Amino acid replacements in the Serratia marcescens haemolysin ShIA define sites involved in activation and secretion

Structural and biophysical characterization of the secreted, β-helical adhesin EtpA of Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a diarrhoeal pathogen associated with high morbidity and mortality especially among young children in developing countries. At present, there is no vaccine for ETEC. One candidate vaccine antigen, EtpA, is a conserved secreted adhesin that binds to the tips of flagellae to bridge ETEC to host intestinal glycans. EtpA is exported through a Gram-negative, two-partner secretion system (TPSS, type Vb) comprised of the secreted EtpA passenger (TpsA) protein and EtpB (TpsB) transporter that is integrated into the outer bacterial membrane. TpsA proteins share a conserved, N-terminal TPS domain followed by an extensive C-terminal domain with divergent sequence repeats. Two soluble, N-terminal constructs of EtpA were prepared and analysed respectively including residues 67 to 447 (EtpA67-447) and 1 to 606 (EtpA1-606). The crystal structure of EtpA67-447 solved at 1.76 Å resolution revealed a right-handed parallel β-helix with two extra-helical hairpins and an N-terminal β-strand cap. Analyses by circular dichroism spectroscopy confirmed the β-helical fold and indicated high resistance to chemical and thermal denaturation as well as rapid refolding. A theoretical AlphaFold model of full-length EtpA largely concurs with the crystal structure adding an extended β-helical C-terminal domain after an interdomain kink. We propose that robust folding of the TPS domain upon secretion provides a template to extend the N-terminal β-helix into the C-terminal domains of TpsA proteins.

Bacterial pore-forming toxins

Pore-forming toxins (PFTs) are widely distributed in both Gram-negative and Gram-positive bacteria. PFTs can act as virulence factors that bacteria utilise in dissemination and host colonisation or, alternatively, they can be employed to compete with rival microbes in polymicrobial niches. PFTs transition from a soluble form to become membrane-embedded by undergoing large conformational changes. Once inserted, they perforate the membrane, causing uncontrolled efflux of ions and/or nutrients and dissipating the protonmotive force (PMF). In some instances, target cells intoxicated by PFTs display additional effects as part of the cellular response to pore formation. Significant progress has been made in the mechanistic description of pore formation for the different PFTs families, but in several cases a complete understanding of pore structure remains lacking. PFTs have evolved recognition mechanisms to bind specific receptors that define their host tropism, although this can be remarkably diverse even within the same family. Here we summarise the salient features of PFTs and highlight where additional research is necessary to fully understand the mechanism of pore formation by members of this diverse group of protein toxins.

The Outer Membrane Took Center Stage

My interest in membranes was piqued during a lecture series given by one of the founders of molecular biology, Max Delbrück, at Caltech, where I spent a postdoctoral year to learn more about protein chemistry. That general interest was further refined to my ultimate research focal point-the outer membrane of Escherichia coli-through the influence of the work of Wolfhard Weidel, who discovered the murein (peptidoglycan) layer and biochemically characterized the first phage receptors of this bacterium. The discovery of lipoprotein bound to murein was completely unexpected and demonstrated that the protein composition of the outer membrane and the structure and function of proteins could be unraveled at a time when nothing was known about outer membrane proteins. The research of my laboratory over the years covered energy-dependent import of proteinaceous toxins and iron chelates across the outer membrane, which does not contain an energy source, and gene regulation by iron, including transmembrane transcriptional regulation.

Two-Partner Secretion: Combining Efficiency and Simplicity in the Secretion of Large Proteins for Bacteria-Host and Bacteria-Bacteria Interactions

Initially identified in pathogenic Gram-negative bacteria, the two-partner secretion (TPS) pathway, also known as Type Vb secretion, mediates the translocation across the outer membrane of large effector proteins involved in interactions between these pathogens and their hosts. More recently, distinct TPS systems have been shown to secrete toxic effector domains that participate in inter-bacterial competition or cooperation. The effects of these systems are based on kin vs. non-kin molecular recognition mediated by specific immunity proteins. With these new toxin-antitoxin systems, the range of TPS effector functions has thus been extended from cytolysis, adhesion, and iron acquisition, to genome maintenance, inter-bacterial killing and inter-bacterial signaling. Basically, a TPS system is made up of two proteins, the secreted TpsA effector protein and its TpsB partner transporter, with possible additional factors such as immunity proteins for protection against cognate toxic effectors. Structural studies have indicated that TpsA proteins mainly form elongated β helices that may be followed by specific functional domains. TpsB proteins belong to the Omp85 superfamily. Open questions remain on the mechanism of protein secretion in the absence of ATP or an electrochemical gradient across the outer membrane. The remarkable dynamics of the TpsB transporters and the progressive folding of their TpsA partners at the bacterial surface in the course of translocation are thought to be key elements driving the secretion process.

Type V Secretion: the Autotransporter and Two-Partner Secretion Pathways

The autotransporter and two-partner secretion (TPS) pathways are used by E. coli and many other Gram-negative bacteria to delivervirulence factors into the extracellular milieu.Autotransporters arecomprised of an N-terminal extracellular ("passenger") domain and a C-terminal β barrel domain ("β domain") that anchors the protein to the outer membrane and facilitates passenger domain secretion. In the TPS pathway, a secreted polypeptide ("exoprotein") is coordinately expressed with an outer membrane protein that serves as a dedicated transporter. Bothpathways are often grouped together under the heading "type V secretion" because they have many features in common and are used for the secretion of structurally related polypeptides, but it is likely that theyhave distinct evolutionary origins. Although it was proposed many years ago that autotransporterpassenger domains are transported across the outer membrane through a channel formed by the covalently linked β domain, there is increasing evidence that additional factors are involved in the translocation reaction. Furthermore, details of the mechanism of protein secretion through the TPS pathway are only beginning to emerge. In this chapter I discussour current understanding ofboth early and late steps in the biogenesis of polypeptides secreted through type V pathways and current modelsofthe mechanism of secretion.

