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

Effects of Natural Rheum tanguticum on the Cell Wall Integrity of Resistant Phytopathogenic Pectobacterium carotovorum subsp. Carotovorum

The abuse of agricultural antibiotics has led to the emergence of drug-resistant phytopathogens. Rifampicin and streptomycin and streptomycin resistance Pectobacterium carotovorum subsp. carotovorum (PccS1) was obtained from pathological plants in a previous experiment. Rheum tanguticum, derived from the Chinese plateau area, exhibits excellent antibacterial activity against PccS1, yet the action mode has not been fully understood. In present text, the cell wall integrity of the PccS1 was tested by the variation of the cellular proteins, SDS polyacrylamide gel electrophoresis (SDS-PAGE), scanning electron microscopy (SEM) and Fourier transform infrared spectrophotometer (FTIR) characteristics. Label-free quantitative proteomics was further used to identify the DEPs in the pathogen response to treatment with Rheum tanguticum Maxim. ex Balf. extract (abbreviated as RTMBE). Based on the bioinformatics analysis of these different expressed proteins (DEPs), RTMBE mainly inhibited some key protein expressions of beta-Lactam resistance, a two-component system and phosphotransferase system. Most of these membrane proteins were extraordinarily suppressed, which was also consistent with the morphological tests. In addition, from the downregulated flagellar motility related proteins, it was also speculated that RTMBE played an essential antibacterial role by affecting the swimming motility of the cells. The results indicated that Rheum tanguticum can be used to attenuate the virulence of the drug-resistant phytopathogenic bacteria.

Membrane Translocation of Folded Proteins

An ever-increasing number of proteins have been shown to translocate across various membranes of bacterial as well as eukaryotic cells in their folded states as a part of physiological and/or pathophysiological processes. Herein we provide an overview of the systems/processes that are established or likely to involve the membrane translocation of folded proteins, such as protein export by the twin-arginine translocation (TAT) system in bacteria and chloroplasts, unconventional protein secretion (UPS) and protein import into the peroxisome in eukaryotes, and the cytosolic entry of proteins (e.g., bacterial toxins) and viruses into eukaryotes. We also discuss the various mechanistic models that have previously been proposed for the membrane translocation of folded proteins including pore/channel formation, local membrane disruption, membrane thinning, and transport by membrane vesicles. Finally, we introduce a newly discovered vesicular transport mechanism, vesicle budding and collapse (VBC), and present evidence that VBC may represent a unifying mechanism that drives some (and potentially all) of folded protein translocation processes.

The Periplasmic Oxidoreductase DsbA Is Required for Virulence of the Phytopathogen Dickeya solani

In bacteria, the DsbA oxidoreductase is a crucial factor responsible for the introduction of disulfide bonds to extracytoplasmic proteins, which include important virulence factors. A lack of proper disulfide bonds frequently leads to instability and/or loss of protein function; therefore, improper disulfide bonding may lead to avirulent phenotypes. The importance of the DsbA function in phytopathogens has not been extensively studied yet. Dickeya solani is a bacterium from the Soft Rot Pectobacteriaceae family which is responsible for very high economic losses mainly in potato. In this work, we constructed a D. solani dsbA mutant and demonstrated that a lack of DsbA caused a loss of virulence. The mutant bacteria showed lower activities of secreted virulence determinants and were unable to develop disease symptoms in a potato plant. The SWATH-MS-based proteomic analysis revealed that the dsbA mutation led to multifaceted effects in the D. solani cells, including not only lower levels of secreted virulence factors, but also the induction of stress responses. Finally, the outer membrane barrier seemed to be disturbed by the mutation. Our results clearly demonstrate that the function played by the DsbA oxidoreductase is crucial for D. solani virulence, and a lack of DsbA significantly disturbs cellular physiology.

Characterization of Pectobacterium carotovorum proteins differentially expressed during infection of Zantedeschia elliotiana in vivo and in vitro which are essential for virulence

The identification of phytopathogen proteins that are differentially expressed during the course of the establishment of an infection is important to better understand the infection process. In vitro approaches, using plant extracts added to culture medium, have been used to identify such proteins, but the biological relevance of these findings for in planta infection are often uncertain until confirmed by in vivo studies. Here, we compared the proteins of Pectobacterium carotovorum ssp. carotovorum strain PccS1 differentially expressed in Luria-Bertani medium supplemented with extracts of the ornamental plant Zantedeschia elliotiana cultivar 'Black Magic' (in vitro) and in plant tissues (in vivo) by two-dimensional electrophoresis coupled with mass spectrometry. A total of 53 differentially expressed proteins (>1.5-fold) were identified (up-regulated or down-regulated in vitro, in vivo or both). Proteins that exhibited increased expression in vivo but not in vitro, or in both conditions, were identified, and deletions were made in a number of genes encoding these proteins, four of which (clpP, mreB, flgK and eda) led to a loss of virulence on Z. elliotiana, although clpP and mreB were later also shown to be reduced in growth in rich and minimal media. Although clpP, flgK and mreB have previously been reported as playing a role in virulence in plants, this is the first report of such a role for eda, which encodes 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase, a key enzyme in Entner-Doudoroff metabolism. The results highlight the value of undertaking in vivo as well as in vitro approaches for the identification of new bacterial virulence factors.

Structure and secretion of CofJ, a putative colonization factor of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) colonize the human gut, causing severe cholera-like diarrhoea. ETEC utilize a diverse array of pili and fimbriae for host colonization, including the Type IVb pilus CFA/III. The CFA/III pilus machinery is encoded on the cof operon, which is similar in gene sequence and synteny to the tcp operon that encodes another Type IVb pilus, the Vibrio cholerae toxin co-regulated pilus (TCP). Both pilus operons possess a syntenic gene encoding a protein of unknown function. In V. cholerae, this protein, TcpF, is a critical colonization factor secreted by the TCP apparatus. Here we show that the corresponding ETEC protein, CofJ, is a soluble protein secreted via the CFA/III apparatus. We present a 2.6 Å resolution crystal structure of CofJ, revealing a large β-sandwich protein that bears no sequence or structural homology to TcpF. CofJ has a cluster of exposed hydrophobic side-chains at one end and structural homology to the pore-forming proteins perfringolysin O and α-haemolysin. CofJ binds to lipid vesicles and epithelial cells, suggesting a role in membrane attachment during ETEC colonization.

