In vitro trimerization of OmpF porin secreted by spheroplasts of Escherichia coli

Proteomic and bioinformatic analyses of proteins in the outer membrane and extracellular compartments and outer membrane vesicles of Candidatus Liberibacter species

Citrus Huanglongbing (HLB) is the most devastating citrus disease in the world. Candidatus Liberibacter asiaticus (Las) is the prevalent HLB pathogen, which is yet to be cultivated. A recent study demonstrates that Las does not contain pathogenicity factors that are directly responsible for HLB symptoms. Instead, Las triggers systemic and chronic immune responses, representing a pathogen-triggered immune disease. Importantly, overproduction of reactive oxygen species (ROS) causes systemic cell death of phloem tissues, thus causing HLB symptoms. Because Las resides in the phloem tissues, it is expected that phloem cell might recognize outer membrane proteins, outer membrane vesicle (OMV) proteins and extracellular proteins of Las to contribute to the immune responses. Because Las has not been cultivated, we used Liberibacter crescens (Lcr) as a surrogate to identify proteins in the OM fraction, OMV proteins and extracellular proteins by liquid chromatography with tandem mass spectrometry (LC–MS/MS). We observed OMVs of Lcr under scanning electron microscope, representing the first experimental evidence that Liberibacter can deliver proteins to the extracellular compartment. In addition, we also further analyzed LC–MS/MS data using bioinformatic tools. Our study provides valuable information regarding the biology of Ca. Liberibacter species and identifies many putative proteins that may interact with host proteins in the phloem tissues.

Reconstitution of Bam Complex-Mediated Assembly of a Trimeric Porin into Proteoliposomes

Many integral membrane proteins form oligomeric complexes, but the assembly of these structures is poorly understood. Here, we show that the assembly of OmpC, a trimeric porin that resides in the Escherichia coli outer membrane (OM), can be reconstituted in vitro. Although we observed the insertion of both urea-denatured and in vitro-synthesized OmpC into pure lipid vesicles at physiological pH, the protein assembled only into dead-end dimers. In contrast, in vitro-synthesized OmpC was inserted into proteoliposomes that contained the barrel assembly machinery (Bam) complex, a conserved heterooligomer that catalyzes protein integration into the bacterial OM, and folded into heat-stable trimers by passing through a short-lived dimeric intermediate. Interestingly, complete OmpC assembly was also dependent on the addition of lipopolysaccharide (LPS), a glycolipid located exclusively in the OM. Our results strongly suggest that trimeric porins form through a stepwise process that requires the integration of the protein into the OM in an assembly-competent state. Furthermore, our results provide surprising evidence that interaction with LPS is required not only for trimerization but also for the productive insertion of individual subunits into the lipid bilayer.

Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex

The major class of integral proteins found in the outer membrane (OM) of E. coli and Salmonella adopt a β-barrel conformation (OMPs). OMPs are synthesized in the cytoplasm with a typical signal sequence at the amino terminus, which directs them to the secretion machinery (SecYEG) located in the inner membrane for translocation to the periplasm. Chaperones such as SurA, or DegP and Skp, escort these proteins across the aqueous periplasm protecting them from aggregation. The chaperones then deliver OMPs to a highly conserved outer membrane assembly site termed the Bam complex. In E. coli, the Bam complex is composed of an essential OMP, BamA, and four associated OM lipoproteins, BamBCDE, one of which, BamD, is also essential. Here we provide an overview of what we know about the process of OMP assembly and outline the various hypotheses that have been proposed to explain how proteins might be integrated into the asymmetric OM lipid bilayer in an environment that lacks obvious energy sources. In addition, we describe the envelope stress responses that ensure the fidelity of OM biogenesis and how factors, such as phage and certain toxins, have coopted this essential machine to gain entry into the cell.

An In Vitro Assay for Outer Membrane Protein Assembly by the BAM Complex

To elucidate the mechanism of a biochemical process it is often essential to reconstitute the reaction in vitro using the minimal set of factors required to drive the reaction to completion. Here, we describe a method to reconstitute the folding and membrane integration of bacterial outer membrane (OM) proteins that have a characteristic β-barrel structure. In this method the BAM complex, a heteroligomer that catalyzes the membrane integration of β-barrel proteins, is first purified and inserted into small lipid vesicles. Denatured OM proteins are then assembled and integrated into the vesicles in the presence of a molecular chaperone called SurA.

The lipopolysaccharide export pathway in Escherichia coli: structure, organization and regulated assembly of the Lpt machinery

The bacterial outer membrane (OM) is a peculiar biological structure with a unique composition that contributes significantly to the fitness of Gram-negative bacteria in hostile environments. OM components are all synthesized in the cytosol and must, then, be transported efficiently across three compartments to the cell surface. Lipopolysaccharide (LPS) is a unique glycolipid that paves the outer leaflet of the OM. Transport of this complex molecule poses several problems to the cells due to its amphipatic nature. In this review, the multiprotein machinery devoted to LPS transport to the OM is discussed together with the challenges associated with this process and the solutions that cells have evolved to address the problem of LPS biogenesis.

On the essentiality of lipopolysaccharide to Gram-negative bacteria

Lipopolysaccharide is a highly acylated saccharolipid located on the outer leaflet of the outer membrane of Gram-negative bacteria. Lipopolysaccharide is critical to maintaining the barrier function preventing the passive diffusion of hydrophobic solutes such as antibiotics and detergents into the cell. Lipopolysaccharide has been considered an essential component for outer membrane biogenesis and cell viability based on pioneering studies in the model Gram-negative organisms Escherichia coli and Salmonella. With the isolation of lipopolysaccharide-null mutants in Neisseria meningitidis, Moraxella catarrhalis, and most recently in Acinetobacter baumannii, it has become increasingly apparent that lipopolysaccharide is not an essential outer membrane building block in all organisms. We suggest the accumulation of toxic intermediates, misassembly of essential outer membrane porins, and outer membrane stress response pathways that are activated by mislocalized lipopolysaccharide may collectively contribute to the observed strain-dependent essentiality of lipopolysaccharide.