Import and export of bacterial protein toxins

The paper provides a short overview of three investigated bacterial protein toxins, colicin M (Cma) of Escherichia coli, pesticin (Pst) of Yersinia pestis and hemolysin (ShlAB) of Serratia marcescens. Cma and Pst are exceptional among colicins in that they kill bacteria by degrading the murein (peptidoglycan). Both are released into the medium and bind to specific receptor proteins in the outer membrane of sensitive E. coli cells. Subsequently they are translocated into the periplasm by an energy-consuming process using the proton motive force. For transmembrane translocation the colicins unfold and refold in the periplasm. In the case of Cma the FkpA peptidyl prolyl cis-trans isomerase/chaperone is required. ShlA is secreted and activated through ShlB in the outer membrane by a type Vb secretion mechanism.

Secretion and activation of the Serratia marcescens hemolysin by structurally defined ShlB mutants

The ShlA hemolysin of Serratia marcescens is secreted across the outer membrane by the ShlB protein; ShlB belongs to the two-partner secretion system (type Vb), a subfamily of the Omp85 outer membrane protein assembly and secretion superfamily. During secretion, ShlA is converted from an inactive non-hemolytic form into an active hemolytic form. The structure of ShlB is predicted to consist of the N-terminal α-helix H1, followed by the two polypeptide-transport-associated domains POTRA P1 and P2, and the β-barrel of 16 β-strands. H1 is inserted into the pore of the β-barrel in the outer membrane; P1 and P2 are located in the periplasm. To obtain insights into the secretion and activation of ShlA by ShlB, we isolated ShlB mutants impaired in secretion and/or activation. The triple H1 P1 P2 mutant did not secrete ShlA. The P1 and P2 deletion derivatives secreted reduced amounts of ShlA, of which P1 showed some hemolysis, whereas P2 was inactive. Deletion of loop 6 (L6), which is conserved among exporters of the Omp85 family, compromised activation but retained low secretion. Secretion-negative mutants generated by random mutagenesis were located in loop 6. The inactive secreted ShlA derivatives were complemented in vitro to active ShlA by an N-terminal ShlA fragment (ShlA242) secreted by ShlB. Deletion of H1 did not impair secretion of hemolytic ShlA. The study defines domains of ShlB which are important for ShlA secretion and activation.

Two-partner secretion: as simple as it sounds?

The two-partner secretion (TPS) pathway is a branch of type V secretion. TPS systems are dedicated to the secretion across the outer membrane of long proteins that form extended β-helices. They are composed of a 'TpsA' cargo protein and a 'TpsB' transporter, which belongs to the Omp85 superfamily. This basic design can be supplemented by additional components in some TPS systems. X-ray structures are available for the conserved TPS domain of several TpsA proteins and for one TpsB transporter. However, the molecular mechanisms of two-partner secretion remain to be deciphered, and in particular, the specific role(s) of the TPS domain and the conformational dynamics of the TpsB transporter. Deciphering the TPS pathway may reveal functional features of other transporters of the Omp85 superfamily.

System specificity of the TpsB transporters of coexpressed two-partner secretion systems of Neisseria meningitidis

The two-partner secretion (TPS) systems of Gram-negative bacteria consist of a large secreted exoprotein (TpsA) and a transporter protein (TpsB) located in the outer membrane. TpsA targets TpsB for transport across the membrane via its ∼30-kDa TPS domain located at its N terminus, and this domain is also the minimal secretory unit. Neisseria meningitidis genomes encode up to five TpsAs and two TpsBs. Sequence alignments of TPS domains suggested that these are organized into three systems, while there are two TpsBs, which raised questions on their system specificity. We show here that the TpsB2 transporter of Neisseria meningitidis is able to secrete all types of TPS domains encoded in N. meningitidis and the related species Neisseria lactamica but not domains of Haemophilus influenzae and Pseudomonas aeruginosa. In contrast, the TpsB1 transporter seemed to be specific for its cognate N. meningitidis system and did not secrete the TPS domains of other meningococcal systems. However, TpsB1 did secrete the TPS2b domain of N. lactamica, which is related to the meningococcal TPS2 domains. Apparently, the secretion depends on specific sequences within the TPS domain rather than the overall TPS domain structure.

Virulence determinants involved in differential host niche adaptation of Neisseria meningitidis and Neisseria gonorrhoeae

Neisseria meningitidis and Neisseria gonorrhoeae are the only pathogenic species of the genus Neisseria. Although these two species are closely related, they specialized on survival in completely different environments within the human host-the nasopharynx in the case of N. meningitidis versus the urogenital tract in the case of N. gonorrhoeae. The genetic background of these differences has not yet been determined. Here, we present a comparison of all characterized transcriptional regulators in these species, delineating analogous functions and disclosing differential functional developments of these DNA-binding proteins with a special focus on the recently characterized regulator FarR and its contribution to divergent host niche adaptation in the two Neisseria spp. Furthermore, we summarize the present knowledge on two-partner secretion systems in meningococci, highlighting their overall expression among meningococcal strains in contrast to the complete absence in gonococci. Concluding, the decisive role of these two entirely different factors in host niche adaptation of the two human pathogenic Neisseria species is depicted, illuminating another piece of the puzzle to locate the molecular basis of their differences in preferred colonization sites and pathogenicity.

