytes to express costimulatory B7-2 molecules and to proliferate

The neisserial porins are the major protein components of the outer membrane of the pathogenic Neisseria (N. meningitidis and N. gonorrhoeae). They have been shown to be able to enhance the immune response to poorly immunogenic substances (e.g., polysaccharides, peptides, glycolipids, etc.). To explore the basis of their potent adjuvant activity, the effect of the neisserial porins on T-B cell interactions and T cell costimulation was examined. Neisserial porins increased the surface expression of the costimulatory ligand B7-2 (CD86) but did not affect the expression of B7-1 (CD80). In addition, incubation with the neisserial porins increased the T lymphocyte costimulatory ability of B lymphocytes, which was inhibited by anti-B7-2 but not anti-B7-1 monoclonal antibodies. Upregulation of B7-2 on the surface of B lymphocytes may be the mechanism behind the immunopotentiating activity of neisserial porins.

N-terminal amino acid sequences of the major outer membrane proteins from a Neisseria meningitidis group B strain isolated in Brazil

The four dominant outer membrane proteins (46, 38, 33 and 28 kDa) were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in a semi-purified preparation of vesicle membranes of a Neisseria meningitidis (N44/89, B:4:P1.15:P5.5,7) strain isolated in Brazil. The N-terminal amino acid sequence for the 46 kDa and 28 kDa proteins matched that reported by others for class 1 and 5 proteins respectively, whereas the sequence (25 amino acids) for the 38 kDa (class 3) protein was similar to class 1 meningococcal proteins. The sequence for the 33 kDa (class 4) was unique and not homologous to any known protein.

Cloning and characterization of the meningococcal polyphosphate kinase gene: production of polyphosphate synthesis mutants

The pathogenic Neisseria species accumulate polyphosphate to levels between 10 and 20% of their total phosphate content. However, the significance of this compound for the growth and pathogenicity of these species is not understood. A previous report (C.R. Tinsley, B.N. Manjula, and E.C. Gotschlich, Infect. Immun. 61:3703-3710, 1993) describes the purification of polyphosphate kinase, the enzyme responsible for synthesis of polyphosphate, from Neisseria meningitidis BNCV. By use of probes based on the amino acid sequence of the purified enzyme, the structural gene ppk has been cloned and sequenced. The coding sequence is 2,055 bp long and codes for a protein of 77.2 kDa. The open reading frame of the cloned gene was interrupted by the insertion of a kanamycin resistance cassette, and ppk mutants were obtained in both Neisseria gonorrhoeae and N. meningitidis by transformation with the recombinant plasmid. Amounts of polyphosphate in the ppk mutants were reduced to between 2 and 10% of wild-type levels. The mutants grew less vigorously than wild-type organisms in vitro and showed a striking increase in sensitivity to killing by human serum.

Analysis of Neisseria meningitidis class 3 outer membrane protein gene variable regions and type identification using genetic techniques

The class 3 porin proteins of Neisseria meningitidis stimulate bactericidal antibodies and express serotype-specific antigenic epitopes. Sequence analysis of porB genes for the class 3 proteins revealed regions of variability that map to surface-exposed loops. To evaluate the relationship between serotype and variable-region (VR) genotype, sequences from the 11 class 3-expressing serotype strains and 3 additional serotype 4 strains were analyzed by molecular techniques. Multiple-sequence alignment revealed a limited number of unique sequences at each of four VRs (VR1 to VR4), ranging from four unique sequences at VR1 to seven sequence patterns at VR2 and VR4. Serotype-specific VR sequences were found in each of the four VRs, suggesting that each VR has immunologic importance. Five serotypes had at least one VR sequence that was unique. Three serotypes which had sequences in common with other serotypes at each VR were distinguished by examining multiple VRs. Serotype 3 was identical to serotype 19 at each VR, and serotype 8 was identical to serotype 18 at each VR. Serotypes 4 and 21 were identical at VR1 and significantly different at VR3 and VR4. A subpopulation of serotype 4 strains with a unique VR2 sequence was identified. The serotypes which were grouped with closely related or identical sequences at one VR were grouped with different serotypes at other VRs consistent with the pattern of genetic mosaicism described for the porA (class 1 protein) gene. Hybridization assays demonstrated the ability to identify VR genotypes and distinguish serotypes using biotin-labelled oligonucleotide probes. This information may be useful in strain selection for vaccine development, in epidemiologic studies to determine the prevalence of the individual VR genotype (especially among nonserotypeable strains) and, combined with PCR, in the identification of culture-negative suspected meningococcal cases.