Reassembly of an integral oligomeric membrane protein OmpF porin in n-octyl beta-D: -glucopyranoside-lipids mixtures

The denatured monomers of an integral membrane protein OmpF porin were refolded and reassembled into its sodium dodecyl sulfate-resistant trimer in mixtures of n-octyl beta-D: -glucopyranoside and lipids. Effective reassembly was observed with a yield of 60-70% when the denatured monomers (0.1 mg/mL) were solubilized at 25 degrees C for 24 h in a refolding medium (pH 6.9) containing 7 mg/mL n-octyl beta-D: -glucopyranoside, 1 mg/mL sodium dodecyl sulfate and 2-2.5 mg/mL soybean asolectin. The reassembled species was characterized in the presence of sodium dodecyl sulfate by physicochemical methods. Low-angle laser light scattering measurements revealed that the molecular weight of the reassembled species is 115,000 +/- 3,500 which corresponds to that of the trimer of this protein. Circular dichroism spectra suggested that the reassembled species is composed of the same beta-structure as the native one. Synchrotron radiation small-angle X-ray scattering measurements confirmed that the reassembled species is a trimer that has the same compactness as the native one.

Biogenesis of the gram-negative bacterial outer membrane

The cell envelope of gram-negative bacteria consists of two membranes, the inner and the outer membrane, that are separated by the periplasm. The outer membrane consists of phospholipids, lipopolysaccharides, integral membrane proteins, and lipoproteins. These components are synthesized in the cytoplasm or at the inner leaflet of the inner membrane and have to be transported across the inner membrane and through the periplasm to assemble eventually in the correct membrane. Recent studies in Neisseria meningitidis and Escherichia coli have led to the identification of several machineries implicated in these transport and assembly processes.

Physicochemical characterization of the reassembled dimer of an integral membrane protein OmpF porin

The in vitro reassembled species of OmpF porin, which was renatured from its denatured monomer using n-octyl-beta-D-glucopyranoside, was characterized by low-angle laser light scattering photometry, circular dichroism spectroscopy and synchrotron radiation small-angle X-ray scattering measurements. The light scattering measurement reconfirmed that the reassembled species was the dimer of the protein. Circular dichroism spectra of the reassembled dimer showed a native-like beta-structure. A small-angle X-ray scattering measurement indicated that the size of the reassembled dimer was nearly equal to that of the native trimer under the present experimental conditions. In a thermal denaturation experiment followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the reassembled dimer was less stable than the native trimer.

Arrangement of the translocator of the autotransporter adhesin involved in diffuse adherence on the bacterial surface

Autotransporters of gram-negative bacteria are single-peptide secretion systems that consist of a functional N-terminal alpha-domain ("passenger") fused to a C-terminal beta-domain ("translocator"). How passenger proteins are translocated through the outer membrane has not been resolved, and at present essentially three different models are discussed. In the widely accepted "hairpin model" the passenger proteins are translocated through a channel formed by the beta-barrel of the translocator that is integrated in the outer membrane. This model has been challenged by a recent proposal for a general autotransporter model suggesting that there is a hexameric translocation pore that is generated by the oligomerization of six beta-domains. A third model suggests that conserved Omp85 participates in autotransporter integration and passenger protein translocation. To examine these models, in this study we investigated the presence of putative oligomeric structures of the translocator of the autotransporter adhesin involved in diffuse adherence (AIDA) in vivo by cross-linking techniques. Furthermore, the capacity of isolated AIDA fusion proteins to form oligomers was studied in vitro by several complementary analytical techniques, such as analytical gel filtration, electron microscopy, immunogold labeling, and cross-linking of recombinant autotransporter proteins in which different passenger proteins were fused to the AIDA translocator. Our results show that the AIDA translocator is mostly present as a monomer. Only a fraction of the AIDA autotransporter was found to form dimers on the bacterial surface and in solution. Higher-order structures, such as hexamers, were not detected either in vivo or in vitro and can therefore be excluded as functional moieties for the AIDA autotransporter.

The periplasmic chaperone SurA exploits two features characteristic of integral outer membrane proteins for selective substrate recognition

The Escherichia coli periplasmic chaperone and peptidyl-prolyl isomerase (PPIase) SurA facilitates the maturation of outer membrane porins. Although the PPIase activity exhibited by one of its two parvulin-like domains is dispensable for this function, the chaperone activity residing in the non-PPIase regions of SurA, a sizable N-terminal domain and a short C-terminal tail, is essential. Unlike most cytoplasmic chaperones SurA is selective for particular substrates and recognizes outer membrane porins synthesized in vitro much more efficiently than other proteins. Thus, SurA may be specialized for the maturation of outer membrane proteins. We have characterized the substrate specificity of SurA based on its natural, biologically relevant substrates by screening cellulose-bound peptide libraries representing outer membrane proteins. We show that two features are critical for peptide binding by SurA: specific patterns of aromatic residues and the orientation of their side chains, which are found more frequently in integral outer membrane proteins than in other proteins. For the first time this sufficiently explains the capability of SurA to discriminate between outer membrane protein and non-outer membrane protein folding intermediates. Furthermore, peptide binding by SurA requires neither an active PPIase domain nor the presence of proline, indicating that the observed substrate specificity relates to the chaperone function of SurA. Finally, we show that SurA is capable of associating with the outer membrane. Together, our data support a model in which SurA is specialized to interact with non-native periplasmic outer membrane protein folding intermediates and to assist in their maturation from early to late outer membrane-associated steps.