The Pseudomonas aeruginosa patatin-like protein PlpD is the archetype of a novel Type V secretion system

We discovered a novel secreted protein by Pseudomonas aeruginosa, PlpD, as a member of the bacterial lipolytic enzyme family of patatin-like proteins (PLPs). PlpD is synthesized as a single molecule consisting of a secreted domain fused to a transporter domain. The N-terminus of PlpD includes a classical signal peptide followed by the four PLP conserved blocks that account for its lipase activity. The C-terminus consists of a POTRA (polypeptide transport-associated) motif preceding a putative 16-stranded beta-barrel similar to those of TpsB transporters of Type Vb secretion system. We showed that the C-terminus remains inserted into the outer membrane while the patatin moiety is secreted. The association between a TpsB component and a passenger protein is a unique hybrid organization that we propose to classify as Type Vd. More than 200 PlpD orthologues exist among pathogenic and environmental bacteria, which suggests that bacteria secrete numerous PLPs using this newly defined mechanism.

Sequential translocation of an Escherchia coli two-partner secretion pathway exoprotein across the inner and outer membranes

In Gram-negative bacteria, a variety of high molecular weight 'exoproteins' are translocated across the outer membrane (OM) via the two-partner secretion (TPS) pathway by interacting with a dedicated transporter. It is unclear, however, whether the translocation of exoproteins across the OM is coupled to their translocation across the inner membrane (IM). To address this question, we separated the production of an Escherichia coli O157:H7 exoprotein (OtpA) and its transporter (OtpB) temporally by placing otpA and otpB under the control of distinct regulatable promoters. We found that when both full-length and truncated forms of OtpA were expressed prior to OtpB, a significant fraction of the exoprotein was secreted. The results indicate that OtpA can be translocated into the periplasm and briefly remain secretion-competent. Furthermore, by engineering cysteine residues into OtpA and using disulphide bond formation as a reporter of periplasmic localization, we obtained additional evidence that the C-terminus of OtpA enters the periplasm before the N-terminus is translocated across the OM even when OtpA and OtpB are expressed simultaneously. Taken together, our results demonstrate that the translocation of a TPS exoprotein across the OM can occur independently from its translocation across the IM.

Structural and functional studies of truncated hemolysin A from Proteus mirabilis

In this study we analyzed the structure and function of a truncated form of hemolysin A (HpmA265) from Proteus mirabilis using a series of functional and structural studies. Hemolysin A belongs to the two-partner secretion pathway. The two-partner secretion pathway has been identified as the most common protein secretion pathway among Gram-negative bacteria. Currently, the mechanism of action for the two-partner hemolysin members is not fully understood. In this study, hemolysis experiments revealed a unidirectional, cooperative, biphasic activity profile after full-length, inactive hemolysin A was seeded with truncated hemolysin A. We also solved the first x-ray structure of a TpsA hemolysin. The truncated hemolysin A formed a right-handed parallel beta-helix with three adjoining segments of anti-parallel beta-sheet. A CXXC disulfide bond, four buried solvent molecules, and a carboxyamide ladder were all located at the third complete beta-helix coil. Replacement of the CXXC motif led to decreased activity and stability according to hemolysis and CD studies. Furthermore, the crystal structure revealed a sterically compatible, dry dimeric interface formed via anti-parallel beta-sheet interactions between neighboring beta-helix monomers. Laser scanning confocal microscopy further supported the unidirectional interconversion of full-length hemolysin A. From these results, a model has been proposed, where cooperative, beta-strand interactions between HpmA265 and neighboring full-length hemolysin A molecules, facilitated in part by the highly conserved CXXC pattern, account for the template-assisted hemolysis.

Autotransporter and Two-Partner Secretion: Delivery of Large-Size Virulence Factors by Gram-Negative Bacterial Pathogens

A number of protein secretion mechanisms have been identified in gram-negative pathogens. Many of these secretion systems are dependent upon the Sec translocase for protein export from the cytoplasm into the periplasm and then utilize other mechanisms for transport from the periplasm through the outer membrane. In this article, we review secretion similarities between autotransporter and two-partner secretion systems, and we report similarities between the autotransporter secretion mechanism with that of intimin/invasins. Considering that many secreted proteins are virulence factors, a better understanding of their secretion mechanisms will aid in the development of disease treatments and new bacterial vaccines.

The 'P-usher', a novel protein transporter involved in fimbrial assembly and TpsA secretion

We identified a new bacterial transporter, the Pseudomonas aeruginosa CupB3 protein, which is an outer membrane usher involved in pili assembly. In CupB3, the usher domain has fused during evolution with a POTRA (polypeptide-transport-associated)-like domain found in TpsB transporters of two-partner secretion systems. In TpsBs, the POTRA captures the TpsA passenger, which is then transported across the outer membrane through the TpsB beta-barrel. We named CupB3 a 'P-usher' for POTRA-like domain-containing usher. We showed that CupB3 assembles CupB1 fimbrial subunits into pili and secretes CupB5, a TpsA-like protein. The CupB3 usher domain has the function of a TpsB beta-barrel in CupB5 translocation. We revealed that the POTRA-like domain is neither essential for CupB1 fimbriae assembly nor for cell surface exposition of CupB5, but is crucial to coordinate bona fide transport of CupB1 and CupB5 through the usher domain. The P-usher defines a novel transport pathway involving a molecular machine made with old spare parts.

The Structure of the Haemophilus influenzae HMW1 Pro-piece Reveals a Structural Domain Essential for Bacterial Two-partner Secretion

In pathogenic Gram-negative bacteria, many virulence factors are secreted via the two-partner secretion pathway, which consists of an exoprotein called TpsA and a cognate outer membrane translocator called TpsB. The HMW1 and HMW2 adhesins are major virulence factors in nontypeable Haemophilus influenzae and are prototype two-partner secretion pathway exoproteins. A key step in the delivery of HMW1 and HMW2 to the bacterial surface involves targeting to the HMW1B and HMW2B outer membrane translocators by an N-terminal region called the secretion domain. Here we present the crystal structure at 1.92 A of the HMW1 pro-piece (HMW1-PP), a region that contains the HMW1 secretion domain and is cleaved and released during HMW1 secretion. Structural analysis of HMW1-PP revealed a right-handed beta-helix fold containing 12 complete parallel coils and one large extra-helical domain. Comparison of HMW1-PP and the Bordetella pertussis FHA secretion domain (Fha30) reveals limited amino acid homology but shared structural features, suggesting that diverse TpsA proteins have a common structural domain required for targeting to cognate TpsB proteins. Further comparison of HMW1-PP and Fha30 structures may provide insights into the keen specificity of TpsA-TpsB interactions.

