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

Triplin: Functional Probing of Its Structure and the Dynamics of the Voltage-Gating Process

Gram-negative bacteria have a large variety of channel-forming proteins in their outer membrane, generally referred to as porins. Some display weak voltage dependence. A similar trimeric channel former, named Triplin, displays very steep voltage dependence, rivaling that responsible for the electrical excitability of mammals, and high inter-subunit cooperativity. We report detailed insights into the molecular basis for these very unusual properties explored at the single-molecule level. By using chemical modification to reduce the charge on the voltage sensors, they were shown to be positively charged structures. Trypsin cleavage of the sensor eliminates voltage gating by cleaving the sensor. From asymmetrical addition of these reagents, the positively charged voltage sensors translocate across the membrane and are, thus, responsible energetically for the steep voltage dependence. A mechanism underlying the cooperativity was also identified. Theoretical calculations indicate that the charge on the voltage sensor can explain the rectification of the current flowing through the open pores if it is located near the pore mouth in the open state. All results support the hypothesis that one of the three subunits is oriented in a direction opposite to that of the other two. These properties make Triplin perhaps the most complex pore-forming molecular machine described to date.

The potential utility of liposomes for Neisseria vaccines

INTRODUCTION

Species of the genus Neisseria are important global pathogens. Neisseria gonorrhoeae (gonococcus) causes the sexually transmitted disease gonorrhea and Neisseria meningitidis (meningococcus) causes meningitis and sepsis. Liposomes are self-assembled spheres of phospholipid bilayers enclosing a central aqueous space, and they have attracted much interest and use as a delivery vehicle for Neisseria vaccine antigens.

AREAS COVERED

A brief background on Neisseria infections and the success of licensed meningococcal vaccines are provided. The absence of a gonococcal vaccine is highlighted. The use of liposomes for delivering Neisseria antigens and adjuvants, for the purposes of generating specific immune responses, is reviewed. The use of other lipid-based systems for antigen and adjuvant delivery is examined briefly.

EXPERT OPINION

With renewed interest in developing a gonococcal vaccine, liposomes remain an attractive option for delivering antigens. The discipline of nanotechnology provides additional nanoparticle-based options for gonococcal vaccine development. Future work would be needed to tailor the composition of liposomes and other nanoparticles to the specific vaccine antigen(s), in order to generate optimal anti-gonococcal immune responses. The potential use of liposomes and other nanoparticles to deliver anti-gonococcal compounds to treat infections also should be explored further.

Heterogeneous Idealization of Ion Channel Recordings - Open Channel Noise

We propose a new model-free segmentation method for idealizing ion channel recordings. This method is designed to deal with heterogeneity of measurement errors. This in particular applies to open channel noise which, in general, is particularly difficult to cope with for model-free approaches. Our methodology is able to deal with lowpass filtered data which provides a further computational challenge. To this end we propose a multiresolution testing approach, combined with local deconvolution to resolve the lowpass filter. Simulations and statistical theory confirm that the proposed idealization recovers the underlying signal very accurately at presence of heterogeneous noise, even when events are shorter than the filter length. The method is compared to existing approaches in computer experiments and on real data. We find that it is the only one which allows to identify openings of the PorB porine at two different temporal scales. An implementation is available as an R package.

High-resolution experimental and computational electrophysiology reveals weak β-lactam binding events in the porin PorB

The permeation of most antibiotics through the outer membrane of Gram-negative bacteria occurs through porin channels. To design drugs with increased activity against Gram-negative bacteria in the face of the antibiotic resistance crisis, the strict constraints on the physicochemical properties of the permeants imposed by these channels must be better understood. Here we show that a combination of high-resolution electrophysiology, new noise-filtering analysis protocols and atomistic biomolecular simulations reveals weak binding events between the β-lactam antibiotic ampicillin and the porin PorB from the pathogenic bacterium Neisseria meningitidis. In particular, an asymmetry often seen in the electrophysiological characteristics of ligand-bound channels is utilised to characterise the binding site and molecular interactions in detail, based on the principles of electro-osmotic flow through the channel. Our results provide a rationale for the determinants that govern the binding and permeation of zwitterionic antibiotics in porin channels.

