Two distinct steps in pullulanase secretion by Escherichia coli K12

InvL, an Invasin-Like Adhesin, Is a Type II Secretion System Substrate Required for Acinetobacter baumannii Uropathogenesis

Acinetobacter baumannii is an opportunistic pathogen of growing concern, as isolates are commonly multidrug resistant. While A. baumannii is most frequently associated with pulmonary infections, a significant proportion of clinical isolates come from urinary sources, highlighting its uropathogenic potential. The type II secretion system (T2SS) of commonly used model Acinetobacter strains is important for virulence in various animal models, but the potential role of the T2SS in urinary tract infection (UTI) remains unknown. Here, we used a catheter-associated UTI (CAUTI) model to demonstrate that a modern urinary isolate, UPAB1, requires the T2SS for full virulence. A proteomic screen to identify putative UPAB1 T2SS effectors revealed an uncharacterized lipoprotein with structural similarity to the intimin-invasin family, which serve as type V secretion system (T5SS) adhesins required for the pathogenesis of several bacteria. This protein, designated InvL, lacked the β-barrel domain associated with T5SSs but was confirmed to require the T2SS for both surface localization and secretion. This makes InvL the first identified T2SS effector belonging to the intimin-invasin family. InvL was confirmed to be an adhesin, as the protein bound to extracellular matrix components and mediated adhesion to urinary tract cell lines in vitro. Additionally, the invL mutant was attenuated in the CAUTI model, indicating a role in Acinetobacter uropathogenesis. Finally, bioinformatic analyses revealed that InvL is present in nearly all clinical isolates belonging to international clone 2, a lineage of significant clinical importance. In all, we conclude that the T2SS substrate InvL is an adhesin required for A. baumannii uropathogenesis. IMPORTANCE While pathogenic Acinetobacter can cause various infections, we recently found that 20% of clinical isolates come from urinary sources. Despite the clinical relevance of Acinetobacter as a uropathogen, few virulence factors involved in urinary tract colonization have been defined. Here, we identify a novel type II secretion system effector, InvL, which is required for full uropathogenesis by a modern urinary isolate. Although InvL has predicted structural similarity to the intimin-invasin family of autotransporter adhesins, InvL is predicted to be anchored to the membrane as a lipoprotein. Similar to other invasin homologs, however, we demonstrate that InvL is a bona fide adhesin capable of binding extracellular matrix components and mediating adhesion to urinary tract cell lines. In all, this work establishes InvL as an adhesin important for Acinetobacter's urinary tract virulence and represents the first report of a type II secretion system effector belonging to the intimin-invasin family.

Secretion of bacterial lipoproteins: through the cytoplasmic membrane, the periplasm and beyond

Bacterial lipoproteins are peripherally anchored membrane proteins that play a variety of roles in bacterial physiology and virulence in monoderm (single membrane-enveloped, e.g., gram-positive) and diderm (double membrane-enveloped, e.g., gram-negative) bacteria. After export of prolipoproteins through the cytoplasmic membrane, which occurs predominantly but not exclusively via the general secretory or Sec pathway, the proteins are lipid-modified at the cytoplasmic membrane in a multistep process that involves sequential modification of a cysteine residue and cleavage of the signal peptide by the signal II peptidase Lsp. In both monoderms and diderms, signal peptide processing is preceded by acylation with a diacylglycerol through preprolipoprotein diacylglycerol transferase (Lgt). In diderms but also some monoderms, lipoproteins are further modified with a third acyl chain through lipoprotein N-acyl transferase (Lnt). Fully modified lipoproteins that are destined to be anchored in the inner leaflet of the outer membrane (OM) are selected, transported and inserted by the Lol (lipoprotein outer membrane localization) pathway machinery, which consists of the inner-membrane (IM) ABC transporter-like LolCDE complex, the periplasmic LolA chaperone and the OM LolB lipoprotein receptor. Retention of lipoproteins in the cytoplasmic membrane results from Lol avoidance signals that were originally described as the "+2 rule". Surface localization of lipoproteins in diderms is rare in most bacteria, with the exception of several spirochetal species. Type 2 (T2SS) and type 5 (T5SS) secretion systems are involved in secretion of specific surface lipoproteins of γ-proteobacteria. In the model spirochete Borrelia burgdorferi, surface lipoprotein secretion does not follow established sorting rules, but remains dependent on N-terminal peptide sequences. Secretion through the outer membrane requires maintenance of lipoproteins in a translocation-competent unfolded conformation, likely through interaction with a periplasmic holding chaperone, which delivers the proteins to an outer membrane lipoprotein flippase. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Complex formation and processing of the minor transformation pilins of Bacillus subtilis

Transformation in most bacteria is dependent on orthologues of Type 2 secretion and Type 4 pilus system proteins. In each system, pilin proteins (major and minor) are required to make the pilus structure and are essential to the process, although the precise roles of the minor pilins remain unclear. We have explored protein-protein interactions among the competence minor pilins of Bacillus subtilis through in vitro binding studies, immunopurification and mass spectrometry. We demonstrate that the minor pilins directly interact, and the minor pilin ComGG interacts with most of the known proteins required for transformation. We find that ComGG requires other ComG proteins for its stabilization and for processing by the pre-pilin peptidase. These observations, C-terminal mutations in ComGG that prevent processing and the inaccessibility of pre-ComGG to externally added protease suggest a model in which pre-ComGG must be associated with other minor pilins for processing to take place. We propose that ComGG does not become a transmembrane protein until after processing. These behaviours contrast with that of pre-ComGC, the major pilin, which is accessible to externally added protease and requires only the peptidase to be processed. The roles of the pilins and of the pilus in transformation are discussed.

Type II secretion: the substrates that won't go away

Type II secretion systems (T2SSs) generally release their substrates into the culture medium. A few T2SS substrates remain anchored to or bound at the surface of the bacteria after secretion. Since they handle already folded proteins, T2SSs are the best way for bacteria to target, at their surface, proteins containing a cofactor, proteins that have to be folded in the cytoplasm or in the periplasm, or multimeric proteins. However, how a T2SS deals with membrane-anchored proteins is not yet understood. While this type of protein has until now been overlooked, new proteomic approaches will facilitate its identification.