Current challenges in autotransport and two-partner protein secretion pathways

The two-partner and autotransport pathways are widely represented in Gram-negative bacteria, essentially among pathogens. Both mediate the translocation of large proteins across the outer membrane. Deciphering the molecular mechanisms of secretion machineries as well as folding of exoproteins in the course of translocation represents a current challenge.

The iron- and temperature-regulated haemolysin YhlA is a virulence factor of Yersinia ruckeri

Yersinia ruckeri causes the enteric redmouth disease or yersiniosis, an important systemic fish infection. In an attempt to dissect the virulence mechanisms of this bacterium, a gene encoding a putative protein involved in the secretion/activation of a haemolysin (yhlB), which had been previously identified by in vivo expression technology, was further analysed. The gene yhlB precedes another ORF (yhlA) encoding a Serratia-type haemolysin. Other toxins belonging to this group have been identified in genomic analyses of human-pathogenic yersiniae, although their role and importance in pathogenicity have not been defined yet. In spite of its being an in vivo-induced gene, the expression of yhlA can be induced under certain in vitro conditions similar to those encountered in the host, as deduced from the results obtained by using a yhlB : : lacZY fusion. Thus, higher levels of expression were obtained at 18 degrees C, the temperature of occurrence of disease outbreaks, than at 28 degrees C, the optimal growth temperature. The expression of the haemolysin also increased under iron-starvation conditions. This confirmed the decisive role of iron and temperature as environmental cues that regulate and coordinate the expression of genes encoding extracellular factors involved in the virulence of Y. ruckeri. LD(50) and cell culture experiments, using yhlB and yhlA insertional mutant strains, demonstrated the participation of the haemolysin in the virulence of Y. ruckeri and also its cytolytic properties against the BF-2 fish cell line. Finally, a screening for the production of haemolytic activity and the presence of yhlB and yhlA genes in 12 Y. ruckeri strains proved once more the genetic homogeneity of this species, since all possessed both haemolytic activity and the yhlB and yhlA genes.

Moraxella catarrhalis strain O35E expresses two filamentous hemagglutinin-like proteins that mediate adherence to human epithelial cells

Two-partner secretion (TPS) systems are a family of proteins being rapidly identified and characterized in a growing number of gram-negative bacteria. TPS systems mediate the secretion of proteins, many involved in virulence traits such as hemolysis, adherence to epithelial cells, inhibition of bacterial growth, and immunomodulation of the host. A TPS system typically consists of a transporter located in the bacterial outer membrane (OM) which is responsible for the recognition and secretion of at least one large exoprotein. Two of the better-characterized TPS systems specify the Bordetella pertussis FHA and Haemophilus influenzae HMW1/HMW2 proteins. We identified three gene products of Moraxella catarrhalis strain O35E that resemble TPS proteins and designated them MhaC (transporter), MhaB1 (exoprotein), and MhaB2 (exoprotein). Western blot analysis using anti-MhaC, or antibodies reacting to both MhaB1 and MhaB2 (MhaB-reactive), revealed that these antigens are expressed in the OM of 63% of isolates tested. Mutations in the mhaC gene specifying the putative transporter of the M. catarrhalis wild-type strains O35E, O12E, and McGHS1 resulted in the absence of MhaB1/MhaB2 in the OM of mutants. These results are therefore consistent with the Mha proteins functioning as a TPS system. Furthermore, we discovered that these mhaC mutants exhibit markedly decreased binding to human epithelial cells relevant to pathogenesis by M. catarrhalis (Chang, HEp2, A549, and/or 16HBE14o(-)). Expression of O12E MhaC and MhaB1 in a nonadherent strain of Escherichia coli was found to increase the adherence of recombinant bacteria to HEp2 monolayers by sevenfold, thereby demonstrating that this M. catarrhalis TPS system directly mediates binding to human epithelial cells. The construction of isogenic mutants in the mhaB1 and mhaB2 genes of strain O35E also suggests that the MhaB proteins play distinct roles in M. catarrhalis adherence.

Characterization of a novel two-partner secretion system in Escherichia coli O157:H7

Gram-negative bacteria contain multiple secretion pathways that facilitate the translocation of proteins across the outer membrane. The two-partner secretion (TPS) system is composed of two essential components, a secreted exoprotein and a pore-forming beta barrel protein that is thought to transport the exoprotein across the outer membrane. A putative TPS system was previously described in the annotation of the genome of Escherichia coli O157:H7 strain EDL933. We found that the two components of this system, which we designate OtpA and OtpB, are not predicted to belong to either of the two major subtypes of TPS systems (hemolysins and adhesins) based on their sequences. Nevertheless, we obtained direct evidence that OtpA and OtpB constitute a bona fide TPS system. We found that secretion of OtpA into the extracellular environment in E. coli O157:H7 requires OtpB and that when OtpA was produced in an E. coli K-12 strain, its secretion was strictly dependent on the production of OtpB. Furthermore, using OtpA/OtpB as a model system, we show that protein secretion via the TPS pathway is extremely rapid.

