Docking and assembly of the type II secretion complex of Vibrio cholerae

Modularization of the type II secretion gene cluster from Xanthomonas euvesicatoria facilitates the identification of a structurally conserved XpsCLM assembly platform complex

Many bacterial pathogens depend on a type II secretion (T2S) system to secrete virulence factors from the periplasm into the extracellular milieu. T2S systems consist of an outer membrane secretin channel, a periplasmic pseudopilus and an inner membrane-associated assembly platform including a cytoplasmic ATPase. The components of T2S systems are often conserved in different bacterial species, however, the architecture of the assembly platform is largely unknown. Here, we analysed predicted assembly platform components of the Xps-T2S system from the plant-pathogenic bacterium Xanthomonas euvesicatoria. To facilitate these studies, we generated a modular xps-T2S gene cluster by Golden Gate assembly of single promoter and gene fragments. The modular design allowed the efficient deletion and replacement of T2S genes and the insertion of reporter fusions. Mutant approaches as well as interaction and crosslinking studies showed that the predicted assembly platform components XpsC, XpsL and XpsM form a trimeric complex which is essential for T2S and associates with the cytoplasmic ATPase XpsE and the secretin XpsD. Structural modeling revealed a similar trimeric architecture of XpsCLM homologs from Pseudomonas, Vibrio and Klebsiella species, despite overall low amino acid sequence similarities. In X. euvesicatoria, crosslinking and fluorescence microscopy studies showed that the formation of the XpsCLM complex is independent of the secretin and vice versa, suggesting that the assembly of the T2S system is a dynamic process which involves the association of preformed subcomplexes.

Defining Assembly Pathways by Fluorescence Microscopy

Bacterial secretion systems are among the largest protein complexes in prokaryotes and display remarkably complex architectures. Their assembly often follows clearly defined pathways. Deciphering these pathways not only reveals how bacteria accomplish to build these large functional complexes but can provide crucial information on the interactions and subcomplexes within secretion systems, their distribution within the bacterium, and even functional insights. Fluorescence microscopy provides a powerful tool for biological imaging, which presents an interesting method to accurately define the biogenesis of macromolecular complexes using fluorescently labeled components. Here, I describe the use of this method to decipher the assembly pathway of bacterial secretion systems.

The mechanism for polar localization of the type IVa pilus machine in Myxococcus xanthus

IMPORTANCE

Type IVa pili (T4aP) are widespread bacterial cell surface structures with important functions in motility, surface adhesion, biofilm formation, and virulence. Different bacteria have adapted different piliation patterns. To address how these patterns are established, we focused on the bipolar localization of the T4aP machine in the model organism Myxococcus xanthus by studying the localization of the PilQ secretin, the first component of this machine that assembles at the poles. Based on experiments using a combination of fluorescence microscopy, biochemistry, and computational structural analysis, we propose that PilQ, and specifically its AMIN domains, binds septal and polar peptidoglycan, thereby enabling polar Tgl localization, which then stimulates PilQ multimerization in the outer membrane. We also propose that the presence and absence of AMIN domains in T4aP secretins contribute to the different piliation patterns across bacteria.

High-efficiency transformation of Streptococcus thermophilus using electroporation

BACKGROUND

Streptococcus thermophilus, one of the most important lactic acid bacteria, is widely used in food fermentation, which is beneficial to improve food quality. However, the current genetic transformation systems are inefficient for S. thermophilus S-3, which hinders its further study.

RESULTS

We developed three electroporation transformation methods for S. thermophilus S-3, and optimized various parameters to enhance the transformation efficiency up to 1.3 × 10⁶  CFU/μg DNA, which was 32-fold higher than that of unoptimized. Additionally, transcriptional analysis showed that a series of competence genes in S. thermophilus S-3 were remarkedly up-regulated after optimization, indicating that improvement of transformation efficiency was attributed to the expression level of competence genes. Furthermore, to prove their potential, expression of competence genes (comEA, cbpD and comX) were employed to increase transformation efficiency. The maximum transformation efficiency was obtained by overexpression of comEA, which was 14-fold higher than that of control.

CONCLUSION

This is the first report of competence gene expression for enhancing transformability in S. thermophilus, which exerts a positive effect on the development of desirable characteristics strains. © 2021 Society of Chemical Industry.

Suppressor Mutations in Type II Secretion Mutants of Vibrio cholerae: Inactivation of the VesC Protease

Genome-wide transposon mutagenesis has identified the genes encoding the T2SS in Vibrio cholerae as essential for viability, but the reason for this is unclear. Mutants with deletions or insertions in these genes can be isolated, suggesting that they have acquired secondary mutations that suppress their growth defect.