The Role of Meningococcal Porin B in Protein-Protein Interactions with Host Cells

Neisseria meningitidis is a Gram-negative diplococcus responsible for bacterial meningitis and fatal sepsis. Ligand-receptor interactions are one of the main steps in the development of neuroinvasion. Porin B (PorB), neisserial outer membrane protein (ligand), binds to host receptors and triggers many cell signalling cascades allowing the meningococcus to damage the host cells or induce immune cells responses via the TLR2-dependent mechanisms. In this paper, we present a brief review of the structure and function of PorB.

Neisseria meningitidis Lacking the Major Porins PorA and PorB Is Viable and Modulates Apoptosis and the Oxidative Burst of Neutrophils

The bacterial pathogen Neisseria meningitidis expresses two major outer-membrane porins. PorA expression is subject to phase-variation (high frequency, random, on-off switching), and both PorA and PorB are antigenically variable between strains. PorA expression is variable and not correlated with meningococcal colonisation or invasive disease, whereas all naturally-occurring strains express PorB suggesting strong selection for expression. We have generated N. meningitidis strains lacking expression of both major porins, demonstrating that they are dispensable for bacterial growth in vitro. The porAB mutant strain has an exponential growth rate similar to the parental strain, as do the single porA or porB mutants, but the porAB mutant strain does not reach the same cell density in stationary phase. Proteomic analysis suggests that the double mutant strain exhibits compensatory expression changes in proteins associated with cellular redox state, energy/nutrient metabolism, and membrane stability. On solid media, there is obvious growth impairment that is rescued by addition of blood or serum from mammalian species, particularly heme. These porin mutants are not impaired in their capacity to inhibit both staurosporine-induced apoptosis and a phorbol 12-myristate 13-acetate-induced oxidative burst in human neutrophils suggesting that the porins are not the only bacterial factors that can modulate these processes in host cells.

Tuning the affinity of anion binding sites in porin channels with negatively charged residues: molecular details for OprP

The cell envelope of the Gram negative opportunistic pathogen Pseudomonas aeruginosa is poorly permeable to many classes of hydrophilic molecules including antibiotics due to the presence of the narrow and selective porins. Here we focused on one of the narrow-channel porins, that is, OprP, which is responsible for the high-affinity uptake of phosphate ions. Its two central binding sites for phosphate contain a number of positively charged amino acids together with a single negatively charged residue (D94). The presence of this negatively charged residue in a binding site for negatively charged phosphate ions is highly surprising due to the potentially reduced binding affinity. The goal of this study was to better understand the role of D94 in phosphate binding, selectivity, and transport using a combination of mutagenesis, electrophysiology, and free-energy calculations. The presence of a negatively charged residue in the binding site is critical for this specific porin OprP as emphasized by the evolutionary conservation of such negatively charged residue in the binding site of several anion-selective porins. Mutations of D94 in OprP to any positively charged or neutral residue increased the binding affinity of phosphate for OprP. Detailed analysis indicated that this anionic residue in the phosphate binding site of OprP, despite its negative charge, maintained energetically favorable phosphate binding sites in the central region of the channel and at the same time decreased residence time thus preventing excessively strong binding of phosphate that would oppose phosphate flux through the channel. Intriguingly mutations of D94 to positively charged residues, lysine and arginine, resulted in very different binding affinities and free energy profiles, indicating the importance of side chain conformations of these positively charged residues in phosphate binding to OprP.