On the path to uncover the bacterial type II secretion system

Gram-negative bacteria have evolved several secretory pathways to release enzymes or toxins into the surrounding environment or into the target cells. The type II secretion system (T2SS) is conserved in Gram-negative bacteria and involves a set of 12 to 16 different proteins. Components of the T2SS are located in both the inner and outer membranes where they assemble into a supramolecular complex spanning the bacterial envelope, also called the secreton. The T2SS substrates transiently go through the periplasm before they are translocated across the outer membrane and exposed to the extracellular milieu. The T2SS is unique in its ability to promote secretion of large and sometimes multimeric proteins that are folded in the periplasm. The present review describes recently identified protein-protein interactions together with structural and functional advances in the field that have contributed to improve our understanding on how the type II secretion apparatus assembles and on the role played by individual proteins of this highly sophisticated system.

The type II secretion system: biogenesis, molecular architecture and mechanism

Many gram-negative bacteria use the sophisticated type II secretion system (T2SS) to translocate a wide range of proteins from the periplasm across the outer membrane. The inner-membrane platform of the T2SS is the nexus of the system and orchestrates the secretion process through its interactions with the periplasmic filamentous pseudopilus, the dodecameric outer-membrane complex and a cytoplasmic secretion ATPase. Here, recent structural and biochemical information is reviewed to describe our current knowledge of the biogenesis and architecture of the T2SS and its mechanism of action.

Functional characterization of pfm in protein secretion and lung infection of Pseudomonas aeruginosa

Lung infections caused by Pseudomonas aeruginosa in cystic fibrosis (CF) patients cause progressive airway obstruction and tissue damage, which is the predominant cause of morbidity and mortality in patients with CF. This paper describes the functional characterization of the pfm gene (open reading frame PA2950) of P. aeruginosa. Using DNA microarrays, we found that the transcriptional levels of type II secretory system genes were significantly reduced in the pfm mutant strain. The type-II-dependent exoprotein LasB could not be secreted normally. The pfm gene was identified as a gene involved in bacterial protein secretion that was critical for the extracellular release of elastase in P. aeruginosa. The abilities to induce lung injury by wild-type and pfm mutant P. aeruginosa were evaluated in a murine acute lung infection model. The results showed that the pathogenicity and virulence of the pfm mutant strain was significantly reduced compared with that of the wild-type strain. The pfm gene and its expression product, as potential new drug targets against P. aeruginosa infection, have important research significance.

Study and design of stability in GH5 cellulases

Thermostable enzymes that hydrolyze lignocellulosic materials provide potential advantages in process configuration and enhancement of production efficiency over their mesophilic counterparts in the bioethanol industry. In this study, the dynamics of β-1,4-endoglucanases (EC: 3.2.1.4) from family 5 of glycoside hydrolases (GH5) were investigated computationally. The conformational flexibility of 12 GH5 cellulases, ranging from psychrophilic to hyperthermophilic, was investigated by molecular dynamics (MD) simulations at elevated temperatures. The results indicated that the protein flexibility and optimum activity temperatures are appreciably correlated. Intra-protein interactions, packing density and solvent accessible area were further examined in crystal structures to investigate factors that are possibly involved in higher rigidity of thermostable cellulases. The MD simulations and the rules learned from analyses of stabilizing factors were used in design of mutations toward the thermostabilization of cellulase C, one of the GH5 endoglucanases. This enzyme was successfully stabilized both chemically and thermally by introduction of a new disulfide cross-link to its highly mobile 56-amino acid subdomain.

The three-dimensional structure of the cytoplasmic domains of EpsF from the type 2 secretion system of Vibrio cholerae

The type 2 secretion system (T2SS), a multi-protein machinery that spans both the inner and the outer membranes of Gram-negative bacteria, is used for the secretion of several critically important proteins across the outer membrane. Here we report the crystal structure of the N-terminal cytoplasmic domain of EpsF, an inner membrane spanning T2SS protein from Vibrio cholerae. This domain consists of a bundle of six anti-parallel helices and adopts a fold that has not been described before. The long C-terminal helix alpha6 protrudes from the body of the domain and most likely continues as the first transmembrane helix of EpsF. Two N-terminal EpsF domains form a tight dimer with a conserved interface, suggesting that the observed dimer occurs in the T2SS of many bacteria. Two calcium binding sites are present in the dimer interface with ligands provided for each site by both subunits. Based on this new structure, sequence comparisons of EpsF homologs and localization studies of GFP fused with EpsF, we propose that the second cytoplasmic domain of EpsF adopts a similar fold as the first cytoplasmic domain and that full-length EpsF, and its T2SS homologs, have a three-transmembrane helix topology.

Crystal structure of the N-terminal domain of the secretin GspD from ETEC determined with the assistance of a nanobody

Secretins are among the largest bacterial outer membrane proteins known. Here we report the crystal structure of the periplasmic N-terminal domain of GspD (peri-GspD) from the type 2 secretion system (T2SS) secretin in complex with a nanobody, the VHH domain of a heavy-chain camelid antibody. Two different crystal forms contained the same compact peri-GspD:nanobody heterotetramer. The nanobody contacts peri-GspD mainly via CDR3 and framework residues. The peri-GspD structure reveals three subdomains, with the second and third subdomains exhibiting the KH fold which also occurs in ring-forming proteins of the type 3 secretion system. The first subdomain of GspD is related to domains in phage tail proteins and outer membrane TonB-dependent receptors. A dodecameric peri-GspD model is proposed in which a solvent-accessible beta strand of the first subdomain interacts with secreted proteins and/or T2SS partner proteins by beta strand complementation.

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.