The type II secretion system (T2SS) is a conserved transport pathway responsible for the secretion of a range of virulence factors by many pathogens, including Vibrio cholerae. Disruption of the T2SS genes in V. cholerae results in loss of secretion, changes in cell envelope function, and growth defects. While T2SS mutants are viable, high-throughput genomic analyses have listed these genes among essential genes. To investigate whether secondary mutations arise as a consequence of T2SS inactivation, we sequenced the genomes of six V. cholerae T2SS mutants with deletions or insertions in either the epsG, epsL, or epsM genes and identified secondary mutations in all mutants. Two of the six T2SS mutants contain distinct mutations in the gene encoding the T2SS-secreted protease VesC. Other mutations were found in genes coding for V. cholerae cell envelope proteins. Subsequent sequence analysis of the vesC gene in 92 additional T2SS mutant isolates identified another 19 unique mutations including insertions or deletions, sequence duplications, and single-nucleotide changes resulting in amino acid substitutions in the VesC protein. Analysis of VesC variants and the X-ray crystallographic structure of wild-type VesC suggested that all mutations lead to loss of VesC production and/or function. One possible mechanism by which V. cholerae T2SS mutagenesis can be tolerated is through selection of vesC-inactivating mutations, which may, in part, suppress cell envelope damage, establishing permissive conditions for the disruption of the T2SS. Other mutations may have been acquired in genes encoding essential cell envelope proteins to prevent proteolysis by VesC.

IMPORTANCE Genome-wide transposon mutagenesis has identified the genes encoding the T2SS in Vibrio cholerae as essential for viability, but the reason for this is unclear. Mutants with deletions or insertions in these genes can be isolated, suggesting that they have acquired secondary mutations that suppress their growth defect. Through whole-genome sequencing and phenotypic analysis of T2SS mutants, we show that one means by which the growth defect can be suppressed is through mutations in the gene encoding the T2SS substrate VesC. VesC homologues are present in other Vibrio species and close relatives, and this may be why inactivation of the T2SS in species such as Vibrio vulnificus, Vibrio sp. strain 60, and Aeromonas hydrophila also results in a pleiotropic effect on their outer membrane assembly and integrity.

Environmental and cellular factors affecting the localization of T6SS proteins in Burkholderia thailandensis

The type VI secretion system (T6SS) injects effector proteins into neighboring bacteria and host cells. Effector translocation is driven by contraction of a tubular sheath in the cytoplasm that expels an inner needle across the cell envelope. The AAA + ATPase ClpV disassembles and recycles the contracted sheath. While ClpV-1-GFP of the Burkholderia T6SS-1, which targets prokaryotic cells, assembles into randomly localized foci, ClpV-5-GFP of the virulence-associated T6SS-5 displays a polar distribution. The mechanisms underlying the localization of T6SSs to a particular site in the bacterial cell are currently unknown. We recently showed that ClpV-5-GFP retains its polar localization in the absence of all T6SS-5 components during infection of host cells. Herein, we set out to identify factors involved in the distribution of ClpV-5 and ClpV-1 in Burkholderia thailandensis. We show that focal assembly and polar localization of ClpV-5-GFP is not dependent on the intracellular host cell environment, known to contain the signal to induce T6SS-5 gene expression. In contrast to ClpV-5-GFP, localization of ClpV-1-GFP was dependent on the cognate T6SS. Foci formation of both ClpV5-GFP and ClpV-1-GFP was decreased by D cycloserine-mediated inhibition of peptidoglycan synthesis while treatment of B. thailandensis with A22 blocking the cytoskeletal protein MreB did not affect assembly of ClpV-5 and ClpV-1 into single discrete foci. Furthermore, we found that surface contact promotes but is not essential for localization of ClpV-5-GFP to the pole whereas expression of clpV-1-gfp appears to be induced by surface contact. In summary, the study provides novel insights into the localization of ClpV ATPases of T6SSs targeting prokaryotic and eukaryotic cells.

Architecture, Function, and Substrates of the Type II Secretion System

The type II secretion system (T2SS) delivers toxins and a range of hydrolytic enzymes, including proteases, lipases, and carbohydrate-active enzymes, to the cell surface or extracellular space of Gram-negative bacteria. Its contribution to survival of both extracellular and intracellular pathogens as well as environmental species of proteobacteria is evident. This dynamic, multicomponent machinery spans the entire cell envelope and consists of a cytoplasmic ATPase, several inner membrane proteins, a periplasmic pseudopilus, and a secretin pore embedded in the outer membrane. Despite the trans-envelope configuration of the T2S nanomachine, proteins to be secreted engage with the system first once they enter the periplasmic compartment via the Sec or TAT export system. Thus, the T2SS is specifically dedicated to their outer membrane translocation. The many sequence and structural similarities between the T2SS and type IV pili suggest a common origin and argue for a pilus-mediated mechanism of secretion. This minireview describes the structures, functions, and interactions of the individual T2SS components and the general architecture of the assembled T2SS machinery and briefly summarizes the transport and function of a growing list of T2SS exoproteins. Recent advances in cryo-electron microscopy, which have led to an increased understanding of the structure-function relationship of the secretin channel and the pseudopilus, are emphasized.

