C-terminal processing of GlyGly-CTERM containing proteins by rhombosortase in Vibrio cholerae

Vibrio cholerae and a subset of other Gram-negative bacteria, including Acinetobacter baumannii, express proteins with a C-terminal tripartite domain called GlyGly-CTERM, which consists of a motif rich in glycines and serines, followed by a hydrophobic region and positively charged residues. Here we show that VesB, a V. cholerae serine protease, requires the GlyGly-CTERM domain, the intramembrane rhomboid-like protease rhombosortase, and the type II secretion system (T2SS) for localization at the cell surface. VesB is cleaved by rhombosortase to expose the second glycine residue of the GlyGly-CTERM motif, which is then conjugated to a glycerophosphoethanolamine-containing moiety prior to engagement with the T2SS and outer membrane translocation. In support of this, VesB accumulates intracellularly in the absence of the T2SS, and surface-associated VesB activity is no longer detected when the rhombosortase gene is inactivated. In turn, when VesB is expressed without an intact GlyGly-CTERM domain, VesB is released to the extracellular milieu by the T2SS and does not accumulate on the cell surface. Collectively, our findings suggest that the posttranslational modification of the GlyGly-CTERM domain is essential for cell surface localization of VesB and other proteins expressed with this tripartite extension.

Targeting the Type II Secretion System: Development, Optimization, and Validation of a High-Throughput Screen for the Identification of Small Molecule Inhibitors

Nosocomial pathogens that develop multidrug resistance present an increasing problem for healthcare facilities. Due to its rapid rise in antibiotic resistance, Acinetobacter baumannii is one of the most concerning gram-negative species. A. baumannii typically infects immune compromised individuals resulting in a variety of outcomes, including pneumonia and bacteremia. Using a murine model for bacteremia, we have previously shown that the type II secretion system (T2SS) contributes to in vivo fitness of A. baumannii. Here, we provide support for a role of the T2SS in protecting A. baumannii from human complement as deletion of the T2SS gene gspD resulted in a 100-fold reduction in surviving cells when incubated with human serum. This effect was abrogated in the absence of Factor B, a component of the alternative pathway of complement activation, indicating that the T2SS protects A. baumannii against the alternative complement pathway. Because inactivation of the T2SS results in loss of secretion of multiple enzymes, reduced in vivo fitness, and increased sensitivity to human complement, the T2SS may be a suitable target for therapeutic intervention. Accordingly, we developed and optimized a whole-cell high-throughput screening (HTS) assay based on secreted lipase activity to identify small molecule inhibitors of the T2SS. We tested the reproducibility of our assay using a 6,400-compound library. With small variation within controls and a dynamic range between positive and negative controls, the assay had a z-factor of 0.65, establishing its suitability for HTS. Our screen identified the lipase inhibitors Orlistat and Ebelactone B demonstrating the specificity of the assay. To eliminate inhibitors of lipase activity and lipase expression, two counter assays were developed and optimized. By implementing these assays, all seven tricyclic antidepressants present in the library were found to be inhibitors of the lipase, highlighting the potential of identifying alternative targets for approved pharmaceuticals. Although no T2SS inhibitor was identified among the compounds that reduced lipase activity by ≥30%, our small proof-of-concept pilot study indicates that the HTS regimen is simple, reproducible, and specific and that it can be used to screen larger libraries for the identification of T2SS inhibitors that may be developed into novel A. baumannii therapeutics.

Measuring In Vitro ATPase Activity for Enzymatic Characterization

Adenosine triphosphate-hydrolyzing enzymes, or ATPases, play a critical role in a diverse array of cellular functions. These dynamic proteins can generate energy for mechanical work, such as protein trafficking and degradation, solute transport, and cellular movements. The protocol described here is a basic assay for measuring the in vitro activity of purified ATPases for functional characterization. Proteins hydrolyze ATP in a reaction that results in inorganic phosphate release, and the amount of phosphate liberated is then quantitated using a colorimetric assay. This highly adaptable protocol can be adjusted to measure ATPase activity in kinetic or endpoint assays. A representative protocol is provided here based on the activity and requirements of EpsE, the AAA+ ATPase involved in Type II Secretion in the bacterium Vibrio cholerae. The amount of purified protein needed to measure activity, length of the assay and the timing and number of sampling intervals, buffer and salt composition, temperature, co-factors, stimulants (if any), etc. may vary from those described here, and thus some optimization may be necessary. This protocol provides a basic framework for characterizing ATPases and can be performed quickly and easily adjusted as necessary.

Zinc coordination is essential for the function and activity of the type II secretion ATPase EpsE

The type II secretion system Eps in Vibrio cholerae promotes the extracellular transport of cholera toxin and several hydrolytic enzymes and is a major virulence system in many Gram‐negative pathogens which is structurally related to the type IV pilus system. The cytoplasmic ATPase EpsE provides the energy for exoprotein secretion through ATP hydrolysis. EpsE contains a unique metal‐binding domain that coordinates zinc through a tetracysteine motif (CXXCX₂₉CXXC), which is also present in type IV pilus assembly but not retraction ATPases. Deletion of the entire domain or substitution of any of the cysteine residues that coordinate zinc completely abrogates secretion in an EpsE‐deficient strain and has a dominant negative effect on secretion in the presence of wild‐type EpsE. Consistent with the in vivo data, chemical depletion of zinc from purified EpsE hexamers results in loss of in vitro ATPase activity. In contrast, exchanging the residues between the two dicysteines with those from the homologous ATPase XcpR from Pseudomonas aeruginosa does not have a significant impact on EpsE. These results indicate that, although the individual residues in the metal‐binding domain are generally interchangeable, zinc coordination is essential for the activity and function of EpsE.

Outer Membrane Vesicle-Mediated Export of Processed PrtV Protease from Vibrio cholerae

Background

Outer membrane vesicles (OMVs) are known to release from almost all Gram-negative bacteria during normal growth. OMVs carry different biologically active toxins and enzymes into the surrounding environment. We suggest that OMVs may therefore be able to transport bacterial proteases into the target host cells. We present here an analysis of the Vibrio cholerae OMV-associated protease PrtV.