Secretion signal of the filamentous haemagglutinin, a model two-partner secretion substrate

The sorting of proteins to their proper subcellular compartment requires specific addressing signals that mediate interactions with ad hoc transport machineries. In Gram-negative bacteria, the widespread two-partner secretion (TPS) pathway is dedicated to the secretion of large, mostly virulence-related proteins. The secreted TpsA proteins carry a characteristic 250-residue-long N-terminal 'TPS domain' essential for secretion, while their TpsB transporters are pore-forming proteins that specifically recognize their respective TpsA partners and mediate their translocation across the outer membrane. However, the nature of the secretion signal has not been elucidated yet. The whooping cough agent Bordetella pertussis secretes its major adhesin filamentous haemagglutinin (FHA) via the TpsB transporter FhaC. In this work, we show specific interactions between an N-terminal fragment of FHA containing the TPS domain and FhaC by using two different techniques, an overlay assay and a pull-down of the complex. FhaC recognizes only non-native conformations of the TPS domain, corroborating the model that in vivo, periplasmic FHA is not yet folded. By generating single amino acid substitutions, we have identified interaction determinants forming the secretion signal. They are found unexpectedly far into the TPS domain and include both conserved and variable residues, which most likely explains the specificity of the TpsA-TpsB interaction. The N-terminal domain of FhaC is involved in the FHA-FhaC interaction, in agreement with its proposed function and periplasmic localization.

Identification of a two-partner secretion locus of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) remains a formidable cause of diarrheal illness worldwide. At present, there is no vaccine that provides broad-based protection against ETEC. A 'phoA-based self-cloning mutagenesis system, TnphoA.ts, employed to identify novel ETEC surface antigens, led to identification of an ETEC two-partner secretion locus (etpBAC) on the pCS1 virulence plasmid of prototype strain H10407. Cloning and expression of etpBAC in recombinant E. coli LMG194(pJY019) resulted in secretion of a high-molecular-weight (HMW) glycosylated exoprotein. This glycoprotein, EtpA, exhibits linear peptide sequence and predicted structural homologies with known HMW adhesins produced by other two-partner secretion loci. Antibodies directed against recombinant EtpA (anti-rEtpA.6H) recognized an HMW protein in culture supernatants of ETEC strains H10407 and LMG194(pJY019) but not in culture supernatant of strain H10407-P, which lacks the 92-kb pCS1 plasmid, or an isogenic etpA mutant. etpA mutants were deficient in adherence to intestinal epithelial cells in vitro, and anti-rEtpA.6H antibodies inhibited association of H10407 with target epithelial cells. Cloning and expression of etpB in recombinant E. coli were sufficient to confer adherence. Screening of multiple ETEC isolates for the etpBAC locus by colony hybridization and by EtpA immunoblotting suggested that EtpA is one of the most common antigens secreted by these pathogens. Together, these results indicate that the newly identified ETEC two-partner secretion locus directs the secretion of a high-molecular-weight glycosylated protein, EtpA, that in concert with the putative EtpB transporter participates in adherence of H10407 to epithelial cells, thereby expanding the repertoire of potential ETEC virulence proteins and vaccine candidates.

Protein secretion and secreted proteins in pathogenicNeisseriaceae

Secreted proteins of pathogenic bacteria are often essential virulence factors. They are involved, for example, in the adherence of the bacteria to host cells or required to suppress the host's defence mechanisms. Until recently, only IgA1 protease had been studied in detail in the NeisseriaceaeNeisseria meningitidis and Neisseria gonorrhoeae. The availability of their genome sequences, however, has boosted research in this area. Here, we present a survey of the secretome of the pathogenic Neisseriaceae, based on the available genome sequences, and the current knowledge of the functions and structures of the secreted proteins. Of the six protein-secretion pathways that are widely disseminated among Gram-negative bacteria, three pathways appear to be present among the Neisseriaceae, i.e. the autotransporter-, the two-partner- and the type I-secretion mechanisms. Comparison of the predicted secretomes reveals a considerable flexibility. As compared with N. meningitidis and the nonpathogen N. lactamica, N. gonorrhoeae appears to have a considerably degenerated secretome, which may reflect its altered niche occupancy. The flexibility of the secretome may be enhanced by the presence of ORFs in the genomes potentially encoding fragments of secreted proteins. We hypothesize that these ORFs may substitute for the corresponding fragments in the full-length genes through genetic recombination, thereby changing the host-cell receptor specificity of the secreted protein.

Identification of Xylella fastidiosa antivirulence genes: hemagglutinin adhesins contribute a biofilm maturation to X. fastidios and colonization and attenuate virulence

Xylella fastidosa, a gram-negative, xylem-limited bacterium, is the causal agent of several economically important plant diseases, including Pierce's disease (PD) and citrus variegated chlorosis (CVC). Until recently, the inability to transform or produce transposon mutants of X. fastidosa had been a major impediment to identifying X. fastidosa genes that mediate pathogen and plant interactions. A random transposon (Tn5) library of X. fastidosa was constructed and screened for mutants showing more severe symptoms and earlier grapevine death (hypervirulence) than did vines infected with the wild type. Seven hypervirulent mutants identified in this screen moved faster and reached higher populations than the wild type in grapevines. These results suggest that X. fastidosa attenuates its virulence in planta and that movement is important in X. fastidosa virulence. The mutated genes were sequenced and none had been described previously as antivirulence genes, although six of them showed similarity with genes of known functions in other organisms. One transposon insertion inactivated a hemagglutinin adhesin gene (PD2118), which we named HxfA. Another mutant in a second putative X. fastidosa hemagglutinin gene, PD1792 (HxfB), was constructed, and further characterization of these hxf mutants suggests that X. fastidosa hemagglutinins mediate contact between X. fastidosa cells, which results in colony formation and biofilm maturation within the xylem vessels.