Assembly and Post-assembly Turnover and Dynamics in the Type III Secretion System

The type III secretion system (T3SS) is one of the largest transmembrane complexes in bacteria, comprising several intricately linked and embedded substructures. The assembly of this nanomachine is a hierarchical process which is regulated and controlled by internal and external cues at several critical points. Recently, it has become obvious that the assembly of the T3SS is not a unidirectional and deterministic process, but that parts of the T3SS constantly exchange or rearrange. This article aims to give an overview on the assembly and post-assembly dynamics of the T3SS, with a focus on emerging general concepts and adaptations of the general assembly pathway.

Fluorescent Proteins, Promoters, and Selectable Markers for Applications in the Lyme Disease Spirochete Borrelia burgdorferi

Lyme disease is the most widely reported vector-borne disease in the United States. Its incidence is rapidly increasing, and disease symptoms can be debilitating. The need to understand the biology of the disease agent, the spirochete Borrelia burgdorferi, is thus evermore pressing. Despite important advances in B. burgdorferi genetics, the array of molecular tools available for use in this organism remains limited, especially for cell biological studies. Here, we adapt a palette of bright and mostly monomeric fluorescent proteins for versatile use and multicolor imaging in B. burgdorferi We also characterize two novel antibiotic selection markers and establish the feasibility of their use in conjunction with extant markers. Last, we describe a set of promoters of low and intermediate strengths that allow fine-tuning of gene expression levels. These molecular tools complement and expand current experimental capabilities in B. burgdorferi, which will facilitate future investigation of this important human pathogen. To showcase the usefulness of these reagents, we used them to investigate the subcellular localization of BB0323, a B. burgdorferi lipoprotein essential for survival in the host and vector environments. We show that BB0323 accumulates at the cell poles and future division sites of B. burgdorferi cells, highlighting the complex subcellular organization of this spirochete.IMPORTANCE Genetic manipulation of the Lyme disease spirochete B. burgdorferi remains cumbersome, despite significant progress in the field. The scarcity of molecular reagents available for use in this pathogen has slowed research efforts to study its unusual biology. Of interest, B. burgdorferi displays complex cellular organization features that have yet to be understood. These include an unusual morphology and a highly fragmented genome, both of which are likely to play important roles in the bacterium's transmission, infectivity, and persistence. Here, we complement and expand the array of molecular tools available for use in B. burgdorferi by generating and characterizing multiple fluorescent proteins, antibiotic selection markers, and promoters of varied strengths. These tools will facilitate investigations in this important human pathogen, as exemplified by the polar and midcell localization of the cell envelope regulator BB0323, which we uncovered using these reagents.

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.

Utilization of Vibrio cholerae as a Model Organism to Screen Natural Product Libraries for Identification of New Antibiotics

The development of antibiotic-resistant bacteria requires increasing research efforts in drug discovery. Vibrio cholerae can be utilized as a model gram-negative enteric pathogen in high- and medium-throughput screening campaigns to identify antimicrobials with different modes of action. In this chapter, we describe methods for the optimal growth of V. cholerae in 384-well plates, preparation of suitable microtiter natural product sample libraries, as well as their screening using measurements of bacterial density and activity of type II secretion-dependent protease as readouts. Concomitant LC-MS/MS profiling and spectral data networking of assay sample libraries facilitate dereplication of putative known and/or nuisance compounds and efficient prioritization of samples containing putative new natural products for further investigation.

Shedding light on biology of bacterial cells

To understand basic principles of living organisms one has to know many different properties of all cellular components, their mutual interactions but also their amounts and spatial organization. Live-cell imaging is one possible approach to obtain such data. To get multiple snapshots of a cellular process, the imaging approach has to be gentle enough to not disrupt basic functions of the cell but also have high temporal and spatial resolution to detect and describe the changes. Light microscopy has become a method of choice and since its early development over 300 years ago revolutionized our understanding of living organisms. As most cellular components are indistinguishable from the rest of the cellular contents, the second revolution came from a discovery of specific labelling techniques, such as fusions to fluorescent proteins that allowed specific tracking of a component of interest. Currently, several different tags can be tracked independently and this allows us to simultaneously monitor the dynamics of several cellular components and from the correlation of their dynamics to infer their respective functions. It is, therefore, not surprising that live-cell fluorescence microscopy significantly advanced our understanding of basic cellular processes. Current cameras are fast enough to detect changes with millisecond time resolution and are sensitive enough to detect even a few photons per pixel. Together with constant improvement of properties of fluorescent tags, it is now possible to track single molecules in living cells over an extended period of time with a great temporal resolution. The parallel development of new illumination and detection techniques allowed breaking the diffraction barrier and thus further pushed the resolution limit of light microscopy. In this review, we would like to cover recent advances in live-cell imaging technology relevant to bacterial cells and provide a few examples of research that has been possible due to imaging.

This article is part of the themed issue ‘The new bacteriology’.