Expression of large amounts of neisserial porin proteins in Escherichia coli and refolding of the proteins into native trimers

Porins from different neisserial strains and species have been shown to have differences in both primary amino acid sequence and biophysical characteristics as observed by functional assays. A closer examination of how the changes in the primary amino acid sequence of Neisseria porin molecules correlate with these observed biophysical changes has been impeded by the inability to easily manipulate the cloned porin genes by modern molecular techniques and then obtain enough of the expressed modified porin protein to purify and use in these biophysical functional assays. In this report, we describe a method by which the genes encoding three different porin proteins, lacking their neisserial promoter and signal sequences, were cloned into an expression plasmid and transformed into Escherichia coli. Upon induction, large amounts of the porin proteins were produced. The expressed porin proteins were then manipulated to regenerate their native trimer structure and purified by standard protein chemistry. Sufficient purified recombinant porin protein was obtained for further antigenic as well as biophysical characterization. This sets the stage for the biophysical characterization of these neisserial porin proteins in more detail.

Molecular characterization of the 98-kilodalton iron-regulated outer membrane protein of Neisseria meningitidis

When grown under iron limitation, Neisseria meningitidis expresses several additional outer membrane proteins (OMPs), which were studied to assess their vaccine potential. Two monoclonal antibodies were obtained against a 98-kDa OMP of strain 2996 (B:2b:P1.2). Cross-reactivity studies revealed that the two antibodies reacted with 44 and 42 of 74 meningococcal strains, respectively. The antibodies did not block the binding of transferrin or lactoferrin to intact cells. The structural gene for the protein, tentatively designated iroA, was isolated and sequenced. Computer analysis revealed homology to the ferric siderophore receptors in the outer membrane of Escherichia coli and to gonococcal transferrin-binding protein 1 (TbpA). The high degree of cross-reactivity and the results of Southern blot analyses, which showed that the iroA gene is also present in strains that did not react with the monoclonal antibodies, suggest that the 98-kDa OMP is well conserved among meningococci and that it is a suitable vaccine candidate. However, the antibodies were not bactericidal in an in vitro assay with human complement.

Oxidation of D-lactate and L-lactate by Neisseria meningitidis: purification and cloning of meningococcal D-lactate dehydrogenase

Neisseria meningitidis was found to contain at least two lactate-oxidizing enzymes. One of these was purified 460-fold from spheroplast membranes and found to be specific primarily for D-lactate, with low-affinity activity for L-lactate. The gene for this enzyme (dld) was cloned, and a dld mutant was constructed by insertional inactivation of the gene. The mutant was unable to grow on D-lactate but retained the ability to grow on L-lactate, providing evidence for a second lactate-oxidizing enzyme with specificity for L-lactate. High-affinity L-lactate-oxidizing activity was detected in intact bacteria of both the dld+ and dld mutant strains. This L-lactate-oxidizing activity was also seen in sonicated bacteria but was reduced substantially on detergent solubilization or on preparation of spheroplast membranes.

Expression of meningococcal epitopes in LamB of Escherichia coli and the stimulation of serosubtype-specific antibody responses