A 20-residue peptide of the inner membrane protein OutC mediates interaction with two distinct sites of the outer membrane secretin OutD and is essential for the functional type II secretion system in Erwinia chrysanthemi

The type II secretion system (T2SS) is widely exploited by proteobacteria to secrete enzymes and toxins involved in bacterial survival and pathogenesis. The outer membrane pore formed by the secretin OutD and the inner membrane protein OutC are two key components of the secretion complex, involved in secretion specificity. Here, we show that the periplasmic regions of OutC and OutD interact directly and map the interaction site of OutC to a 20-residue peptide named OutCsip (secretin interacting peptide, residues 139-158). This peptide interacts in vitro with two distinct sites of the periplasmic region of OutD, one located on the N0 subdomain and another overlapping the N2-N3' subdomains. The two interaction sites of OutD have different modes of binding to OutCsip. A single substitution, V143S, located within OutCsip prevents its interaction with one of the two binding sites of OutD and fully inactivates the T2SS. We show that the N0 subdomain of OutD interacts also with a second binding site within OutC located in the region proximal to the transmembrane segment. We suggest that successive interactions between these distinct regions of OutC and OutD may have functional importance in switching the secretion machine.

Erwinia chrysanthemi iron metabolism: the unexpected implication of the inner membrane platform within the type II secretion system

The type II secretion (T2S) system is an essential device for Erwinia chrysanthemi virulence. Previously, we reported the key role of the OutF protein in forming, along with OutELM, an inner membrane platform in the Out T2S system. Here, we report that OutF copurified with five proteins identified by matrix-assisted laser desorption ionization-time of flight analysis as AcsD, TogA, SecA, Tsp, and DegP. The AcsD protein was known to be involved in the biosynthesis of achromobactin, which is a siderophore important for E. chrysanthemi virulence. The yeast two-hybrid system allowed us to gain further evidence for the OutF-AcsD interaction. Moreover, we showed that lack of OutF produced a pleiotropic phenotype: (i) altered production of the two siderophores of E. chrysanthemi, achromobactin and chrysobactin; (ii) hypersensitivity to streptonigrin, an iron-activated antibiotic; (iii) increased sensitivity to oxidative stress; and (iv) absence of the FbpA-like iron-binding protein in the periplasmic fraction. Interestingly, outE and outL mutants also exhibited similar phenotypes, but, outD and outJ mutants did not. Moreover, using the yeast two-hybrid system, several interactions were shown to occur between components of the T2S system inner membrane platform (OutEFL) and proteins involved in achromobactin production (AcsABCDE). The OutL-AcsD interaction was also demonstrated by Ni(2+) affinity chromatography. These results fully confirm our previous view that the T2S machinery is made up of three discrete blocks. The OutEFLM-forming platform is proposed to be instrumental in two different processes essential for virulence, protein secretion and iron homeostasis.

Towards the identification of type II secretion signals in a nonacylated variant of pullulanase from Klebsiella oxytoca

Pullulanase (PulA) from the gram-negative bacterium Klebsiella oxytoca is a 116-kDa surface-anchored lipoprotein of the isoamylase family that allows growth on branched maltodextrin polymers. PulA is specifically secreted via a type II secretion system. PelBsp-PulA, a nonacylated variant of PulA made by replacing the lipoprotein signal peptide (sp) with the signal peptide of pectate lyase PelB from Erwinia chrysanthemi, was efficiently secreted into the medium. Two 80-amino-acid regions of PulA, designated A and B, were previously shown to promote secretion of beta-lactamase (BlaM) and endoglucanase CelZ fused to the C terminus. We show that A and B fused to the PelB signal peptide can also promote secretion of BlaM and CelZ but not that of nuclease NucB or several other reporter proteins. However, the deletion of most of region A or all of region B, either individually or together, had only a minor effect on PelBsp-PulA secretion. Four independent linker insertions between amino acids 234 and 324 in PelBsp-PulA abolished secretion. This part of PulA, region C, could contain part of the PulA secretion signal or be important for its correct presentation. Deletion of region C abolished PelBsp-PulA secretion without dramatically affecting its stability. PelBsp-PulA-NucB chimeras were secreted only if the PulA-NucB fusion point was located downstream from region C. The data show that at least three regions of PulA contain information that influences its secretion, depending on their context, and that some reporter proteins might contribute to the secretion of chimeras of which they are a part.

Systematic analysis, by the yeast two-hybrid, of protein interaction between components of the type II secretory machinery of Erwinia chrysanthemi

Type II systems allow for the secretion of numerous enzymes and toxins in several Gram-negative pathogens. In Erwinia chrysanthemi, 14 Out proteins are necessary for building the type II apparatus. We performed a systematic two-hybrid analysis to test interactions between the periplasmic regions of the Out proteins. Results obtained using this approach suggested that OutJ (a pseudopilin) was able to interact with (i) OutD, the outer membrane secretin, (ii) OutI, mainly located in the periplasm, and (iii) OutL, an inner membrane protein. Taken together, these results suggest that OutJ is involved in multiple partnerships. Implications of these partnerships in the overall architecture of the type II secretion machinery are discussed.

Characterization of the hrp pathogenicity cluster of Erwinia carotovora subsp. carotovora: high basal level expression in a mutant is associated with reduced virulence

Extracellularly targeted proteins are crucial for virulence of gram-negative phytopathogenic bacteria. Erwinia carotovora subsp. carotovora employs the so-called type II (GSP) pathway to secrete a number of pectinases and cellulases, which cause the typical tissue maceration symptoms of soft-rot disease. The type III (hrp) pathway is the major virulence determinant in the genera Pseudomonas, Ralstonia and Xanthomonas, and in non-macerating species of Erwinia. The hrp cluster was recently partially characterized from E. carotovora sp. carotovora, and shown to affect virulence during early stages of infection. Here we have isolated and characterized 15 hrp genes comprising the remaining part of the cluster. The genes hrpL, hrpXY and hrpS were deduced to be transcribed as separate units, whereas the 11 remaining genes from hrpJ to hrcU form a single large operon. The hrpX gene, which codes for the sensory kinase of the two-component regulatory locus hrpXY was insertionally inactivated by placing a transposon (entranceposon) in the gene. The resulting mutant bacterium expresses the hrp genes at high basal level even in a non-inducing medium. This relative overexpression was shown to be due to the hrpX::entranceposon insertion causing enhanced transcription of the downstream hrpY gene. The hrpX(-)-hrpYC mutant bacterium exhibited a slower growth rate and the appearance of disease symptoms in infected Arabidopsis plants was delayed, as compared to the wild-type strain. The need for hrp gene expression for virulence has been documented in both non-macerating plant pathogens and in soft-rotting Erwinia sp. but this is the first demonstration that high basal-level expression of hrp -regulated genes may actually have a negative impact on disease progress in a susceptible host plant.