The Apoptogenic Toxin AIP56 Is Secreted by the Type II Secretion System of Photobacterium damselae subsp. piscicida

AIP56 (apoptosis-inducing protein of 56 kDa) is a key virulence factor of Photobacterium damselae subsp. piscicida (Phdp), the causative agent of a septicaemia affecting warm water marine fish species. Phdp-associated pathology is triggered by AIP56, a short trip AB toxin with a metalloprotease A domain that cleaves the p65 subunit of NF-κB, an evolutionarily conserved transcription factor that regulates the expression of inflammatory and anti-apoptotic genes and plays a central role in host responses to infection. During infection by Phdp, AIP56 is systemically disseminated and induces apoptosis of macrophages and neutrophils, compromising the host phagocytic defence and contributing to the genesis of pathology. Although it is well established that the secretion of AIP56 is crucial for Phdp pathogenicity, the protein secretion systems operating in Phdp and the mechanism responsible for the extracellular release of the toxin remain unknown. Here, we report that Phdp encodes a type II secretion system (T2SS) and show that mutation of the EpsL component of this system impairs AIP56 secretion. This work demonstrates that Phdp has a functional T2SS that mediates secretion of its key virulence factor AIP56.

The role of intrinsic disorder and dynamics in the assembly and function of the type II secretion system

Many Gram-negative commensal and pathogenic bacteria use a type II secretion system (T2SS) to transport proteins out of the cell. These exported proteins or substrates play a major role in toxin delivery, maintaining biofilms, replication in the host and subversion of host immune responses to infection. We review the current structural and functional work on this system and argue that intrinsically disordered regions and protein dynamics are central for assembly, exo-protein recognition, and secretion competence of the T2SS. The central role of intrinsic disorder-order transitions in these processes may be a particular feature of type II secretion.

Tracking Proteins Secreted by Bacteria: What's in the Toolbox?

Bacteria have acquired multiple systems to expose proteins on their surface, release them in the extracellular environment or even inject them into a neighboring cell. Protein secretion has a high adaptive value and secreted proteins are implicated in many functions, which are often essential for bacterial fitness. Several secreted proteins or secretion machineries have been extensively studied as potential drug targets. It is therefore important to identify the secretion substrates, to understand how they are specifically recognized by the secretion machineries, and how transport through these machineries occurs. The purpose of this review is to provide an overview of the biochemical, genetic and imaging tools that have been developed to evaluate protein secretion in a qualitative or quantitative manner. After a brief overview of the different tools available, we will illustrate their advantages and limitations through a discussion of some of the current open questions related to protein secretion. We will start with the question of the identification of secreted proteins, which for many bacteria remains a critical initial step toward a better understanding of their interactions with the environment. We will then illustrate our toolbox by reporting how these tools have been applied to better understand how substrates are recognized by their cognate machinery, and how secretion proceeds. Finally, we will highlight recent approaches that aim at investigating secretion in real time, and in complex environments such as a tissue or an organism.

Super-Resolution Imaging of Protein Secretion Systems and the Cell Surface of Gram-Negative Bacteria

Gram-negative bacteria have a highly evolved cell wall with two membranes composed of complex arrays of integral and peripheral proteins, as well as phospholipids and glycolipids. In order to sense changes in, respond to, and exploit their environmental niches, bacteria rely on structures assembled into or onto the outer membrane. Protein secretion across the cell wall is a key process in virulence and other fundamental aspects of bacterial cell biology. The final stage of protein secretion in Gram-negative bacteria, translocation across the outer membrane, is energetically challenging so sophisticated nanomachines have evolved to meet this challenge. Advances in fluorescence microscopy now allow for the direct visualization of the protein secretion process, detailing the dynamics of (i) outer membrane biogenesis and the assembly of protein secretion systems into the outer membrane, (ii) the spatial distribution of these and other membrane proteins on the bacterial cell surface, and (iii) translocation of effector proteins, toxins and enzymes by these protein secretion systems. Here we review the frontier research imaging the process of secretion, particularly new studies that are applying various modes of super-resolution microscopy.

Polar N-terminal Residues Conserved in Type 2 Secretion Pseudopilins Determine Subunit Targeting and Membrane Extraction Steps during Fibre Assembly

Bacterial type 2 secretion systems (T2SS), type 4 pili, and archaeal flagella assemble fibres from initially membrane-embedded pseudopilin and pilin subunits. Fibre subunits are made as precursors with positively charged N-terminal anchors, whose cleavage via the prepilin peptidase, essential for pilin membrane extraction and assembly, is followed by N-methylation of the mature (pseudo)pilin N terminus. The conserved Glu residue at position 5 (E5) of mature (pseudo)pilins is essential for assembly. Unlike T4 pilins, where E5 residue substitutions also abolish N-methylation, the E5A variant of T2SS pseudopilin PulG remains N-methylated but is affected in interaction with the T2SS component PulM. Here, biochemical and functional analyses showed that the PulM interaction defect only partly accounts for the PulG^E5A assembly defect. First, PulG^T2A variant, equally defective in PulM interaction, remained partially functional. Furthermore, pseudopilus assembly defect of pulG(E5A) mutant was stronger than that of the pulM deletion mutant. To understand the dominant effect of E5A mutation, we used molecular dynamics simulations of PulG^E5A, methylated PulG^WT (MePulG^WT), and MePulG^E5A variant in a model membrane. These simulations pointed to a key role for an intramolecular interaction between the pseudopilin N-terminal amine and E5 to limit polar interactions with membrane phospholipids. N-methylation of the N-terminal amine further limited its interactions with phospholipid head-groups to facilitate pseudopilin membrane escape. By binding to polar residues in the conserved N-terminal region of PulG, we propose that PulM acts as chaperone to promote pseudopilin recruitment and coordinate its membrane extraction with subsequent steps of the fibre assembly process.