The class 1 outer membrane protein (OMP), a major variable surface antigen of Neisseria meningitidis, is a component of novel meningococcal vaccines currently in field trials. Serological variants of the protein are also used to serosubtype meningococci. Most of the amino acid changes that give rise to antigenic variants of the protein occur in two variable regions (VR1 and VR2) that are thought to form loops on the cell surface. The polymerase chain reaction (PCR) was used to amplify the nucleotide sequences encoding VR1 and VR2 from the chromosomal DNA of N. meningitidis strain M1080. These were cloned in frame into the lamB gene of the Escherichia coli expression vector pAJC264. Whole-cell enzyme-linked immunosorbent assays (ELISAs), using monoclonal antibodies, and SDS-PAGE confirmed that, upon induction, strains of E. coli carrying these constructs expressed hybrid LamB proteins containing the N. meningitidis surface loops. These strains were used to immunize rabbits and the resultant polyclonal antisera reacted specifically with the class 1 OMP of reference strain M1080 (P1.7). Immunogold labelling of meningococcal cells and whole-cell dot-blot analyses with these antisera showed that the variable epitopes were exposed on the cell surface and confirmed that this approach could be used to obtain serosubtype-specific antisera. The binding profiles of the antisera were determined from their reactions with overlapping synthetic peptides and their reactivity compared with that of relevant serosubtype-specific monoclonal antibodies. This approach was used successfully to raise antisera against two other class 1 OMP VR2s. A fourth antiserum raised against a VR2, including the P1.1 epitope, was not subtype specific.

Purification and characterization of protein H, the major porin of Pasteurella multocida

Protein H (B. Lugtenberg, R. van Boxtel, D. Evenberg, M. de Jong, P. Storm, and J. Frik, Infect. Immun. 52:175-182, 1986) is the major polypeptide of the outer membrane of Pasteurella multocida, a bacterium pathogenic for humans and animals. We have purified this protein to homogeneity by size exclusion chromatography after selective extraction with surfactants and demonstrated its pore-forming ability after reincorporation into planar lipid bilayers. In these experiments, the current through the pores was a linear function of the applied voltage in the range of -50 to +50 mV. Voltages beyond +/- 50 mV tended to partially close the channels, giving rise to apparent negative resistances. These observations suggest that protein H channels are probably not voltage regulated in vivo. With the patch clamp technique, single-channel conductance fluctuations of 0.33 nS were recorded in 1 M KCl. Electrophoretic and circular dichroism analyses showed that protein H forms homotrimers stable in sodium dodecyl sulfate at room temperature, with a high content of beta-sheet secondary structure. Upon boiling, the trimers were fully dissociated into monomers with an increase of alpha helix and irregular structure, at the expense of beta sheets. The apparent molecular mass of fully denatured monomers ranged between 37 and 41.8 kDa, depending on the electrophoretic system used for analysis. The trimeric arrangement of protein H was confirmed by image analysis of negatively stained, two-dimensional crystal arrays. This morphological study revealed, in agreement with electrophoretical data, a trimeric structure with an overall diameter of 7.7 nm. Each monomer appeared to contain a pore with an average diameter of 1 nm. Quantitative comparisons revealed that the amino acid composition (hydropathy index of -0.40) and the N-terminal sequence (determined over 36 residues) of protein H are similar to those of bacterial general porins, notably porin P2 of Haemophilus influenzae. We conclude from this set of structural and functional data that protein H of P. multocida is a pore-forming protein related to the superfamily of the nonspecific bacterial porins.

Identification of variable region differences in Neisseria meningitidis class 3 protein sequences among five group B serotypes

Strains of Neisseria meningitidis express one of two porin proteins. These porins have been identified as the class 2 and class 3 proteins, and express serotype-specific epitopes. The gene for the class 3 protein was amplified by the polymerase chain reaction from the DNA of a serotype 4 strain as a 1025 bp fragment. The nucleotide sequence of this gene was determined and compared with two recently published sequences. On the basis of this comparison we have identified two major variable regions in the translated protein sequence, VR1 and VR2, that may be associated with serotype specificity. Three other variable regions were also identified. The sequences in the VR1 and VR2 regions from five additional group B N. meningitidis strains of serotypes 1, 4, 8, 12, and 15, all expressing class 3 proteins, were determined. The VR1 and VR2 regions were variable and were flanked by highly conserved regions among eight different class 3 sequences. These two variable regions of 15 and 9 amino acids are predicted to be in surface-exposed loops.