A putative three-dimensional targeting motif of polygalacturonase (PehA), a protein secreted through the type II (GSP) pathway in Erwinia carotovora

Intramolecular information specifying protein secretion through the type II (GSP) pathway of Gram-negative bacteria was investigated. Two regions of the polygalacturonase (PehA) of Erwinia carotovora containing residues proposed to be included in a targeting motif were located, one close to the C-terminus between residues 342 and 369 and another between residues 84 and 135 in the large central loops. The regions were required together to promote secretion. Further residues in the middle of the protein were required for proper positioning of the regions, suggesting that they were both involved in interaction with the GSP. To our knowledge, this is the first time that a possible three-dimensional targeting motif has been defined. At least one of the motifs comprises a cluster on the surface of the protein. The two motifs are structurally dissimilar, suggesting that there are two distinct recognition regions in the GSP apparatus. Finally, we propose that the targeting motifs are of a complex conformational nature with some variability accommodated, as illustrated by the observation that many mutations exhibited no clear phenotype individually but, in combination, severely compromised secretion.

Type II protein secretion in gram-negative pathogenic bacteria: the study of the structure/secretion relationships of the cellulase Cel5 (formerly EGZ) from Erwinia chrysanthemi

Erwinia chrysanthemi, a Gram-negative plant pathogen, secretes the cellulase Cel5 (formerly EGZ) via the type II secretion pathway (referred to as Out). Cel5 is composed of two domains, a large N-terminal catalytic domain (390 amino acid residues) and a small C-terminal cellulose-binding domain (62 amino acid residues) separated by a linker region. A combination of mutagenesis and structural analysis permitted us to investigate the structure/secretion relationships with respect to the catalytic domain of Cel5. The 3D structure of the catalytic domain was solved by molecular replacement at 2.3 A resolution. Cel5 exhibits the (beta/alpha)8 structural fold and two extra-barrel features. Our previous genetic study based upon tRNA-mediated suppression allowed us to predict positions of importance in the molecule in relation to structure and catalysis. Remarkably, all of the predictions proved to be correct when compared with the present structural information. Mutations of Arg57, which is located at the heart of the catalytic domain, allowed us to test the consequences of structural modifications on the secretion efficiency. The results revealed that secretability imposes remarkably strong constraints upon folding. In particular, an Arg-to-His mutation yielded a species that folded to a stable conformation close to, but distinct from the wild-type, which however was not secretable. We discuss the relationships between folding of a protein in the periplasm, en route to the cell exterior, and presentation of secretion information. We propose that different solutions have been selected for type II secreted exoproteins in order to meet the constraints imposed by their interaction with their respective secretion machineries. We propose that evolutionary pressure has led to the adaptation of different secretion motifs for different type II exoproteins.

Alteration of a single tryptophan residue of the cellulose-binding domain blocks secretion of the Erwinia chrysanthemi Cel5 cellulase (ex-EGZ) via the type II system

Cel5 (formerly known as endoglucanase Z) of Erwinia chrysanthemi is secreted by the Out type II pathway. Previous studies have shown that the catalytic domain (CD), linker region (LR) and cellulose-binding domain (CBD) each contain information needed for secretion. The aim of this work was to further investigate the secretion-related information present in the CBD(Cel5). Firstly(, )deleting a surface-exposed flexible loop had no effect on secretion. This indicated that some structural freedom is tolerated by the type II system. Secondly, mutation of a single tryptophan residue, previously shown to be important for binding to cellulose, i.e. Trp43, was found also to impair secretion. This indicated that the flat cellulose-binding surface of CBD(Cel5 )contains secretion-related information. Thirdly, CBD(Cel5) was substituted by the CBD(EGG) of Alteromonas haloplanctis endoglucanase G, yielding a hybrid protein CD(Cel5)-LR(Cel5)-CBD(EGG) that exhibited 90 % identity with Cel5, including the Trp43 residue. The hybrid protein was not secreted. This indicated that the Trp43 residue is necessary but not sufficient for secretion. Here we propose a model in which the secretion of Cel5 involves a transient intramolecular interaction between the cellulose-binding surface of CBD(Cel5) and a region close to the entry into the active site in CD(Cel5). Once secreted, the protein may then open out to allow the cellulose-binding surface of CBD(Cel5 )to interact with the surface of the cellulose substrate. An implication of this model is that protein molecules fold to a specific secretion-competent conformation prior to secretion that is different from the folding state of the secreted species.

Bacterial type II protein export and pilus biogenesis: more than just homologies?

Protein export by Gram-negative bacteria requires devoted machineries to allow for the passage of hydrolytic enzymes and toxins through the cell envelope. The Type II export machinery has a number of distinct characteristics, which include its role as an extension of Sec-dependent secretion, its ability to recognize and export fully folded substrates efficiently and, perhaps most significantly, the relationship between a subset of its gene products with the Type IV pilus-biogenesis apparatus. An important question is whether we can extrapolate our knowledge, albeit limited, of Type IV pilus biogenesis to understand the structure and function of the Type II export apparatus. This and other questions relating to the energetics of assembly and specificity of the apparatus are addressed in this article.

Membrane association and multimerization of secreton component pulC

The PulC component of the Klebsiella oxytoca pullulanase secretion machinery (the secreton) was found by subcellular fractionation to be associated with both the cytoplasmic (inner) and outer membranes. Association with the outer membrane was independent of other secreton components, including the outer membrane protein PulD (secretin). The association of PulC with the inner membrane is mediated by the signal anchor sequence located close to its N terminus. These results suggest that PulC forms a bridge between the two membranes that is disrupted when bacteria are broken open for fractionation. Neither the signal anchor sequence nor the cytoplasmic N-terminal region that precedes it was found to be required for PulC function, indicating that PulC does not undergo sequence-specific interactions with other cytoplasmic membrane proteins. Cross-linking of whole cells resulted in the formation of a ca. 110-kDa band that reacted with PulC-specific serum and whose detection depended on the presence of PulD. However, antibodies against PulD failed to react with this band, suggesting that it could be a homo-PulC trimer whose formation requires PulD. The data are discussed in terms of the possible role of PulC in energy transduction for exoprotein secretion.

GSP-dependent protein secretion in gram-negative bacteria: the Xcp system of Pseudomonas aeruginosa

Bacteria have evolved several secretory pathways to release proteins into the extracellular medium. In Gram-negative bacteria, the exoproteins cross a cell envelope composed of two successive hydrophobic barriers, the cytoplasmic and outer membranes. In some cases, the protein is translocated in a single step across the cell envelope, directly from the cytoplasm to the extracellular medium. In other cases, outer membrane translocation involves an extension of the signal peptide-dependent pathway for translocation across the cytoplasmic membrane via the Sec machinery. By analogy with the so-called general export pathway (GEP), this latter route, including two separate steps across the inner and the outer membrane, was designated as the general secretory pathway (GSP) and is widely conserved among Gram-negative bacteria. In their great majority, exoproteins use the main terminal branch (MTB) of the GSP, namely the Xcp machinery in Pseudomonas aeruginosa, to reach the extracellular medium. In this review, we will use the P. aeruginosa Xcp system as a basis to discuss multiple aspects of the GSP mechanism, including machinery assembly, exoprotein recognition, energy requirement and pore formation for driving through the outer membrane.