Improving Production of Protease from Pseudoalteromonas sp. CSN423 by Random Mutagenesis

Pseudoalteromonas sp. CSN423, a marine strain, can express a major protease designated as E423 and it was secreted into the supernatant. To improve the protease E423 yield, Pseudoalteromonas sp. CSN423 was subjected to mutagenesis using UV irradiation. Mutant strain with 5.1-fold higher protease yield was isolated and named as Pseudoalteromonas sp. CSN423-M. Three protease bands were detected by zymography with casein as substrate, and results of mass spectrometry (MS) showed that two lower molecular weight protein bands were the same protease but with different mature forms. The entire protease operon was sequenced and no mutation was found. Mutant strain-associated changes of expression levels of protease synthesis and secretion-related genes were determined by quantitative real-time polymerase chain reaction (qRT-PCR). Mutant strain had higher expression of e423 than wild-type strain. Such result was consistent with protease activity profiles. Moreover, the mutant strain had higher transcriptional levels of citrate synthase (cs), α-ketoglutarate decarboxylase (kgd), cytochrome c oxidase subunit I (coxI), tolC, hlyD (membrane protein), luxR3, luxO, and luxT (transcriptional regulator). However, hexokinase (hk), pyruvate dehydrogenase E1 (pd-e1), epsD (membrane protein), and luxR1 remained unchanged, and luxR2 decreased sharply in the mutant. These results suggested that the redox pathway was promoted in the mutant strain, and LuxR family transcriptional regulators in Pseudoalteromonas spp. may play some role in regulating protease expression. Meanwhile, the secretion of extracellular protease was closely related to ABC transport system. These results may shed some light on the molecular mechanism underlying higher yield of protease E423 from Pseudoalteromonas sp. CSN423-M.

GerM is required to assemble the basal platform of the SpoIIIA-SpoIIQ transenvelope complex during sporulation in Bacillus subtilis

Sporulating Bacillus subtilis cells assemble a multimeric membrane complex connecting the mother cell and developing spore that is required to maintain forespore differentiation. An early step in the assembly of this transenvelope complex (called the A-Q complex) is an interaction between the extracellular domains of the forespore membrane protein SpoIIQ and the mother cell membrane protein SpoIIIAH. This interaction provides a platform onto which the remaining components of the complex assemble and also functions as an anchor for cell-cell signalling and morphogenetic proteins involved in spore development. SpoIIQ is required to recruit SpoIIIAH to the sporulation septum on the mother cell side; however, the mechanism by which SpoIIQ specifically localizes to the septal membranes on the forespore side has remained enigmatic. Here, we identify GerM, a lipoprotein previously implicated in spore germination, as the missing factor required for SpoIIQ localization. Our data indicate that GerM and SpoIIIAH, derived from the mother cell, and SpoIIQ, from the forespore, have reciprocal localization dependencies suggesting they constitute a tripartite platform for the assembly of the A-Q complex and a hub for the localization of mother cell and forespore proteins.

Visualization of the Serratia Type VI Secretion System Reveals Unprovoked Attacks and Dynamic Assembly

The Type VI secretion system (T6SS) is a bacterial nanomachine that fires toxic proteins into target cells. Deployment of the T6SS represents an efficient and widespread means by which bacteria attack competitors or interact with host organisms and may be triggered by contact from an attacking neighbor cell as a defensive strategy. Here, we use the opportunist pathogen Serratia marcescens and functional fluorescent fusions of key components of the T6SS to observe different subassemblies of the machinery simultaneously and on multiple timescales in vivo. We report that the localization and dynamic behavior of each of the components examined is distinct, revealing a multi-stage and dynamic assembly process for the T6SS machinery. We also show that the T6SS can assemble and fire without needing a cell contact trigger, defining an aggressive strategy that broadens target range and suggesting that activation of the T6SS is tailored to survival in specific niches.

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.

Exceptionally widespread nanomachines composed of type IV pilins: the prokaryotic Swiss Army knives

Prokaryotes have engineered sophisticated surface nanomachines that have allowed them to colonize Earth and thrive even in extreme environments. Filamentous machineries composed of type IV pilins, which are associated with an amazing array of properties ranging from motility to electric conductance, are arguably the most widespread since distinctive proteins dedicated to their biogenesis are found in most known species of prokaryotes. Several decades of investigations, starting with type IV pili and then a variety of related systems both in bacteria and archaea, have outlined common molecular and structural bases for these nanomachines. Using type IV pili as a paradigm, we will highlight in this review common aspects and key biological differences of this group of filamentous structures.