Structure and function of the PorB porin from disseminating Neisseria gonorrhoeae

The outer membrane of Gram-negative bacteria contains a large number of channel-forming proteins, porins, for the uptake of small nutrient molecules. Neisseria gonorrhoeae PorBIA (PorB of serotype A) are associated with disseminating diseases and mediate a rapid bacterial invasion into host cells in a phosphate-sensitive manner. To gain insights into this structure-function relationship we analysed PorBIA by X-ray crystallography in the presence of phosphate and ATP. The structure of PorBIA in the complex solved at a resolution of 3.3 Å (1 Å=0.1 nm) displays a surplus of positive charges inside the channel. ATP ligand-binding in the channel is co-ordinated by the positively charged residues of the channel interior. These residues ligate the aromatic, sugar and pyrophosphate moieties of the ligand. Two phosphate ions were observed in the structure, one of which clamped by two arginine residues (Arg92 and Arg124) localized at the extraplasmic channel exit. A short β-bulge in β2-strand together with the long L3 loop narrow the barrel diameter significantly and further support substrate specificity through hydrogen bond interactions. Interestingly the structure also comprised a small peptide as a remnant of a periplasmic protein which physically links porin molecules to the peptidoglycan network. To test the importance of Arg92 on bacterial invasion the residue was mutated. In vivo assays of bacteria carrying a R92S mutation confirmed the importance of this residue for host-cell invasion. Furthermore systematic sequence and structure comparisons of PorBIA from Neisseriaceae indicated Arg92 to be unique in disseminating N. gonorrhoeae thereby possibly distinguishing invasion-promoting porins from other neisserial porins.

Voltage dependent closure of PorB class II porin from Neisseria meningitidis investigated using impedance spectroscopy in a tethered bilayer lipid membrane interface

Electrochemical impedance spectroscopy (EIS) was used to characterize voltage-dependent closure of PorB class II (PorBII) porin from Neisseria meningitidis incorporated in a tethered bilayer lipid membrane (tBLM). The tBLM's lower leaflet was fabricated by depositing a self assembled monolayer (SAM) of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) on a gold electrode, and the upper leaflet was formed by depositing1,2-dioleoyl-sn-glycero-3-phoshocholine (DOPC) liposomes. At 0mV bias DC potential, incorporation of PorBII decreased the membrane resistance (R(m)) from 2.5 MΩc m(2) to 0.6 MΩ cm(2), giving a ΔR(m) of 1.9 MΩ cm(2) and a normalized ΔR(m) (ΔR(m) divided by the R(m) of the tBLM without PorBII) of 76%. When the bias DC potential was increased to 200 mV, the normalized ΔR(m) value decreased to 20%. The effect of applied voltage on ΔR(m) was completely reversible, suggesting voltage-dependent closure of PorBII. The voltage dependence of PorBII was further studied in a planar bilayer lipid membrane made from 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhytPC). Following a single insertion event, PorBII exhibited multiple conductance states, with reversible, voltage-dependent closure of PorBII porin occurring at high transmembrane potentials. The trimetric porin closed in three discrete steps, each step corresponding to closure of one conducting monomer unit. The most probable single channel conductance was 4.2 nS. The agreement between results obtained with the tBLM and pBLM platforms demonstrates the utility of EIS to screen channel proteins immobilized in tBLM for voltage-gated behavior.

Computational electrophysiology: the molecular dynamics of ion channel permeation and selectivity in atomistic detail

Presently, most simulations of ion channel function rely upon nonatomistic Brownian dynamics calculations, indirect interpretation of energy maps, or application of external electric fields. We present a computational method to directly simulate ion flux through membrane channels based on biologically realistic electrochemical gradients. In close analogy to single-channel electrophysiology, physiologically and experimentally relevant timescales are achieved. We apply our method to the bacterial channel PorB from pathogenic Neisseria meningitidis, which, during Neisserial infection, inserts into the mitochondrial membrane of target cells and elicits apoptosis by dissipating the membrane potential. We show that our method accurately predicts ion conductance and selectivity and elucidates ion conduction mechanisms in great detail. Handles for overcoming channel-related antibiotic resistance are identified.

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.