Nanobody-aided structure determination of the EpsI:EpsJ pseudopilin heterodimer from Vibrio vulnificus

Pseudopilins form the central pseudopilus of the sophisticated bacterial type 2 secretion systems. The crystallization of the EpsI:EpsJ pseudopilin heterodimer from Vibrio vulnificus was greatly accelerated by the use of nanobodies, which are the smallest antigen-binding fragments derived from heavy-chain only camelid antibodies. Seven anti-EpsI:EpsJ nanobodies were generated and co-crystallization of EpsI:EpsJ nanobody complexes yielded several crystal forms very rapidly. In the structure solved, the nanobodies are arranged in planes throughout the crystal lattice, linking layers of EpsI:EpsJ heterodimers. The EpsI:EpsJ dimer observed confirms a right-handed architecture of the pseudopilus, but, compared to a previous structure of the EpsI:EpsJ heterodimer, EpsI differs 6 degrees in orientation with respect to EpsJ; one loop of EpsJ is shifted by approximately 5A due to interactions with the nanobody; and a second loop of EpsJ underwent a major change of 17A without contacts with the nanobody. Clearly, nanobodies accelerate dramatically the crystallization of recalcitrant protein complexes and can reveal conformational flexibility not observed before.

The coiled-coil protein-binding motif in Fusarium verticillioides Fsr1 is essential for maize stalk rot virulence

Fusarium verticillioides (Sacc.) Nirenberg (teleomorph Gibberella moniliformis Wineland) is one of the key pathogens of maize stalk rot disease. However, a clear understanding of stalk rot pathogenesis is still lacking. Previously, we identified the F. verticillioides FSR1 gene, which plays a key role in fungal virulence and sexual mating. The predicted Fsr1 protein contains multiple protein-binding domains, namely a caveolin-binding domain, a coiled-coil structure, and a calmodulin-binding motif at the N terminus and a WD40 repeat domain at the C terminus. Fsr1 shares significant similarity to a family of striatin proteins that play a critical role in cellular mechanisms that regulate a variety of developmental processes. Significantly, FSR1 function is conserved in Fusarium graminearum, where it also plays a direct role in pathogenesis. In this study, our goal was to determine the motif(s) in Fsr1 that are directly associated with fungal virulence. We complemented the FSR1 knockout (Deltafsr1) strain with mutated versions of the FSR1 gene, and determined that the Fsr1 C-terminal WD40 repeat domain is dispensable for vegetative growth and maize stalk rot virulence. We also examined the potential link between FSR1-mediated virulence and cell wall-degrading enzyme (alpha-amylase, pectinase and cellulase) activities. Further characterization of the N-terminal region revealed that the coiled-coil structure is essential for virulence in F. verticillioides. The coiled-coil domain is involved in a variety of protein-protein interactions in eukaryotic systems, and thus we hypothesize that the interaction between Fsr1 and the putative Fsr1-binding protein triggers downstream gene signalling that is associated with F. verticillioides virulence.

The crystal structure of a binary complex of two pseudopilins: EpsI and EpsJ from the type 2 secretion system of Vibrio vulnificus

Type II secretion systems (T2SS) translocate virulence factors from the periplasmic space of many pathogenic bacteria into the extracellular environment. The T2SS of Vibrio cholerae and related species is called the extracellular protein secretion (Eps) system that consists of a core of multiple copies of 11 different proteins. The pseudopilins, EpsG, EpsH, EpsI, EpsJ and EpsK, are five T2SS proteins that are thought to assemble into a pseudopilus, which is assumed to interact with the outer membrane pore, and may actively participate in the export of proteins. We report here biochemical evidence that the minor pseudopilins EpsI and EpsJ from Vibrio species interact directly with one another. Moreover, the 2.3 A resolution crystal structure of a complex of EspI and EpsJ from Vibrio vulnificus represents the first atomic resolution structure of a complex of two different pseudopilin components from the T2SS. Both EpsI and EpsJ appear to be structural extremes within the family of type 4a pilin structures solved to date, with EpsI having the smallest, and EpsJ the largest, "variable pilin segment" seen thus far. A high degree of sequence conservation in the EpsI:EpsJ interface indicates that this heterodimer occurs in the T2SS of a large number of bacteria. The arrangement of EpsI and EpsJ in the heterodimer would correspond to a right-handed helical character of proteins assembled into a pseudopilus.

Purification and characterization of the N-terminal domain of ExeA: a novel ATPase involved in the type II secretion pathway of Aeromonas hydrophila

Aeromonas hydrophila secretes a number of degradative enzymes and toxins into the external milieu via the type II secretory pathway or secreton. ExeA is an essential component of this system and is necessary for the localization and/or multimerization of the secretin ExeD. ExeA contains two sequence motifs characteristic of the Walker superfamily of ATPases. Previous examination of substitution derivatives altered in these motifs suggested that ATP binding or hydrolysis is required for ExeAB complex formation and subsequent secretion function. To directly examine ExeA function, the N-terminal cytoplasmic domain of ExeA with the addition of a C-terminal hexahistidine tag (cytExeA) was overproduced in Escherichia coli and purified by metal chelate affinity and anion-exchange chromatographic techniques. Purified preparations of cytExeA exhibited ATPase activity in the presence of several divalent cations, Mg2+ being the preferred cation, with an optimum reaction temperature of approximately 37 to 42 degrees C and an optimum pH of 7 to 8. cytExeA exhibited an apparent K(m) for Mg-ATP of 0.22 mM and a V(max) of 0.72 nmol min(-1) mg(-1) of protein. cytExeA displayed low specificity for nucleoside triphosphate substrates and was significantly inhibited by F-type ATPase inhibitors. Gel filtration analyses of cytExeA, ExeA, and ExeAB indicated that ExeA dimerizes and forms a very large complex with ExeB. These findings support a model whereby ExeAB utilizes energy derived from ATP hydrolysis to facilitate the correct localization and multimerization of the ExeD secretin.