Methodology/Principal Findings

In this study, we demonstrated that PrtV was secreted from the wild type V. cholerae strain C6706 via the type II secretion system in association with OMVs. By immunoblotting and electron microscopic analysis using immunogold labeling, the association of PrtV with OMVs was examined. We demonstrated that OMV-associated PrtV was biologically active by showing altered morphology and detachment of cells when the human ileocecum carcinoma (HCT8) cells were treated with OMVs from the wild type V. cholerae strain C6706 whereas cells treated with OMVs from the prtV isogenic mutant showed no morphological changes. Furthermore, OMV-associated PrtV protease showed a contribution to bacterial resistance towards the antimicrobial peptide LL-37.

Conclusion/Significance

Our findings suggest that OMVs released from V. cholerae can deliver a processed, biologically active form of PrtV that contributes to bacterial interactions with target host cells.

Functional and structural characterization of Vibrio cholerae extracellular serine protease B, VesB

The chymotrypsin subfamily A of serine proteases consists primarily of eukaryotic proteases, including only a few proteases of bacterial origin. VesB, a newly identified serine protease that is secreted by the type II secretion system in Vibrio cholerae, belongs to this subfamily. VesB is likely produced as a zymogen because sequence alignment with trypsinogen identified a putative cleavage site for activation and a catalytic triad, His-Asp-Ser. Using synthetic peptides, VesB efficiently cleaved a trypsin substrate, but not chymotrypsin and elastase substrates. The reversible serine protease inhibitor, benzamidine, inhibited VesB and served as an immobilized ligand for VesB affinity purification, further indicating its relationship with trypsin-like enzymes. Consistent with this family of serine proteases, N-terminal sequencing implied that the propeptide is removed in the secreted form of VesB. Separate mutagenesis of the activation site and catalytic serine rendered VesB inactive, confirming the importance of these features for activity, but not for secretion. Similar to trypsin but, in contrast to thrombin and other coagulation factors, Na(+) did not stimulate the activity of VesB, despite containing the Tyr(250) signature. The crystal structure of catalytically inactive pro-VesB revealed that the protease domain is structurally similar to trypsinogen. The C-terminal domain of VesB was found to adopt an immunoglobulin (Ig)-fold that is structurally homologous to Ig-folds of other extracellular Vibrio proteins. Possible roles of the Ig-fold domain in stability, substrate specificity, cell surface association, and type II secretion of VesB, the first bacterial multidomain trypsin-like protease with known structure, are discussed.

Hexamers of the type II secretion ATPase GspE from Vibrio cholerae with increased ATPase activity

The type II secretion system (T2SS), a multiprotein machinery spanning two membranes in Gram-negative bacteria, is responsible for the secretion of folded proteins from the periplasm across the outer membrane. The critical multidomain T2SS assembly ATPase GspE(EpsE) had not been structurally characterized as a hexamer. Here, four hexamers of Vibrio cholerae GspE(EpsE) are obtained when fused to Hcp1 as an assistant hexamer, as shown with native mass spectrometry. The enzymatic activity of the GspE(EpsE)-Hcp1 fusions is ∼20 times higher than that of a GspE(EpsE) monomer, indicating that increasing the local concentration of GspE(EpsE) by the fusion strategy was successful. Crystal structures of GspE(EpsE)-Hcp1 fusions with different linker lengths reveal regular and elongated hexamers of GspE(EpsE) with major differences in domain orientation within subunits, and in subunit assembly. SAXS studies on GspE(EpsE)-Hcp1 fusions suggest that even further variability in GspE(EpsE) hexamer architecture is likely.

Fluorescence microscopy and proteomics to investigate subcellular localization, assembly, and function of the type II secretion system

Investigation of secretion systems is often critical to understanding the virulence mechanisms of bacterial pathogens. With estimates as high as 30-40% of proteins secreted or localized to the cell envelope, information about the subcellular localization and organization of secretion complexes and identification and functional characterization of their substrates are key steps toward understanding these intricate systems. Here we describe a protocol using fluorescent live-cell imaging of fusion proteins that can provide a powerful tool to potentially examine the localization, assembly, and role of each component in the secretion complex. In addition, we describe protocols for the identification of secreted substrates using 1D SDS-PAGE coupled with nano-liquid chromatography (LC) and tandem mass spectrometry (MS/MS), and isobaric tagging for absolute quantification (iTRAQ) coupled with two-dimensional LC and MS/MS. Both experimental approaches are applicable to any similar study of membrane transport systems.

Structural and functional studies on the interaction of GspC and GspD in the type II secretion system

Type II secretion systems (T2SSs) are critical for secretion of many proteins from Gram-negative bacteria. In the T2SS, the outer membrane secretin GspD forms a multimeric pore for translocation of secreted proteins. GspD and the inner membrane protein GspC interact with each other via periplasmic domains. Three different crystal structures of the homology region domain of GspC (GspC^HR) in complex with either two or three domains of the N-terminal region of GspD from enterotoxigenic Escherichia coli show that GspC^HR adopts an all-β topology. N-terminal β-strands of GspC and the N0 domain of GspD are major components of the interface between these inner and outer membrane proteins from the T2SS. The biological relevance of the observed GspC–GspD interface is shown by analysis of variant proteins in two-hybrid studies and by the effect of mutations in homologous genes on extracellular secretion and subcellular distribution of GspC in Vibrio cholerae. Substitutions of interface residues of GspD have a dramatic effect on the focal distribution of GspC in V. cholerae. These studies indicate that the GspC^HR–GspD^N0 interactions observed in the crystal structure are essential for T2SS function. Possible implications of our structures for the stoichiometry of the T2SS and exoprotein secretion are discussed.

Many bacterial pathogens affecting humans, animals and plants export diverse proteins across the cell membranes into the medium surrounding the bacteria. Some of these secreted proteins are involved in pathogenesis. One example is cholera toxin secreted by the bacterium Vibrio cholerae, a causative agent of cholera. The sophisticated type II secretion system is responsible for moving this toxin, and several other proteins, across the outer membrane. Here, we studied the interaction between the outer membrane pore of the type II secretion system, the secretin GspD, and the inner membrane protein GspC. We have solved three crystal structures of complexes between the interacting domains and identified critical contacts in the GspC–GspD interface. We also showed the importance of these contacts for assembly of the secretion system and for secretion of proteins by V. cholerae. Our studies provide a major piece in the puzzle of how the type II secretion system is assembled and how it functions. One day this knowledge might allow us to design compounds which interfere with this secretion process. Such compounds would be useful in the battle against bacteria affecting human health.

