Two-partner secretion in Gram-negative bacteria: a thrifty, specific pathway for large virulence proteins

Prevalence of pathogenicity island IICFT073 genes among extraintestinal clinical isolates of Escherichia coli

Uropathogenic Escherichia coli is the most common cause of urinary tract infection (UTI). Cystitis in women is by far the most common UTI; pyelonephritis in both sexes and prostatitis in men are more severe but are less frequent complaints. The ability of E. coli to cause UTI is associated with specific virulence determinants, some of which are encoded on pathogenicity islands (PAI). One such PAI (PAI IICFT073), of the prototypical uropathogenic E. coli strain CFT073, contains 116 open reading frames, including iron-regulated genes, carbohydrate biosynthetic genes, the serine protease autotransporter picU, a two-partner secretion system, a type I secretion system, mobility genes, and a large number of hypothetical genes. To determine the association of PAI IICFT073 with UTI, PCR was used to examine the prevalence of the five virulence-associated loci among the ECOR collection and a collection of E. coli isolated from patients with cystitis, pyelonephritis, prostatitis, or septicemia. All PAI IICFT073 loci were found to be more prevalent among the B2 phylogenetic group than any other group within the ECOR collection and among invasive prostatitis strains than were cystitis or pyelonephritis strains. These data support the theory that clinical isolates causing prostatitis are more virulent than those producing cystitis or pyelonephritis in women.

Protein secretion systems in Fusobacterium nucleatum: Genomic identification of Type 4 piliation and complete Type V pathways brings new insight into mechanisms of pathogenesis

Recent genomic analyses of the two sequenced strains F. nucleatum subsp. nucleatum ATCC 25586 and F. nucleatum subsp. vincentii ATCC 49256 suggested that the major protein secretion systems were absent. However, such a paucity of protein secretion systems is incongruous with F. nucleatum pathogenesis. Moreover, the presence of one or more such systems has been described for every other Gram-negative organism sequenced to date. In this investigation, the question of protein secretion in F. nucleatum was revisited. In the current study, the absence in F. nucleatum of a twin-arginine translocation system (TC #2.A.64.), a Type III secretion system (TC #3.A.6.), a Type IV secretion system (TC #3.A.7.) and a chaperone/usher pathway (TC #1.B.11.) was confirmed. However, contrary to previous findings, our investigations indicated that a Type I protein secretion system was also absent from F. nucleatum. In contrast, members of the holin family (TC #1.E) and the machinery required for a Type 4 piliation/fimbriation system (TC #3.A.15.2.) were identified using a variety of bioinformatic tools. Furthermore, a complete range of proteins resembling members of the Type V secretion pathway, i.e., the Type Va (autotransporter; TC #1.B.12.), Type Vb (two-partner secretion system; TC #1.B.20.) and Type Vc (YadA-like trimeric autotransporter; TC #1.B.42.), was found. This work provides new insight into the protein secretion and virulence mechanisms of F. nucleatum.

Recombinant protein secretion in Escherichia coli

The secretory production of recombinant proteins by the Gram-negative bacterium Escherichia coli has several advantages over intracellular production as inclusion bodies. In most cases, targeting protein to the periplasmic space or to the culture medium facilitates downstream processing, folding, and in vivo stability, enabling the production of soluble and biologically active proteins at a reduced process cost. This review presents several strategies that can be used for recombinant protein secretion in E. coli and discusses their advantages and limitations depending on the characteristics of the target protein to be produced.

Conserved pore-forming regions in polypeptide-transporting proteins

Transport of solutes and polypeptides across membranes is an essential process for every cell. In the past, much focus has been placed on helical transporters. Recently, the beta-barrel-shaped transporters have also attracted some attention. The members of this family are found in the outer bacterial membrane and the outer membrane of endosymbiotically derived organelles. Here we analyze the features and the evolutionary development of a specified translocator family, namely the beta-barrel-shaped polypeptide-transporters. We identified sequence motifs, which characterize all transporters of this family, as well as motifs specific for a certain subgroup of proteins of this class. The general motifs are related to the structural composition of the pores. Further analysis revealed a defined distance of two motifs to the C-terminal portion of the proteins. Furthermore, the evolutionary relationship of the proteins and the motifs are discussed.

Crystallization of truncated hemolysin A from Proteus mirabilis

Proteus species are second only to Escherichia coli as the most common causative agent of Gram-negative bacteria-based urinary-tract infections and many harbor several virulence factors that provide inherent uropathogenicity. One virulence factor stems from a two-partner secretion pathway comprised of hemolysin A and hemolysin B; upon hemolysin B-dependent secretion, hemolysin A becomes activated. This system is distinct from the classic type I secretion pathway exemplified by the hemolysin system within Escherichia coli. In order to describe the mechanism by which hemolysin A is activated for pore formation, an amino-terminal truncated form capable of complementing the non-secreted full-length hemolysin A and thereby restoring hemolytic activity has been constructed, expressed and purified. A room-temperature data set has been collected to 2.5 A resolution. The crystal belongs to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 34.47, b = 58.40, c = 119.74 A. The asymmetric unit is expected to contain a single monomer, which equates to a Matthews coefficient of 1.72 A3 Da(-1) and a solvent content of 28.3%.