External loops at the C terminus of Erwinia chrysanthemi pectate lyase C are required for species-specific secretion through the out type II pathway

The type II secretion system (main terminal branch of the general secretion pathway) is used by diverse gram-negative bacteria to secrete extracellular proteins. Proteins secreted by this pathway are synthesized with an N-terminal signal peptide which is removed upon translocation across the inner membrane, but the signals which target the mature proteins for secretion across the outer membrane are unknown. The plant pathogens Erwinia chrysanthemi and Erwinia carotovora secrete several isozymes of pectate lyase (Pel) by the out-encoded type II pathway. However, these two bacteria cannot secrete Pels encoded by heterologously expressed pel genes from the other species, suggesting the existence of species-specific secretion signals within these proteins. The functional cluster of E. chrysanthemi out genes carried on cosmid pCPP2006 enables Escherichia coli to secrete E. chrysanthemi, but not E. carotovora, Pels. We exploited the high sequence similarity between E. chrysanthemi PelC and E. carotovora Pel1 to construct 15 hybrid proteins in which different regions of PelC were replaced with homologous sequences from Pell. The differential secretion of these hybrid proteins by E. coli(pCPP2006) revealed M118 to D175 and V215 to C329 as regions required for species-specific secretion of PelC. We propose that the primary targeting signal is contained within the external loops formed by G274 to C329 but is dependent on residues in M118 to D170 and V215 to G274 for proper positioning.

The requirement for DsbA in pullulanase secretion is independent of disulphide bond formation in the enzyme

Results from previous studies have suggested that an intramolecular disulphide bond in the exoprotein pullulanase is needed for its recognition and transport across the outer membrane. This interpretation of the data is shown here to be incorrect: pullulanase devoid of all potential disulphide bonds is secreted with apparently the same efficiency as the wild-type protein. Furthermore, the periplasmic disulphide bond, oxidoreductase DsbA, previously shown to catalyse the formation of a disulphide bond in pullulanase and to decrease its transit time in the periplasm, is shown here to be required for the rapid secretion of pullulanase devoid of disulphide bonds. Several possible explanations for the role of DsbA in pullulanase secretion are discussed.

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

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

Specific interaction between OutD, an Erwinia chrysanthemi outer membrane protein of the general secretory pathway, and secreted proteins

OutD is an outer membrane component of the main terminal branch of the general secretory pathway (GSP) in Erwinia chrysanthemi. We analyzed the interactions of OutD with other components of the GSP (Out proteins) and with secreted proteins (PelB, EGZ and PemA). OutD is stabilized by its interaction with another GSP component, OutS. The 62 C-terminal amino acids of OutD are necessary for this interaction. In vivo formation of OutD multimers, up to tetramers, was proved after the dissociation in mild conditions of the OutD aggregates formed in the outer membrane. Thus, OutD could form a channel-like structure in the outer membrane. We showed that OutD is stabilized in vivo when co-expressed with Out-secreted proteins. This stabilization results from the formation of complexes that were detected in experiments of co-immunoprecipitation and co-sedimentation in sucrose density gradients. The presence of the N-terminal part of OutD is required for this interaction. The interaction between OutD and the secreted protein PelB was confirmed in vitro, suggesting that no other component of the GSP is required for this recognition. No interaction was observed between the E. carotovora PelC and the E. chrysanthemi OutD. Thus, the interaction between GspD and the secreted proteins present in the periplasm could be the key to the specificity of the secretion machinery and a trigger for that process.

Pullulanase: model protein substrate for the general secretory pathway of gram-negative bacteria

Pullulanase of Klebsiella oxytoca is one of a wide variety of extracellular proteins that are secreted by Gram-negative bacteria by the complex main terminal branch (MTB) of the general secretory pathway. The roles of some of the 14 components of the MTB are now becoming clear. In this review it is proposed that most of these proteins form a complex, the secretion, that spans the cell envelope to control the opening and closing of channel in the outer membrane. Progress toward the goal of testing this model is reviewed.

Characterization of EspC, a 110-kilodalton protein secreted by enteropathogenic Escherichia coli which is homologous to members of the immunoglobulin A protease-like family of secreted proteins

Enteropathogenic Escherichia coli (EPEC) secretes at least five proteins. Two of these proteins, EspA and EspB (previously called EaeB), activate signal transduction pathways in host epithelial cells. While the role of the other three proteins (39, 40, and 110 kDa) remains undetermined, secretion of all five proteins is under the control of perA, a known positive regulator of several EPEC virulence factors. On the basis of amino-terminal protein sequence data, we cloned and sequenced the gene which encodes the 110-kDa secreted protein and examined its possible role in EPEC signaling and interaction with epithelial cells. In accordance with the terminology used for espA and espB, we called this gene espC, for EPEC-secreted protein C. We found significant homology between the predicted EspC protein sequence and a family of immunoglobulin A (IgA) protease-like proteins which are widespread among pathogenic bacteria. Members of this protein family are found in avian pathogenic Escherichia coli (Tsh), Haemophilus influenzae (Hap), and Shigella flexneri (SepA). Although these proteins and EspC do not encode IgA protease activity, they have considerable homology with IgA protease from Neisseria gonorrhoeae and H. influenzae and appear to use a export system for secretion. We found that genes homologous to espC also exist in other pathogenic bacteria which cause attaching and effacing lesions, including Hafnia alvei biotype 19982, Citrobacter freundii biotype 4280, and rabbit diarrheagenic E. coli (RDEC-1). Although these strains secrete various proteins similar in molecular size to the proteins secreted by EPEC, we did not detect secretion of a 110-kDa protein by these strains. To examine the possible role of EspC in EPEC interactions with epithelial cells, we constructed a deletion mutant in espC by allelic exchange and characterized the mutant by standard tissue culture assays. We found that EspC is not necessary for mediating EPEC-induced signal transduction in HeLa epithelial cells and does not play a role in adherence or invasion of tissue culture cells.