Using type IV pili as a paradigm, we review common genetic, structural and mechanistic features (many) as well as differences (few) of the exceptionally widespread and functionally versatile prokaryotic nano-machines composed of type IV pilins.

The Type II secretion system delivers matrix proteins for biofilm formation by Vibrio cholerae

Gram-negative bacteria have evolved several highly dedicated pathways for extracellular protein secretion, including the type II secretion (T2S) system. Since substrates secreted via the T2S system include both virulence factors and degradative enzymes, this secretion system is considered a major survival mechanism for pathogenic and environmental species. Previous analyses revealed that the T2S system mediates the export of ≥ 20 proteins in Vibrio cholerae, a human pathogen that is indigenous to the marine environment. Here we demonstrate a new role in biofilm formation for the V. cholerae T2S system, since wild-type V. cholerae was found to secrete the biofilm matrix proteins RbmC, RbmA, and Bap1 into the culture supernatant, while an isogenic T2S mutant could not. In agreement with this finding, the level of biofilm formation in a static microtiter assay was diminished in T2S mutants. Moreover, inactivation of the T2S system in a rugose V. cholerae strain prevented the development of colony corrugation and pellicle formation at the air-liquid interface. In contrast, extracellular secretion of the exopolysaccharide VPS, an essential component of the biofilm matrix, remained unaffected in the T2S mutants. Our results indicate that the T2S system provides a mechanism for the delivery of extracellular matrix proteins known to be important for biofilm formation by V. cholerae. Because the T2S system contributes to the pathogenicity of V. cholerae by secreting proteins such as cholera toxin and biofilm matrix proteins, elucidation of the molecular mechanism of T2S has the potential to lead to the development of novel preventions and therapies.

DNA transport across the outer and inner membranes of naturally transformable Vibrio cholerae is spatially but not temporally coupled

The physiological state of natural competence for transformation allows certain bacteria to take up free DNA from the environment and to recombine such newly acquired DNA into their chromosomes. However, even though conserved components that are required to undergo natural transformation have been identified in several naturally competent bacteria, our knowledge of the underlying mechanisms of the DNA uptake process remains very limited. To better understand these mechanisms, we investigated the competence-mediated DNA transport in the naturally transformable pathogen Vibrio cholerae. Previously, we used a cell biology-based approach to experimentally address an existing hypothesis, which suggested the competence protein ComEA plays a role in the DNA uptake process across the outer membrane of Gram-negative bacteria. Here, we extended this knowledge by investigating the dynamics of DNA translocation across both membranes. More precisely, we indirectly visualized the transfer of the external DNA from outside the cell into the periplasm followed by the shuttling of the DNA into the cytoplasm. Based on these data, we conclude that for V. cholerae, the DNA translocation across the outer and inner membranes is spatially but not temporally coupled.

As a mode of horizontal gene transfer, natural competence for transformation has contributed substantially to the plasticity of genomes and to bacterial evolution. Natural competence is often a tightly regulated process and is induced by diverse environmental cues. This is in contrast to the mechanistic aspects of the DNA translocation event, which are most likely conserved among naturally transformable bacteria. However, the DNA uptake process is still not well understood. We therefore investigated how external DNA reaches the cytosol of the naturally transformable bacterium V. cholerae. More specifically, we provide evidence that the DNA translocation across the membranes is spatially but not temporally coupled. We hypothesize that this model also applies to other competent Gram-negative bacteria and that our study contributes to the general understanding of this important biological process.

The type II secretion pathway in Vibrio cholerae is characterized by growth phase-dependent expression of exoprotein genes and is positively regulated by σE

Vibrio cholerae, an etiological agent of cholera, circulates between aquatic reservoirs and the human gastrointestinal tract. The type II secretion (T2S) system plays a pivotal role in both stages of the lifestyle by exporting multiple proteins, including cholera toxin. Here, we studied the kinetics of expression of genes encoding the T2S system and its cargo proteins. We have found that under laboratory growth conditions, the T2S complex was continuously expressed throughout V. cholerae growth, whereas there was growth phase-dependent transcriptional activity of genes encoding different cargo proteins. Moreover, exposure of V. cholerae to different environmental cues encountered by the bacterium in its life cycle induced transcriptional expression of T2S. Subsequent screening of a V. cholerae genomic library suggested that σ(E) stress response, phosphate metabolism, and the second messenger 3',5'-cyclic diguanylic acid (c-di-GMP) are involved in regulating transcriptional expression of T2S. Focusing on σ(E), we discovered that the upstream region of the T2S operon possesses both the consensus σ(E) and σ(70) signatures, and deletion of the σ(E) binding sequence prevented transcriptional activation of T2S by RpoE. Ectopic overexpression of σ(E) stimulated transcription of T2S in wild-type and isogenic ΔrpoE strains of V. cholerae, providing additional support for the idea that the T2S complex belongs to the σ(E) regulon. Together, our results suggest that the T2S pathway is characterized by the growth phase-dependent expression of genes encoding cargo proteins and requires a multifactorial regulatory network to ensure appropriate kinetics of the secretory traffic and the fitness of V. cholerae in different ecological niches.