Identification of meningococcal serosubtypes by polymerase chain reaction

The polymerase chain reaction was used as the basis of a novel typing method for Neisseria meningitidis. Southern hybridization experiments demonstrated that it was possible to identify genes encoding different serological variants of the meningococcal class 1 outer membrane protein by probing with polymerase chain reaction products corresponding to known epitopes. A set of 14 defined variable regions was prepared in bacteriophage M13mp19 by the cloning of polymerase chain reaction products. The phage were dot blotted onto membrane filters, which were used as targets for hybridization of radiolabeled amplified class 1 outer membrane protein genes. Thus, the presence of many different subtype-specific epitopes could be investigated in one experiment. This technique was evaluated with a set of serological reference strains, mainly of serogroup B organisms, and provided an alternative, rapid, and comprehensive typing system that was capable of distinguishing known serosubtypes and also of defining currently untypeable strains independently of sodium dodecyl sulfate-polyacrylamide gel electrophoresis or serological analysis. An additional advantage of this technique was that in the case of an unknown serosubtype (i.e., one that did not hybridize with any of the known samples), the DNA amplified from the original sample could be used to determine the nucleotide sequence of the novel serosubtype and to clone the corresponding variable region into bacteriophage M13. It may be possible to develop this procedure for the diagnostic detection and typing of meningococci directly from clinical samples even when culture is not possible because of antibiotic treatment of an acute case.

Molecular analysis of the serotyping antigens of Neisseria meningitidis

Molecular approaches to the rapid analysis of the serotyping antigens of Neisseria meningitidis, the class 2 and 3 outer membrane proteins (OMPs), were developed, evaluated, and used to study 12 antigenic variants of these proteins. A primer set for the polymerase chain reaction (PCR) amplification of the genes encoding these antigens was devised. Low-stringency amplification of meningococcal chromosomal DNA with this primer set resulted in the amplification of two products from each strain, whereas at higher stringencies only one product was amplified in most strains. Southern hybridization techniques and restriction analyses were used to differentiate the PCR products amplified at high stringencies from strains expressing class 2 or class 3 OMPs; these PCR products were further characterized by the determination of their nucleotide sequences, confirming that they represented the amplified class 2 and class 3 OMP genes. Analyses of these and other nucleotide sequences enabled the construction of a phenogram illustrating the interrelationships between Neisseria OMP genes. The comparative analysis of deduced amino acid sequences revealed conserved and variable regions of the proteins; the latter probably correspond to surface loops on the protein and hence are potentially exposed to the immune system. Further analyses of the primary structures of these related porins from Neisseria species enabled construction of models of the secondary structure of these antigens and comparison of these models with those previously published. The methods reported in the present work are rapid reproducible procedures for the analysis of antigenic variants of these proteins.

Identification and characterization of a novel insertion sequence, IS1106, downstream of the porA gene in B15 Neisseria meningitidis

Examination of Neisseria meningitidis strains associated with endemic meningococcal disease demonstrated differences in the number of copies of a repetitive sequence. Characterization of a copy of this repetitive sequence present in B15 strains has revealed the presence of a novel insertion sequence (IS1106) located within a complex repetitive region downstream of the gene for the major surface antigen (porA). IS1106 has a length of 1137 bp and is flanked by 36bp inverted repeats. Two open reading frames (ORF1 and ORF2) are present in opposite strands in codon-codon register with ORF2 entirely located within ORF1. The predicted protein from ORF1 demonstrates homology with the 5A protein of IS5 (Kroger and Hobom, 1982). Strains from two independent outbreaks of B15 meningococcal disease in the UK were found to contain the same genomic deletion removing a copy of IS1106 downstream of the porA gene.

A vaccine carrier derived from Neisseria meningitidis with mitogenic activity for lymphocytes

Protein carriers vary in their ability to increase the immunogenicity of poorly immunogenic or T-lymphocyte-independent antigens. We examined one such carrier, the outer membrane protein complex derived from Neisseria meningitidis serogroup B strain B11, in an attempt to determine why this outer membrane protein complex was more immunogenic in young infants and in relevant animal models than two other carriers used in conjugates made with Haemophilus influenzae type b polysaccharide, a T-cell-independent antigen. A single protein of the outer membrane protein complex, the class 2 porin protein, was purified and shown to function as a T-helper lymphocyte carrier protein. Unexpectedly, it was also found to have mitogenic activity for lymphocytes that was not due to lipopolysaccharide. This mitogenic activity appears to date to be unique to this carrier protein of the carrier proteins tested and may contribute to the ability of the H. influenzae type b conjugate vaccine made with the outer membrane protein complex to generate IgG anti-polysaccharide antibody responses in mice and infant monkeys and protective immune responses in infants less than 6 months of age.