Identification of two regions of Klebsiella oxytoca pullulanase that together are capable of promoting beta-lactamase secretion by the general secretory pathway

Pullulanase (PulA) is a 116 kDa amylolytic lipoprotein secreted by the Gram-negative bacterium Klebsiella oxytoca via the general secretory pathway. A deletion strategy was used in an attempt to determine the nature and the location of the secretion signal(s) in PulA presumed to be necessary for its specific secretion. The starting material was a gene fusion coding for an efficiently secreted PulA-beta-lactamase hybrid protein. Successive series of exonuclease III-generated deletions were used to remove internal segments of PulA from this hybrid. A simple plate test allowed the identification of truncated hybrids that retained beta-lactamase activity and that were secreted. Two non-adjacent regions, A and B (78 and 80 amino acids, respectively), were together necessary and sufficient to promote beta-lactamase translocation across the outer membrane. Secretion of PulA itself was markedly reduced when either of these regions was deleted, and was completely abolished when both regions were eliminated.

Complementation of deletion mutations in a cloned functional cluster of Erwinia chrysanthemi out genes with Erwinia carotovora out homologues reveals OutC and OutD as candidate gatekeepers of species-specific secretion of proteins via the type II pathway

The type II or Sec-dependent secretion system is used by diverse Gram-negative bacteria for secretion of extracellular proteins. Of the 12-15 proteins involved in secretion, the requirement for many has not been demonstrated and little is known about their functions in the secretion process. The plant pathogens Erwinia chrysanthemi and Erwinia carotovora secrete extra-cellular pectate lyases (Pels) using the type II or Out pathway. However, these two bacteria cannot secrete Pels encoded by heterologously expressed genes from the other species, suggesting the presence of species-specific recognition factors in the Out systems of the two Erwinia species. We previously reported the isolation of a cosmid clone, pCPP2OO6, from E. chrysanthemi EC16, which enables Escherichia coil to secrete heterologously expressed E. chrysanthemi Pels. Sequencing in a region required for secretion revealed the presence of 12 genes, outC-M and outO. We report here the construction of functionally non-polar mutations in each gene in the outC-M operon and outS and outB using a polA(ts) strain of E. coli to facilitate homologous recombination between out genes carrying deletions and their wild-type copies on pCPP2006. By testing for complementation of each deletion with wild-type out genes from E. chrysanthemi EC16 and E. carotovora SCRI193 we have demonstrated that: (i) each out gene is required for secretion of E. chrysanthemi PelE from E. coli with the exception of outH; (ii) each mutation can be complemented by its homologue from E. carotovora, except for outC and outD; (iii) outC and outD from E. carotovora do not confer secretion of Pel1 on the E. chrysanthemi Out system; and (iv) Pel1 secretion can be conferred on the E. chrysanthemi Out system by the presence of outC-M, S and B from E. carotovora. The data suggest that OutC and OutD are gatekeepers of the Out system involved in recognition of Pels targeted for secretion but that OutC and OutD from E. carotovora cannot be successfully assembled into the E. chrysanthemi Out system.

XpsD, an outer membrane protein required for protein secretion by Xanthomonas campestris pv. campestris, forms a multimer

XpsD is an outer membrane lipoprotein, required for the secretion of extracellular enzymes by Xanthomonas campestris pv. campestris. Our previous studies indicated that when the xpsD gene was interrupted by transposon Tn5, extracellular enzymes were accumulated in the periplasm (Hu, N.-T., Hung, M.-N., Chiou, S.-J., Tang, F., Chiang, D.-C. Huang, H.-Y. and Wu, C.-Y. (1992) J. Bacteriol. 174, 2679-2687). In this study, we constructed a series of substitutions and deletion mutant xpsD genes to investigate the roles of NH2- and COOH-terminal halves of XpsD in protein secretory function. Among these secretion defective xpsD mutations, one group (encoded by pCD105, pYLA, pKdA6, and pKD2) caused secretion interference when co-expressed with wild type xpsD, but the other (encoded by pMH7, pKdPs, and pKDT) did not. Cross-linking studies and gel filtration chromatography analysis indicated that the wild type XpsD protein forms a multimer in its native state. Similar gel filtration analysis of xpsD mutants revealed positive correlations between multimer formation and secretion interfering properties exerted by the mutant XpsD proteins in the parental strain XC1701. Those mutant XpsD proteins (encoded by pCD105, pYL4, pKdA6, and pKD2) that caused secretion interference formed multimers that are similar to the wild type XpsD multimers and those (encoded by pMH7, pKdPs, and pKDT) that did not formed smaller ones. Furthermore, gel filtration and anion exchange chromatography analyses indicated that the wild type XpsD protein co-fractionated with XpsD (delta 29-428) or XpsD (delta 448-650) protein but not with XpsD (delta 74-303) or XpsD (delta 553-759) protein. We propose that the mutant XpsD (delta 29-428) protein caused secretion interference primarily by forming mixed nonfunctional multimers with the wild type XpsD protein in XC1701 (pCD105), whereas the mutant XpsD (delta 74-303) did so by competing for unknown factor(s) in XC1701(pYL4).

Bacteriocin release proteins: mode of action, structure, and biotechnological application

The mechanism by which Gram-negative bacteria like Escherichia coli secrete bacteriocins into the culture medium is unique and quite different from the mechanism by which other proteins are translocated across the two bacterial membranes, namely through the known branches of the general secretory pathway. The release of bacteriocins requires the expression and activity of a so-called bacteriocin release protein and the presence of the detergent-resistant phospholipase A in the outer membrane. The bacteriocin release proteins are highly expressed small lipoproteins which are synthesized with a signal peptide that remains stable and which accumulates in the cytoplasmic membrane after cleavage. The combined action of these stable, accumulated signal peptides, the lipid-modified mature bacteriocin release proteins (BRPs) and phospholipase A cause the release of bacteriocins. The structure and mode of action of these BRPs as well as their application in the release of heterologous proteins by E. coli is described in this review.