In vivo cross-linking of EpsG to EpsL suggests a role for EpsL as an ATPase-pseudopilin coupling protein in the Type II secretion system of Vibrio cholerae

The type II secretion system is a multi-protein complex that spans the cell envelope of Gram-negative bacteria and promotes the secretion of proteins, including several virulence factors. This system is homologous to the type IV pilus biogenesis machinery and contains five proteins, EpsG-K, termed the pseudopilins that are structurally homologous to the type IV pilins. The major pseudopilin EpsG has been proposed to form a pilus-like structure in an energy-dependent process that requires the ATPase, EpsE. A key remaining question is how the membrane-bound EpsG interacts with the cytoplasmic ATPase, and if this is a direct or indirect interaction. Previous studies have established an interaction between the bitopic inner membrane protein EpsL and EpsE; therefore, in this study we used in vivo cross-linking to test the hypothesis that EpsG interacts with EpsL. Our findings suggest that EpsL may function as a scaffold to link EpsG and EpsE and thereby transduce the energy generated by ATP hydrolysis to support secretion. The recent discovery of structural homology between EpsL and a protein in the type IV pilus system implies that this interaction may be conserved and represent an important functional interaction for both the type II secretion and type IV pilus systems.

Long helical filaments are not seen encircling cells in electron cryotomograms of rod-shaped bacteria

How rod-shaped bacteria form and maintain their shape is an important question in bacterial cell biology. Results from fluorescent light microscopy have led many to believe that the actin homolog MreB and a number of other proteins form long helical filaments along the inner membrane of the cell. Here we show using electron cryotomography of six different rod-shaped bacterial species, at macromolecular resolution, that no long (> 80 nm) helical filaments exist near or along either surface of the inner membrane. We also use correlated cryo-fluorescent light microscopy (cryo-fLM) and electron cryo-tomography (ECT) to identify cytoplasmic bundles of MreB, showing that MreB filaments are detectable by ECT. In light of these results, the structure and function of MreB must be reconsidered: instead of acting as a large, rigid scaffold that localizes cell-wall synthetic machinery, moving MreB complexes may apply tension to growing peptidoglycan strands to ensure their orderly, linear insertion.

Proteomic analysis of the Vibrio cholerae type II secretome reveals new proteins, including three related serine proteases

The type II secretion (T2S) system is responsible for extracellular secretion of a broad range of proteins, including toxins and degradative enzymes that play important roles in the pathogenesis and life cycle of many gram-negative bacteria. In Vibrio cholerae, the etiological agent of cholera, the T2S machinery transports cholera toxin, which induces profuse watery diarrhea, a hallmark of this life-threatening disease. Besides cholera toxin, four other proteins have been shown to be transported by the T2S machinery, including hemagglutinin protease, chitinase, GbpA, and lipase. Here, for the first time, we have applied proteomic approaches, including isotope tagging for relative and absolute quantification coupled with multidimensional liquid chromatography and tandem mass spectrometry, to perform an unbiased and comprehensive analysis of proteins secreted by the T2S apparatus of the V. cholerae El Tor strain N16961 under standard laboratory growth conditions. This analysis identified 16 new putative T2S substrates, including sialidase, several proteins participating in chitin utilization, two aminopeptidases, TagA-related protein, cytolysin, RbmC, three hypothetical proteins encoded by VCA0583, VCA0738, and VC2298, and three serine proteases VesA, VesB, and VesC. Focusing on the initial characterization of VesA, VesB, and VesC, we have confirmed enzymatic activities and T2S-dependent transport for each of these proteases. In addition, analysis of single, double, and triple protease knock-out strains indicated that VesA is the primary protease responsible for processing the A subunit of cholera toxin during in vitro growth of the V. cholerae strain N16961.

Oligomerization of EpsE coordinates residues from multiple subunits to facilitate ATPase activity

EpsE is an ATPase that powers transport of cholera toxin and hydrolytic enzymes through the Type II secretion (T2S) apparatus in the gram-negative bacterium, Vibrio cholerae. On the basis of structures of homologous Type II/IV secretion ATPases and our biochemical data, we believe that EpsE is active as an oligomer, likely a hexamer, and the binding, hydrolysis, and release of nucleotide cause EpsE to undergo dynamic structural changes, thus converting chemical energy to mechanical work, ultimately resulting in extracellular secretion. The conformational changes that occur as a consequence of nucleotide binding would realign conserved arginines (Arg(210), Arg(225), Arg(320), Arg(324), Arg(336), and Arg(369)) from adjoining domains and subunits to complete the active site around the bound nucleotide. Our data suggest that these arginines are essential for ATP hydrolysis, although their roles in shaping the active site of EpsE are varied. Specifically, we have shown that replacements of these arginine residues abrogate the T2S process due to a reduction of ATPase activity yet do not have any measurable effect on nucleotide binding or oligomerization of EpsE. We have further demonstrated that point mutations in the EpsE intersubunit interface also reduce ATPase activity without disrupting oligomerization, strengthening the idea that residues from multiple subunits must precisely interact in order for EpsE to be sufficiently active to support T2S. Our findings suggest that the action of EpsE is similar to that of other Type II/IV secretion ATPase family members, and thus these results may be widely applicable to the family as a whole.

Calcium is essential for the major pseudopilin in the type 2 secretion system

The pseudopilus is a key feature of the type 2 secretion system (T2SS) and is made up of multiple pseudopilins that are similar in fold to the type 4 pilins. However, pilins have disulfide bridges, whereas the major pseudopilins of T2SS do not. A key question is therefore how the pseudopilins, and in particular, the most abundant major pseudopilin, GspG, obtain sufficient stability to perform their function. Crystal structures of Vibrio cholerae, Vibrio vulnificus, and enterohemorrhagic Escherichia coli (EHEC) GspG were elucidated, and all show a calcium ion bound at the same site. Conservation of the calcium ligands fully supports the suggestion that calcium ion binding by the major pseudopilin is essential for the T2SS. Functional studies of GspG with mutated calcium ion-coordinating ligands were performed to investigate this hypothesis and show that in vivo protease secretion by the T2SS is severely impaired. Taking all evidence together, this allows the conclusion that, in complete contrast to the situation in the type 4 pili system homologs, in the T2SS, the major protein component of the central pseudopilus is dependent on calcium ions for activity.