Murine cross-reactive T-cell epitopes of Neisseria meningitidis outer membrane proteins

Five non-covalent vaccines of outer membrane proteins (OMPs) complexed to capsular polysaccharide were prepared from Neisseria meningitidis serogroup B strains. Each vaccine contained distinct serotype (class 2/3) and subtype (class 1) OMPs. The cross-reactivity of the T-cell response to the meningococcal vaccine-associated proteins was examined in an in vitro T-cell proliferative assay, following antigenic priming of mice with one of these vaccines (MB6:P1.6) or with its purified class 1 (subtype P1.6) and class 2 (serotype 6) proteins. Cross-reactive T-cell epitopes were found in all five vaccine preparations on both the class 1 and class 2/3 OMPs. Priming of mice with the subtype P1.6 N-terminal peptide led to a significant but small increase in T-cell proliferation with the MB6:P1.6 vaccine.

Cloning and expression in Escherichia coli of opc, the gene for an unusual class 5 outer membrane protein from Neisseria meningitidis (meningococci/surface antigen)

A genomic library was constructed in a lambda gt11 vector using chromosomal DNA from a meningococcal serogroup A strain and plaques expressing the class 5C protein were recognized by screening with specific monoclonal antibodies. The opc insert was subcloned into a multicopy plasmid which induced expression of that protein in Escherichia coli as a surface-exposed major outer membrane protein. The nucleotide sequence of opc is typical of an outer membrane protein with a promoter and terminator region, a leader peptide which is cleaved during expression and a complete open reading frame. Unlike other meningococcal class 5 proteins or gonococcal P.II proteins, the sequence did not contain any pentanucleotide repeats and the sequence showed little homology to these other functionally related proteins. However, the predicted amino acid sequence of the mature protein for opc showed 27% similarity to that for a second opa gene cloned from the same meningococcal strain. This is the first report of cloning and expression of a functional meningococcal gene encoding a class 5 outer membrane protein in E. coli.

Topology of outer membrane porins in pathogenic Neisseria spp

In Escherichia coli, membrane-spanning amphipathic beta-sheet structures are characteristic of many outer membrane proteins. By applying the principles that have been recognized for them to the four classes of neisserial porins, we have constructed a model for the topology of the porins within the outer membrane. This model predicts eight surface-exposed loops, both in the meningococcal class 1 and 2 proteins and in the gonococcal PIA and PIB proteins. The transmembrane sequences are highly conserved among these porins and are able to form an amphipathic beta-sheet structure. The surface-exposed hydrophilic loops show extensive variation in both length and sequence. Experimental evidence in support of this model has been obtained by using antisera against synthetic peptides which correspond to surface-exposed loops in class 1 and 2 proteins. Thus, binding to the cell surface was observed with antibodies against loops 1, 4, and 5 of class 1 and loops 1 and 5 of class 2. In class 1, these loops are the longest ones and show the highest sequence diversity among strains of different subtypes. Mapping of epitopes recognized by monoclonal antibodies with bactericidal activity has also provided strong support for the model. The epitopes are located in loops 1 and 4 of class 1 protein, loop 5 of PIB, and loop 6 of PIA. A nonbactericidal antibody that binds only weakly to whole cells was shown to recognize loop 3 of PIB. These results suggest that the longest loops are immunodominant, provide the binding sites for bactericidal antibodies, and display the greatest variation among different strains.