Protein secretion through autotransporter and two-partner pathways

Two distinct protein secretion pathways, the autotransporter (AT) and the two-partner secretion (TPS) pathways are characterized by their apparent simplicity. Both are devoted to the translocation across the outer membrane of mostly large proteins or protein domains. As implied by their name, AT proteins contain their own transporter domain, covalently attached to the C-terminal extremity of the secreted passenger domain, while TPS systems are composed of two separate proteins, with TpsA being the secreted protein and TpsB its specific transporter. In both pathways, the secreted proteins are exported in a Sec-dependent manner across the inner membrane, after which they cross the outer membrane with the help of their cognate transporters. The AT translocator domains and the TpsB proteins constitute distinct families of protein-translocating, outer membrane porins of Gram-negative bacteria. Both types of transporters insert into the outer membrane as beta-barrel proteins possibly forming oligomeric pores in the case of AT and serve as conduits for their cognate secreted proteins or domains across the outer membrane. Translocation appears to be folding-sensitive in both pathways, indicating that AT passenger domains and TpsA proteins cross the periplasm and the outer membrane in non-native conformations and fold progressively at the cell surface. A major difference between AT and TPS pathways arises from the manner by which specificity is established between the secreted protein and its transporter. In AT, the covalent link between the passenger and the translocator domains ensures the translocation of the former without the need for a specific molecular recognition between the two modules. In contrast, the TPS pathway has solved the question of specific recognition between the TpsA proteins and their transporters by the addition to the TpsA proteins of an N-proximal module, the conserved TPS domain, which represents a hallmark of the TPS pathway.

Membrane Targeting of a Bacterial Virulence Factor Harbouring an Extended Signal Peptide

Filamentous haemagglutinin (FHA) is the major adhesin of Bordetella pertussis, the whooping cough agent. FHA is synthesized as a 367-kDa precursor harbouring a remarkably long signal peptide with an N-terminal extension that is conserved among related virulence proteins. FHA is secreted via the two-partner secretion pathway that involves transport across the outer membrane by a cognate transporter protein. Here we have analyzed the mechanism by which FHA is targeted to, and translocated across, the inner membrane. Studies were performed both in vitro using Escherichia coli inside-out inner membrane vesicles and in vivo by pulse-chase labelling of Bordetella pertussis cells. The data collectively indicate that like classical periplasmic and outer membrane proteins, FHA requires SecA and SecB for its export through the SecYEG translocon in the inner membrane. Although short nascent chains of FHA were found to cross-link to signal recognition particle (SRP), we did not obtain indication for an SRP-dependent, co-translational membrane targeting provoked by the FHA signal sequence. Our results rule out that the extended signal peptide of FHA determines a specific mode of membrane targeting but rather suggest that it might influence the export rate at the inner membrane.

Biogenesis of the Gram-negative bacterial outer membrane

Gram-negative bacteria are bounded by two membranes. The outer membrane consists of phospholipids, lipopolysaccharides, lipoproteins and integral outer membrane proteins, all of which are synthesized in the cytoplasm. Recently, much progress has been made in the elucidation of the mechanisms of transport of these molecules over the inner membrane, through the periplasm and into the outer membrane, in part by exploiting the extraordinary capacity of Neisseria meningitidis to survive without lipopolysaccharide.

An unusual signal peptide facilitates late steps in the biogenesis of a bacterial autotransporter

Bacterial autotransporters are proteins that use a C-terminal porin-like domain to facilitate the transport of an upstream "passenger domain" across the outer membrane. Although autotransporters are translocated across the inner membrane (IM) via the Sec pathway, some of them contain exceptionally long signal peptides distinguished by a unique N-terminal sequence motif. In this study, we used the Escherichia coli O157:H7 autotransporter EspP as a model protein to investigate the function of the unusual signal peptides. We found that removal of the N-terminal motif or replacement of the EspP signal peptide did not affect translocation of the protein across the IM. Remarkably, modification of the signal peptide caused EspP to misfold in the periplasm and blocked transport of the passenger domain across the outer membrane. Further analysis suggested that the EspP signal peptide transits slowly through the Sec machinery. Based on these results, we propose that the unusual signal peptides not only function as targeting signals, but also prevent misfolding of the passenger domain in the periplasm by transiently tethering it to the IM.

Major structural differences and novel potential virulence mechanisms from the genomes of multiple campylobacter species

Sequencing and comparative genome analysis of four strains of Campylobacter including C. lari RM2100, C. upsaliensis RM3195, and C. coli RM2228 has revealed major structural differences that are associated with the insertion of phage- and plasmid-like genomic islands, as well as major variations in the lipooligosaccharide complex. Poly G tracts are longer, are greater in number, and show greater variability in C. upsaliensis than in the other species. Many genes involved in host colonization, including racR/S, cadF, cdt, ciaB, and flagellin genes, are conserved across the species, but variations that appear to be species specific are evident for a lipooligosaccharide locus, a capsular (extracellular) polysaccharide locus, and a novel Campylobacter putative licABCD virulence locus. The strains also vary in their metabolic profiles, as well as their resistance profiles to a range of antibiotics. It is evident that the newly identified hypothetical and conserved hypothetical proteins, as well as uncharacterized two-component regulatory systems and membrane proteins, may hold additional significant information on the major differences in virulence among the species, as well as the specificity of the strains for particular hosts.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.

Evidence for conservation of architecture and physical properties of Omp85-like proteins throughout evolution

Omp85-like proteins represent a family of proteins involved in protein translocation, and they are present in all domains of life, except archaea. In eukaryotes, Omp85-like proteins have been demonstrated to form tetrameric pore-forming complexes that interact directly with their substrate proteins. Studies performed with bacterial Omp85-like proteins have demonstrated pore-forming activity but no evidence of multimerization. In this article, we characterize the Haemophilus influenzae HMW1B protein, an Omp85-like protein that has been demonstrated to be critical for secretion of the H. influenzae HMW1 adhesin. Analysis of purified protein by biochemical and electron microscopic techniques revealed that HMW1B forms a tetramer. Examination using liposome-swelling assays demonstrated that HMW1B has pore-forming activity, with a pore size of approximately equal to 2.7 nm. Far-Western blot analysis established that HMW1B interacts with the N terminus of HMW1. These results provide evidence that a bacterial Omp85-like protein forms a tetramer and interacts directly with a substrate protein, suggesting that the architecture and physical properties of Omp85-like proteins have been conserved throughout evolution.