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.

Compromised outer membrane integrity in Vibrio cholerae Type II secretion mutants

The type II secretion (T2S) system of Vibrio cholerae is a multiprotein complex that spans the cell envelope and secretes proteins important for pathogenesis as well as survival in different environments. Here we report that, in addition to the loss of extracellular secretion, removal or inhibition of expression of the T2S genes, epsC-N, results in growth defects and a broad range of alterations in the outer membrane that interfere with its barrier function. Specifically, the sensitivity to membrane-perturbing agents such as bile salts and the antimicrobial peptide polymyxin B is increased, and periplasmic constituents leak out into the culture medium. As a consequence, the sigma(E) stress response is induced. Furthermore, due to the defects caused by inactivation of the T2S system, the Deltaeps deletion mutant of V. cholerae strain N16961 is incapable of surviving the passage through the infant mouse gastrointestinal tract. The growth defect and leaky outer membrane phenotypes are suppressed when the culture medium is supplemented with 5% glucose or sucrose, although the eps mutants remain sensitive to membrane-damaging agents. This suggests that the sugars do not restore the integrity of the outer membrane in the eps mutant strains per se but may provide osmoprotective functions.

Identification of a novel targeting sequence for regulated secretion in the serine protease inhibitor neuroserpin

Ns (neuroserpin) is a member of the serpin (serine protease inhibitor) gene family that is primarily expressed within the central nervous system. Its principal target protease is tPA (tissue plasminogen activator), which is thought to contribute to synaptic plasticity and to be secreted in a stimulus-dependent manner. In the present study, we demonstrate in primary neuronal cultures that Ns co-localizes in LDCVs (large dense core vesicles) with the regulated secretory protein chromogranin B. We also show that Ns secretion is regulated and can be specifically induced 4-fold by secretagogue treatment. A novel 13-amino-acid sorting signal located at the C-terminus of Ns is identified that is both necessary and sufficient to target Ns to the regulated secretion pathway. Its deletion renders Ns no longer responsive to secretagogue stimulation, whereas PAI-Ns [Ns (neuroserpin)-PAI-1 (plasminogen activator inhibitor-1) chimaera appending the last 13 residues of Ns sequence to the C-terminus of PAI-1] shifts PAI-1 secretion into a regulated secretory pathway.

Synergistic stimulation of EpsE ATP hydrolysis by EpsL and acidic phospholipids

EpsE is a cytoplasmic component of the type II secretion system in Vibrio cholerae. Through ATP hydrolysis and an interaction with the cytoplasmic membrane protein EpsL, EpsE supports secretion of cholera toxin across the outer membrane. In this study, we have determined the effect of the cytoplasmic domain of EpsL (cyto-EpsL) and purified phospholipids on the ATPase activity of EpsE. Acidic phospholipids, specifically cardiolipin, bound the copurified EpsE/cyto-EpsL complex and stimulated its ATPase activity 30-130-fold, whereas the activity of EpsE alone was unaffected. Removal of the last 11 residues (residues 243-253) from cyto-EpsL prevented cardiolipin binding as well as stimulation of the ATPase activity of EpsE. Further mutagenesis of the C-terminal region of the EpsL cytoplasmic domain adjacent to the predicted transmembrane helix suggested that this region participates in fine tuning the interaction of EpsE with the cytoplasmic membrane and influences the oligomerization state of EpsE thereby stimulating its ATPase activity and promoting extracellular secretion in V. cholerae.

Type II secretion: from structure to function

Gram-negative bacteria use the type II secretion system to transport a large number of secreted proteins from the periplasmic space into the extracellular environment. Many of the secreted proteins are major virulence factors in plants and animals. The components of the type II secretion system are located in both the inner and outer membranes where they assemble into a multi-protein, cell-envelope spanning, complex. This review discusses recent progress, particularly newly published structures obtained by X-ray crystallography and electron microscopy that have increased our understanding of how the type II secretion apparatus functions and the role that individual proteins play in this complex system.

The X-ray structure of the type II secretion system complex formed by the N-terminal domain of EpsE and the cytoplasmic domain of EpsL of Vibrio cholerae

Gram-negative bacteria use type II secretion systems for the transport of virulence factors and hydrolytic enzymes through the outer membrane. These sophisticated multi-protein complexes reach from the pore in the outer membrane via the pseudopilins in the periplasm and a multi-protein inner-membrane sub-complex, to an ATPase in the cytoplasm. The human pathogen Vibrio cholerae uses such a secretion machinery, called the Eps-system, for the export of its major virulence factor cholera toxin into the intestinal tract of the human host. Here, we describe the 2.4 A structure of the hetero-tetrameric complex of the N-terminal domain of the ATPase EpsE and the cytoplasmic domain of the inner membrane protein EpsL, which constitute the major cytoplasmic components of the Eps-system. A stable fragment of EpsE in complex with the cytoplasmic domain of EpsL was identified via limited proteolysis and facilitated the crystallization of the complex. This first structure of a complex between two different proteins of the type II secretion system reveals that the N-terminal domain of EpsE and the cytoplasmic domain of EpsL form a hetero-tetramer, in which EpsL is the central dimer and EpsE binds on the periphery. The dimer of EpsL in this complex is very similar to the dimer seen in the crystal structure of the native cytoplasmic domain of EpsL, suggesting a possible physiological relevance despite a relatively small 675 A2 buried solvent accessible surface. The N-terminal domain of EpsE, which forms a compact domain with an alpha+beta-fold, places its helix alpha2 in a mostly hydrophobic cleft between domains II and III of EpsL burying 1700 A2 solvent accessible surface. This extensive interface involves several residues whose hydrophobic or charged nature is well conserved and is therefore likely to be of general importance in type II secretion systems.