Bacterial porin disrupts mitochondrial membrane potential and sensitizes host cells to apoptosis

The bacterial PorB porin, an ATP-binding β-barrel protein of pathogenic Neisseria gonorrhoeae, triggers host cell apoptosis by an unknown mechanism. PorB is targeted to and imported by host cell mitochondria, causing the breakdown of the mitochondrial membrane potential (ΔΨ_m). Here, we show that PorB induces the condensation of the mitochondrial matrix and the loss of cristae structures, sensitizing cells to the induction of apoptosis via signaling pathways activated by BH3-only proteins. PorB is imported into mitochondria through the general translocase TOM but, unexpectedly, is not recognized by the SAM sorting machinery, usually required for the assembly of β-barrel proteins in the mitochondrial outer membrane. PorB integrates into the mitochondrial inner membrane, leading to the breakdown of ΔΨ_m. The PorB channel is regulated by nucleotides and an isogenic PorB mutant defective in ATP-binding failed to induce ΔΨ_m loss and apoptosis, demonstrating that dissipation of ΔΨ_m is a requirement for cell death caused by neisserial infection.

PorB is a bacterial porin that plays an important role in the pathogenicity of Neisseria gonorrhoeae. Upon infection with these bacteria, PorB is transported into mitochondria of infected cells, causing the loss of mitochondrial membrane potential and eventually leading to apoptotic cell death. Here, we show that PorB enters mitochondria through the TOM complex, similar to other mitochondria-targeted proteins, but then bypasses the SAM complex machinery that assembles all other porin-like proteins into the outer mitochondrial membrane. This leads to the accumulation of PorB in the intermembrane space and the integration of a fraction of PorB into the inner mitochondrial membrane (IMM). In the IMM, ATP-regulated pores are formed, leading to dissipation of membrane potential and the loss of cristae structure in affected mitochondria, the necessary first steps in induction of apoptosis. Our work offers, for the first time, a detailed analysis of the mechanism by which PorB targets and damages host cell mitochondria.

Junctin and triadin each activate skeletal ryanodine receptors but junctin alone mediates functional interactions with calsequestrin

Normal Ca(2+) signalling in skeletal muscle depends on the membrane associated proteins triadin and junctin and their ability to mediate functional interactions between the Ca(2+) binding protein calsequestrin and the type 1 ryanodine receptor in the lumen of the sarcoplasmic reticulum. This important mechanism conserves intracellular Ca(2+) stores, but is poorly understood. Triadin and junctin share similar structures and are lumped together in models of interactions between skeletal muscle calsequestrin and ryanodine receptors, however their individual roles have not been examined at a molecular level. We show here that purified skeletal ryanodine receptors are similarly activated by purified triadin or purified junctin added to their luminal side, although a lack of competition indicated that the proteins act at independent sites. Surprisingly, triadin and junctin differed markedly in their ability to transmit information between skeletal calsequestrin and ryanodine receptors. Purified calsequestrin inhibited junctin/triadin-associated, or junctin-associated, ryanodine receptors and the calsequestrin re-associated channel complexes were further inhibited when luminal Ca(2+) fell from 1mM to <or=100 microM, as seen with native channels (containing endogenous calsequestrin/triadin/junctin). In contrast, skeletal calsequestrin had no effect on the triadin/ryanodine receptor complex and the channel activity of this complex increased when luminal Ca(2+) fell, as seen with purified channels prior to triadin/calsequestrin re-association. Therefore in this cell free system, junctin alone mediates signals between luminal Ca(2+), skeletal calsequestrin and skeletal ryanodine receptors and may curtail resting Ca(2+) leak from the sarcoplasmic reticulum. We suggest that triadin serves a different function which may dominate during excitation-contraction coupling.

Neisseria meningitidis PorB, a TLR2 ligand, induces an antigen-specific eosinophil recall response: potential adjuvant for helminth vaccines?

Efficacious adjuvants are important components of new vaccines. The neisserial outer membrane protein, PorB, is a TLR2 ligand with unique adjuvant activity. We demonstrate that PorB promotes Th2-skewed cellular immune response to the model Ag, OVA, in mice, including Ag-specific recall eosinophil recruitment to the peritoneum. PorB induces chemokine secretion by myeloid cells using both TLR2-dependent and -independent mechanisms, suggesting that anatomical distribution of TLR2(+) cells may not be a limiting factor for potential vaccine strategies. The results from this study suggest that PorB, and other TLR2 ligands, may be ideal for use against pathogens where eosinophilia may be protective, such as parasitic helminths.