Interleukin-1 beta secretion. A possible multistep process that is regulated in a cell type-specific manner

In a prior study, we found that the processed form of human interleukin-1 beta (mature IL-1 beta) is secreted to a significantly greater extent than the precursor form of the protein, indicating that the precursor domain acts in some manner to reduce the secretory potential of the protein. In view of this observation, we sought to define the sequence(s) in the IL-1 beta precursor that limit the secretion of the protein as well as the sequences in the mature protein that promote secretion. The P388D1 murine macrophage cell line and the Jurkat human T-cell line were transiently transfected with cDNA expression vectors encoding truncated forms of human precursor IL-1 beta proteins, lacking either the first 76, 94, 99, or 104 amino acids. The removal of increasing numbers of precursor amino acid residues resulted in a graded increase in the secretion of the truncated precursor IL-1 beta proteins from both cell lines. The minimal region of the precursor sequence required to inhibit the optimal secretion of IL-1 beta occurs between amino acids 100 and 104 for P388D1 cells and 95-99 for Jurkat cells. Deletion of the amino acids within these regions increased the secretion level of the truncated proteins to that of mature IL-1 beta. Mutagenesis of the mature IL-1 beta sequence revealed that a region of basic amino acids may play an important role in the optimal secretion of mature IL-1 beta in P388D1 cells, but not in Jurkat cells. Based on the differences in the structural requirements for IL-1 beta secretion in P388D1 and Jurkat cell lines, it is likely that the secretion of IL-1 beta may be subject to multiple levels of regulation that are differentially operative in different cell types.

Detection and subcellular localization of three Ptl proteins involved in the secretion of pertussis toxin from Bordetella pertussis

The ptl locus of Bordetella pertussis contains eight open reading frames which are predicted to encode proteins (PtlA to PtlH) that are essential for secretion of pertussis toxin from the bacterium and which are members of a family of transport proteins found in other types of bacteria. We have detected PtlE, PtlF, and PtlG in immunoblots of extracts of B. pertussis by using antibodies raised to fusion proteins consisting of maltose-binding protein and the individual Ptl proteins. These proteins have apparent molecular weights similar to those predicted by DNA sequence analysis. Cell fractionation studies indicated that all three Ptl proteins are associated with the membranes of B. pertussis, suggesting that the Ptl proteins form a gate or channel which facilitates transport of pertussis toxin. Cell extracts of other Bordetella spp. were probed with antibodies to Ptl proteins for the presence of these transport proteins. Neither Bordetella parapertussis nor Bordetella bronchiseptica contained detectable levels of PtlE or PtlF. This lack of detectable Ptl protein may provide an explanation for previous observations which indicated that introduction of the genes encoding pertussis toxin subunits from B. pertussis into other Bordetella spp. results in production of the toxin but not secretion of the toxin.

The toxin-co-regulated pilus of Vibrio cholerae O1: a model for type 4 pilus biogenesis?

The toxin-co-regulated pilus (TCP), an important colonization factor of Vibrio cholerae, is similar to the type 4 pilus produced by a variety of pathogenic Gram-negative bacteria. The putative translocation and assembly machinery of TCP has broad similarities with known pilin and nonpilin export mechanisms.

Alginate lyase from Klebsiella pneumoniae, subsp. aerogenes: gene cloning, sequence analysis and high-level production in Escherichia coli

The alyA gene, encoding a secreted guluronate-specific alginate lyase (Aly) from Klebsiella pneumoniae subsp. aerogenes type 25, has been cloned. DNA sequence analysis reveals two possible translation start sites for the precursor form of Aly and a long open reading frame (ORF) predicted to encode a 287-amino-acid (aa) mature form of Aly, in agreement with N-terminal aa sequence analysis of the protein. Aly has a calculated molecular mass of 31.4 kDa, in good agreement with SDS-PAGE analysis, and a calculated pI of 9.39. Comparison of the deduced aa sequence with a mannuronate-specific lyase from a marine bacterium reveals 19.3% identity and 28.8% similarity with a 9-aa conserved region close to the C terminus, probably of functional or structural significance. There is no obvious sequence similarity with pectate lyases which also catalyse a beta-elimination reaction. Heterologous expression of K. pneumoniae alyA in Escherichia coli yields 10 mg of Aly per litre of culture supernatant, apparently due to non-specific release from the periplasm.

Molecular characterization of PulE, a protein required for pullulanase secretion

pulE, one of 14 genes specifically required for pullulanase secretion in Klebsiella oxytoca, codes for a putative nucleotide-binding protein. Subcellular fractionation indicated that the majority of PulE in Escherichia coli cells expressing all 14 secretion genes is mainly associated with the cytoplasmic membrane through both hydrophobic and non-hydrophobic interactions. Mutational analysis revealed that one of the two regions of PulE that are conserved in many nucleotide-binding proteins (Walker box A) is essential for pullulanase secretion. Likewise, mutations that removed aspartate residues from each of two regions immediately downstream from the Walker box A also reduced secretion. These aspartate-rich regions are highly conserved in all 16 known PulE homologues but not in any other nucleotide-binding proteins. Altogether, these results indicate that PulE might belong to a new family of nucleotide-binding proteins. The protein could not be cross-linked to the photoactivatable ATP analogue azido-ATP, however. Most pulE point or deletion mutations which prevented pullulanase secretion exhibited transdominance when expressed at high levels in cells producing wild-type PulE protein. Evidence presented suggests that PulE might be a homodimer.

Periplasmic disulphide bond formation is essential for cellulase secretion by the plant pathogen Erwinia chrysanthemi

Secretion to the cell exterior of cellulase EGZ and of at least six pectinases enables the Gram-negative Erwinia chrysanthemi to cause severe plant disease. The C-terminal cellulose-binding domain (CBD) of EGZ was found to contain a disulphide bond which forms, in the periplasm, between residues Cys-325 and Cys-382. Dithiothreitol (DTT)-treatment of native EGZ showed that the disulphide bond was dispensable, both for catalysis and cellulose binding. Adding DTT to E. chrysanthemi cultures led to immediate arrest of secretion of EGZ which accumulated in the periplasm where the CBD was eventually proteolysed. Site-directed mutagenesis that affected Cys residues involved in disulphide bond formation resulted in molecules that were catalytically active and able to bind to cellulose but were no longer secreted. Instead they accumulated in the periplasm. Interestingly, the region around EGZ Cys-325 is conserved in two pectinases secreted by the same pathway as EGZ. We conclude that the conserved Cys, and possibly adjacent residues, bear essential information for EGZ to be secreted and that periplasmic disulphide bond formation is an obligatory step which provides a pre-folded functional form of EGZ with secretion competence.

Protein uptake into E. coli during Bdellovibrio infection. A process of reverse secretion?

Bdellovibrio bacteriovorus is a small bacterial parasite that infects other Gram-negative bacteria, resides in the periplasm of the host cell, and utilizes host macromolecules as a source of nutrients. Evidence is summarized suggesting that B. bacteriovorus secretes proteases and nucleases synthesized in its own cytoplasm that are targeted to the cytoplasm of the host cell. Possible mechanisms for this trans-trimembrane protein transport process are discussed.