Identification of specific in vivo-induced (ivi) genes in Yersinia ruckeri and analysis of ruckerbactin, a catecholate siderophore iron acquisition system

This work reports the utilization of an in vivo expression technology system to identify in vivo-induced (ivi) genes in Yersinia ruckeri after determination of the conditions needed for its selection in fish. Fourteen clones were selected, and the cloned DNA fragments were analyzed after partial sequencing. In addition to sequences with no significant similarity, homology with genes encoding proteins putatively involved in two-component and type IV secretion systems, adherence, specific metabolic functions, and others were found. Among these sequences, four were involved in iron acquisition through a catechol siderophore (ruckerbactin). Thus, unlike other pathogenic yersiniae producing yersiniabactin, Y. ruckeri might be able to produce and utilize only this phenolate. The genetic organization of the ruckerbactin biosynthetic and uptake loci was similar to that of the Escherichia coli enterobactin gene cluster. Genes rucC and rupG, putative counterparts of E. coli entC and fepG, respectively, involved in the biosynthesis and transport of the iron siderophore complex, respectively, were analyzed further. Thus, regulation of expression by iron and temperature and their presence in other Y. ruckeri siderophore-producing strains were confirmed for these two loci. Moreover, 50% lethal dose values 100-fold higher than those of the wild-type strain were obtained with the rucC isogenic mutant, showing the importance of ruckerbactin in the pathogenesis caused by this microorganism.

Identification of a novel two-partner secretion system from Burkholderia pseudomallei

Two adjacent genes, bpaA and bpaB, whose products display significant similarity to a number of two-partner secretion (TPS) systems have been identified in Burkholderia pseudomallei strain 08, but are absent from the closely related avirulent species B. thailandensis. They possess a number of sequence features characteristic of TPS systems, including the presence of an NPNGI motif in a region of BpaA which strongly resembles a TPS secretion domain. BpaA is a very large protein (approximately 530 kDa) and contains three repeats, each 600-800-amino acids long. Putative membrane-spanning regions in BpaB were identified through alignment with TpsB family members, and this also revealed an N-terminal extension not found in other TpsB proteins. The bpaA gene was found to be absent from the majority of B. pseudomallei strains. It appears that bpaAB are located within a putative genomic island that is inserted in close proximity to a methionine tRNA(CAT)-encoding gene. Expression of BpaA was undetectable in cells grown in laboratory media. However, owing to the similarity of BpaA to known adhesin molecules, a potential role of BpaA in virulence was investigated in cell culture and in an animal model, but no evidence for such a role was found in these test systems.

Getting out: protein traffic in prokaryotes

Protein secretion systems in prokaryotes are increasingly shifting from being considered as experimental models for 'more complex' processes (i.e. eukaryotes) to being a major source of key biological questions in their own right. The pathways by which proteins move between compartments or insert into membranes in prokaryotic cells are certainly less numerous than in eukaryotes (though not dramatically so). However, the quality and complexity of bacterial protein targeting systems indicate that virtually all mechanistic problems associated with protein traffic were solved very efficiently well before eukaryotes appeared on the Earth crust. Indeed, recent studies have both increased the number of known prokaryotic protein traffic systems and indicated new layers of complexity for those that were already well characterized. This report describes some recent developments in bacterial protein traffic that were presented at two meetings in the autumn of 2003.

The autotransporter secretion system

The type V secretion system includes the autotransporter family, the two-partner system and the Oca family. The autotransporter secretion process involving first the translocation of the precursor through the inner membrane and then its translocation through the outer membrane via a pore formed by a beta-barrel is reviewed.

The general secretory pathway: a general misnomer?

The term general secretory pathway (GSP) has been usurped and misused in the literature over the past few years. The concept of GSP is discussed from an historical perspective, and the definitions of the general export pathway (GEP), the main terminal branch (MTB) of the GSP, the unified GSP nomenclature and the type II, IV and V secretion pathways are also described to show how they have fuelled the confusion. By putting the record straight and using novel findings within the field of bacterial protein secretion, we hope to bring clarity to this area of science and prevent further promulgation of incorrect terminologies.

The louse-borne human pathogen Bartonella quintana is a genomic derivative of the zoonotic agent Bartonella henselae

We present the complete genomes of two human pathogens, Bartonella quintana (1,581,384 bp) and Bartonella henselae (1,931,047 bp). The two pathogens maintain several similarities in being transmitted by insect vectors, using mammalian reservoirs, infecting similar cell types (endothelial cells and erythrocytes) and causing vasculoproliferative changes in immunocompromised hosts. A primary difference between the two pathogens is their reservoir ecology. Whereas B. quintana is a specialist, using only the human as a reservoir, B. henselae is more promiscuous and is frequently isolated from both cats and humans. Genome comparison elucidated a high degree of overall similarity with major differences being B. henselae specific genomic islands coding for filamentous hemagglutinin, and evidence of extensive genome reduction in B. quintana, reminiscent of that found in Rickettsia prowazekii. Both genomes are reduced versions of chromosome I from the highly related pathogen Brucella melitensis. Flanked by two rRNA operons is a segment with similarity to genes located on chromosome II of B. melitensis, suggesting that it was acquired by integration of megareplicon DNA in a common ancestor of the two Bartonella species. Comparisons of the vector-host ecology of these organisms suggest that the utilization of host-restricted vectors is associated with accelerated rates of genome degradation and may explain why human pathogens transmitted by specialist vectors are outnumbered by zoonotic agents, which use vectors of broad host ranges.