Structure of the Complex of the Colicin E2 R-domain and Its BtuB Receptor

The crystal structure of the complex of the BtuB receptor and the 135-residue coiled-coil receptor-binding R-domain of colicin E3 (E3R135) suggested a novel mechanism for import of colicin proteins across the outer membrane. It was proposed that one function of the R-domain, which extends along the outer membrane surface, is to recruit an additional outer membrane protein(s) to form a translocon for passage colicin activity domain. A 3.5-A crystal structure of the complex of E2R135 and BtuB (E2R135-BtuB) was obtained, which revealed E2R135 bound to BtuB in an oblique orientation identical to that previously found for E3R135. The only significant difference between the two structures was that the bound coiled-coil R-domain of colicin E2, compared with that of colicin E3, was extended by two and five residues at the N and C termini, respectively. There was no detectable displacement of the BtuB plug domain in either structure, implying that colicin is not imported through the outer membrane by BtuB alone. It was concluded that the oblique orientation of the R-domain of the nuclease E colicins has a function in the recruitment of another member(s) of an outer membrane translocon. Screening of porin knock-out mutants showed that either OmpF or OmpC can function in such a translocon. Arg(452) at the R/C-domain interface in colicin E2 was found have an essential role at a putative site of protease cleavage, which would liberate the C-terminal activity domain for passage through the outer membrane translocon.

Minimum length requirement of the flexible N-terminal translocation subdomain of colicin E3

The 315-residue N-terminal T domain of colicin E3 functions in translocation of the colicin across the outer membrane through its interaction with outer membrane proteins including the OmpF porin. The first 83 residues of the T domain are known from structure studies to be disordered. This flexible translocation subdomain contains the TolB box (residues 34 to 46) that must cross the outer membrane in an early translocation event, allowing the colicin to bind to the TolB protein in the periplasm. In the present study, it was found that cytotoxicity of the colicin requires a minimum length of 19 to 23 residues between the C terminus (residue 46) of the TolB box and the end of the flexible subdomain (residue 83). Colicin E3 molecules of sufficient length display normal binding to TolB and occlusion of OmpF channels in vitro. The length of the N-terminal subdomain is critical because it allows the TolB box to cross the outer membrane and interact with TolB. It is proposed that the length constraint is a consequence of ordered structure in the downstream segment of the T domain (residues 84 to 315) that prevents its insertion through the outer membrane via a translocation pore that includes OmpF.

Porin-mediated antibiotic resistance in Neisseria gonorrhoeae: ion, solute, and antibiotic permeation through PIB proteins with penB mutations

Neisseria gonorrhoeae has two porins, PIA and PIB, whose genes (porA and porB, respectively) are alleles of a single por locus. We recently demonstrated that penB mutations at positions 120 and 121 in PIB, which are presumed to reside in loop 3 that forms the pore constriction zone, confer intermediate-level resistance to penicillin and tetracycline (M. Olesky, M. Hobbs, and R. A. Nicholas, Antimicrob. Agents Chemother. 46:2811-2820, 2002). In the present study, we investigated the electrophysiological properties as well as solute and antibiotic permeation rates of recombinant PIB proteins containing penB mutations (G120K, G120D/A121D, G120P/A121P, and G120R/A121H). In planar lipid bilayers, the predominant conducting state of each porin variant was 30 to 40% of the wild type, even though the anion selectivity and maximum channel conductance of each PIB variant was similar to that of the wild type. Liposome-swelling experiments revealed no significant differences in the permeation of sugars or beta-lactam antibiotics through the wild type or PIB variants. Although these results are seemingly contradictory with the ability of these variants to increase antibiotic resistance, they are consistent with MIC data showing that these porin mutations confer resistance only in strains containing an mtrR mutation, which increases expression of the MtrC-MtrD-MtrE efflux pump. Moreover, both the mtrR and penB mutations were required to decrease in vivo permeation rates below those observed in the parental strain containing either mtrR or porin mutations alone. Thus, these data demonstrate a novel mechanism of porin-mediated resistance in which mutations in PIB have no affect on antibiotic permeation alone but instead act synergistically with the MtrC-MtrD-MtrE efflux pump in the development of antibiotic resistance in gonococci.