Construction of minimum size cellulase (Cel5Z) from Pectobacterium chrysanthemi PY35 by removal of the C-terminal region

Pectobacterium chrysanthemi PY35 secretes the endoglucanase Cel5Z, an enzyme of the glycoside hydrolase family 5. Cel5Z is a 426 amino acid, signal peptide (SP)-containing protein composed of two domains: a large N-terminal catalytic domain (CD; 291 amino acids) and a small C-terminal cellulose binding domain (CBD; 62 amino acids). These two domains are separated by a 30 amino acid linker region (LR). A truncated cel5Z gene was constructed with the addition of a nonsense mutation that removes the C-terminal region of the protein. A truncated Cel5Z protein, consisting of 280 amino acid residues, functioned as a mature enzyme despite the absence of the SP, 11 amino acid CD, LR, and CBD region. In fact, this truncated Cel5Z protein showed an enzymatic activity 80% higher than that of full-length Cel5Z. However, cellulase activity was undetectable in mature Cel5Z proteins truncated to less than 280 amino acids.

Biogenesis of Fe-S cluster by the bacterial Suf system: SufS and SufE form a new type of cysteine desulfurase

Biosynthesis of iron-sulfur clusters (Fe-S) depends on multiprotein systems. Recently, we described the SUF system of Escherichia coli and Erwinia chrysanthemi as being important for Fe-S biogenesis under stressful conditions. The SUF system is made of six proteins: SufC is an atypical cytoplasmic ABC-ATPase, which forms a complex with SufB and SufD; SufA plays the role of a scaffold protein for assembly of iron-sulfur clusters and delivery to target proteins; SufS is a cysteine desulfurase which mobilizes the sulfur atom from cysteine and provides it to the cluster; SufE has no associated function yet. Here we demonstrate that: (i) SufE and SufS are both cystosolic as all members of the SUF system; (ii) SufE is a homodimeric protein; (iii) SufE forms a complex with SufS as shown by the yeast two-hybrid system and by affinity chromatography; (iv) binding of SufE to SufS is responsible for a 50-fold stimulation of the cysteine desulfurase activity of SufS. This is the first example of a two-component cysteine desulfurase enzyme.

SufA from Erwinia chrysanthemi. Characterization of a scaffold protein required for iron-sulfur cluster assembly

SufA is a component of the recently discovered suf operon, which has been shown to play an important function in bacteria during iron-sulfur cluster biosynthesis and resistance to oxidative stress. The SufA protein from Erwinia chrysanthemi, a Gram-negative plant pathogen, has been purified to homogeneity and characterized. It is a homodimer with the ability to assemble rather labile [2Fe-2S] and [4Fe-4S] clusters as shown by Mössbauer spectroscopy. These clusters can be transferred to apoproteins such as ferredoxin or biotin synthase during a reaction that is not inhibited by bathophenanthroline, an iron chelator. Cluster assembly in these proteins is much more efficient when iron and sulfur are provided by holoSufA than by free iron sulfate and sodium sulfide. We propose the function of SufA is that of a scaffold protein for [Fe-S] cluster assembly and compare it to IscA, a member of the isc operon also involved in cluster biosynthesis in both prokaryotes and eukaryotes. Mechanistic and physiological implications of these results are also discussed.

Rhamnogalacturonate lyase RhiE is secreted by the out system in Erwinia chrysanthemi

Supernatants of rhamnose-induced Erwinia chrysanthemi strain 3937 cultures contain a principal secreted protein named RhiE. A rhiE mutant has been found among a set of rhamnose-induced MudI1681 lacZ fusions. RhiE is a 62-kDa protein that has rhamnogalacturonate lyase activity on rhamnogalacturonan I (RG-I). It does not require a divalent cation for its activity and has an optimal pH of 6.0. rhiE expression is strongly induced in the presence of rhamnose but is also regulated by PecT and Crp, two regulators of the transcription of pectinolytic enzyme genes. RhiE is secreted through the type II Out secretion pathway. RhiE has no disulfide bond. The absence of RhiE secretion in a dsb mutant indicated that disulfide bond formation is required for the biogenesis of the secretion apparatus. RhiE was searched for in several E. chrysanthemi strains by using antibodies, and it was found to be present in one-third of the strains tested. However, the reduced virulence of the rhiE mutant indicates that degradation of the RG-I region of pectin is important for full virulence of E. chrysanthemi.

SufC: an unorthodox cytoplasmic ABC/ATPase required for [Fe-S] biogenesis under oxidative stress

Proteins containing [Fe-S] clusters perform essential functions in all domains of life. Previously, we identified the sufABCDSE operon as being necessary for virulence of the plant pathogen Erwinia chrysanthemi. In addition, we collected preliminary evidence that the sufABCDSE operon might be involved in the assembly of [Fe-S] clusters. Of particular interest are the sufB, sufC and sufD genes, which are conserved among Eubacteria, Archaea, plants and parasites. The present study establishes SufC as an unorthodox ATPase of the ABC superfamily that is located in the cytosol, wherein it interacts with both SufB and SufD. Moreover, under oxidative stress conditions, SufC was found to be necessary for the activity of enzymes containing oxygen-labile [Fe-S] clusters, but dispensable for glutamate synthase, which contains an oxidatively stable [Fe-S] cluster. Lastly, we have shown SufBCD to be essential for iron acquisition via chrysobactin, a siderophore of major importance in virulence. We discuss a model wherein the SufBCD proteins contribute to bacterial pathogenicity via their role in the assembly of [Fe-S] clusters under oxidative stress and iron limitation.

Identification of XcpP domains that confer functionality and specificity to the Pseudomonas aeruginosa type II secretion apparatus

Gram-negative bacteria have evolved several types of secretion mechanisms to release proteins into the extracellular medium. One such mechanism, the type II secretory system, is a widely conserved two-step process. The first step is the translocation of signal peptide-bearing exoproteins across the inner membrane. The second step, the translocation across the outer membrane, involves the type II secretory apparatus or secreton. The secretons are made up of 12-15 proteins (Gsp) depending on the organism. Even though the systems are conserved, heterologous secretion is mostly species restricted. Moreover, components of the secreton are not systematically exchangeable, especially with distantly related microorganisms. In closely related species, two components, the GspC and GspD (secretin) family members, confer specificity for substrate recognition and/or secreton assembly. We used Pseudomonas aeruginosa as a model organism to determine which domains of XcpP (GspC member) are involved in specificity. By constructing hybrids between XcpP and OutC, the Erwinia chrysanthemi homologue, we identified a region of 35 residues that was not exchangeable. We showed that this region might influence the stability of the XcpYZ secreton subcomplex. Remarkably, XcpP and OutC have domains, coiled-coil and PDZ, respectively, which exhibit the same function but that are structurally different. Those two domains are exchangeable and we provided evidence that they are involved in the formation of homomultimeric complexes of XcpP.