Disassembly of F-actin cytoskeleton after interaction of Bacillus cereus with fully differentiated human intestinal Caco-2 cells

In the present study, the role of direct procaryote-eucaryote interactions in the virulence of Bacillus cereus was investigated. As a model of human enterocytes, differentiated Caco-2 cells were used. Infection of fully differentiated Caco-2 cells with B. cereus in the exponential phase of growth, in order to minimize the concentration of spores or sporulating microorganisms, shows that a strain-dependent cytopathic effect develops. Interestingly, addition of 3-h-old cultures of some strains resulted in complete detachment of the cultured cells after a 3-h infection whereas no such effect was found after a 3-h infection with 16-h-old cultures. Infection of enterocyte-like cells with B. cereus leads to disruption of the F-actin network and necrosis. Even though the effect of secreted factors cannot be ruled out, direct eucaryote-procaryote interaction seems to be necessary. In addition, we observed that some B. cereus strains were able to be internalized in Caco-2 cells. Our findings add a new insight into the mechanisms of virulence of B. cereus in the context of intestinal infection.

The LspB protein is involved in the secretion of the LspA1 and LspA2 proteins by Haemophilus ducreyi

The LspA1 and LspA2 proteins of Haemophilus ducreyi 35000 are two very large macromolecules that can be detected in concentrated culture supernatant fluid. Both of these proteins exhibit homology with the N-terminal region of the Bordetella pertussis filamentous hemagglutinin (FHA), which is involved in secretion of the latter macromolecule. The lspA2 open reading frame is flanked upstream by a gene, lspB, that encodes a predicted protein with homology to the B. pertussis FhaC outer membrane protein that is involved in secretion of FHA across the outer membrane. The H. ducreyi lspB gene encodes a protein with a predicted molecular mass of 66,573 Da. Reverse transcription-PCR analysis suggested that the lspB gene was transcribed together with the lspA2 gene on a single mRNA transcript. Polyclonal H. ducreyi LspB antiserum reacted with a 64-kDa antigen present in the Sarkosyl-insoluble cell envelope fraction of H. ducreyi 35000, which indicated that the LspB protein is likely an outer membrane protein. Concentrated culture supernatant fluids from H. ducreyi lspB and lspA1 lspB mutants did not contain detectable LspA1 and detectable LspA2, respectively. However, complementation of the lspB mutant with the wild-type lspB gene on a plasmid restored LspB protein expression and resulted in release of detectable amounts of the LspA1 protein into culture supernatant fluid. When evaluated in the temperature-dependent rabbit model of infection, the lspB mutant was attenuated in the ability to cause lesions and was never recovered in a viable form from lesions. These results indicated that the H. ducreyi LspB protein is involved in secretion of the LspA1 and LspA2 proteins across the outer membrane.

The crystal structure of filamentous hemagglutinin secretion domain and its implications for the two-partner secretion pathway

Filamentous hemagglutinin (FHA), the major 230-kDa adhesin of the whooping cough agent Bordetella pertussis, is one of the most efficiently secreted proteins in Gram-negative bacteria. FHA is secreted by means of the two-partner secretion (TPS) pathway. Several important human, animal, and plant pathogens also secrete adhesins and other virulence factors by using this mode of secretion. A TPS system is composed of two separate proteins, with TpsA the secreted protein and TpsB its associated specific outermembrane transporter. All TPS-secreted proteins contain a distinctive N-proximal module essential for secretion, the TPS domain. We report here the 1.7- A structure of a functionally secreted 30-kDa N-terminal fragment of FHA. It reveals that the TPS domain folds into a beta-helix, with three extrahelical motifs, a beta-hairpin, a four-stranded beta-sheet, and an N-terminal capping, mostly formed by the nonconserved regions of the TPS domain. The structure thus explains why the TPS domain is able to initiate folding of the beta-helical motifs that form the central domain of the adhesin, because it is itself a beta-helical scaffold. It also contains less conserved extrahelical regions most likely involved in specific properties, such as the recognition of the outer-membrane transporter. This structure is representative of the TPS domains found so far in >100 secreted proteins from pathogenic bacteria. It also provides a mechanistic insight into how protein folding may be linked to secretion in the TPS pathway.

Omp85, an evolutionarily conserved bacterial protein involved in outer-membrane-protein assembly

The insertion of proteins into membranes generally requires the assistance of membrane proteins. A protein, designated Omp85 in Neisseria meningitidis, was shown to be required for the assembly of bacterial outer-membrane proteins. The protein is essential for the viability of the bacteria and is ubiquitous among Gram-negative bacteria. Omp85 depletion results in the accumulation of aggregates of unfolded outer-membrane proteins, and we argue that Omp85 is directly involved in outer-membrane-protein assembly. Omp85 shows sequence similarity with Toc75 of the chloroplast protein-import machinery, suggesting a common evolutionary origin.