Energetics of membrane protein folding and stability

The critical role of membrane proteins in a myriad of biological and physiological functions has spawned numerous investigations over the past several decades with the long-term goal of identifying the molecular origins and energetic forces that stabilize these proteins within the membrane. Parallel structural and thermodynamics studies on several systems have provided significant insight regarding the driving forces governing folding, assembly, insertion, and translocation of membrane proteins. The present review surveys families of membrane-associated proteins including alpha-helical and beta-barrel structures, viral surface receptors, and pore-forming toxins, citing representative proteins within each of these classes for further scrutiny in terms of structure-function relationships and global conformational stability. This overview presents seminal findings from pioneering studies on the energetics of membrane protein folding and stability to modern techniques that are exploiting the use of molecular genetics and single molecule studies. An overall consensus regarding the molecular origins of membrane protein stability is that a number of intrinsic properties resemble features of soluble proteins, yet there are distinct energetic differences arising from specific intra- and intermolecular interactions within the membrane. The combined efforts from structural, energetics, and dynamics approaches offer unique insights and improve our fundamental understanding of the driving forces dictating membrane protein folding and stability.

Meningococcal porin PorB binds to TLR2 and requires TLR1 for signaling

TLR2 plays a key role in the initiation of the cellular innate immune responses by a wide range of bacterial products. TLRs signaling, including TLR2 and its coreceptors TLR1 and TLR6, is mediated by a number of specific ligands. Although many of the TLR-mediated cell signaling pathways have been elucidated in the past few years, the molecular mechanisms that lead to cell activation are still poorly understood. In this study, we investigate the interaction of PorB from Neisseria meningitidis with TLR2 and describe the direct binding of a bacterial protein to TLR2 for the first time. Using labeled PorB, we demonstrate its binding to TLR2 both in its soluble form in vitro, and when it is over-expressed on the surface of human embryonic kidney 293 cells. We also show that TLR2-mediated binding of PorB is directly related to cellular activation. In addition, using 293 cells expressing the chimeric TLR2/TLR1 and TLR2/TLR6 complexes, we report the selectivity of PorB binding to the TLR2/TLR1 heterodimer, which is required for initiating signaling in transfected 293 cells and in murine B cells. Together, these data provide new evidence that TLR2 recognizes PorB through direct binding, and that PorB-induced cell activation is mediated by a TLR2/TLR1 complex.

Colicin occlusion of OmpF and TolC channels: outer membrane translocons for colicin import

The interaction of colicins with target cells is a paradigm for protein import. To enter cells, bactericidal colicins parasitize Escherichia coli outer membrane receptors whose physiological purpose is the import of essential metabolites. Colicins E1 and E3 initially bind to the BtuB receptor, whose beta-barrel pore is occluded by an N-terminal globular "plug". The x-ray structure of a complex of BtuB with the coiled-coil BtuB-binding domain of colicin E3 did not reveal displacement of the BtuB plug that would allow passage of the colicin (Kurisu, G., S. D. Zakharov, M. V. Zhalnina, S. Bano, V. Y. Eroukova, T. I. Rokitskaya, Y. N. Antonenko, M. C. Wiener, and W. A. Cramer. 2003. Nat. Struct. Biol. 10:948-954). This correlates with the inability of BtuB to form ion channels in planar bilayers, shown in this work, suggesting that an additional outer membrane protein(s) is required for colicin import across the outer membrane. The identity and interaction properties of this OMP were analyzed in planar bilayer experiments.OmpF and TolC channels in planar bilayers were occluded by colicins E3 and E1, respectively, from the trans-side of the membrane. Occlusion was dependent upon a cis-negative transmembrane potential. A positive potential reversibly opened OmpF and TolC channels. Colicin N, which uses only OmpF for entry, occludes OmpF in planar bilayers with the same orientation constraints as colicins E1 and E3. The OmpF recognition sites of colicins E3 and N, and the TolC recognition site of colicin E1, were found to reside in the N-terminal translocation domains. These data are considered in the context of a two-receptor translocon model for colicin entry into cells.