Identification and characterization of a two-component hemolysin from Edwardsiella ictaluri

The channel catfish pathogen Edwardsiella ictaluri possesses hemolysin activity, and strains that are adapted for growth in fish tend to have greater hemolysin activity than strains that are adapted for in vitro growth conditions. To investigate its potential role in virulence, an isogenic hemolysin mutant strain of E. ictaluri R4383 was constructed by transposon mutagenesis. Sequencing of the chromosomal insertion site identified two genes, designated eihA and eihB, that encode proteins with homology to the Serratia family of two-component hemolysins. EihB is similar to the secretion/activation proteins from this family, and EihA is similar to the cytolysin proteins from this family. Bacterial challenge in channel catfish fingerlings did not show a significant difference in virulence between the wild type E. ictaluri strain and the hemolysin deficient E. ictaluri mutant strain.

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.

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.

A role for histone-like protein H1 (H-NS) in the regulation of hemolysin expression by Serratia marcescens

The histone-like protein H1 (H-NS) is an abundant structural component of the bacterial nucleoid and influences many cellular processes including recombination, transcription and transposition. Mutations in the hns gene encoding H-NS are highly pleiotropic, affecting the expression of many unrelated genes. We have studied the role of H-NS on the regulation of hemolysin gene expression in Serratia marcescens. The Escherichia coli hns mutant carrying S. marcescens hemolysin genes on a plasmid constructed by ligation of the 3.2-kb HindIII-SacI fragment of pR02 into pBluescriptIIKS, showed a high level of expression of this hemolytic factor. To determine the osmoregulation of wild-type and hns defective mutants the cells were grown to mid-logarithmic phase in LB medium with 0.06 or 0.3 M NaCl containing ampicillin and kanamycin, whereas to analyze the effect of pH on hemolysin expression, the cells were grown to late-logarithmic phase in LB medium buffered with 0.1 M Tris-HCl, pH 4.5 to 8.0. To assay growth phase-related hemolysin production, bacterial cells were grown in LB medium supplemented with ampicillin and kanamycin. The expression of S. marcescens hemolysin genes in wild-type E. coli and in an hns-defective derivative at different pH and during different growth phases indicated that, in the absence of H-NS, the expression of hemolysin did not vary with pH changes or growth phases. Furthermore, the data suggest that H-NS may play an important role in the regulation of hemolysin expression in S. marcescens and its effect may be due to changes in DNA topology influencing transcription and thus the amount of hemolysin expression. Implications for the mechanism by which H-NS influences gene expression are discussed.

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 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.

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 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.

Activation of Serratia marcescens hemolysin through a conformational change

For Serratia marcescens, secreted hemolysin/cytotoxin is not only secreted but also activated by an outer membrane protein. Excluding posttranslational processing by mass spectrometry, the conformation of active and inactive ShlA derivatives strongly differed in electrophoretic mobilities, gel permeation chromatography, sensitivity to trypsin, circular dichroism, and intrinsic fluorescence. We concluded that ShlB interacts with ShlA during secretion and imposes a conformational change in ShlA to form the active hemolysin.

Epitope analysis of the FanC subunit protein of the K99 (F5) fimbriae of enterotoxigenic Escherichia coli using a recombinant fusion technique

We have used a recombinant approach to characterise the B- and T-cell epitopes of FanC, the major subunit polypeptide of K99 (F5) fimbriae of enterotoxigenic Escherichia coli strains. This involved the fusion of FanC and its carboxy-terminal truncated derivatives to a reporter, the E. coli alkaline phosphatase (PhoA), generating stable, recombinant fusions. The B-cell epitopes of FanC were characterised by Western blotting of FanC::PhoA fusion proteins with a polyclonal mouse antiserum directed against K99 fimbrial antigen, and with a panel of monoclonal antibodies generated to the K99 antigen. An attempt to characterise the T-cell epitopes of the fimbrial subunit was made by standard in vitro T-cell proliferation assay. Our results suggest that the B-cell epitopes of FanC are likely to be continuous, with a potentially immunodominant epitope at the carboxy-terminus. However, T-cell proliferation assays with the FanC::PhoA fusion proteins did not indicate any immunodominant T-cell epitope(s). We hypothesise that fusion of FanC peptides to PhoA had resulted in altered folding of the peptides for antibody and T-cell recognition, highlighting the potential problems and drawbacks of the recombinant fusion technique in defining the epitopes of certain proteins.

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.

Identification of a locus involved in systemic dissemination of Yersinia enterocolitica

A putative LysR-type transcriptional activator, Hre20, was identified previously in an in vivo expression technology screen designed to identify factors which are expressed early during infection by Yersinia enterocolitica (G. M. Young and V. L. Miller, Mol. Microbiol. 25:319-328, 1997). An insertion in hre20, now designated rscR, resulted in increased splenic dissemination of bacteria during infection in a BALB/c mouse model. A nonpolar mutation was generated in rscR, and examination of this strain in the BALB/c mouse model demonstrated that the mutation in rscR was responsible for the increased dissemination to the spleen that was seen in the original experiments. RscR is homologous to the LysR family of transcriptional regulators; thus, a screen was undertaken to identify genes regulated by RscR. A strain containing an insertion in the chromosomal rscR gene and carrying rscR on a plasmid under the control of the inducible araBAD promoter was mutagenized with an mTn5Km-2 transposon containing a promoterless lacZY. Eighteen insertions were identified which appeared to respond to levels of RscR, and these were classified into four allelic groups based on Southern blot hybridization analysis. Representative members were sequenced from three allelic groups. Sequencing revealed insertions in an ORF with no known homologues, a homologue of OmpF of Serratia marcescens, and a locus (designated rscBAC) with similarity to the hmwABC locus of Haemophilus influenzae. The hmwABC locus promotes adherence of H. influenzae to host cells (S. J. Barenkamp and J. W. St. Geme III, Infect. Immun. 62:3320-3328, 1994; J. W. St. Geme III, S. Falkow, and S. J. Barenkamp, Proc. Natl. Acad. Sci. USA 90:2875-2879, 1993). A strain containing a deletion mutant of rscA, the hmwA homologue, exhibits increased splenic dissemination of bacteria during infection in a BALB/c mouse model, similar to the rscR mutant. This suggests that the phenotype of an rscR mutant is due to the loss of RscA.