The structure of the cytoplasmic domain of EpsL, an inner membrane component of the type II secretion system of Vibrio cholerae: an unusual member of the actin-like ATPase superfamily

The type II secretion system (T2SS) is used by several Gram-negative bacteria for the secretion of hydrolytic enzymes and virulence factors across the outer membrane. In these secretion systems, a complex of 12-15 so-called "Gsp proteins" spans from a regulatory ATPase in the cytoplasm, via several signal or energy transducing proteins in the inner membrane and the pseudopilins in the periplasm, to the actual pore in the outer membrane. The human pathogen Vibrio cholerae employs such an assembly, called the Eps system, for the export of its major virulence factor, cholera toxin, from its periplasm into the lumen of the gastro-intestinal tract of the host. Here, we report the atomic structure of the major cytoplasmic domain of the inner membrane-spanning EpsL protein from V. cholerae. EpsL is the binding partner of the regulatory ATPase EpsE as well as of EpsM and pseudopilins, and is therefore a critical link between the cytoplasmic and the periplasmic part of the Eps-system. The 2.7A resolution structure was determined by a combination of Se-Met multiple anomalous dispersion (MAD) and multiple isomorphous replacement with anomalous scattering (MIRAS) phasing methods. The 28kDa cytoplasmic domain of EpsL (cyto-EpsL) consists of three beta-sheet-rich domains. With domains I and III similar to the RNaseH-fold, cyto-EpsL unexpectedly shows structural homology with the superfamily of actin-like ATPases. cyto-EpsL, however, is an unusual member of this superfamily as it misses the canonical actin domains 1B and 2B, which are common yet variable in this superfamily. Moreover, cyto-EpsL has an additional domain II, which has the topology of an SHS2-fold module. Within the superfamily this fold module has been observed only for domain 1C of the cell division protein FtsA, in which it mediates protein-protein interactions. This domain II displays great flexibility and contributes to a pronounced negatively charged canyon on the surface of cyto-EpsL. Functional data as well as structural homology and sequence conservation suggest that domain II interacts with EpsE, the major cytoplasmic binding partner of EpsL.

Molecular analysis of the Vibrio cholerae type II secretion ATPase EpsE

The type II secretion system is a macromolecular assembly that facilitates the extracellular translocation of folded proteins in gram-negative bacteria. EpsE, a member of this secretion system in Vibrio cholerae, contains a nucleotide-binding motif composed of Walker A and B boxes that are thought to participate in binding and hydrolysis of ATP and displays structural homology to other transport ATPases. Here we demonstrate that purified EpsE is an Mg2+-dependent ATPase and define optimal conditions for the hydrolysis reaction. EpsE displays concentration-dependent activity, which may suggest that the active form is oligomeric. Size exclusion chromatography showed that the majority of purified EpsE is monomeric; however, detailed analyses of specific activities obtained following gel filtration revealed the presence of a small population of active oligomers. We further report that EpsE binds zinc through a tetracysteine motif near its carboxyl terminus, yet metal displacement assays suggest that zinc is not required for catalysis. Previous studies describing interactions between EpsE and other components of the type II secretion pathway together with these data further support the hypothesis that EpsE functions to couple energy to the type II apparatus, thus enabling secretion.

Crystal structure of the extracellular protein secretion NTPase EpsE of Vibrio cholerae

Type II secretion systems consist of an assembly of 12-15 Gsp proteins responsible for transporting a variety of virulence factors across the outer membrane in several pathogenic bacteria. In Vibrio cholerae, the major virulence factor cholera toxin is secreted by the Eps Type II secretion apparatus consisting of 14 Eps proteins. One of these, EpsE, is a cytoplasmic putative NTPase essential for the functioning of the Eps system and member of the GspE subfamily of Type II secretion ATPases. The crystal structure of a truncated form of EpsE in nucleotide-liganded and unliganded state has been determined, and reveals a two-domain architecture with the four characteristic sequence "boxes" of the GspE subfamily clustering around the nucleotide-binding site of the C-domain. This domain contains two C-terminal subdomains not reported before in this superfamily of NTPases. One of these subdomains contains a four-cysteine motif that appears to be involved in metal binding as revealed by anomalous difference density. The EpsE subunits form a right-handed helical arrangement in the crystal with extensive and conserved contacts between the C and N domains of neighboring subunits. Combining the most conserved interface with the quaternary structure of the C domain in a distant homolog, a hexameric model for EpsE is proposed which may reflect the assembly of this critical protein in the Type II secretion system. The nucleotide ligand contacts both domains in this model. The N2-domain-containing surface of the hexamer appears to be highly conserved in the GspE family and most likely faces the inner membrane interacting with other members of the Eps system.

Tissue-type plasminogen activator induces opening of the blood-brain barrier via the LDL receptor-related protein

The regulation of cerebrovascular permeability is critical for normal brain homeostasis, and the "breakdown" of the blood-brain barrier (BBB) is associated with the development of vasogenic edema and intracranial hypertension in a number of neurological disorders. In this study we demonstrate that an increase in endogenous tissue-type plasminogen activator (tPA) activity in the perivascular tissue following cerebral ischemia induces opening of the BBB via a mechanism that is independent of both plasminogen (Plg) and MMP-9. We also show that injection of tPA into the cerebrospinal fluid in the absence of ischemia results in a rapid dose-dependent increase in vascular permeability. This activity is not seen with urokinase-type Plg activator (uPA) but is induced in Plg-/- mice, confirming that the effect is Plg-independent. However, the activity is blocked by antibodies to the LDL receptor-related protein (LRP) and by the LRP antagonist, receptor-associated protein (RAP), suggesting a receptor-mediated process. Together these studies demonstrate that tPA is both necessary and sufficient to directly increase vascular permeability in the early stages of BBB opening, and suggest that this occurs through a receptor-mediated cell signaling event and not through generalized degradation of the vascular basement membrane.