Regional sequence homologies in starch-degrading enzymes

The enzymatic hydrolysis of starch, consisting of linear (amylose) and branched (amylopectin) glucose polymers, is catalyzed by alpha-, beta- and glucoamylases (gamma-amylases), cyclodextrinases, alpha-glucosidases, and debranching enzymes. Saccharomyces cerevisiae cannot utilize starch. Our laboratory has previously co-expressed the Bacillus amyloliquefaciens alpha-amylase (AMY) and the Saccharomyces diastaticus glucoamylase (STA2) genes in S. cerevisiae. A gene encoding a debranching enzyme (pullulanase) from Klebsiella pneumoniae ATCC15050 was cloned and its nucleotide sequence determined. This gene will be co-expressed with the alpha- and gamma-amylase to produce an amylolytic S. cerevisiae strain. Extensive data base comparisons of the K. pneumoniae pullulanase amino-acid sequence with the amino-acid sequences of other debranching enzymes and alpha-, beta- and gamma-amylases (from bacteria, yeasts, higher fungi and higher eukaryotes), indicated that these debranching enzymes have amino-acid regions similar to those found in alpha-amylases. The conserved regions in alpha-amylases comprise key residues that are implicated in substrate binding, catalysis, and calcium binding and are as follows. Region 1: DVVINH; region 2: GFRLDAAKH and region 4: FVDNHD. When comparing conserved regions, no similarity could be detected between debranching enzymes and beta- and gamma-amylases.

The general secretory pathway of Klebsiella oxytoca: no evidence for relocalization or assembly of pilin-like PulG protein into a multiprotein complex

It has been proposed that the four type IV pilin-like proteins that are required for extracellular protein secretion by the general secretory pathway (GSP) might assemble into a trans-periplasm complex resembling a type IV pilus. To test this idea, we examined the subcellular distribution and oligomeric state of PulG, one of the type IV pilin-like proteins required for pullulanase secretion in Klebsiella oxytoca. Fractionation of Escherichia coli cells carrying a single copy of each pul gene showed that PulG protein was located in two distinct envelope fractions corresponding to the outer and cytoplasmic membranes. The protein was partially released by treating the membranes with Triton X-100 + EDTA or at high pH, but not by Triton X-100 alone or by 8 M urea, 6 M guanidine hydrochloride or 1 M NaCl. Like type IV pilins, non-sedimentable PulG that had been released from the membranes at high pH could be sedimented by centrifugation when the pH was lowered. Treatment of whole cells, sphaeroplasts or isolated membranes with a cleavable cross-linking agent produced mainly PulG homodimers. Previous studies showed that both PulO, which cleaves and N-methylates the PulG precursor, and PulE, a putative ATP-binding protein, share extensive sequence identity with proteins known to be required for type IV pilus processing and assembly. However, mutations which disrupted either pulE or pulO, or indeed the complete absence of all other components of the pullulanase secretion apparatus, had little or no effect on any of the properties of PulG protein described above. We conclude that there is no evidence that PulG protein assembles into a stable multiprotein complex or that processing of the PulG precursor causes a detectable change in its subcellular distribution.

Common components in the assembly of type 4 fimbriae, DNA transfer systems, filamentous phage and protein-secretion apparatus: a general system for the formation of surface-associated protein complexes

The Pseudomonas aeruginosa genes pilB-D and pilQ are necessary for the assembly of type 4 fimbriae. Homologues of these genes and of the subunit (pilin) gene have been described in various different bacterial species, but not always in association with type 4 fimbrial biosynthesis and function. Pil-like proteins are also involved in protein secretion, DNA transfer by conjugation and transformation, and morphogensis of filamentous bacteriophages. It seems likely that the Pil homologues function in the processing and export of proteins resembling type 4 fimbrial subunits, and in their organization into fimbrial-like structures. These may either be true type 4 fimbriae, or components of protein complexes which act in the transport of macromolecules (DNA or protein) into or out of the cell. Some PilB-like and PilQ-like proteins are apparently also involved in the assembly of non-type 4 polymeric structures (filamentous phage virions and conjugative pili). The diverse studies summarized in this review are providing insight into an extensive infrastructural system which appears to be utilized in the formation of a variety of cell surface-associated complexes.

The role of the lipoprotein sorting signal (aspartate +2) in pullulanase secretion

The analyses of hybrid proteins and of deletion and insertion mutations reveal that the only amino acid at the amino-proximal end of the cell surface lipoprotein pullulanase that is specifically required for its extracellular secretion is an aspartate at position +2, immediately after the fatty acylated amino-terminal cysteine. To see whether the requirement for this amino acid is related to its proposed role as a cytoplasmic membrane lipoprotein sorting signal, we used sucrose gradient floatation analysis to determine the subcellular location of pullulanase variants (with or without the aspartate residue) that accumulated in cells lacking the pullulanase-specific secretion genes. A non-secretable pullulanase variant with a serine at position +2 cofractionated mainly with the major peak of outer membrane porin. In contrast, most (55%) of a pullulanase variant with an aspartate at position +2 cofractionated with slightly lighter fractions that contained small proportions of both outer membrane porin and the cytoplasmic membrane marker NADH oxidase. Only 5% of this pullulanase variant cofractionated with the major NADH oxidase peak, while the rest (c. 40%) remained at the bottom of the gradient in fractions totally devoid of porin and NADH oxidase. When analysed by sedimentation through sucrose gradients, however, a large proportion of this variant was recovered from fractions near the top of the gradient that also contained the major NADH oxidase peak. When this peak fraction was applied to a floatation gradient the pullulanase activity remained at the bottom while the NADH oxidase floated to the top.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcellular localization of seven VirB proteins of Agrobacterium tumefaciens: implications for the formation of a T-DNA transport structure

Plant cell transformation by Agrobacterium tumefaciens involves the transfer of a single-stranded DNA-protein complex (T-complex) from the bacterium to the plant cell. One of the least understood and important aspects of this process is how the T-complex exits the bacterium. The eleven virB gene products have been proposed to specify the DNA export channel on the basis of their predicted hydrophobicity. To determine the cellular localization of the VirB proteins, two different cell fractionation methods were employed to separate inner and outer membranes. Seven VirB-specific antibodies were used on Western blots (immunoblots) to detect the proteins in the inner and outer membranes and soluble (containing cytoplasm and periplasm) fractions. VirB5 was in both the inner membrane and cytoplasm. Six of the VirB proteins were detected in the membrane fractions only. Three of these, VirB8, VirB9, and VirB10, were present in both inner and outer membrane fractions regardless of the fractionation method used. Three additional VirB proteins, VirB1, VirB4, and VirB11, were found mainly in the inner membrane fraction by one method and were found in both inner and outer membrane fractions by a second method. These results confirm the membrane localization of seven VirB proteins and strengthen the hypothesis that VirB proteins are involved in the formation of a T-DNA export channel or gate. That most of the VirB proteins analyzed are found in both inner and outer membrane fractions suggest that they form a complex pore structure that spans both membranes, and their relative amounts in the two membrane fractions reflect their differential sensitivity to the experimental conditions.