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

A collection of large virulence exoproteins, including Ca2+-independent cytolysins, an iron acquisition protein and several adhesins, are secreted by the two-partner secretion (TPS) pathway in various Gram-negative bacteria. The hallmarks of the TPS pathway are the presence of an N-proximal module called the 'secretion domain' in the exoproteins that we have named the TpsA family, and the channel-forming beta-barrel transporter proteins we refer to as the TpsB family. The genes for cognate exoprotein and transporter protein are usually organized in an operon. Specific secretion signals are present in a highly conserved region of the secretion domain of TpsAs. TpsBs probably serve as specific receptors of the TpsA secretion signals and as channels for the translocation of the exoproteins across the outer membrane. A subfamily of transporters also mediates activation of their cognate cytolysins upon secretion. The exoproteins are synthesized as precursors with an N-terminal cleavable signal peptide, and a subset of them carries an extended signal peptide of unknown function. According to our current model, the exoproteins are probably translocated across the cytoplasmic membrane in a Sec-dependent fashion, and their signal peptide is probably processed by a LepB-type signal peptidase. The N-proximal secretion domain directs the exoproteins towards their transporters early, so that translocation across both membranes is coupled. The exoproteins transit through the periplasm in an extended conformation and fold progressively at the cell surface before eventually being released into the extracellular milieu. Several adhesins also undergo extensive proteolytic processing upon secretion. The genes of many new TpsAs and TpsBs are found in recently sequenced genomes, suggesting that the TPS pathway is widespread.

Mapping of binding domains of nontypeable Haemophilus influenzae HMW1 and HMW2 adhesins

Nontypeable Haemophilus influenzae is an important cause of localized respiratory tract disease, which begins with colonization of the upper respiratory mucosa. In previous work we reported that the nontypeable H. influenzae HMW1 and HMW2 proteins are high-molecular-weight nonpilus adhesins responsible for attachment to human epithelial cells, an essential step in the process of colonization. Interestingly, although HMW1 and HMW2 share significant sequence similarity, they display distinct cellular binding specificities. In order to map the HMW1 and HMW2 binding domains, we generated a series of complementary HMW1-HMW2 chimeric proteins and examined the ability of these proteins to promote in vitro adherence by Escherichia coli DH5alpha. Using this approach, we localized the HMW1 and HMW2 binding domains to an approximately 360-amino-acid region near the N terminus of the mature HMW1 and HMW2 proteins. Experiments with maltose-binding protein fusion proteins containing segments of either HMW1 or HMW2 confirmed these results and suggested that the fully functional binding domains may be conformational structures that require relatively long stretches of sequence. Of note, the HMW1 and HMW2 binding domains correspond to areas of maximal sequence dissimilarity, suggesting that selective advantage associated with broader adhesive potential has been a major driving force during H. influenzae evolution. These findings should facilitate efforts to develop a subcomponent vaccine effective against nontypeable H. influenzae disease.

ShlB mutants of Serratia marcescens allow uncoupling of activation and secretion of the ShlA hemolysin

The ShlB protein in the outer membrane of Serratia marcescens secretes hemolytic ShlA protein into the culture medium. In the absence of ShlB, nonhemolytic ShlA remains in the periplasm. ShlB mutants were isolated in which secretion was uncoupled from activation. Mutants with a tetrapeptide insertion after residues 136 or 224 of mature ShlB and a mutant with an insertion after residue 154 and a deletion secreted inactive ShlA. In vitro, secreted nonhemolytic ShlA was converted into hemolytic ShlA by isolated wild-type ShlB and by complementation with an N-terminal ShlA fragment of 255 residues (ShlA-255). The isolation of secretion-competent, but activation-negative mutants indicates that secretion alone is not sufficient for activation of ShlA. Rather, ShlB is required for activation and secretion, and the mutants define sites in ShlB which are involved in activation. According to a predicted transmembrane model of ShlB, the mutations that retain secretion competence but abolish activation competence are located in the most prominent surface loop and the following transmembrane loop. In one tetrapeptide insertion mutant, ShlB-332, most of the ShlA remained cell-associated in an inactive form and low amounts (6%) were hemolytic. Secreted inactive ShlA(o) was completely degraded by trypsin, in contrast to hemolytic ShlA, which was cleaved into two fragments of 60 and 100 kDa. This result indicates that the conformational change from a highly trypsin-sensitive to a highly trypsin-resistant protein with only a single cleavage site in a polypeptide of 1,578 residues occurs upon activation of ShlA and not during secretion.

Novel topological features of FhaC, the outer membrane transporter involved in the secretion of the Bordetella pertussis filamentous hemagglutinin

Many pathogenic Gram-negative bacteria secrete virulence factors across the cell envelope into the extracellular milieu. The secretion of filamentous hemagglutinin (FHA) by Bordetella pertussis depends on the pore-forming outer membrane protein FhaC, which belongs to a growing family of protein transporters. Protein alignment and secondary structure predictions indicated that FhaC is likely to be a beta-barrel protein with an odd number of transmembrane beta-strands connected by large surface loops and short periplasmic turns. The membrane topology of FhaC was investigated by random insertion of the c-Myc epitope and the tobacco etch virus protease-specific cleavage sequence. FhaC was fairly permissive to short linker insertions. Furthermore, FhaC appeared to undergo conformational changes upon FHA secretion. Surface detection of the inserted sequences indicated that several predicted loops in the C-terminal moiety as well as the N terminus of the protein are exposed. However, a large surface-predicted region in the N-terminal moiety of FhaC was inaccessible from the surface. In addition, the activity and the stability of the protein were affected by insertions in that region, indicating that it may have important structural and/or functional roles. The surface exposure of the N terminus and the presence of an odd number of beta-strands are novel features for beta-barrel outer membrane proteins.