Lipopolysaccharide Transport to the Bacterial Outer Membrane in Spheroplasts

The mechanism of lipopolysaccharide (LPS) transport in Gram-negative bacteria from the inner membrane to the outer membrane is largely unknown. Here, we investigated the possibility that LPS transport proceeds via a soluble intermediate associated with a periplasmic chaperone analogous to the Lol-dependent transport mechanism of lipoproteins. Whereas newly synthesized lipoproteins could be released from spheroplasts of Escherichia coli upon addition of a periplasmic extract containing LolA, de novo synthesized LPS was not released. We demonstrate that LPS synthesized de novo in spheroplasts co-fractionated with the outer membranes and that this co-fractionation was dependent on the presence in the spheroplasts of a functional MsbA protein, the protein responsible for the flip-flop of LPS across the inner membrane. The outer membrane localization of the LPS was confirmed by its modification by the outer membrane enzyme CrcA (PagP). We conclude that a substantial amount of LPS was translocated to the outer membrane in spheroplasts, suggesting that transport proceeds via contact sites between the two membranes. In contrast to LPS, de novo synthesized phospholipids were not transported to the outer membrane in spheroplasts. Apparently, LPS and phospholipids have different requirements for their transport to the outer membrane.

Outer membrane proteins of pathogenic spirochetes

Pathogenic spirochetes are the causative agents of several important diseases including syphilis, Lyme disease, leptospirosis, swine dysentery, periodontal disease and some forms of relapsing fever. Spirochetal bacteria possess two membranes and the proteins present in the outer membrane are at the site of interaction with host tissue and the immune system. This review describes the current knowledge in the field of spirochetal outer membrane protein (OMP) biology. What is known concerning biogenesis and structure of OMPs, with particular regard to the atypical signal peptide cleavage sites observed amongst the spirochetes, is discussed. We examine the functions that have been determined for several spirochetal OMPs including those that have been demonstrated to function as adhesins, porins or to have roles in complement resistance. A detailed description of the role of spirochetal OMPs in immunity, including those that stimulate protective immunity or that are involved in antigenic variation, is given. A final section is included which covers experimental considerations in spirochetal outer membrane biology. This section covers contentious issues concerning cellular localization of putative OMPs, including determination of surface exposure. A more detailed knowledge of spirochetal OMP biology will hopefully lead to the design of new vaccines and a better understanding of spirochetal pathogenesis.

In vivo reconstitution of the FhuA transport protein of Escherichia coli K-12

The FhuA protein in the outer membrane of Escherichia coli actively transports ferrichrome and the antibiotics albomycin and rifamycin CGP 4832 and serves as a receptor for the phages T1, T5, and phi80 and for colicin M and microcin J25. The crystal structure reveals a beta-barrel with a globular domain, the cork, which closes the channel formed by the barrel. Genetic deletion of the cork resulted in a beta-barrel that displays no FhuA activity. A functional FhuA was obtained by cosynthesis of separately encoded cork and the beta-barrel domain, each endowed with a signal sequence, which showed that complementation occurs after secretion of the fragments across the cytoplasmic membrane. Inactive complete mutant FhuA and an FhuA fragment containing 357 N-proximal amino acid residues complemented the separately synthesized wild-type beta-barrel to form an active FhuA. Previous claims that the beta-barrel is functional as transporter and receptor resulted from complementation by inactive complete FhuA and the 357-residue fragment. No complementation was observed between the wild-type cork and complete but inactive FhuA carrying cork mutations that excluded the exchange of cork domains. The data indicate that active FhuA is reconstituted extracytoplasmically by insertion of separately synthesized cork or cork from complete FhuA into the beta-barrel, and they suggest that in wild-type FhuA the beta-barrel is formed prior to the insertion of the cork.

Characterization of the refolding and reassembly of an integral membrane protein OmpF porin by low-angle laser light scattering photometry coupled with high-performance gel chromatography

The refolding and reassembly of an integral membrane protein OmpF porin denatured in sodium dodecylsulfate (SDS) into its stable species by the addition of n-octyl-beta-D-glucopyranoside (OG) have been studied by means of circular dichroism (CD) spectroscopy and low-angle laser light scattering photometry coupled with high-performance gel chromatography. The minimal concentration where change in the secondary structure was induced by the addition of OG was found to be 6.0 mg/ml in CD experiments. A species unfolded further than the SDS-denatured form of this protein was observed at an early stage (5-15 min) of refolding just above the minimal OG concentration. In addition, the CD spectrum of protein species obtained above the minimal OG concentration showed that the protein is composed of a beta-structure which is different from the native structure of this protein. In light scattering experiments, no changes in molecular assemblies were observed when the OG concentration was below its minimal refolding concentration determined by CD measurements. Above the minimal concentration, a compact monomeric species was observed when denatured OmpF porin was incubated for 5 min at 25 degrees C in a refolding medium containing 1 mg/ml SDS and 7 mg/ml OG, and then injected into columns equilibrated with the refolding medium. After an incubation of 24 h before injection into the columns, predominant dimerization of this protein was observed in addition to incorrect aggregation.

A rapid selective extraction procedure for the outer membrane protein (OmpF) from Escherichia coli

Porins are essential pore-forming proteins found in the outer membrane of several gram-negative bacteria. Investigating the relationships between molecular structure and function involves an extremely time-consuming and labor-intensive purification procedure. We report a method for rapid extraction of the outer membrane protein, OmpF, from freeze-dried Escherichia coli cells using valeric acid, alleviating the effort and time in sample preparation. Extraction results in a highly enriched fraction containing OmpF as 76% of the total protein content. The apparent molecular mass determined by SDS-PAGE mobility was 38,900, similar to that of the monomeric form of OmpF. N-terminal sequencing yielded 23 amino acids with 100% identity to the published OmpF sequence. The trimeric form of OmpF was observed in unheated samples run on SDS-PAGE and analysis of these samples by periodic acid/silver staining revealed the presence of unbound lipopolysaccharides. Furthermore, this method should prove useful for isolating other outer membrane proteins.