Regulation of seizure spreading by neuroserpin and tissue-type plasminogen activator is plasminogen-independent

Tissue-type plasminogen activator (tPA) is a highly specific serine proteinase expressed in the CNS during events that require neuronal plasticity. In this study we demonstrate that endogenous tPA mediates the progression of kainic acid-induced (KA-induced) seizures by promoting the synchronization of neuronal activity required for seizure spreading, and that, unlike KA-induced cell death, this activity is plasminogen-independent. Specifically, seizure induction by KA injection into the amygdala induces tPA activity and cell death in both hippocampi, and unilateral treatment of rats with neuroserpin, a natural inhibitor of tPA in the brain, enhances neuronal survival in both hippocampi. Inhibition of tPA within the hippocampus by neuroserpin treatment does not prevent seizure onset but instead markedly delays the progression of seizure activity in both rats and wild-type mice. In tPA-deficient mice, seizure progression is significantly delayed, and neuroserpin treatment does not further delay seizure spreading. In contrast, plasminogen-deficient mice show a pattern of seizure spreading and a response to neuroserpin that is similar to that of wild-type animals. These findings indicate that tPA acts on a substrate other than plasminogen and that the effects of neuroserpin on seizure progression and neuronal cell survival are mediated through the inhibition of tPA.

Directed polar secretion of protease from single cells of Vibrio cholerae via the type II secretion pathway

Bacteria have long been thought of as little more than sacks of homogeneously distributed enzymes. However, recent cytological studies indicate that bacteria are compartmentalized with proteins involved in processes such as cell division, motility, chemotaxis, and development located at distinct sites. We have used the green fluorescent protein as a reporter to determine the cellular distribution of the extracellular protein secretion (eps)-encoded type II secretion complex responsible for extracellular secretion of cholera toxin and hemagglutinin/protease in Vibrio cholerae. Real-time monitoring of green fluorescent protein fused to EpsM in living cells indicated that, like the single polar flagellum, the Eps complex is located at the old pole after cell division. Eps-dependent protease secretion was also visualized in single cells by fluorescence microscopy by using intramolecularly quenched casein. This analysis demonstrated that active protease secretion is focused at the poles and colocalizes with the site of the polar Eps apparatus. These results suggest that the type II secretion complex is responsible for directed delivery of virulence factors during cholera pathogenesis.

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.

Characterization of the multimeric Eps complex required for cholera toxin secretion

Vibrio cholerae causes diarrheal disease through colonization of the small intestine. A critical aspect of V. cholerae pathogenesis is its ability to actively secrete cholera toxin to the extracellular environment. This occurs via the type II secretion pathway, where the toxin subunits are first transported to the periplasm through the Sec pathway. Following folding and assembly the toxin is then translocated across the outer membrane by a specialized Extracellular Protein Secretion (Eps) machinery encoded by at least 13 genes. Although the Eps proteins are believed to form a secretion apparatus that spans both membranes, cholera toxin is thought to engage this complex first in the periplasm. In order to determine the organization of the Eps apparatus and to understand the mechanism of secretion, the Eps apparatus has been dissected and three of the components, EpsE, EpsL and EpsM, have been purified and characterized. They were shown to form a stable, multiprotein complex spanning the cytoplasmic membrane.

Neuroserpin reduces cerebral infarct volume and protects neurons from ischemia-induced apoptosis

Neuroserpin, a recently identified inhibitor of tissue-type plasminogen activator (tPA), is primarily localized to neurons within the central nervous system, where it is thought to regulate tPA activity. In the present study neuroserpin expression and its potential therapeutic benefits were examined in a rat model of stroke. Neuroserpin expression increased in neurons surrounding the ischemic core (ischemic penumbra) within 6 hours of occlusion of the middle cerebral artery and remained elevated during the first week after the ischemic insult. Injection of neuroserpin directly into the brain immediately after infarct reduced stroke volume by 64% at 72 hours compared with control animals. In untreated animals both tPA and urokinase-type plasminogen activator (uPA) activity was significantly increased within the region of infarct by 6 hours after reperfusion. Activity of tPA then decreased to control levels by 72 hours, whereas uPA activity continued to rise and was dramatically increased by 72 hours. Both tPA and uPA activity were significantly reduced in neuroserpin-treated animals. Immunohistochemical staining of basement membrane laminin with a monoclonal antibody directed toward a cryptic epitope suggested that proteolysis of the basement membrane occurred as early as 10 minutes after reperfusion and that intracerebral administration of neuroserpin significantly reduced this proteolysis. Neuroserpin also decreased apoptotic cell counts in the ischemic penumbra by more than 50%. Thus, neuroserpin may be a naturally occurring neuroprotective proteinase inhibitor, whose therapeutic administration decreases stroke volume most likely by inhibiting proteinase activity and subsequent apoptosis associated with focal cerebral ischemia/reperfusion. (Blood. 2000;96:569-576)

Convergence of the secretory pathways for cholera toxin and the filamentous phage, CTXphi

Virulence of Vibrio cholerae depends on secretion of cholera toxin (CT), which is encoded within the genome of a filamentous phage, CTXphi. Release of CT is mediated by the extracellular protein secretion (eps) type II secretion system. Here, the outer membrane component of this system, EpsD, was shown to be required for secretion of the phage as well. Thus, EpsD plays a role both in pathogenicity and in horizontal transfer of a key virulence gene. Genomic analysis suggests that additional filamentous phages also exploit chromosome-encoded outer membrane channels.

Two regions of EpsL involved in species-specific protein-protein interactions with EpsE and EpsM of the general secretion pathway in Vibrio cholerae

Extracellular secretion of proteins via the type II or general secretion pathway in gram-negative bacteria requires the assistance of at least 12 gene products that are thought to form a complex apparatus through which secreted proteins are translocated. Although this apparatus is specifically required only for the outer membrane translocation step during transport across the bacterial cell envelope, it is believed to span both membranes. The EpsE, EpsL, and EpsM proteins of the type II apparatus in Vibrio cholerae are thought to form a trimolecular complex that is required to either control the opening and closing of the secretion pore or to transduce energy to the site of outer membrane translocation. EpsL is likely to play an important role in this relay by interacting with both the cytoplasmic EpsE protein and the cytoplasmic membrane protein EpsM, which is predominantly exposed on the periplasmic side of the membrane. We have now extended this model and mapped the separate regions within EpsL that contain the EpsE and EpsM binding domains. By taking advantage of the species specificity of the type II pathway, we have used chimeric proteins composed of EpsL and its homologue, ExeL, from Aeromonas hydrophila together with either EpsE or its Aeromonas homologue, ExeE, to complement the secretion defect in both epsL and exeL mutant strains. These studies have mapped the species-specific EpsE binding site to the N-terminal cytoplasmic region between residues 57 and 216 of EpsL. In addition, the species-specific EpsM binding site was mapped to the C-terminal half of EpsL by coimmunoprecipitation of EpsM with different EpsL-ExeL chimeras. This site is present in the region between amino acids 216 and 296, which contains the predicted membrane-spanning segment of EpsL.