Structure of the translocator domain of a bacterial autotransporter

Autotransporters are virulence-related proteins of Gram-negative bacteria that are secreted via an outer-membrane-based C-terminal extension, the translocator domain. This domain supposedly is sufficient for the transport of the N-terminal passenger domain across the outer membrane. We present here the crystal structure of the in vitro-folded translocator domain of the autotransporter NalP from Neisseria meningitidis, which reveals a 12-stranded beta-barrel with a hydrophilic pore of 10 x 12.5 A that is filled by an N-terminal alpha-helix. The domain has pore activity in vivo and in vitro. Our data are consistent with the model of passenger-domain transport through the hydrophilic channel within the beta-barrel, and inconsistent with a model for transport through a central channel formed by an oligomer of translocator domains. However, the dimensions of the pore imply translocation of the secreted domain in an unfolded form. An alternative model, possibly covering the transport of folded domains, is that passenger-domain transport involves the Omp85 complex, the machinery required for membrane insertion of outer-membrane proteins, on which autotransporters are dependent.

Lessons learned from the genome analysis of ralstonia solanacearum

Ralstonia solanacearum is a devastating plant pathogen with a global distribution and an unusually wide host range. This bacterium can also be free-living as a saprophyte in water or in the soil in the absence of host plants. The availability of the complete genome sequence from strain GMI1000 provided the basis for an integrative analysis of the molecular traits determining the adaptation of the bacterium to various environmental niches and pathogenicity toward plants. This review summarizes current knowledge and speculates on some key bacterial functions, including metabolic versatility, resistance to metals, complex and extensive systems for motility and attachment to external surfaces, and multiple protein secretion systems. Genome sequence analysis provides clues about the evolution of essential virulence genes such as those encoding the Type III secretion system and related pathogenicity effectors. It also provided insights into possible mechanisms contributing to the rapid adaptation of the bacterium to its environment in general and to its interaction with plants in particular.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Surface anchoring of bacterial subtilisin important for maturation function

Many extracytoplasmic proteins undergo proteolytic processing during secretion, which is essential to their maturation. These post-translational modifications are carried out by specific enzymes whose subcellular localization is important for function. We have described a maturation subtilisin in Gram-negative Bordetella pertussis, the autotransporter SphB1. SphB1 catalyses the maturation of the precursor of the adhesin filamentous haemagglutinin (FHA) at the bacterial surface, in addition to the processing of its own precursor. Here, we show that the outer membrane anchor of SphB1 is crucial to its function, as evidenced by the lack of FHA maturation in a strain releasing a variant of SphB1 into the milieu. In contrast, surface association is not required for automaturation of SphB1. The surface retention of mature SphB1 is mediated by lipidation of the protein. The tethered protease appears to be stabilized by unusual Gly- and Pro-rich motifs at the N-terminus of the protein. This represents a new mode of localization for a protease involved in protein secretion.

Secretins of Pseudomonas aeruginosa: large holes in the outer membrane

Pseudomonas aeruginosa produces a large number of exoproteins, ranging from the ADP-ribosyltransferases exotoxin A and ExoS to degradative enzymes, such as elastase and chitinase. As it is a gram-negative bacterium, P. aeruginosa must be able to transport these exoproteins across both membranes of the cell envelope. In addition, also proteins that are part of cellular appendages, such as type IV pili and flagella, have to cross the cell envelope. Whereas the majority of the proteins transported across the inner membrane are dependent on the Sec channel, the systems for translocation across the outer membrane seem to be more diverse. Gram-negative bacteria have invented a number of different strategies during the course of evolution to achieve this goal. Although these transport machineries seem to be radically different, many of them actually depend on a member of the secretin protein family for their function. Recent results show that secretins form a large complex in the outer membrane, which constitutes the actual translocation channel. Understanding the working mechanism of this protein translocation channel could open up new strategies to target molecular machineries at the heart of many important virulence factors.

Mutations in the lspA1 and lspA2 genes of Haemophilus ducreyi affect the virulence of this pathogen in an animal model system

Haemophilus ducreyi 35000HP contains two genes, lspA1 and lspA2, whose predicted protein products have molecular weights of 456,000 and 543,000, respectively (C. K. Ward, S. R. Lumbley, J. L. Latimer, L. D. Cope, and E. J. Hansen, J. Bacteriol. 180:6013-6022, 1998). We have constructed three H. ducreyi 35000HP mutants containing antibiotic resistance cartridges in one or both of the lspA1 and lspA2 open reading frames. Western blot analysis using LspA1- and LspA2-specific monoclonal antibodies indicated that the wild-type parent strain 35000HP expressed LspA1 protein that was readily detectable in culture supernatant fluid together with a barely detectable amount of LspA2 protein. The lspA2 mutant 35000HP.2 expressed LspA1 protein that was detectable in culture supernatant fluid and no LspA2 protein. In contrast, the H. ducreyi lspA1 mutant 35000HP.1, which did not express the LspA1 protein, expressed a greater quantity of the LspA2 protein than did the wild-type parent strain. The lspA1 lspA2 double mutant 35000HP.12 expressed neither LspA1 nor LspA2. The three mutant strains adhered to human foreskin fibroblasts and to a human keratinocyte cell line in vitro at a level that was not significantly different from that of the wild-type strain 35000HP. Lack of expression of the LspA1 protein by both the lspA1 mutant and the lspA1 lspA2 double mutant was associated with an increased tendency to autoagglutinate. When evaluated in the temperature-dependent rabbit model for chancroid, the lspA1 lspA2 double mutant was substantially less virulent than the wild-type strain 35000HP. The results of these studies indicated that H. ducreyi requires both the LspA1 and LspA2 proteins to be fully virulent in this animal model for experimental chancroid.