Effect of various mild surfactants on the reassembly of an oligomeric integral membrane protein OmpF porin

Reassembly of OmpF porin from its denatured monomer into the sodium dodecyl sulfate-resistant species was investigated by using 27 kinds of mild surfactants. Polyethyleneoxide-type surfactants with a hydrophilic-lipophilic balance value of 10.8-14.6 induced the trimerization of denatured OmpF porin. Dimerization and trimerization were induced by non-polyethyleneoxide-type mild surfactants that are generally used for membrane protein solubilization. The dependence of surfactant concentrations on reassembly was estimated to obtain a minimal concentration required for the reassembly of the protein. Extensive reassembly (to approximately 85% yield) into dimer (a putative assembly intermediate) was observed at a protein concentration of 0.05 mg/ml in 7 mg/ml n-octyl-beta-D-glucopyranoside and 1 mg/ml sodium dodecyl sulfate. This condition will be useful for the studies of the dimer and dimerization of OmpF porin. The role of mixed micelle system on the protein renaturation was discussed.

Identification of phospholipids as new components that assist in the in vitro trimerization of a bacterial pore protein

The in vitro trimerization of folded monomers of the bacterial pore protein PhoE, into its native-like, heat- and SDS-stable form requires incubations with isolated cell envelopes and Triton X-100. The possibility that membranes could be isolated that are enriched in assembly factors required for assembly of the pore protein was now investigated. Fractionation of total cell envelopes of Escherichia coli via various techniques indeed revealed the existence of membrane fractions with different capacities to support assembly in vitro. Fractions containing mainly inner membrane vesicles supported the formation of trimers that were associated with these membrane vesicles. However, only a proportion of these trimers were heat- and SDS-stable and these were formed with slow kinetics. In contrast, fractions containing mainly outer membrane vesicles supported formation of high amounts of heat-stable trimers with fast kinetics. We identified phospholipids as active assembly components in these membranes that support trimerization of folded monomers in a process with similar characteristics as observed with inner membrane vesicles. Furthermore, phospholipids strongly stimulate the kinetics of trimerization and increase the final yield of heat-stable trimers in the context of outer membranes. We propose that lipopolysaccharides stabilize the assembly competent state of folded monomers as a lipochaperone. Phospholipids are involved in converting the folded monomer into new assembly competent intermediate with a short half-life that will form heat-stable trimers most efficiently in the context of outer membrane vesicles. These results provide biochemical evidence for the involvement of different lipidic components at distinct stages of the porin assembly process.

The assembly pathway of outer membrane protein PhoE of Escherichia coli

The assembly of the wild-type and several mutant forms of the trimeric outer membrane porin PhoE of Escherichia coli was investigated in vitro and in vivo. In in vivo pulse-chase experiments, approximately half of the wild-type PhoE molecules assembled within the 30-s pulse in the native conformation in the cell envelope. The other half of the molecules followed slower kinetics, and three intermediates in this multistep assembly process were detected: a soluble trypsin-sensitive monomer, a trypsin-sensitive monomeric form that was loosely associated with the cell envelope and a metastable trimer, which was integrated into the membranes and converted to the stable trimeric configuration within minutes. The metastable trimers disassembled during sample preparation for standard SDS/PAGE into folded monomers. In vitro, the isolated PhoE protein could efficiently be folded in the presence of N,N-dimethyldodecylamine-N-oxide (LDAO). A mutant PhoE protein, DeltaF330, which lacks the C-terminal phenylalanine residue, mainly followed the slower kinetic pathway observed in vivo, resulting in increased amounts of the various assembly intermediates. It appears that the DeltaF330 mutant protein is intrinsically able to fold, because it was able to fold in vitro with LDAO with similar efficiencies as the wild-type protein. Therefore, we propose that the conserved C-terminal Phe is (part of) a sorting signal, directing the protein efficiently to the outer membrane. Furthermore, we analysed a mutant protein with a hydrophilic residue introduced at the hydrophobic side of one of the membrane-spanning amphipathic beta strands. The assembly of this mutant protein was not affected in vivo or in vitro in the presence of LDAO. However, it was not able to form folded monomers in a previously established in vitro folding system, which requires the presence of lipopolysaccharides and Triton. Hence, a folded monomer might not be a true assembly intermediate of PhoE in vivo.

The biogenesis and assembly of bacterial membrane proteins

Bacterial proteins in the inner and outer membranes differ dramatically in their architecture. Although both types of proteins are transported across the inner membrane through a common pore, recent studies have identified distinct factors that target them to transport sites and catalyze proper folding.

Physicochemical evidence that Treponema pallidum TroA is a zinc-containing metalloprotein that lacks porin-like structure

Although TroA (Tromp1) was initially reported to be a Treponema pallidum outer membrane protein with porin-like properties, subsequent studies have suggested that it actually is a periplasmic substrate-binding protein involved in the transport of metals across the treponemal cytoplasmic membrane. Here we conducted additional physicochemical studies to address the divergent viewpoints concerning this protein. Triton X-114 phase partitioning of recombinant TroA constructs with or without a signal sequence corroborated our prior contention that the native protein's amphiphilic behavior is due to its uncleaved leader peptide. Whereas typical porins are trimers with extensive beta-barrel structure, size exclusion chromatography and circular dichroism spectroscopy revealed that TroA was a monomer and predominantly alpha-helical. Neutron activation, atomic absorption spectroscopy, and anomalous X-ray scattering all demonstrated that TroA binds zinc in a 1:1 molar stoichiometric ratio. TroA does not appear to possess structural features consistent with those of bacterial porins.

Targeting and assembly of periplasmic and outer-membrane proteins in Escherichia coli

Escherichia coli must actively transport many of its proteins to extracytoplasmic compartments such as the periplasm and outer membrane. To perform this duty, E. coli employs a collection of Sec (secretion) proteins that catalyze the translocation of various polypeptides through the inner membrane. After translocation across the inner membrane, periplasmic and outer-membrane proteins are folded and targeted to their appropriate destinations. Here we review our knowledge of protein translocation across the inner membrane. We also discuss the various signal transduction systems that monitor extracytoplasmic protein folding and targeting, and we consider how these signal transduction systems may ultimately control these processes.