Expression of the Klebsiella pneumoniae pullulanase-encoding gene in Saccharomyces cerevisiae

A 3800-base pair (bp) DNA fragment encoding the mature pullulanase from Klebsiella pneumoniae was inserted between two different yeast expression-secretion cassettes and an yeast gene terminator. These cassettes were cloned into an yeast centromeric plasmid YCplacIII and transformed into laboratory strains of Saccharomyces cerevisiae. Transcription initiation signals were derived from the mating pheromone alpha-factor (MF alpha 1p) and alcohol dehydrogenase (ADC1p) gene promoters. Secretion of pullulanase was directed by the leader sequence of the yeast mating pheromone alpha-factor (MF alpha 1s). Transcription termination was effected by the yeast tryptophan synthase gene terminator (TRP5T). Southernblot analysis confirmed the presence of pulA in transformed yeasts and Northern-blot analysis revealed the presence of PUL1 mRNA. A pullulan agarose assay indicated the extracellular production of biologically active pullulanase by S. cerevisiae.

The Chlamydia trachomatis Mip-like protein is a lipoprotein

The Mip-like protein of Chlamydia trachomatis is similar to the Mip protein of Legionella pneumophila and may be equally important for the initiation of intracellular infection. This article presents data which identify the chlamydial Mip-like protein as a lipoprotein. The amino acid sequence of the Mip-like protein contains a signal peptidase II recognition sequence, as is seen in procaryotic lipoproteins. Palmitic acid was incorporated into the recombinant chlamydial Mip-like protein. Globomycin, known to inhibit signal peptidase II, inhibited processing of the recombinant Mip-like protein. Labelling of chlamydial organisms with palmitic acid revealed incorporation into the native Mip-like protein.

Mutagenesis of cellulase EGZ for studying the general protein secretory pathway in Erwinia chrysanthemi

Extracellular secretion of endoglucanase Z (EGZ) from Erwinia chrysanthemi is mediated by the so-called Out general secretion pathway and, presumably, involves recognition of EGZ-carried structural information by one or more of the Out proteins. Investigating the relationships between structure and secretability of EGZ was the purpose of the present work. EGZ is made of two independent domains, located at the N- and C-proximal sides, separated by a Ser/Thr-rich region, which are responsible for catalysis and cellulose-binding, respectively. The existence of a secretion region ('targeting signal') was investigated by studying the secretability of modified EGZ derivatives. These resulted from deletion or peptide insertion and were designed by using the domain organization cited above as a guide. Catalytic and/or cellulose-binding tests showed that all proteins exhibited at least a functional EGZ domain while immunoblot analyses confirmed that neither the insertions nor the deletions led to grossly misfolded proteins. In contrast, all of the proteins lost their secretability in E. chrysanthemi. This suggested that at least two secretion motifs existed, one lying within each functional domain. The role of the Ser/Thr-rich linker region was subsequently tested. Accordingly, two proteins containing a linker region whose length was increased by the addition of 8 and 18 additional residues and one protein lacking the linker region were studied. All three exhibited endoglucanase activity and cellulose-binding ability, confirming the independence of the domains within the context of EGZ/polysaccharide interaction. In contrast, none was secreted by E. chrysanthemi.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of the outer membrane vesicles from Vibrio cholerae and Vibrio parahaemolyticus

We found numerous small vesicles released from the cell by thin sectioning of the plate culture of Vibrio cholerae and V. parahaemolyticus fixed with the freeze-substitution technique. From the broth media of exponentially growing bacteria we could collect the vesicles by the centrifugation but not enough without fixation. The vesicles are encompassed with a membrane structure similar to the outer membrane of these bacteria. The anti-O (Inaba) serum reacted with the surface of the vesicles and the inside of the vesicle are generally filled with an electron-dense mass.

Translocation of a folded protein across the outer membrane in Escherichia coli

A mutation in the Escherichia coli dsbA gene (coding for a periplasmic disulfide oxidoreductase) reduces the rate of disulfide bond formation in the enzyme pullulanase and also reduces the rate at which the enzyme is secreted to the cell surface, as measured by protease accessibility. The enzyme did not become protease accessible when disulfide bond formation was completely prevented in the mutant strain by carboxymethylation. These results indicate that a disulfide bond may be required for, and certainly does not impede, the translocation of pullulanase across the outer membrane. Since it is unlikely that a disulfide bond could be formed and then reduced again in the periplasm, these results would appear to strengthen the argument that pullulanase polypeptides fold into or close to their final conformation before they are transported across the outer membrane. It is suggested that this might be a feature common to all proteins that are secreted by other Gram-negative bacteria by a pullulanase-like pathway.

Cloning and characterization of a gene required for the secretion of extracellular enzymes across the outer membrane by Xanthomonas campestris pv. campestris

Nonpathogenic mutants of Xanthomonas campestris pv. campestris, generated from transposon mutagenesis, accumulated extracellular polygalacturonate lyase, alpha-amylase, and endoglucanase in the periplasm. The transposon Tn5 was introduced by a mobilizable, suicidal plasmid, pSUP2021 or pEYDG1. Genomic banks of wild-type X. campestris pv. campestris, constructed on the broad-host-range, mobilizable cosmid pLAFR1 or pLAFR3, were conjugated with one of the mutants, designated XC1708. Recombinant plasmids isolated by their ability to complement XC1708 can be classified into two categories. One, represented by pLASC3, can complement some mutants, whereas the other, represented by a single plasmid, pLAHH2, can complement all of the other mutants. Restriction mapping showed that the two recombinant plasmids shared an EcoRI fragment of 8.9 kb. Results from subcloning, deletion mapping, and mini-Mu insertional mutation of the 8.9-kb EcoRI fragment suggested that a 4.2-kb fragment was sufficient to complement the mutant XC1708. Sequence analysis of this 4.2-kb fragment revealed three consecutive open reading frames (ORFs), ORF1, ORF2, and ORF3. Hybridization experiments showed that Tn5 in the genome of XC1708 and other mutants complemented by pLASC3 was located in ORF3, which could code for a protein of 83.5 kDa. A signal peptidase II processing site was identified at the N terminus of the predicted amino acid sequence. Sequence homology of 51% was observed between the amino acid sequences predicted from ORF3 and the pulD gene of Klebsiella species.