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.

Maturation of Pseudomonas aeruginosa Elastase

Elastase of Pseudomonas aeruginosa is synthesized as a preproenzyme. After propeptide-mediated folding in the periplasm, the proenzyme is autoproteolytically processed, prior to translocation of both the mature enzyme and the propeptide across the outer membrane. The formation of the two disulfide bonds present in the mature enzyme was examined by studying the expression of the wild-type enzyme and of alanine for cysteine mutant derivatives in the authentic host and in dsb mutants of Escherichia coli. It appeared that the two disulfide bonds are formed successively. First, DsbA catalyzes the formation of the disulfide bond between Cys-270 and Cys-297 within the proenzyme. This step is essential for the subsequent autoproteolytic processing to occur. The second disulfide bond between Cys-30 and Cys-57 is formed more slowly and appears to be formed after processing of the proenzyme, and its formation is catalyzed by DsbA as well. This second disulfide bond appeared to be required for the full proteolytic activity of the enzyme and contributes to its stability.

Biology of type II secretion

The type II secretion pathway or the main terminal branch of the general secretion pathway, as it has also been referred to, is widely distributed among Proteobacteria, in which it is responsible for the extracellular secretion of toxins and hydrolytic enzymes, many of which contribute to pathogenesis in both plants and animals. Secretion through this pathway differs from most other membrane transport systems, in that its substrates consist of folded proteins. The type II secretion apparatus is composed of at least 12 different gene products that are thought to form a multiprotein complex, which spans the periplasmic compartment and is specifically required for translocation of the secreted proteins across the outer membrane. This pathway shares many features with the type IV pilus biogenesis system, including the ability to assemble a pilus-like structure. This review discusses recent findings on the organization of the secretion apparatus and the role of its various components in secretion. Different models for pilus-mediated secretion through the gated pore in the outer membrane are also presented, as are the possible properties that determine whether a protein is recognized and secreted by the type II pathway.

Disulfide bond formation in secreton component PulK provides a possible explanation for the role of DsbA in pullulanase secretion

When expressed in Escherichia coli, the 15 Klebsiella oxytoca pul genes that encode the so-called Pul secreton or type II secretion machinery promote pullulanase secretion and the assembly of one of the secreton components, PulG, into pili. Besides these pul genes, efficient pullulanase secretion also requires the host dsbA gene, encoding a periplasmic disulfide oxidoreductase, independently of disulfide bond formation in pullulanase itself. Two secreton components, the secretin pilot protein PulS and the minor pseudopilin PulK, were each shown to posses an intramolecular disulfide bond whose formation was catalyzed by DsbA. PulS was apparently destabilized by the absence of its disulfide bond, whereas PulK stability was not dramatically affected either by a dsbA mutation or by the removal of one of its cysteines. The pullulanase secretion defect in a dsbA mutant was rectified by overproduction of PulK, indicating reduced disulfide bond formation in PulK as the major cause of the secretion defect under the conditions tested (in which PulS is probably present in considerable excess of requirements). PulG pilus formation was independent of DsbA, probably because PulK is not needed for piliation.

Exchange of Xcp (Gsp) secretion machineries between Pseudomonas aeruginosa and Pseudomonas alcaligenes: species specificity unrelated to substrate recognition

Pseudomonas aeruginosa and Pseudomonas alcaligenes are gram-negative bacteria that secrete proteins using the type II or general secretory pathway, which requires at least 12 xcp gene products (XcpA and XcpP to -Z). Despite strong conservation of this secretion pathway, gram-negative bacteria usually cannot secrete exoproteins from other species. Based on results obtained with Erwinia, it has been proposed that the XcpP and/or XcpQ homologs determine this secretion specificity (M. Linderberg, G. P. Salmond, and A. Collmer, Mol. Microbiol. 20:175-190, 1996). In the present study, we report that XcpP and XcpQ of P. alcaligenes could not substitute for their respective P. aeruginosa counterparts. However, these complementation failures could not be correlated to species-specific recognition of exoproteins, since these bacteria could secrete exoproteins of each other. Moreover, when P. alcaligenes xcpP and xcpQ were expressed simultaneously in a P. aeruginosa xcpPQ deletion mutant, complementation was observed, albeit only on agar plates and not in liquid cultures. After growth in liquid culture the heat-stable P. alcaligenes XcpQ multimers were not detected, whereas monomers were clearly visible. Together, our results indicate that the assembly of a functional Xcp machinery requires species-specific interactions between XcpP and XcpQ and between XcpP or XcpQ and another, as yet uncharacterized component(s).

Disulfide bond in Pseudomonas aeruginosa lipase stabilizes the structure but is not required for interaction with its foldase

Pseudomonas aeruginosa secretes a 29-kDa lipase which is dependent for folding on the presence of the lipase-specific foldase Lif. The lipase contains two cysteine residues which form an intramolecular disulfide bond. Variant lipases with either one or both cysteines replaced by serines showed severely reduced levels of extracellular lipase activity, indicating the importance of the disulfide bond for secretion of lipase through the outer membrane. Wild-type and variant lipase genes fused to the signal sequence of pectate lyase from Erwinia carotovora were expressed in Escherichia coli, denatured by treatment with urea, and subsequently refolded in vitro. Enzymatically active lipase was obtained irrespective of the presence or absence of the disulfide bond, suggesting that the disulfide bond is required neither for correct folding nor for the interaction with the lipase-specific foldase. However, cysteine-to-serine variants were more readily denatured by treatment at elevated temperatures and more susceptible to proteolytic degradation by cell lysates of P. aeruginosa. These results indicate a stabilizing function of the disulfide bond for the active conformation of lipase. This conclusion was supported by the finding that the disulfide bond function could partly be substituted by a salt bridge constructed by changing the two cysteine residues to arginine and aspartate, respectively.