The bacterial porin superfamily: sequence alignment and structure prediction

The porins of Gram-negative bacteria are responsible for the 'molecular sieve' properties of the outer membrane. They form large water-filled channels which allow the diffusion of hydrophilic molecules into the periplasmic space. Owing to the strong hydrophilicity of their amino acid sequence and the nature of their secondary structure (beta strands), conventional hydropathy methods for predicting membrane topology are useless for this class of protein. The large number of available porin amino acid sequences was exploited to improve the accuracy of the prediction in combination with tools detecting amphipathicity of secondary structure. Using the constraints of beta-sheet structure these porins are predicted to contain 16 membrane-spanning strands, 14 of which are common to the two (enteric and the neisserial) porin subfamilies.

Class-3 porin protein of Neisseria meningitidis: cloning and structure of the gene

The gene coding for the class-3 protein of Neisseria meningitidis was cloned and sequenced. The deduced amino acid (aa) sequence was highly homologous (50-78%) to those of other neisserial porin proteins. Alignment of the aa sequence of five neisserial porin proteins pinpointed several regions of identity or near identity. These are assumed to be membrane-spanning beta-strands. A comparison of the homologies between these neisserial porins showed that the class-3 protein is most closely related to the Neisseria gonorrhoeae P1A protein.

Cloning and sequencing of the structural gene for the porin protein of Bordetella pertussis

Bordetella pertussis produces a porin protein which is a prominent outer membrane component found in both virulent and avirulent strains. N-terminal amino acid analysis of purified B. pertussis porin was performed and this amino acid sequence was used to design an oligonucleotide that was then utilized to screen a lambda gt11 library containing randomly sheared fragments of DNA from B. pertussis strain 347. One clone, lambda BpPor, was identified and subcloned into pUC18. A portion of the DNA insert in this subclone, pBpPor1, was sequenced and shown to contain the N-terminal region of the structural porin gene. This truncated gene sequence was used to design an additional oligonucleotide that was used to identify a clone, pBpPor2, which overlapped with pBpPor1 and contained a termination codon. The structural gene deduced from this sequence would encode a 365-amino-acid polypeptide with a predicted mass of 39,103 daltons. The predicted product also contains a signal sequence of 20 residues that is similar to that found in other porin genes. The predicted B. pertussis porin protein sequence contains regions that are homologous to regions found in porins expressed by Neisseria species and Escherichia coli, including the presence of phenylalanine as the carboxy-terminal amino acid. DNA hybridization studies indicated that both virulent and avirulent strains of B. pertussis contain only one copy of this gene and that Bordetella bronchiseptica and Bordetella parapertussis contain a similar gene.

Carboxy-terminal phenylalanine is essential for the correct assembly of a bacterial outer membrane protein

Bacterial outer membrane proteins are supposed to span the membrane repeatedly, mostly in the form of amphipathic beta-sheets. The last ten C-terminal amino acid residues of PhoE protein are supposed to form such a membrane-spanning segment. Deletion of this segment completely prevents incorporation into the outer membrane. Comparison of the last ten amino acid residues of other outer membrane proteins from different Gram-negative bacteria revealed the presence of a potential amphipathic beta-sheet with hydrophobic residues at positions 1 (Phe), 3 (preferentially Tyr), 5, 7 and 9 from the C terminus, in the vast majority of these proteins. Since such sequences were not detected at the C termini of periplasmic proteins, it appears to be possible to discriminate between the majority of outer membrane proteins and periplasmic proteins on the basis of sequence data. The highly conserved phenylalanine at the C termini of outer membrane proteins suggests an important function for this amino acid in assembly into the outer membrane. Site-directed mutagenesis was applied to study the role of the C-terminal Phe in PhoE protein assembly. All mutant proteins were correctly incorporated into the outer membrane to some extent, but the efficiency of the process was severely affected. It appears that both the hydrophobicity and the aromatic nature of Phe are of importance.