Affinity of the periplasmic chaperone Skp of Escherichia coli for phospholipids, lipopolysaccharides and non-native outer membrane proteins. Role of Skp in the biogenesis of outer membrane protein

The Skp protein of Escherichia coli has been proposed to be a periplasmic molecular chaperone involved in the biogenesis of outer membrane proteins. In this study, evidence is obtained that Skp exists in two different states characterized by their different sensitivity to proteases. The conversion between these states can be modulated in vitro by phospholipids, lipopolysaccharides and bivalent cations. Skp is able to associate with and insert into phospholipid membranes in vitro, indicating that it may associate with phospholipids in the inner and/or outer membrane in vivo. In addition, it interacts specifically with outer membrane proteins that are in their non-native state. We propose that Skp is required in vivo for the efficient targeting of unfolded outer membrane proteins to the membrane.

Folding-based suppression of extracytoplasmic toxicity conferred by processing-defective LamB

We have utilized processing-defective derivatives of the outer membrane maltoporin, LamB, to study protein trafficking functions in the cell envelope of Escherichia coli. Our model proteins contain amino acid substitutions in the consensus site for cleavage by signal peptidase. As a result, the signal sequence is cleaved with reduced efficiency, effectively tethering the precursor protein to the inner membrane. These mutant porins are toxic when secreted to the cell envelope. Furthermore, strains producing these proteins exhibit altered outer membrane permeability, suggesting that the toxicity stems from some perturbation of the cell envelope (J. H. Carlson and T. J. Silhavy, J. Bacteriol. 175:3327-3334, 1993). We have characterized a multicopy suppressor of the processing-defective porins that appears to act by a novel mechanism. Using fractionation experiments and conformation-specific antibodies, we found that the presence of this multicopy suppressor allowed the processing-defective LamB precursors to be folded and localized to the outer membrane. Analysis of the suppressor plasmid revealed that these effects are mediated by the presence of a truncated derivative of the polytopic inner membrane protein, TetA. The suppression mediated by TetA' is independent of the CpxA/CpxR regulon and the sigma E regulon, both of which are involved in regulating protein trafficking functions in the cell envelope.

Electroinsertion of glycophorin A in interdigitation-fusion giant unilamellar lipid vesicles

Previously we demonstrated that transmembrane back insertion of glycophorin A, a solubilizable intrinsic protein, can be obtained in dipalmitoylphosphatidylcholine multilamellar vesicles, MLVs, by electropulsation (Raffy, S., and Teissié, J. (1995) Eur. J. Biochem. 230, 722-732). Here we report that transmembrane back insertion of protein is obtained by electropulsion of unilamellar giant vesicles, termed interdigitation-fusion vesicles (IFVs), which are better membrane models than MLVs due to their unilamellarity. Electropulsation promotes a field-dependent local permeabilization of the lipid layer, as shown by the associated leakage of entrapped calcein. Glycophorin insertion is assayed by immunofluorescence. Electroinsertion is obtained by pulsing the vesicle/protein mixture. Glycophorin insertion is observed under more drastic electrical conditions than needed for permeabilization. Direct observation of glycophorin insertion in the vesicles under a microscope shows a localized process in agreement with the theoretical prediction. A quantitative evaluation of the immunofluorescence pattern shows that insertion was higher on one side of the vesicle than on the other. This suggests that an electrophoretic movement of the solubilized glycophorin could take place during electropulsation. Insertion of glycophorin, a prefolded intrinsic protein, is then obtained in the lipid bilayer brought transiently to the electropermeabilized state.

Folding of a bacterial outer membrane protein during passage through the periplasm

The transport of bacterial outer membrane proteins to their destination might be either a one-step process via the contact zones between the inner and outer membrane or a two-step process, implicating a periplasmic intermediate that inserts into the membrane. Furthermore, folding might precede insertion or vice versa. To address these questions, we have made use of the known 3D-structure of the trimeric porin PhoE of Escherichia coli to engineer intramolecular disulfide bridges into this protein at positions that are not exposed to the periplasm once the protein is correctly assembled. The mutations did not interfere with the biogenesis of the protein, and disulfide bond formation appeared to be dependent on the periplasmic enzyme DsbA, which catalyzes disulfide bond formation in the periplasm. This proves that the protein passes through the periplasm on its way to the outer membrane. Furthermore, since the disulfide bonds create elements of tertiary structure within the mutant proteins, it appears that these proteins are at least partially folded before they insert into the outer membrane.

Filamentous phage assembly: variation on a protein export theme

Biogenesis of both filamentous phage and type-IV pili involves the assembly of many copies of a small, integral inner membrane protein (the phage major coat protein or pilin) into a helical, tubular array that passes through the outer membrane. The occurrence of related proteins required for assembly and export in both systems suggests that there may be similarities at the mechanistic level as well. This report summarizes the properties of filamentous phage and the proteins required for their assembly, with particular emphasis on features they may share with bacterial protein export and pilus biogenesis systems, and it presents evidence that supports the hypothesis that one of the phage proteins functions as an outer membrane export channel.

Recent progress and future directions in studies of the main terminal branch of the general secretory pathway in Gram-negative bacteria--a review

The main terminal branch (MTB) of the general secretory pathway is used by a wide variety of Gram- bacteria to transport exoproteins from the periplasm to the outside milieu. Recent work has led to the identification of the function of two of its 14 (or more) components: an enzyme with type-IV prepilin peptidase activity and a chaperone-like protein required for the insertion of another of the MTB components into the outer membrane. Despite these important discoveries, little tangible progress has been made towards identifying MTB components that determine secretion specificity (presumably by binding to cognate exoproteins) or which form the putative channel through which exoproteins are transported across the outer membrane. However, the idea that the single integral outer membrane component of the MTB could line the wall of this channel, and the intriguing possibility that other components of the MTB form a rudimentary type-IV pilus-like structure that might span the periplasm both deserve more careful examination. Although Escherichia coli K-12 does not normally secrete exoproteins, its chromosome contains an apparently complete set of genes coding for MTB components. At least two of these genes code for functional proteins, but the operon in which twelve of the genes are located does not appear to be expressed. We are currently searching for conditions which allow these genes to be expressed with the eventual aim of identifying the protein(s) that E. coli K-12 can secrete.