Direct interaction of the EpsL and EpsM proteins of the general secretion apparatus in Vibrio cholerae

The general secretion pathway of gram-negative bacteria is responsible for extracellular secretion of a number of different proteins, including proteases and toxins. This pathway supports secretion of proteins across the cell envelope in two distinct steps, in which the second step, involving translocation through the outer membrane, is assisted by at least 13 different gene products. Two of these components, the cytoplasmic membrane proteins EpsL and EpsM of Vibrio cholerae, have been purified and characterized. Based on gel filtration analysis, both purified EpsM(His)6 and wild-type EpsL present in an Escherichia coli Triton X-100 extract are dimeric proteins. EpsL and EpsM were also found to interact directly and form a Triton X-100 stable complex that could be precipitated with either anti-EpsL or anti-EpsM antibodies. In addition, when the L and M proteins were coexpressed in E. coli, they formed a stable complex and protected each other from proteolytic degradation, indicating that these two proteins interact in vivo and that no other Eps protein is required for their association. Since EpsL is predicted to contain a large cytoplasmic domain, while EpsM is predominantly exposed on the periplasmic side, we speculate that these components might be part of a structure that is involved in bridging the inner and outer membranes. Furthermore, since EpsL has previously been shown to interact with the autophosphorylating cytoplasmic membrane protein EpsE, we hypothesize that this trimolecular complex might be involved in regulating the opening and closing of the secretion pore and/or transducing energy to the site of outer membrane translocation.

Neuroserpin, a brain-associated inhibitor of tissue plasminogen activator is localized primarily in neurons. Implications for the regulation of motor learning and neuronal survival

A cDNA clone for the serine proteinase inhibitor (serpin), neuroserpin, was isolated from a human whole brain cDNA library, and recombinant protein was expressed in insect cells. The purified protein is an efficient inhibitor of tissue type plasminogen activator (tPA), having an apparent second-order rate constant of 6. 2 x 10(5) M-1 s-1 for the two-chain form. However, unlike other known plasminogen activator inhibitors, neuroserpin is a more effective inactivator of tPA than of urokinase-type plasminogen activator. Neuroserpin also effectively inhibited trypsin and nerve growth factor-gamma but reacted only slowly with plasmin and thrombin. Northern blot analysis showed a 1.8 kilobase messenger RNA expressed predominantly in adult human brain and spinal cord, and immunohistochemical studies of normal mouse tissue detected strong staining primarily in neuronal cells with occasionally positive microglial cells. Staining was most prominent in the ependymal cells of the choroid plexus, Purkinje cells of the cerebellum, select neurons of the hypothalamus and hippocampus, and in the myelinated axons of the commissura. Expression of tPA within these regions is reported to be high and has previously been correlated with both motor learning and neuronal survival. Taken together, these data suggest that neuroserpin is likely to be a critical regulator of tPA activity in the central nervous system, and as such may play an important role in neuronal plasticity and/or maintenance.

General secretion pathway (eps) genes required for toxin secretion and outer membrane biogenesis in Vibrio cholerae

The general secretion pathway (GSP) of Vibrio cholerae is required for secretion of proteins including chitinase, enterotoxin, and protease through the outer membrane. In this study, we report the cloning and sequencing of a DNA fragment from V. cholerae, containing 12 open reading frames, epsC to -N, which are similar to GSP genes of Aeromonas, Erwinia, Klebsiella, Pseudomonas, and Xanthomonas spp. In addition to the two previously described genes, epsE and epsM (M. Sandkvist, V. Morales, and M. Bagdasarian, Gene 123: 81-86, 1993; L. J. Overbye, M. Sandkvist, and M. Bagdasarian, Gene 132:101-106, 1993), it is shown here that epsC, epsF, epsG, and epsL also encode proteins essential for GSP function. Mutations in the eps genes result in aberrant outer membrane protein profiles, which indicates that the GSP, or at least some of its components, is required not only for secretion of soluble proteins but also for proper outer membrane assembly. Several of the Eps proteins have been identified by use of the T7 polymerase-promoter system in Escherichia coli. One of them, a pilin-like protein, EpsG, was analyzed also in V. cholerae and found to migrate as two bands on polyacrylamide gels, suggesting that in this organism it might be processed or otherwise modified by a prepilin peptidase. We believe that TcpJ prepilin peptidase, which processes the subunit of the toxin-coregulated pilus, TcpA, is not involved in this event. This is supported by the observations that apparent processing of EpsG occurs in a tcpJ mutant of V. cholerae and that, when coexpressed in E. coli, TcpJ cannot process EpsG although the PilD peptidase from Neisseria gonorrhoeae can.

Secretion of recombinant proteins by Gram-negative bacteria

During the past few years, significant progress has been made towards our understanding of the molecular mechanisms governing the translocation of proteins through bacterial cell membranes. Successful attempts in promoting the secretion of recombinant proteins by employing this knowledge and by empirical efforts have been registered. However, a further in-depth understanding of membrane-translocation mechanisms is required before predictable manipulations of secretion systems can be made to secrete native recombinant proteins that are not naturally targeted to the extracellular compartment.

Interaction between the autokinase EpsE and EpsL in the cytoplasmic membrane is required for extracellular secretion in Vibrio cholerae

Vibrio cholerae secretes a number of proteins important for virulence, including cholera toxin. This process requires the products of the eps genes which have homologues in genera such as Aeromonas, Klebsiella and Pseudomonas and are thought to form a membrane-associated multiprotein complex. Here we show that the putative nucleotide-binding protein EpsE is associated with and stabilized by the cytoplasmic membrane via interaction with EpsL. Analysis of fusion proteins between EpsE and the homologous ExeE from Aeromonas hydrophila demonstrates that the N-terminus of EpsE contains the EpsL binding domain and determines species specificity. An intact Walker A box, commonly found in ATP-binding proteins, is required for activity of EpsE in vivo and for autophosphorylation of purified EpsE in vitro. These results indicate that both the kinase activity of EpsE as well as its ability to interact with the putative cytoplasmic membrane protein EpsL are required for translocation of toxin across the outer membrane in Vibrio cholerae.