DsbA and DsbC affect extracellular enzyme formation in Pseudomonas aeruginosa

DsbA and DsbC proteins involved in the periplasmic formation of disulfide bonds in Pseudomonas aeruginosa were identified and shown to play an important role for the formation of extracellular enzymes. Mutants deficient in either dsbA or dsbC or both genes were constructed, and extracellular elastase, alkaline phosphatase, and lipase activities were determined. The dsbA mutant no longer produced these enzymes, whereas the lipase activity was doubled in the dsbC mutant. Also, extracellar lipase production was severely reduced in a P. aeruginosa dsbA mutant in which an inactive DsbA variant carrying the mutation C34S was expressed. Even when the lipase gene lipA was constitutively expressed in trans in a lipA dsbA double mutant, lipase activity in cell extracts and culture supernatants was still reduced to about 25%. Interestingly, the presence of dithiothreitol in the growth medium completely inhibited the formation of extracellular lipase whereas the addition of dithiothreitol to a cell-free culture supernatant did not affect lipase activity. We conclude that the correct formation of the disulfide bond catalyzed in vivo by DsbA is necessary to stabilize periplasmic lipase. Such a stabilization is the prerequisite for efficient secretion using the type II pathway.

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.

Protein secretion mechanisms in Gram-negative bacteria

Gram-negative bacteria have developed a variety of secretion pathways to secrete toxins and enzymes into the extracellular medium. These pathways are very different with respect to their functional mechanism and complexity, and each system has its own advantages and limitations, regarding the number, size, folding state and fate of their substrates. Pseudomonas aeruginosa secretes many different proteins into the extracellular medium, using at least four secretion pathways. Most of the exoproteins are secreted via the type II system, composed of the 12 Xcp proteins. The only outer membrane protein of the system, XcpQ, belongs to a large family of proteins, designated secretins, which participate in a variety of different transport processes. Other Xcp proteins, XcpT-X, show homology to the subunits of the retractile type IV pili. Further analogies between the type II system and the assembly of retractile pili suggest a mechanism for type II secretion, in which a pilus-like structure, composed of XcpT-X, facilitates the transport of exoproteins through the channel formed by the secretin XcpQ.

Influence of deletions within domain II of exotoxin A on its extracellular secretion from Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative bacterium that secretes many proteins into the extracellular medium via the Xcp machinery. This pathway, conserved in gram-negative bacteria, is called the type II pathway. The exoproteins contain information in their amino acid sequence to allow targeting to their secretion machinery. This information may be present within a conformational motif. The nature of this signal has been examined for P. aeruginosa exotoxin A (PE). Previous studies failed to identify a common minimal motif required for Xcp-dependent recognition and secretion of PE. One study identified a motif at the N terminus of the protein, whereas another one found additional information at the C terminus. In this study, we assess the role of the central PE domain II composed of six alpha-helices (A to F). The secretion behavior of PE derivatives, individually deleted for each helix, was analyzed. Helix E deletion has a drastic effect on secretion of PE, which accumulates within the periplasm. The conformational rearrangement induced in this variant is predicted from the three-dimensional PE structure, and the molecular modification is confirmed by gel filtration experiments. Helix E is in the core of the molecule and creates close contact with other domains (I and III). Deletion of the surface-exposed helix F has no effect on secretion, indicating that no secretion information is contained in this helix. Finally, we concluded that disruption of a structured domain II yields an extended form of the molecule and prevents formation of the conformational secretion motif.

Probing secretion and translocation of a beta-autotransporter using a reporter single-chain Fv as a cognate passenger domain

The mechanism of protein secretion mediated by the beta-domain of the Neisseria gonorrhoeae IgA protease, a paradigm of a family of secreted polypeptides of Gram-negative bacteria called autotransporters, has been examined using a single-chain antibody (scFv) as a reporter passenger domain to monitor the translocation process. Fusion of a scFv to the beta-module of the IgA protease allowed us to investigate the passage of the chimeric protein through the periplasm, its insertion into the outer membrane and the movement of the N-terminal moiety towards the cell surface. As the binding activity of the scFv to its target antigen is entirely dependent on the formation of disulphide bonds, the relationship between secretion, folding and formation of S-S bridges could be analysed in detail. In contrast to the current notion that only an unfolded N-passenger domain can be translocated through the beta-domain, our results show that the scFv is able to pass through the outer membrane, albeit at a threefold reduced level, in an active conformation with its disulphide bonds preformed in the periplasm through the action of the DsbA product. These data call for a re-evaluation of the prevailing model for secretion of the N-domain of autotransporters.

Structure-function analysis of XcpP, a component involved in general secretory pathway-dependent protein secretion in Pseudomonas aeruginosa

The general secretory pathway of Pseudomonas aeruginosa is required for the transport of signal peptide-containing exoproteins across the cell envelope. After completion of the Sec-dependent translocation of exoproteins across the inner membrane and cleavage of the signal peptide, the Xcp machinery mediates translocation across the outer membrane. This machinery consists of 12 components, of which XcpQ (GspD) is the sole outer membrane protein. XcpQ forms a multimeric ring-shaped structure, with a central opening through which exoproteins could pass to reach the medium. Surprisingly, all of the other Xcp proteins are located in or are associated with the cytoplasmic membrane. This study is focused on the characteristics of one such cytoplasmic membrane protein, XcpP. An xcpP mutant demonstrated that the product of this gene is indeed an essential element of the P. aeruginosa secretion machinery. Construction and analysis of truncated forms of XcpP made it possible to define essential domains for the function of the protein. Some of these domains, such as the N-terminal transmembrane domain and a coiled-coil structure identified at the C terminus of XcpP, may be involved in protein-protein interaction during the assembly of the secretory apparatus.