Pleiotropic effects of a mutation in rfaC on Escherichia coli hemolysin

Several genes involved in the lipopolysaccharide (LPS) biosynthetic pathway have been shown to affect the expression or activity of Escherichia coli hemolysin (Hly), a secreted cytotoxin that is the prototype of the RTX family of toxins. To further study this relationship, E. coli K-12 strains harboring mutations in the LPS biosynthetic genes rfaS, rfaQ, rfaJ, rfaP, and rfaC were transformed with a recombinant plasmid harboring the hlyCABD operon and examined for their effects on extracellular expression and hemolytic activity. A mutation in rfaC that affected both extracellular expression and activity of Hly was studied in greater detail. This mutation led to a growth-phase-dependent decrease up to 16-fold in the steady-state level of extracellular HlyA, although transcription and secretion of HlyA were decreased no more than 2-fold. Specific hemolytic activity in toxin produced from the rfaC mutant strain was significantly reduced, in a growth-phase-dependent manner. With the rfaC gene supplied in trans, both the decreased expression and activity of Hly were restored to wild-type levels. Hly from the rfaC mutant strain exhibited much slower kinetics of hemolysis, a more rapid rate of decay of activity, and greater formation of apparently inactive HlyA-containing aggregates in culture supernatants than was exhibited in the wild-type strain. A model is proposed for a physical interaction between LPS and Hly in which LPS with intact inner core participates in forming or maintaining an active conformation of Hly and helps to protect it from aggregation or degradation.

TolA central domain interacts with Escherichia coli porins

TolA is an inner membrane protein with three domains: a transmembrane N-terminus and periplasmic central and C-terminal domains. The interaction of TolA with outer membrane porins of Escherichia coli was investigated. Western blot analyses of cell extracts with anti-TolA antibodies indicated that TolA forms high molecular weight complexes specifically with trimeric OmpF, OmpC, PhoE and LamB, but not with OmpA. The interaction of purified TolA domains with purified porins was also studied. TolA interacted with OmpF, PhoE and LamB porins via its central domain, but not with either their denatured monomeric forms or OmpA. Moreover, the presence or absence of lipopolysaccharides associated with trimeric porins did not modify the interactions. These results suggest that the specific interaction of TolA with outer membrane porins might be relevant to the function of Tol proteins.

Lipopolysaccharides and divalent cations are involved in the formation of an assembly-competent intermediate of outer-membrane protein PhoE of E.coli

To identify the requirements for the biogenesis of outer-membrane proteins in Gram-negative bacteria, the sorting and assembly of the trimeric, pore-forming protein PhoE was studied in vitro. Purified lipopolysaccharide (LPS) in combination with low amounts of Triton X-100 and divalent cations induced the formation of folded monomers. LPS of deep-rough strains was far less efficient in the formation of folded monomers than wild-type LPS was. These folded monomers could be converted into heat-stable trimers upon addition of outer membranes and higher amounts of Triton X-100. Trimerization could precede the insertion step. These in vitro data suggest that the assembly in vivo proceeds sequentially by (i) formation of a folded monomer by interaction with LPS; (ii) sorting of the folded monomers to assembly sites in the outer membrane; (iii) trimerization; and (iv) insertion.

Secondary structure of the outer membrane proteins OmpA of Escherichia coli and OprF of Pseudomonas aeruginosa

When purified without the use of ionic detergents, both OmpA and OprF proteins contained nearly 20% alpha-helical structures, which disappeared completely upon the addition of sodium dodecyl sulfate. This result suggests that the proteins fold in a similar manner, with an N-terminal, membrane-spanning beta-barrel domain and a C-terminal, globular, periplasmic domain.

Secondary structure of the outer membrane proteins OmpA of Escherichia coli and OprF of Pseudomonas aeruginosa

When purified without the use of ionic detergents, both OmpA and OprF proteins contained nearly 20% alpha-helical structures, which disappeared completely upon the addition of sodium dodecyl sulfate. This result suggests that the proteins fold in a similar manner, with an N-terminal, membrane-spanning beta-barrel domain and a C-terminal, globular, periplasmic domain.

Purification and refolding of recombinant Haemophilus influenzae type b porin produced in Bacillus subtilis

The major diffusion channel in the outer membrane of Haemophilus influenzae type b (Hib) is porin (341 amino acids; Mr 37 782). The Hib porin gene was cloned and overexpressed in Bacillus subtilis. Recombinant Hib porin (Bac porin), having aggregated into inclusion bodies, was purified under denaturing conditions and subsequently refolded. To compare Bac porin that is intrinsically devoid of lipooligosaccharides versus native Hib porin, the properties of Bac porin were assessed by the following four criteria: circular dichroism spectroscopy, channel formation in planar bilayers, resistance to trypsin digestion and formation of the conformational epitope recognized by an anti-Hib porin monoclonal antibody. We conclude that in the absence of lipooligosaccharides, Bac porin was refolded into a functional form which closely resembled the structure of Hib porin.

In vitro insertion and assembly of outer membrane protein PhoE of Escherichia coli K-12 into the outer membrane. Role of Triton X-100

The assembly of the in vitro synthesized outer membrane protein PhoE into purified outer membranes was investigated. The assembly appeared to be strongly stimulated by the presence of low amounts of Triton X-100 (optimal 0.08%, w/v). The role of Triton X-100 in the in vitro system was further examined. Pretreating outer membranes with Triton X-100 did not make the membranes competent for correct assembly, indicating that the detergent did not act on the membrane but at the protein level. PhoE became assembly-incompetent with a half-life of approximately 12 min and 90 s at 37 degrees C in the absence and presence, respectively, of 0.08% Triton X-100. Apparently, Triton X-100 induces an assembly-competent state in the PhoE protein with a very short half-life. Furthermore, the efficiency of correct assembly of PhoE was greatly reduced when outer membranes of deep rough lipopolysaccharide mutants were used, indicating an important role of lipopolysaccharides in the assembly of the porin.