Specificity of the protein secretory apparatus: secretion of the heat-labile enterotoxin B subunit pentamers by different species of gram- bacteria

The B-subunit pentamer(s) (EtxBp) of Escherichia coli heat-labile enterotoxin (LT) are secreted from Vibrio cholerae via the general secretion pathway (GSP), but remain periplasmic in E. coli. In order to determine if other Gram- bacteria were also able to secrete the ExtBp, the etxB gene, which encodes EtxB was introduced into different bacteria. Of the bacteria examined, most species of Vibrio and Aeromonas were able to secrete this protein through the outer membrane; other Gram- genera, including Erwinia, Klebsiella and Xanthomonas were not, even though they encode GSP genes homologous to those of V. cholerae. Thus, the ability to recognize the EtxBp as a secretable protein is confined to bacteria that were identified as being closely related to V. cholerae by examination of their 5S rRNA [MacDonell and Colwell, Syst. Appl. Microbiol. 6 (1985) 171-182].

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

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

Genes required for extracellular secretion of enterotoxin are clustered in Vibrio cholerae

Pleiotropic transposon insertion mutants of Vibrio cholerae that are unable to secrete enterotoxin, HA/protease and chitinase through the outer membrane have been isolated. The gene, epsM, responsible for complementation of two of the Tn5 insertion mutations was sequenced. It encodes a putative cytoplasmic membrane protein of 18.5 kDa that exhibits similarity to proteins required for extracellular secretion of pullulanase, pectate lyase or elastase in other Gram-bacteria. It is present on a 15-kb DNA fragment from the V. cholerae genome, containing the epsE gene that was previously shown to be required for secretion of cholera toxin [Sandkvist et al., Gene 123 (1993) 81-86]. Partial reading frames flanking epsM also demonstrated similarity to genes required for extracellular secretion of pullulanase in Klebsiella oxytoca.

A protein required for secretion of cholera toxin through the outer membrane of Vibrio cholerae

A gene essential for the secretion of cholera toxin from the periplasm of Vibrio cholerae into the extracellular medium has been isolated and its nucleotide sequence determined. It encodes a cytoplasmic protein of 56 kDa that exhibits a high degree of similarity to gene products required for extracellular protein secretion in several other Gram- organisms. Sequence similarities in its potential ATP-binding site suggest that the protein may act as an energy provider or signal transducer in the process of extracellular secretion.

Intermolecular interactions between the A and B subunits of heat-labile enterotoxin from Escherichia coli promote holotoxin assembly and stability in vivo

Cholera toxin and the related heat-labile enterotoxin (LT) produced by Escherichia coli consist of a holotoxin of one A subunit and five B subunits (AB5). Here we investigate the domains of the A subunit (EtxA) of E. coli LT which influence the events of B-subunit (EtxB) oligomerization and the formation of a stable AB5 holotoxin complex. We show that the C-terminal 14 amino acids of the A subunit comprise two functional domains that differentially affect oligomerization and holotoxin stability. Deletion of the last 14 amino acids (-14) from the A subunit resulted in a molecule that was significantly impaired in its capacity to promote the assembly of a mutant B subunit, EtxB191.5. In contrast, deletion of the last four amino acids (-4) from the A subunit gave a molecule that retained such a capacity. This suggests that C-terminal residues within the -14 to -4 region of the A subunit are important for promoting the oligomerization of EtxB. In addition, we demonstrate that the truncated A subunit lacking the last 4 amino acids was unable to form a stable AB5 holotoxin complex even though it promoted B-subunit oligomerization. This suggests that the last 4 residues of the A subunit function as an "anchoring" sequence responsible for maintaining the stability of A/B subunit interaction during holotoxin assembly. These data represent an important example of how intermolecular interactions between polypeptides in vivo can modulate the folding and assembly of a macromolecular complex.

Minimal deletion of amino acids from the carboxyl terminus of the B subunit of heat-labile enterotoxin causes defects in its assembly and release from the cytoplasmic membrane of Escherichia coli

Minimal alterations at the carboxyl terminus of the B subunit (EtxB) of heat-labile enterotoxin from Escherichia coli were found to have a marked effect on the assembly and release of this polypeptide into the periplasm. Nine mutant EtxB polypeptides were obtained by genetic manipulation of the 3'-end of the etxB gene using Bal31 nuclease digestion and codon substitution. A correlation was observed between the magnitude of the changes introduced at the carboxyl terminus and the extent to which the mutant polypeptides were defective in assembly and release. Some of the mutant B subunits, exemplified by those in which the last 2 amino acids had been deleted or in which the last 4 residues had been replaced by three different ones, were found to be only partially defective, with a proportion being associated with the periplasmic face of the cytoplasmic membrane and the remainder being exported to the periplasm. The portion associated with membranes was detected as monomers on sodium dodecyl sulfate-polyacrylamide gels, whereas the portion exported to the periplasm were detected as assembled oligomers. We conclude that the last few amino acids at the carboxyl terminus of EtxB exert a profound influence on the assembly and release of the B subunit from the cytoplasmic membrane during export in E. coli.

Alterations at the carboxyl terminus change assembly and secretion properties of the B subunit of Escherichia coli heat-labile enterotoxin

The gene encoding the B subunit of heat-labile enterotoxin (etxB) was mutated at its 3' end by targeted addition of random nucleotide sequences. Gene products from five mutated etxB genes, all of which were shown to encode B subunits with short carboxy-terminal amino acid extensions, were analyzed with respect to a range of functional and structural properties. One class of altered B subunits, exemplified by EtxB124 and EtxB138, which both have seven extra amino acid residues, were found to be specifically defective in their ability to stably associate with A subunits and form holotoxin. Other altered B subunits were less subtlely affected by extensions at their C termini and were, in addition to their failure to associate with A subunits, unable to translocate into the periplasm of Escherichia coli, to pentamerize, or to bind to GM1 ganglioside. This suggests that the carboxy-terminal domain of EtxB mediates A subunit-B subunit interaction.