Enhancement of expression and apparent secretion of Erwinia chrysanthemi endoglucanase (encoded by celZ) in Escherichia coli B

Escherichia coli B has been engineered as a biocatalyst for the conversion of lignocellulose into ethanol. Previous research has demonstrated that derivatives of E. coli B can produce high levels of Erwinia chrysanthemi endoglucanase (encoded by celZ) as a periplasmic product and that this enzyme can function with commercial fungal cellulase to increase ethanol production. In this study, we have demonstrated two methods that improve celZ expression in E. coli B. Initially, with a low-copy-number vector, two E. coli glycolytic gene promoters (gap and eno) were tested and found to be less effective than the original celZ promoter. By screening 18,000 random fragments of Zymomonas mobilis DNA, a surrogate promoter was identified which increased celZ expression up to sixfold. With this promoter, large polar inclusion bodies were clearly evident in the periplasmic space. Sequencing revealed that the most active surrogate promoter is derived from five Sau3A1 fragments, one of which was previously sequenced in Z. mobilis. Visual inspection indicated that this DNA fragment contains at least five putative promoter regions, two of which were confirmed by primer extension analysis. Addition of the out genes from E. chrysanthemi EC16 caused a further increase in the production of active enzyme and facilitated secretion or release of over half of the activity into the extracellular environment. With the most active construct, of a total of 13,000 IU of active enzyme per liter of culture, 7,800 IU was in the supernatant. The total active endoglucanase was estimated to represent 4 to 6% of cellular protein.

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.

Conditional mutations in OutE and OutL block exoenzyme secretion across the Erwinia carotovora outer membrane

The phytopathogen Erwinia carotovora subspecies carotovora secretes pectinases and cellulase via the general secretory pathway, a process requiring at least 13 proteins encoded by the out gene cluster. By exploiting delta::Tn5, a generalised transducing phage (psi KP) and localised mutagenesis of the out gene cluster, we have produced a histidine auxotroph and 19 new secretory mutants, including two (HJN1003 and HJN1004) which were conditional (temperature sensitive) for secretion. All of the mutants accumulated pectinases and cellulase in the periplasm, but in the case of HJN1003 and HJN1004, only at the restrictive temperature. HJN1003 and HJN1004 were complemented by the outE and outL wild-type genes, respectively, and both mutant alleles were cloned and sequenced to reveal single missense substitutions. HJN1003 carries an Arg166 to His alteration in OutE and HJN1004 carries a Pro159 to Leu alteration in OutL. Topology mapping of OutL using a beta-lactamase probe confirmed that OutL is a type II bitopic trans-inner membrane protein and that the mutated Pro159 residue in HJN1004 is located in the cytoplasmic domain of OutL. Hence, the secretion of exoenzymes across the outer membrane is critically dependent on the conformation of secretory components located at the cytoplasmic face of the inner membrane.

Evidence that a globular conformation is not compatible with FhaC-mediated secretion of the Bordetella pertussis filamentous haemagglutinin

The 220 kDa Bordetella pertussis filamentous haemagglutinin (FHA) is the major extracellular protein of this organism. It is exported using a signal peptide-dependent pathway, and its secretion depends on one specific outer membrane accessory protein, FhaC. In this work, we have investigated the influence of conformation on the FhaC-mediated secretion of FHA using an 80kDa N-terminal FHA derivative, Fha44. In contrast to many signal peptide-dependent secretory proteins, no soluble periplasmic intermediate of Fha44 could be isolated. In addition, cell-associated Fha44 synthesized in the absence of FhaC did not remain competent for extracellular secretion upon delayed expression of FhaC, indicating that the translocation steps across the cytoplasmic and the outer membrane might be coupled. A chimeric protein, in which the globular B subunit of the cholera toxin, CtxB, was fused at the C-terminus of Fha44, was not secreted in B. pertussis or in Escherichia coli expressing FhaC. The hybrid protein was only secreted when both disulphide bond-forming cysteines of CtxB were replaced by serines or when it was produced in DsbA- E. coli. The Fha44 portion of the secretion-incompetent hybrid protein was partly exposed on the cell surface. These results argue that the Fha44-CtxB hybrid protein transited through the periplasmic space, where disulphide bond formation is specifically catalysed, and that secretion across the outer membrane was initiated. The folded CtxB portion prevented extracellular release of the hybrid, in contrast to the more flexible CtxB domain devoid of cysteines. We propose a secretion model whereby Fha44 transits through the periplasmic space on its way to the cell surface and initiates its translocation through the outer membrane before being released from the cytoplasmic membrane. Coupling of Fha44 translocation across both membranes would delay the acquisition of its folded structure until the protein emerges from the outer membrane. Such a model would be consistent with the extensive intracellular proteolysis of FHA derivatives in B. pertussis.

Macromolecular assembly and secretion across the bacterial cell envelope: type II protein secretion systems

A decade ago, Pugsley and colleagues reported the existence of a large region of Klebsiella DNA, distinct from the Klebsiella gene encoding pullulanase, which was necessary for secretion of this enzyme to the cell surface in Escherichia coli (d'Enfert et al., 1987a,b). The pul genes it contained proved to be the tip of an iceberg. The sequences reported before 1992 (d'Enfert et al., 1987a,b; d'Enfert & Pugsley, 1989; Pugsley & Reyss, 1990; Reyss & Pugsley, 1990) included only one gene (pulD) that matched any sequence in the data base; a 220 amino acid residue segment of PulD was 32% identical with a portion of the filamentous phage-encoded protein, pIV. But by the time the sequence of the 18.8 kb DNA fragment that contained the pul genes had been completed (Possot et al., 1992), reports of sets of homologous genes in several species of Gram-negative plant and animal pathogens had appeared. For the most part, these gene clusters were cloned by their ability to complement mutants that produced, but failed to secrete, proteins normally found in the extracellular milieu; when tested, the mutants showed reduced pathogenicity or were totally avirulent. The secreted proteins included hydrolytic enzymes such as cellulase and pectinase from plant pathogens, and proteases and toxins from animal pathogens. The multi-gene family necessary for secretion of these enzymes is now known as the type II system or the main terminal branch (MTB) of the general secretion pathway (GSP). As summarized by Pugsley et al. (1997), the current tally includes type II systems from Klebsiella oxytoca (pul), Erwinia chrysanthemi and carotovora (out), Xanthomonas campestris (xps), Pseudomonas aeruginosa (xcp), Aeromonas hydrophila (exe), and Vibrio cholerae (eps). A second type II system (sps) necessary for deposition of the S-layer on the cell surface in A. hydrophila is more similar to the X. campestris than A. hydrophila genes (Thomas & Trust, 1995). The biggest surprise has been the discovery of a complete set of type II secretion genes in E. coli K12. The E. coli genes are not expressed under normal growth conditions, and a search is underway to find inducing conditions and secretion substrates (Francetic & Pugsley, 1996). Impressive progress has already been made in defining components of the pathway. What remains to be understood in mechanistic detail is how this protein secretion system functions.