Comparison of the class 1 outer membrane proteins of eight serological reference strains of Neisseria meningitidis

Primers suitable for the amplification of the gene encoding the class 1 outer membrane protein of Neisseria meningitidis by the polymerase chain reaction (PCR) were designed from published DNA sequences and used to study the gene in eight meningococcal strains of different serogroup, serotype and subtype. At high annealing stringency one product, shown to correspond to the class 1 protein gene, was amplified from each strain. For three strains an additional smaller product, provisionally identified as the gene encoding the class 3 outer membrane protein, was amplified at lower annealing stringencies. Nucleotide sequence analysis of the PCR products corresponding to the class 1 proteins established the differences in the primary structure of the proteins between each of the subtypes and other outer-membrane proteins from Neisseria spp. These differences impose constraints on possible structural models of these proteins. Most amino acid sequence variation occurred in two domains of between 8 and 17 amino acids; there was an additional region which varied mainly between classes of outer membrane protein and there were nine conserved regions. Using appropriate primers it was possible to distinguish between class 1 outer membrane protein genes from strains of different subtypes by the PCR.

A conserved domain on enterobacterial porin protein analysed by monoclonal antibody

Broadly cross-reactive monoclonal antibodies (MAbs) against enterobacterial outer membrane (OM) porin (Po) protein were isolated after immunization of BALB/c mice with whole cells of E. coli 055:B5. MAbs (n = 6) of the IgG class but of four different isotypes were studied. Based on a competition ELISA, all of the MAbs were directed against one and the same Po protein domain (Po I). The MAbs cross-reacted with 72 of 74 strains from 10 different genera of the Enterobacteriaceae. One Morganella and one Salmonella strain showed no cross-reactivity. Also, nine strains of various Neisseria spp. cross-reacted while 21 strains of various other nonenteric Gram-negative bacteria showed no cross-reactivity. The Po I sites were inaccessible in intact homologous bacteria but partially accessible in the OM. Digestion of OM with lysozyme or lysostaphin affected the accessibility of the Po I sites in OMs of various enterobacteria. Lysostaphin strongly enhanced the immunoaccessibility, whereas lysozyme had lesser effects. The enzymes also affected the binding by Neisseria OMs of the anti-Po I MAb. The Po I site was immunogenic both in humans and rabbits. The data indicate that Po I is an important Po protein domain, and that the effects of peptidoglycan-degrading enzymes must be considered in studies of Po protein domains.

Meningococcal vaccines--present and future

Over 20 years after the development of the meningococcal A and C vaccines, an effective vaccine against Neisseria meningitidis group B is still lacking. Major obstacles in the development of a B vaccine have been the remarkable capacity of the organism to evade the immune defences of the host and the lack of a predictive animal model. Three group B vaccines based on outer membrane proteins have been, or are currently being, evaluated in field trials. Nevertheless, a number of important questions remain such as the identity of the active components, the degree of efficacy against heterologous group B subtypes, and the duration of protection. In addition, work on a variety of alternative approaches to a group B vaccine is rapidly progressing. Among these are use of chemically modified group B polysaccharide, synthetic or natural lipopolysaccharide epitopes, synthetic peptides corresponding to bactericidal epitopes on the class 1 outer membrane protein, and iron binding proteins. Although each of these approaches has some problems associated with it, the prospects remain good for an effective solution to the group B problem.

Two distinct outer membrane serotype subcomplexes of Neisseria meningitidis serogroup A

A 350-kilodalton serotype outer membrane complex containing the class 1, 3, and 4 outer membrane proteins was isolated from serogroup A Neisseria meningitidis. Partial denaturation yielded two serotype subcomplexes containing the class 3 and 1 proteins (85 kilodaltons) and the class 3 and 4 proteins (94 kilodaltons), respectively.

Isolation of Neisseria meningitidis mutants deficient in class 1 (porA) and class 3 (porB) outer membrane proteins

The class 1 major outer membrane protein of Neisseria meningitidis is a serious candidate for a meningococcal vaccine. To facilitate studies on the function of this protein, mutants were isolated that lacked this protein or the structurally related class 3 protein. These mutants were obtained by using the antibody-dependent bactericidal action of the complement system. The class 1 protein-deficient strain grew normally in vitro, whereas growth of the class 3 protein-deficient strain was slightly retarded. The class 3 protein-deficient strain displayed increased resistance to the antibiotics tetracycline and cefsulodin, which is consistent with the proposed role of the protein as a pore-forming protein. The class 1 protein was purified to homogeneity from the class 3 protein-deficient strain. Lipid bilayer experiments revealed that this protein also formed pores. The class 1 protein pores were cation selective.