Proteome analysis of extracellular proteins regulated by the las and rhl quorum sensing systems in Pseudomonas aeruginosa PAO1

The las and rhl quorum sensing (QS) systems regulate the expression of several genes in response to cell density changes in Pseudomonas aeruginosa. Many of these genes encode surface-associated or secreted virulence factors. Proteins from stationary phase culture supernatants were collected from wild-type and P. aeruginosa PAO1 mutants deficient in one or more of the lasRI, rhlRI and vfr genes and analysed using two-dimensional gel electrophoresis. All mutants released significantly lower amounts of protein than the wild-type. Protein spot patterns from each strain were compared using image analysis and visible spot differences were identified using mass spectrometry. Several previously unknown QS-regulated proteins were characterized, including an aminopeptidase (PA2939), an endoproteinase (PrpL) and a unique 'hypothetical' protein (PA0572), which could not be detected in the culture supernatants of Deltalas mutants, although they were unaffected in Deltarhl mutants. Chitin-binding protein (CbpD) and a hypothetical protein (PA4944) with similarity to host factor I (HF-I) could not be detected when any of the lasRI or rhlRI genes were disrupted. Fourteen proteins were present at significantly greater levels in the culture supernatants of QS mutants, suggesting that QS may also negatively control the expression of some genes. Increased levels of two-partner secretion exoproteins (PA0041 and PA4625) were observed and may be linked to increased stability of their cognate transporters in a QS-defective background. Known QS-regulated extracellular proteins, including elastase (lasB), LasA protease (lasA) and alkaline metalloproteinase (aprA) were also detected.

Protein secretion systems of Pseudomonas aeruginosa and P fluorescens

Gram-negative bacteria have evolved numerous systems for the export of proteins across their dual-membrane envelopes. Three of these systems (types I, III and IV) secrete proteins across both membranes in a single energy-coupled step. Four systems (Sec, Tat, MscL and Holins) secrete only across the inner membrane, and four systems [the main terminal branch (MTB), fimbrial usher porin (FUP), autotransporter (AT) and two-partner secretion families (TPS)] secrete only across the outer membrane. We have examined the genome sequences of Pseudomonas aeruginosa PAO1 and Pseudomonas fluorescens Pf0-1 for these systems. All systems except type IV were found in P. aeruginosa, and all except types III and IV were found in P. fluorescens. The numbers of each such system were variable depending on the system and species examined. Biochemical and physiological functions were assigned to these systems when possible, and the structural constituents were analyzed. Available information regarding the mechanisms of transport and energy coupling as well as physiological functions is summarized. This report serves to identify and characterize protein secretion systems in two divergent pseudomonads, one an opportunistic human pathogen, the other a plant symbiont.

Two-step purification of the outer membrane transporter and activator protein ShlB from Escherichia coli using internally His6-tagged constructs

ShlB from Serratia marcescens was isolated and purified from a porin-deficient Escherichia coli BL21 strain using a combination of detergent extraction, affinity and ion-exchange chromatography. An internal histidine affinity tag was introduced that did not interfere with activity. At each stage of the purification scheme biological activity of the ShlB protein was assessed. Using this scheme, several His(6)-tagged mutants of ShlB were purified to electrophoretic homogeneity.

Comparative genomics identifies the genetic islands that distinguish Neisseria meningitidis, the agent of cerebrospinal meningitis, from other Neisseria species

Neisseria meningitidis colonizes the nasopharynx and, unlike commensal Neisseria species, is capable of entering the bloodstream, crossing the blood-brain barrier, and invading the meninges. The other pathogenic Neisseria species, Neisseria gonorrhoeae, generally causes an infection which is localized to the genitourinary tract. In order to investigate the genetic basis of this difference in disease profiles, we used a strategy of genomic comparison. We used DNA arrays to compare the genome of N. meningitidis with those of N. gonorrhoeae and Neisseria lactamica, a commensal of the nasopharynx. We thus identified sequences conserved among a representative set of virulent strains which are either specific to N. meningitidis or shared with N. gonorrhoeae but absent from N. lactamica. Though these bacteria express dramatically different pathogenicities, these meningococcal sequences were limited and, in contrast to what has been found in other pathogenic bacterial species, they are not organized in large chromosomal islands.

HecA, a member of a class of adhesins produced by diverse pathogenic bacteria, contributes to the attachment, aggregation, epidermal cell killing, and virulence phenotypes of Erwinia chrysanthemi EC16 on Nicotiana clevelandii seedlings

Erwinia chrysanthemi is representative of a broad class of bacterial pathogens that are capable of inducing necrosis in plants. The E. chrysanthemi EC16 hecA gene predicts a 3,850-aa member of the Bordetella pertussis filamentous hemagglutinin family of adhesins. A hecATn7 mutant was reduced in virulence on Nicotiana clevelandii seedlings after inoculation without wounding. Epifluorescence and confocal laser-scanning microscopy observations of hecA and wild-type cells expressing the green fluorescent protein revealed that the mutant is reduced in its ability to attach and then form aggregates on leaves and to cause an aggregate-associated killing of epidermal cells. Cell killing also depended on production of the major pectate lyase isozymes and the type II, but not the type III, secretion pathway in E. chrysanthemi. HecA homologs were found in bacterial pathogens of plants and animals and appear to be unique to pathogens and universal in necrogenic plant pathogens. Phylogenetic comparison of the conserved two-partner secretion domains in the proteins and the 16S rRNA sequences in respective bacteria revealed the two datasets to be fundamentally incongruent, suggesting horizontal acquisition of these genes. Furthermore, hecA and its two homologs in Yersinia pestis had a G+C content that was 10% higher than that of their genomes and similar to that of plant pathogenic Ralstonia, Xylella, and Pseudomonas spp. Our data suggest that filamentous hemagglutinin-like adhesins are broadly important virulence factors in both plant and animal pathogens.