Does VDAC insert into membranes in random orientation?

It is widely accepted that voltage-dependent anion-selective channel (VDAC) inserts into planar lipid bilayers in a random orientation. This is in contrast to the well-documented oriented insertion of various channel-forming proteins. Because of the potential importance of this issue, we have examined the orientation of VDAC inserted in membranes. The time constants of the VDAC-current relaxation in response to applied positive and negative voltage pulses were used to characterize the channel orientation. We have found that VDAC channels can be separated into two groups according to differences in the time constant ratio. The difference in time constant ratio between the two main groups of VDAC channels was quantitative, and not qualitative as would be expected for opposite topologies. This finding allows us to hypothesize that both groups of VDAC channels possess a qualitatively similar asymmetry with respect to the localization of voltage-gated domains and, consequently, with respect to its entire molecular structure. The probability of having each type of VDAC channel conformation is predetermined by the protein structure in aqueous solution. A striking resemblance between asymmetry in voltage sensitivity at the single-channel and multi-channel levels was also demonstrated. The first inserted channel seems to direct subsequent insertions of channels with a similar conformation.

The role of porins in neisserial pathogenesis and immunity

Neisseria meningitidis and Neisseria gonorrhoeae are Gram-negative pathogenic bacteria responsible for bacterial meningitis and septicemia, and the sexually transmitted disease gonorrhea, respectively. 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 (APCs). Their effect on the immune response is mediated by upregulation of the costimulatory molecule B7-2 (CD86) on the surface of APCs, an effect that is Toll-like receptor 2- and MyD88-dependent. The effect of neisserial porins on the immune system also involves interaction with components of the complement cascade. Furthermore, neisserial porins co-localize with mitochondria of target cells, where they appear to modulate apoptosis.

Immunization with the recombinant PorB outer membrane protein induces a bactericidal immune response against Neisseria meningitidis

Infections with Neisseria meningitidis are characterized by life-threatening meningitis and septicemia. The meningococcal porin proteins from serogroup B meningococci have been identified as candidates for inclusion in vaccines to prevent such infections. In this study, we investigated the vaccine potential of the PorB porin protein free of other meningococcal components. The porB gene from a strain of Neisseria meningitidis expressing the class 3 outer membrane porin protein (PorB3) was cloned into the pRSETB vector, and the protein was expressed at high levels in a heterologous host Escherichia coli. The recombinant protein was purified to homogeneity by affinity chromatography and used for immunization after incorporation into liposomes and into micelles composed either of zwitterionic detergent or nondetergent sulfobetaine. The immunogenicity of these preparations was compared to recombinant PorB protein adsorbed to Al(OH)(3) adjuvant as a control. Although sera raised against the protein adsorbed to Al(OH)(3) reacted with the purified recombinant protein, sera raised against liposomes and micelles showed greater activity with native protein, as measured by enzyme immunoassay with outer membranes and by whole-cell immunofluorescence. Reactivity with native protein was considerably enhanced by incorporation of the adjuvant monophosphoryl lipid A into the liposome or micelle preparations. Recognition of the native protein was in a serotype-specific manner and was associated with the ability of the antisera to promote high levels of serotype-specific complement-mediated killing of meningococci. These results demonstrate that the PorB protein should be considered as a component of a vaccine designed to prevent serogroup B meningococcal infection.