Essential role of a sodium dodecyl sulfate-resistant protein IV multimer in assembly-export of filamentous phage

Filamentous phage f1 encodes protein IV (pIV), a protein essential for phage morphogenesis that localizes to the outer membrane of Escherichia coli, where it is found as a multimer of 10 to 12 subunits. Introduction of internal His or Strep affinity tags at different sites in pIV interfered with its function to a variable extent. A spontaneous second-site suppressor mutation in gene IV allowed several different insertion mutants to function. The identical mutation was also isolated as a suppressor of a multimerization-defective missense mutation. A high-molecular-mass pIV species is the predominant form of pIV present in cells. This species is stable in 4% sodium dodecyl sulfate at temperatures up to 65 degrees C and is largely preserved at 100 degrees C in Laemmli protein sample buffer containing 4% sodium dodecyl sulfate. The suppressor mutation makes the high-molecular-mass form of wild-type pIV extremely resistant to dissociation, and it stabilizes the high-molecular-mass form of several mutant pIV proteins to extents that correlate with their level of function. Mixed multimers of pIV(f1) and pIV(Ike) also remain associated during heating in sodium dodecyl sulfate-containing buffers. Thus, sodium dodecyl sulfate- and heat-resistant high-molecular-mass pIV is derived from pIV multimer and reflects the physiologically relevant form of the protein essential for assembly-export.

Insertion of an outer membrane protein in Escherichia coli requires a chaperone-like protein

Only one of the characterized components of the main terminal branch of the general secretory pathway (GSP) in Gram-negative bacteria, GspD, is an integral outer membrane protein that could conceivably form a channel to permit protein transport across this membrane. PulD, a member of the GspD protein family required for pullulanase secretion by Klebsiella oxytoca, is shown here to form outer membrane-associated complexes which are not readily dissociated by SDS treatment. The outer membrane association of PulD is absolutely dependent on another component of the GSP, the outer membrane-anchored lipoprotein PulS. Furthermore, the absence of PulS resulted in limited proteolysis of PulD and caused induction of the so-called phage shock response, as measured by increased expression of the pspA gene. We propose that PulS may be the first member of a new family of periplasmic chaperones that are specifically required for the insertion of a group of outer membrane proteins into this membrane. PulS is only the second component of the main terminal branch of the GSP for which a precise function can be proposed.

Role of sterols in the functional reconstitution of water-soluble mitochondrial porins from different organisms

Experiments were performed on lipid bilayer membranes with water-soluble mitochondrial porins from different eukaryotic organisms, such as Dictyostelium discoideum, Paramecium, and rat liver, to study the requirements of functional reconstitution of the porins. The water-soluble porins lost their associated lipids and sterols and are unable to form channels in lipid bilayer membranes. We demonstrate that the water-soluble porins regain their channel-forming ability after preincubation of the polypeptides with sterols in the presence of detergents. Mitochondrial porin from Dictyostelium discoideum maintained after this procedure its original properties, in particular the voltage dependence. Water-soluble mitochondrial porins from Paramecium tetraurelia and from rat liver were also activated upon preincubation with different sterols in detergent but showed voltage-dependences that were different from those of detergent-purified porins. Furthermore, the voltage dependence depended on the sterol used for preincubation. Interestingly, the preincubation with sterols can likewise be used to activate detergent-purified mitochondrial porins that may have lost associated sterol during isolation and purification procedures.

Avirulence of rough mutants of Shigella flexneri: requirement of O antigen for correct unipolar localization of IcsA in the bacterial outer membrane

Mutations in the lipopolysaccharide (LPS) of Shigella spp. result in attenuation of the bacteria in both in vitro and in vivo models of virulence, although the precise block in pathogenesis is not known. We isolated defined mutations in two genes, galU and rfe, which directly affect synthesis of the LPS of S. flexneri 2a, in order to determine more precisely the step in virulence at which LPS mutants are blocked. The galU and rfe mutants invaded HeLa cells but failed to generate the membrane protrusions (fireworks) characteristic of intracellular motility displayed by wild-type shigellae. Furthermore, the galU mutant was unable to form plaques on a confluent monolayer of eucaryotic cells and the rfe mutant generated only tiny plaques. These observations indicated that the mutants were blocked in their ability to spread from cell to cell. Western immunoblot analysis of expression of IcsA, the protein essential for intracellular motility and intercellular spread, demonstrated that both mutants synthesized IcsA, although they secreted less of the protein to the extracellular medium than did the wild-type parent. More strikingly, the LPS mutants showed aberrant surface localization of IcsA. Unlike the unipolar localization of IcsA seen in the wild-type parent, the galU mutant expressed the protein in a circumferential fashion. The rfe mutant had an intermediate phenotype in that it displayed some localization of IcsA at one pole while also showing diffuse localization around the bacterium. Given the known structures of the LPS of wild-type S. flexneri 2a, the rfe mutant, and the galU mutant, we hypothesize that the core and O-antigen components of LPS are critical elements in the correct unipolar localization of IcsA. These observations indicate a more precise role for LPS in Shigella pathogenesis.

Assembly of LamB and OmpF in deep rough lipopolysaccharide mutants of Escherichia coli K-12

Assembly of the OmpF and LamB proteins was kinetically retarded in deep rough lipopolysaccharide mutants of Escherichia coli K-12. OmpF assembly was affected at the step of conversion of metastable trimers to stable trimers, whereas LamB assembly was influenced both at the monomer-to-metastable trimer and metastable-to-stable trimer steps. These assembly defects were reversed in the presence of the sfaA1 and sfaB3 suppressor alleles, which were isolated by using ompF assembly mutants.