Peptide linkage mapping of the Agrobacterium tumefaciens vir-encoded type IV secretion system reveals protein subassemblies

Numerous bacterial pathogens use type IV secretion systems (T4SS) to deliver virulence factors directly to the cytoplasm of plant, animal, and human host cells. Here, evidence for interactions among components of the Agrobacterium tumefaciens vir-encoded T4SS is presented. The results derive from a high-resolution yeast two-hybrid assay, in which a library of small peptide domains of T4SS components was screened for interactions. The use of small peptides overcomes problems associated with assaying for interactions involving membrane-associated proteins. We established interactions between VirB11 (an inner membrane pore-forming protein), VirB9 (a periplasmic protein), and VirB7 (an outer membrane-associated lipoprotein and putative pilus component). We provide evidence for an interaction pathway, among conserved members of a T4SS, spanning the A. tumefaciens envelope and including a potential pore protein. In addition, we have determined interactions between VirB1 (a lytic transglycosylase likely involved in the local remodeling of the peptidoglycan) and primarily VirB8, but also VirB4, VirB10, and VirB11 (proteins likely to assemble the core structure of the T4SS). VirB4 interacts with VirB8, VirB10, and VirB11, also establishing a connection to the core components. The identification of these interactions suggests a model for assembly of the T4SS.

Eighty-kilodalton N-terminal moiety of Bordetella pertussis filamentous hemagglutinin: adherence, immunogenicity, and protective role

Bordetella pertussis, the etiological agent of whooping cough, produces a number of factors, such as toxins and adhesins, that are required for full expression of virulence. Filamentous hemagglutinin (FHA) is the major adhesin of B. pertussis. It is a protein of approximately 220 kDa, found both associated at the bacterial cell surface and secreted into the extracellular milieu. Despite its importance in B. pertussis pathogenesis and its inclusion in most acellular pertussis vaccines, little is known about the functional importance of individual domains in infection and in the induction of protective immunity. In this study, we analyzed the role of the approximately 80-kDa N-terminal domain of FHA, designated Fha44, in B. pertussis adherence, colonization, and immunogenicity. Although Fha44 contains the complete heparan sulfate-binding domain, it is not sufficient for adherence to epithelial cells or macrophages. It also cannot replace FHA during colonization of the mouse respiratory tract. Infection with a B. pertussis strain producing Fha44 instead of FHA does not induce anti-FHA antibodies, whereas such antibodies can readily be induced by intranasal administration of purified Fha44. In addition, mice immunized with purified Fha44 were protected against challenge with wild-type B. pertussis, indicating that Fha44 contains protective epitopes. Compared to FHA, Fha44 is much smaller and much more soluble and is therefore easier to purify and to store. These advantages may perhaps warrant considering Fha44 for inclusion in acellular pertussis vaccines.

The PhlA hemolysin from the entomopathogenic bacterium Photorhabdus luminescens belongs to the two-partner secretion family of hemolysins

Photorhabdus is an entomopathogenic bacterium symbiotically associated with nematodes of the family Heterorhabditidae. Bacterial hemolysins found in numerous pathogenic bacteria are often virulence factors. We describe here the nucleotide sequence and the molecular characterization of the Photorhabdus luminescens phlBA operon, a locus encoding a hemolysin which shows similarities to the Serratia type of hemolysins. It belongs to the two-partner secretion (TPS) family of proteins. In low-iron conditions, a transcriptional induction of the phlBA operon was observed by using the chloramphenicol acetyltransferase reporter gene, causing an increase in PhlA hemolytic activity compared to iron-rich media. A spontaneous phase variant of P. luminescens was deregulated in phlBA transcription. The phlA mutant constructed by allelic exchange remained highly pathogenic after injection in the lepidopteran Spodoptera littoralis, indicating that PhlA hemolysin is not a major virulence determinant. Using the gene encoding green fluorescent protein as a reporter, phlBA transcription was observed in hemolymph before insect death. We therefore discuss the possible role of PhlA hemolytic activity in the bacterium-nematode-insect interactions.

Comparative genomic analysis of plant-associated bacteria

This review deals with a comparative analysis of seven genome sequences from plant-associated bacteria. These are the genomes of Agrobacterium tumefaciens, Mesorhizobium loti, Sinorhizobium meliloti, Xanthomonas campestris pv campestris, Xanthomonas axonopodis pv citri, Xylella fastidiosa, and Ralstonia solanacearum. Genome structure and the metabolism pathways available highlight the compromise between the genome size and lifestyle. Despite the recognized importance of the type III secretion system in controlling host compatibility, its presence is not universal in all necrogenic pathogens. Hemolysins, hemagglutinins, and some adhesins, previously reported only for mammalian pathogens, are present in most organisms discussed. Different numbers and combinations of cell wall degrading enzymes and genes to overcome the oxidative burst generally induced by the plant host are characterized in these genomes. A total of 19 genes not involved in housekeeping functions were found common to all these bacteria.

Protein-translocating outer membrane porins of Gram-negative bacteria

Five families of outer membrane porins that function in protein secretion in Gram-negative bacteria are currently recognized. In this report, these five porin families are analyzed from structural and phylogenetic standpoints. They are the fimbrial usher protein (FUP), outer membrane factor (OMF), autotransporter (AT), two-partner secretion (TPS) and outer membrane secretin (Secretin) families. All members of these families in the current databases were identified, and all full-length homologues were multiply aligned for structural and phylogenetic analyses. The organismal distribution of homologues in each family proved to be unique with some families being restricted to proteobacteria and others being widespread in other bacterial kingdoms as well as eukaryotes. The compositions of and size differences between subfamilies provide evidence for specific orthologous relationships, which agree with available functional information and intra-subfamily phylogeny. The results reveal that horizontal transfer of genes encoding these proteins between phylogenetically distant organisms has been exceptionally rare although transfer within select bacterial kingdoms may have occurred. The resultant in silico analyses are correlated with available experimental evidence to formulate models relevant to the structures and evolutionary origins of these proteins.