Distinct docking and stabilization steps of the Pseudopilus conformational transition path suggest rotational assembly of type IV pilus-like fibers

The closely related bacterial type II secretion (T2S) and type IV pilus (T4P) systems are sophisticated machines that assemble dynamic fibers promoting protein transport, motility, or adhesion. Despite their essential role in virulence, the molecular mechanisms underlying helical fiber assembly remain unknown. Here, we use electron microscopy and flexible modeling to study conformational changes of PulG pili assembled by the Klebsiella oxytoca T2SS. Neural network analysis of 3,900 pilus models suggested a transition path toward low-energy conformations driven by progressive increase in fiber helical twist. Detailed predictions of interprotomer contacts along this path were tested by site-directed mutagenesis, pilus assembly, and protein secretion analyses. We demonstrate that electrostatic interactions between adjacent protomers (P-P+1) in the membrane drive pseudopilin docking, while P-P+3 and P-P+4 contacts determine downstream fiber stabilization steps. These results support a model of a spool-like assembly mechanism for fibers of the T2SS-T4P superfamily.

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.

ComEA is essential for the transfer of external DNA into the periplasm in naturally transformable Vibrio cholerae cells

The DNA uptake of naturally competent bacteria has been attributed to the action of DNA uptake machineries resembling type IV pilus complexes. However, the protein(s) for pulling the DNA across the outer membrane of Gram-negative bacteria remain speculative. Here we show that the competence protein ComEA binds incoming DNA in the periplasm of naturally competent Vibrio cholerae cells thereby promoting DNA uptake, possibly through ratcheting and entropic forces associated with ComEA binding. Using comparative modeling and molecular simulations, we projected the 3D structure and DNA-binding site of ComEA. These in silico predictions, combined with in vivo and in vitro validations of wild-type and site-directed modified variants of ComEA, suggested that ComEA is not solely a DNA receptor protein but plays a direct role in the DNA uptake process. Furthermore, we uncovered that ComEA homologs of other bacteria (both Gram-positive and Gram-negative) efficiently compensated for the absence of ComEA in V. cholerae, suggesting that the contribution of ComEA in the DNA uptake process might be conserved among naturally competent bacteria.

Horizontal gene transfer (HGT) plays a key role in transferring genetic information from one organism to another. Natural competence for transformation is one of three modes of HGT used by bacteria to promote the uptake of free DNA from the surrounding. The human pathogen Vibrio cholerae enters such a competence state upon growth on chitinous surfaces, which represent its natural niche in the aquatic environment. Whereas we have gained a reasonable understanding on how the competence phenotype is regulated in V. cholerae we are only at the beginning of deciphering the mechanistic aspects of the DNA uptake process. In this study, we characterize the competence protein ComEA. We show that ComEA is transported into the periplasm of V. cholerae and that it is required for the uptake of DNA across the outer membrane. We demonstrate that ComEA aggregates around incoming DNA in vivo and that the binding of DNA is dependent on specific residues within a conserved helix-hairpin-helix motif. We propose a model indicating that the DNA uptake process across the outer membrane might be driven through ratcheting and entropic forces associated with ComEA binding.

Outside-in assembly pathway of the type IV pilus system in Myxococcus xanthus

Type IV pili (T4P) are ubiquitous bacterial cell surface structures that undergo cycles of extension, adhesion, and retraction. T4P function depends on a highly conserved envelope-spanning macromolecular machinery consisting of 10 proteins that localizes polarly in Myxococcus xanthus. Using this localization, we investigated the entire T4P machinery assembly pathway by systematically profiling the stability of all and the localization of eight of these proteins in the absence of other T4P machinery proteins as well as by mapping direct protein-protein interactions. Our experiments uncovered a sequential, outside-in pathway starting with the outer membrane (OM) PilQ secretin ring. PilQ recruits a subcomplex consisting of the inner membrane (IM) lipoprotein PilP and the integral IM proteins PilN and PilO by direct interaction with the periplasmic domain of PilP. The PilP/PilN/PilO subcomplex recruits the cytoplasmic PilM protein, by direct interaction between PilN and PilM, and the integral IM protein PilC. The PilB/PilT ATPases that power extension/retraction localize independently of other T4P machinery proteins. Thus, assembly of the T4P machinery initiates with formation of the OM secretin ring and continues inwards over the periplasm and IM to the cytoplasm.