Identification of an additional member of the secretin superfamily of proteins in Pseudomonas aeruginosa that is able to function in type II protein secretion

Pseudomonas aeruginosa is a prolific exporter of virulence factors and contains three of the four protein secretion systems that have been described in gram-negative bacteria. The P. aeruginosa type II general secretory pathway (GSP) is used to export the largest number of proteins from this organism, including lipase, phospholipase C, alkaline phosphatase, exotoxin A, elastase and LasA. Although these exoproteins contain no sequence similarity, they are specifically and efficiently transported by the secretion apparatus. Bacterial homologues of XcpQ (GspD), the only outer membrane component of this system, have been proposed to play the role of gatekeeper, by presumably interacting and recognizing the exported substrates to allow their passage through the outer membrane. While determining the phenotype of nonpolar deletions in each of the xcp genes, we have shown that a deletion of the P. aeruginosa strain K xcpQ does not completely abolish protein secretion. As the proposed function of XcpQ should be requisite for secretion, we searched for additional factors that could carry out this role. A cosmid DNA library from a PAK strain deleted for xcpP-Z was tested for its ability to increase protein secretion by screening for enhanced growth on lipid agar, a medium that selects for the secretion of lipase. In this manner, we have identified an XcpQ homologue, XqhA, that is solely responsible for the residual export observed in a deltaxcpQ strain, although it is not required for efficient secretion in wild-type P. aeruginosa. We have also demonstrated that this protein is capable of recognizing all of the exoproteins of P. aeruginosa, arguing against the proposed role of members of the secretin family as determinants of specificity.

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.

Formation of oligomeric rings by XcpQ and PilQ, which are involved in protein transport across the outer membrane of Pseudomonas aeruginosa

Pseudomonas aeruginosa is able to translocate proteins across both membranes of the cell envelope. Many of these proteins are transported via the type II secretion pathway and adopt their tertiary conformation in the periplasm, which implies the presence of a large transport channel in the outer membrane. The outer membrane protein, XcpQ, which is involved in transport of folded proteins across the outer membrane of P. aeruginosa, was purified as a highly stable homomultimer. Insertion and deletion mutagenesis of xcpQ revealed that the C-terminal part of XcpQ is sufficient for the formation of the multimer. However, linker insertions in the N-terminal part can disturb complex formation completely. Furthermore, complex formation is strictly correlated with lethality, caused by overexpression of xcpQ. Electron microscopic evaluation of the XcpQ multimers revealed large, ring-shaped structures with an apparent central cavity of 95 A. Purified PilQ, a homologue of XcpQ involved in the biogenesis of type IV pili, formed similar structures. However, the apparent cavity formed by PilQ was somewhat smaller, 53 A. The size of this cavity could allow for the transport of intact type IV pili.

Characterization of a periplasmic peptidyl-prolyl cis-trans isomerase in Erwinia chrysanthemi

The main determinant of the plant pathogen Erwinia chrysanthemi virulence is the production of extracellular enzymes, mainly pectate lyases. Adjacent to a pectate lyase encoding locus, we identified the gene rotA supposed to encode a folding catalyst. Overproduction of the protein and assay of activity using a synthetic substrate, confirmed that rotA encodes a periplasmic peptidyl-prolyl cis-trans isomerase. rotA disruption provokes no change in cell morphology, cell viability, growth rate or stability of the extracellular and periplasmic proteins. In addition, this mutation does not alter the activity of the pectate lyases, their stability in the periplasm during the transitory step of secretion or their recognition by the Out secretory system. rotA expression was followed using a rotA::uidA transcriptional fusion. Some environmental conditions, such as temperature variations and nitrogen starvation, modulate rotA expression. In contrast to the E. coli rotA gene, the E. chrysanthemi rotA possesses only one promoter and is not controlled by the CRP global regulator.

Making and breaking disulfide bonds

It is now well established that protein folding requires the assistance of folding helpers in vivo. The formation or isomerization of disulfide bonds in proteins is a slow process requiring catalysis. In nascent polypeptide chains the cysteine residues are in the thiol form. The formation of the disulfide bonds usually occurs simultaneously with the folding of the polypeptide, which means in the endoplasmic reticulum of eukaryotes or in the periplasm of Gram-negative bacteria. In prokaryotes, the existence of redox proteins involved in the formation of disulfide bonds containing proteins has recently been revealed in the periplasm. The discovery of these redox proteins through various genetic approaches will be summarized, as well as the most recent insights regarding their biochemical and biological activities.

Biogenesis of the bundle-forming pilus of enteropathogenic Escherichia coli: reconstitution of fimbriae in recombinant E. coli and role of DsbA in pilin stability--a review

Enteropathogenic Escherichia coli (EPEC) adhere to tissue culture cells in a distinct pattern known as localized adherence (LA). We have defined two loci necessary for LA. A plasmid-encoded gene cluster encodes bundlin, the major structural subunit of a type-IV fimbria called the bundle-forming pilus (BFP), a prepilin peptidase necessary for processing of pre-bundlin to its mature form, and twelve other proteins. Under the control of an exogenous promoter, these 14 genes are sufficient for the biogenesis of BFP in a heterologous E. coli host. The chromosomal gene dsbA, which encodes a periplasmic disulfide-bond oxidoreductase, is also required for LA. In the absence of DsbA protein, bundlin is made but rapidly degraded. Pre-bundlin is also rapidly degraded in the absence of DsbA, suggesting that the prepilin is a transcytoplasmic protein simultaneously accessible to enzymes on both sides of the inner membrane. These studies offer a fresh perspective on the biogenesis of type-IV pili.

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.