Use of colicin-based genetic tools for studying bacterial protein transport

Transport of proteins across the envelope of Gram-negative bacteria is a very challenging domain of investigation, which involves membrane-embedded proteinaceous complexes at which specific targeting occurs. These transporters (translocon or secreton) have been studied both with genetics and biochemistry. In this review we report recent developments that should help to identify novel interactions that exist within these complexes, and to decipher the signals that specifically direct transported proteins to the cognate system. These developments are exclusively based on the re-routing of colicins to these molecular machineries. The re-routing induces a lethal situation in the case of efficient or inefficient transport, depending on the system, thus creating a genetic tool for selection of mutations that correct or generate a transport default.

Molecular and cellular determinants of non-typeable Haemophilus influenzae adherence and invasion

Non-typeable Haemophilus influenzae is a common cause of human disease and initiates infection by colonizing the upper respiratory tract. Based on information from histopathologic specimens and in vitro studies with human cells and tissues in culture, non-typeable H. influenzae is capable of efficient adherence and appreciable invasion, properties that facilitate the process of colonization. A number of adhesive factors exist, each recognizing a distinct host cell structure and influencing cellular binding specificity. In addition, at least three invasion pathways exist, including one resembling macropinocytosis, a second mediated via the PAF receptor and a third involving beta-glucan receptors. Organisms are also capable of disrupting cell-cell junctions and passing between cells to the subepithelial space.

Genome sequence of the plant pathogen Ralstonia solanacearum

Ralstonia solanacearum is a devastating, soil-borne plant pathogen with a global distribution and an unusually wide host range. It is a model system for the dissection of molecular determinants governing pathogenicity. We present here the complete genome sequence and its analysis of strain GMI1000. The 5.8-megabase (Mb) genome is organized into two replicons: a 3.7-Mb chromosome and a 2.1-Mb megaplasmid. Both replicons have a mosaic structure providing evidence for the acquisition of genes through horizontal gene transfer. Regions containing genetically mobile elements associated with the percentage of G+C bias may have an important function in genome evolution. The genome encodes many proteins potentially associated with a role in pathogenicity. In particular, many putative attachment factors were identified. The complete repertoire of type III secreted effector proteins can be studied. Over 40 candidates were identified. Comparison with other genomes suggests that bacterial plant pathogens and animal pathogens harbour distinct arrays of specialized type III-dependent effectors.

Polymorphic proteins of Chlamydia spp.--autotransporters beyond the Proteobacteria

Gram-negative bacteria secrete a variety of proteins to the cell surface and beyond, a process with many inherent difficulties. An exceptionally widespread answer to these problems is the type V (or autotransporter) secretion pathway. By exploiting the data made available by bacterial genome sequencing, we have discovered that the previously described polymorphic proteins of Chlamydia spp. resemble members of the autotransporter family, and we suggest that they follow the same secretion pathway.

Subtilisin-like autotransporter serves as maturation protease in a bacterial secretion pathway

Proteins of Gram-negative bacteria destined to the extracellular milieu must cross the two cellular membranes and then fold at the appropriate time and place. The synthesis of a precursor may be a strategy to maintain secretion competence while preventing aggregation or premature folding (especially for large proteins). The secretion of 230 kDa filamentous haemagglutinin (FHA) of Bordetella pertussis requires the synthesis and the maturation of a 367 kDa precursor that undergoes the proteolytic removal of its approximately 130 kDa C-terminal intramolecular chaperone domain. We have identified a specific protease, SphB1, responsible for the timely maturation of the precursor FhaB, which allows for extracellular release of FHA. SphB1 is a large exported protein with a subtilisin-like domain and a C-terminal domain typical of bacterial autotransporters. SphB1 is the first described subtilisin-like protein that serves as a specialized maturation protease in a secretion pathway of Gram-negative bacteria. This is reminiscent of pro-protein convertases of eukaryotic cells.

Type IV secretion: intercellular transfer of macromolecules by systems ancestrally related to conjugation machines

Bacterial conjugation systems are highly promiscuous macromolecular transfer systems that impact human health significantly. In clinical settings, conjugation is exceptionally problematic, leading to the rapid dissemination of antibiotic resistance genes and other virulence traits among bacterial populations. Recent work has shown that several pathogens of plants and mammals - Agrobacterium tumefaciens, Bordetella pertussis, Helicobacter pylori and Legionella pneumophila - have evolved secretion pathways ancestrally related to conjugation systems for the purpose of delivering effector molecules to eukaryotic target cells. Each of these systems exports distinct DNA or protein substrates to effect a myriad of changes in host cell physiology during infection. Collectively, secretion pathways ancestrally related to bacterial conjugation systems are now referred to as the type IV secretion family. The list of putative type IV family members is increasing rapidly, suggesting that macromolecular transfer by these systems is a widespread phenomenon in nature.

Bordetella pertussis, molecular pathogenesis under multiple aspects

Recent studies, including those based on genomics, have demonstrated that besides toxins and adhesins, Bordetella pertussis uses many additional virulence determinants. Most of them are part of the BvgAS regulon, although some, in particular iron-uptake systems, are independent of BvgAS. They are regulated by iron, although in one case, the production of a siderophore receptor could be linked to the BvgAS regulon.