Secretion of virulence determinants by the general secretory pathway in gram-negative pathogens: an evolving story

Secretion of proteins by the general secretory pathway (GSP) is a two-step process requiring the Sec translocase in the inner membrane and a separate substrate-specific secretion apparatus for translocation across the outer membrane. Gram-negative bacteria with pathogenic potential use the GSP to deliver virulence factors into the extracellular environment for interaction with the host. Well-studied examples of virulence determinants using the GSP for secretion include extracellular toxins, pili, curli, autotransporters, and crystaline S-layers. This article reviews our current understanding of the GSP and discusses examples of terminal branches of the GSP which are utilized by factors implicated in bacterial virulence.

Maturation and secretion of the non-typable Haemophilus influenzae HMW1 adhesin: roles of the N-terminal and C-terminal domains

Non-typable Haemophilus influenzae is a common cause of human disease and initiates infection by colonizing the upper respiratory tract. The non-typable H. influenzae HMW1 and HMW2 adhesins mediate attachment to human epithelial cells, an essential step in the process of colonization. HMW1 and HMW2 have an unusual N-terminus and undergo cleavage of a 441-amino-acid N-terminal fragment during the course of their maturation. Following translocation across the outer membrane, they remain loosely associated with the bacterial surface, except for a small amount that is released extracellularly. In the present study, we localized the signal sequence to the first 68 amino acids, which are characterized by a highly charged region from amino acids 1-48, followed by a more typical signal peptide with a predicted leader peptidase cleavage site after the amino acid at position 68. Additional experiments established that the SecA ATPase and the SecE translocase are essential for normal export and demonstrated that maturation involves cleavage first between residues 68 and 69, via leader peptidase, and next between residues 441 and 442. Site-directed mutagenesis revealed that HMW1 processing, secretion and extracellular release are dependent on amino acids in the region between residues 150 and 166 and suggested that this region interacts with the HMW1B outer membrane translocator. Deletion of the C-terminal end of HMW1 resulted in augmented extracellular release and elimination of HMW1-mediated adherence, arguing that the C-terminus may serve to tether the adhesin to the bacterial surface. These observations suggest that the HMW proteins are secreted by a variant form of the general secretory pathway and provide insight into the mechanisms of secretion of a growing family of Gram-negative bacterial exoproteins.

Cloning and characterisation of the Pasteurella multocida ahpA gene responsible for a haemolytic phenotype in Escherichia coli

Haemolysins are membrane-damaging agents which have been described as bacterial virulence factors due to their ability to lyse erythrocytes and other host cells, and therefore inducing a greater inflammatory response (Elliott et al., 1998). Pasteurella multocida was found to be haemolytic under anaerobic conditions. In this study, we cloned and characterised a P. multocida gene, designated ahpA, which conferred a haemolytic phenotype on Escherichia coli when incubated under anaerobic conditions. A deletion was introduced into the ahpA open reading frame which abolished the haemolytic phenotype. The clone containing ahpA showed erythrocyte specificity, causing haemolysis of bovine and equine erythrocytes, and demonstrated weak haemolysis on ovine erythrocytes. Upon further investigation, AhpA was found to affect the expression of the E. coli K-12 latent haemolysin, SheA, under anaerobic conditions.

Two-step fast protein liquid chromatographic purification of the Serratia marcescens hemolysin and peptide mapping with mass spectrometry

The pore forming toxin of Serratia marcescens (ShlA) is secreted and activated by an outer membrane protein (ShlB). Activation of inactive ShlA (termed ShlA*) by ShlB is dependent on phosphatidylethanolamine (PE). Activation may be a covalent modification of ShlA. To test this hypothesis, the responsible activation domain (in the N-terminal 255 amino acids of ShlA) was isolated from whole bacteria with 8 M urea in an inactive form (ShlA-255*) and from the culture supernatant in an active form (ShlA-255), followed by a two-step purification by anion-exchange chromatography and gel permeation chromatography. Comparison of a tryptic peptide map of both forms with subsequent electrospray mass spectrometry (ES-MS) and sequencing by tandem ES-MS revealed no modification. These data imply that ShlB presumably imposes a conformation on ShlA-255 that triggers activity.

The haemolysin-secreting ShlB protein of the outer membrane of Serratia marcescens: determination of surface-exposed residues and formation of ion-permeable pores by ShlB mutants in artificial lipid bilayer membranes

The ShlB protein in the outer membrane of Serratia marcescens is the only protein known to be involved in secretion of the ShlA protein across the outer membrane. At the same time, ShlB converts ShlA into a haemolytic and a cytolytic toxin. Surface-exposed residues of ShlB were determined by reaction of an M2 monoclonal antibody with the M2 epitope DYKDDDDK inserted at 25 sites along the entire ShlB polypeptide. The antibody bound to the M2 epitope at 17 sites in intact cells, which indicated surface exposure of the epitope, and to 23 sites in isolated outer membranes. Two insertion mutants contained no ShlB(M2) protein in the outer membrane. The ShlB derivatives activated and/or secreted ShlA. To gain insights into the secretion mechanism, we studied whether highly purified ShlB and ShlB deletion derivatives formed pores in artificial lipid bilayer membranes. Wild-type ShlB formed channels with very low single channel conductance that rarely assumed an open channel configuration. In contrast, open channels with a considerably higher single channel conductance were observed with the deletion mutants ShlB(Delta65-186), ShlB(Delta87-153), and ShlB(Delta126-200). ShlB(Delta126-200) frequently formed permanently open channels, whereas the conductance caused by ShlB(Delta65-186) and ShlB(Delta87-153) did not assume a stationary value, but fluctuated rapidly between open and closed configurations. The results demonstrate the orientation of large portions of ShlB in the outer membrane and suggest that ShlB may function as a specialized pore through which ShlA is secreted.