Transport across the bacterial outer membrane

Diffusion of small molecules across the outer membrane of gram-negative bacteria may occur through protein channels and through lipid bilayer domains. Among protein channels, many examples of trimeric porins, which produce water-filled diffusion channels, are known. Although the channels are nonspecific, the diffusion rates of solutes are often drastically affected by their gross physicochemical properties, such as size, charge, or lipophilicity, because the channel has a dimension not too different from that of the diffusing solutes. In the last few years, the structures of three such porins have been solved by X-ray crystallography. It is now known that a monomer unit traverses the membrane 16 times as beta-strands, and one of the external loop folds back into the channel to produce a narrow constriction. Most of the static properties of the channel, such as the pore size and the position of the amino acids that produce the constriction, can now be explained by the three-dimensional structure. Controversy, however, still surrounds the issue of whether there are dynamic modulation of the channel properties in response to pH, ionic strength, or membrane potential, and of whether such responses are physiological. More recently, two examples of monomeric porins have been identified. These porins allow a very slow diffusion of solutes, but the reason for this low permeability is still unclear. Finally, channels with specific binding sites facilitate the diffusion of specific classes of nutrients, often those compounds that are too large to penetrate rapidly through the porin channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of temperature-sensitive assembly mutants of heat-labile enterotoxin B subunits

Deletions or substitutions of amino acids at the carboxyl-terminus of the heat-labile enterotoxin B subunit (EtxB) affect its assembly into pentamers in a temperature-dependent manner. At 42 degrees C, the mutations prevent the B subunits from achieving their final pentameric structure resulting in membrane association of the monomers. However, mutant B subunits produced at 30 degrees C assemble, in the periplasm, into pentamers that remain stable when transferred to 42 degrees C, indicating that the mutant pentamers are stable under conditions where their formation is inhibited. The mutant pentamers are, similarly to wild-type pentamers, SDS-resistant and stable, in vitro, at temperatures up to 65 degrees C. This suggests that although the C-terminal amino acids are part of the subunit interface, they appear not to contribute significantly to the stability of the final pentameric complex, but are instead essential for the formation or stabilization of an assembly intermediate in the pentamerization process. Single second site mutations suppress the assembly defect of mutant EtxB191.5, which carries substitutions at its C-terminus. The Thr-->Ile replacement at position 75 in the alpha 2-helix probably restores the van der Waals contact between residues 75 and 101, which had been greatly reduced by the Met-->Leu substitution at position 101 in the beta 6-strand of EtxB191.5. Interaction between the alpha 2-helix and beta 6-strand which contains the C-terminus probably stabilizes a conformation essential for assembly and is therefore required for the formation of pentamers.

Location and unusual membrane topology of the immunity protein of the Escherichia coli phage T4

The immunity protein (Imm) encoded by the Escherichia coli phage T4 effects exclusion of phage superinfecting cells already infected with T4. The 83-residue polypeptide possesses two long lipophilic areas (from residues 3 to 32 and 37 to 65) interrupted by a hydrophilic stretch including two positively charged residues. The charge distribution of the protein very strongly suggested that it is a plasma membrane protein with the C terminus facing the periplasm. While it could be shown that the expected location was correct, fusions of Imm to alkaline phosphatase or beta-galactosidase showed that the C terminus was at the cytosolic side of the membrane. Also, concerning function, there was almost no structural specificity to this part of the protein. Even removal of the two positively charged residues did not completely abolish function. Evidence suggesting that Imm is associated with the membrane at specific sites is presented. It is proposed that Imm is localized to the membrane with the help of a receptor and that, therefore, it does not follow the established rules for the topology of other membrane proteins. The results also suggest that Imm acts indirectly, possibly by altering the conformation of a component of a phage DNA injection site.

OmpF assembly mutants of Escherichia coli K-12: isolation, characterization, and suppressor analysis

This paper describes a novel genetic method used to isolate mutations that alter proper assembly of OmpF in the outer membrane. The thermolabile nature of assembly intermediates allowed selection of temperature-sensitive mutations within the ompF gene. A variant allele of ompF (ompF-Dex) was used because it provided a convenient selectable phenotype (Dex+). Assembly mutants were isolated in two steps. First, amber mutations were obtained that mapped in ompF-Dex. This resulted in a Dex- phenotype. Starting with these Dex- strains, Dex+ revertants were isolated. Mutants that displayed a temperature-sensitive Dex+ phenotype were further characterized. Three such mutants possessed a single substitution within ompF that reverted the nonsense codon to a sense codon which replaced W214 with either an E or Q and Y231 with a Q residue in the mature OmpF protein. All three mutant OmpF proteins showed an assembly defect. This defect led to a substantial reduction in the amount of stable OmpF trimers with the concomitant increase of a high-molecular-weight form of OmpF which migrated at the top of the gel. Suppressor mutations were sought that corrected the assembly defect of OmpF. These extragenic suppressor mutations were mapped at 45 min on the Escherichia coli chromosome. The suppressor mutations displayed no allele specificity and were recessive to the wild-type allele. In the presence of a suppressor, mutant stable trimers appeared in an almost normal manner. The appearance of stable trimers concurred with a substantial loss of the high-molecular-weight OmpF species. At this stage, it is not clear whether the high-molecular-weight species of OmpF is a normal assembly intermediate or a dead-end assembly product. The results presented in this study raise the intriguing possibility of a chaperone-like activity for the wild-type suppressor gene product.

Analysis of the structure and subcellular location of filamentous phage pIV

The gene IV protein of filamentous bacteriophages is an integral membrane protein required for phage assembly and export. A series of gene IV::phoA fusion, gene IV deletion, and gene IV missense mutations have been isolated and characterized. The alkaline phosphatase activity of the fusion proteins suggests that pIV lacks a cytoplasmic domain. Cell fractionation studies indicate that the carboxy-terminal half of pIV mediates its assembly into the membrane, although there is no single, discrete membrane localization domain. The properties of gene IV missense and deletion mutants, combined with an analysis of the similarities between pIVs from various filamentous phage and related bacterial export-mediating proteins, suggest that the amino-terminal half of pIV consists of a periplasmic substrate-binding domain that confers specificity to the assembly-export system.