Neisseria meningitidis porin PorB interacts with mitochondria and protects cells from apoptosis

Neisserial porins are strong immune adjuvants and B cell activators. The effect of neisserial porin PorB on activation-induced cell death was investigated, as a potential additional mechanism of the porin's immunopotentiating ability. Neisserial porins interact with target cells to localize intracellularly in the mitochondrial compartment without negatively affecting cellular survival. Pretreatment with Neisseria meningitidis PorB porin decreased or abrogated the mitochondrial damage induced by apoptotic stimuli. In addition, end stage determinants of apoptosis, including DNA breakdown, were diminished by PorB. Immunoprecipitation experiments revealed that PorB interacts with the mitochondrial porin VDAC (voltage-dependent anion channel). The mechanism of the antiapoptotic effect of neisserial porins could be explained by the protein-protein interaction of PorB with VDAC, similar to the interaction of VDAC with antiapoptotic Bcl-2 proteins, resulting in an enhancement of cell survival and continued activation of B cells.

Host cell invasion by pathogenic Neisseriae

As outlined in this review, various experimental techniques have been employed in an attempt to understand neisserial pathogenesis. In vitro genetic analysis has been used to study the genetic basis for the structural variability of cell surface components. Transformed or primary epithelial cell cultures have provided the simplest model to analyze bacterial adherence and invasion, while the infection of polarized epithelial monolayers, fallopian tube and nasopharyngeal organ cultures, and ureteral tissue have each been used to more closely represent the events which occur in vivo. Finally, the in vivo infection of human volunteers with N. gonorrhoeae has provided a powerful means to confirm and expand the results obtained in vitro. By these various approaches, a number of neisserial adhesins (i.e. pilli, Opa, Opc and P36) and additional putative virulence determinants which affect bacterial adherence and invasion into host cells (i.e. LOS, capsule, PorB) have been identified. Clearly, neisserial surface variation serves as an adaptive mechanism which can modulate tissue tropism, immune evasion and survival in the changing host environment. Important progress has been made in recent years with respect to the host cellular receptors and subsequent signal transduction processes which are involved in neisserial adherence, invasion and transcytosis. This has led to the identification of (i) CD46 as a receptor for pilus which allows adherence to epithelial and endothelial cells, (ii) HSPGs, in cooperation with vitronectin and fibronectin, as receptors for a particular subset of Opa proteins and Opc, which may both mediate invasion into most epithelial and endothelial cells, and (iii) CD66 as the receptors for most Opa variants, potentially being involved in cellular interactions including adherence, invasion and transcytosis with epithelial, endothelial and phagocytic cells. As most of these data have been obtained using transformed cell lines growing in vitro, attempts must be made to translate these basic observations into a more natural situation. It can be expected that the successful ongoing integration of laboratory findings from the various infection models with human volunteer studies will further increase our understanding of the biology of neisserial infection. Perhaps the most difficult but also most rewarding challenge for the future will be to use volunteer studies to identify and understand the role of host factors which are important for the infectious process. Hopefully, insights gained from each of these studies will reveal new and useful strategies for the preventive and/or therapeutic intervention into infection and disease by these fascinating microbes.

Biochemical and biophysical characterization of in vitro folded outer membrane porin PorA of Neisseria meningitidis

Two subtypes of the outer membrane porin PorA of Neisseria meningitidis, P1.6 and P1.7,16, were folded in vitro after overexpression in, and isolation from Escherichia coli. The PorA porins could be folded efficiently by quick dilution in an appropriate buffer containing the detergent n-dodecyl-N, N-dimethyl-1-ammonio-3-propanesulphonate. Although the two PorA porins are highly homologous, they required different acidities for optimal folding, that is, a pH above the pI was needed for efficient folding. Furthermore, whereas trimers of PorA P1.7,16 were almost completely stable in 2% sodium dodecyl sulphate (SDS), those of P1.6 dissociated in the presence of SDS. The higher electrophoretic mobility of the in vitro folded porins could be explained by the stable association of the RmpM protein to the porins in vivo. This association of RmpM contributes to the stability of the porins. The P1.6 pores were moderately cation-selective and displayed a single-channel conductance of 2.8 nS in 1 M KCl. The PorA P1.6 pores, but not the PorA P1.7,16 pores, showed an unusual non-linear dependence of the single-channel conductance on the salt concentration of the subphase. We hypothesize that a cluster of three negatively charged residues in L5 of P1.6 is responsible for the higher conductance at low salt concentrations.

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.

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.