DNA-uptake machinery of naturally competent Vibrio cholerae

Natural competence for transformation is a mode of horizontal gene transfer that is commonly used by bacteria to take up DNA from their environment. As part of this developmental program, so-called competence genes, which encode the components of a DNA-uptake machinery, are expressed. Several models have been proposed for the DNA-uptake complexes of competent bacteria, and most include a type IV (pseudo)pilus as a core component. However, cell-biology-based approaches to visualizing competence proteins have so far been restricted to Gram-positive bacteria. Here, we report the visualization of a competence-induced pilus in the Gram-negative bacterium Vibrio cholerae. We show that piliated cells mostly contain a single pilus that is not biased toward a polar localization and that this pilus colocalizes with the outer membrane secretin PilQ. PilQ, on the other hand, forms several foci around the cell and occasionally colocalizes with the dynamic cytoplasmic-traffic ATPase PilB, which is required for pilus extension. We also determined the minimum competence regulon of V. cholerae, which includes at least 19 genes. Bacteria with mutations in those genes were characterized with respect to the presence of surface-exposed pili, DNA uptake, and natural transformability. Based on these phenotypes, we propose that DNA uptake in naturally competent V. cholerae cells occurs in at least two steps: a pilus-dependent translocation of the incoming DNA across the outer membrane and a pilus-independent shuttling of the DNA through the periplasm and into the cytoplasm.

A dodecameric ring-like structure of the N0 domain of the type II secretin from enterotoxigenic Escherichia coli

In many bacteria, secretins from the type II secretion system (T2SS) function as outer membrane gated channels that enable passage of folded proteins from the periplasm into the extracellular milieu. Cryo-electron microscopy of the T2SS secretin GspD revealed previously the dodecameric cylindrical architecture of secretins, and crystal structures of periplasmic secretin domains showed a modular domain organization. However, no high-resolution experimental data has as yet been provided about how the entire T2SS secretin or its domains are organized in a cylindrical fashion. Here we present a crystal structure of the N0 domain of the T2SS secretin GspD from enterotoxigenic Escherichia coli containing a helix with 12 subunits per turn. The helix has an outer diameter of ∼125Å and a pitch of only 24Å which suggests a model of a cylindrical dodecameric N0 ring whose dimensions correspond with the cryo-electron microscopy map of Vibrio cholerae GspD. The N0 domain is known to interact with the HR domain of the inner membrane T2SS protein GspC. When the new N0 ring model is combined with the known N0·HR crystal structure, a dodecameric double-ring of twelve N0-HR heterodimers is obtained. In contrast, the previously observed compact N0-N1 GspD module is not compatible with the N0 ring. Interestingly, a N0-N1 T3SS homolog is compatible with forming a N0-N1 dodecameric ring, due to a different N0-vs-N1 orientation. This suggests that the dodecameric N0 ring is an important feature of T2SS secretins with periplasmic domains undergoing considerable motions during exoprotein translocation.

Development of a quantitative assay amenable for high-throughput screening to target the type II secretion system for new treatments against plant-pathogenic bacteria

Plant-pathogenic bacteria are the causative agents of diseases in important agricultural crops and ornamental plants. The severe economic burden of these diseases requires seeking new approaches for their control, particularly because phytopathogenic bacteria are often resistant to available treatments. The type II secretion (T2S) system is a key virulence factor used by major groups of phytopathogenic bacteria. The T2S machinery transports many hydrolytic enzymes responsible for degradation of the plant cell wall, thus enabling successful colonization and dissemination of the bacteria in the plant host. The genetic inactivation of the T2S system leads to loss of virulence, which strongly suggests that targeting the T2S could enable new treatments against plant-pathogenic bacteria. Accordingly, we have designed and optimized an assay to identify small-molecule inhibitors of the T2S system. This assay uses a double parametric output: measurement of bacterial growth and the enzymatic activity of cellulase, which is secreted via the T2S pathway in our model organism Dickeya dadantii. The assay was evaluated by screening natural extracts, culture filtrates isolated from rhizosphere bacteria, and a collection of pharmaceutically active compounds in LOPAC(1280). The calculated Z' values of 0.63, 0.63, and 0.58, respectively, strongly suggest that the assay is applicable for a high-throughput screening platform.

Secretion of TcpF by the Vibrio cholerae toxin-coregulated pilus biogenesis apparatus requires an N-terminal determinant

Type IV pili are important for microcolony formation, biofilm formation, twitching motility, and attachment. We and others have shown that type IV pili are important for protein secretion across the outer membrane, similar to type II secretion systems. This study explored the relationship between protein secretion and pilus formation in Vibrio cholerae. The toxin-coregulated pilus (TCP), a type IV pilus required for V. cholerae pathogenesis, is necessary for the secretion of the colonization factor TcpF (T. J. Kirn, N. Bose, and R. K. Taylor, Mol. Microbiol. 49:81-92, 2003). This phenomenon is not unique to V. cholerae; secreted virulence factors that are dependent on the presence of components of the type IV pilus biogenesis apparatus for secretion have been reported with Dichelobacter nodosus (R. M. Kennan, O. P. Dhungyel, R. J. Whittington, J. R. Egerton, and J. I. Rood, J. Bacteriol. 183:4451-4458, 2001) and Francisella tularensis (A. J. Hager et al., Mol. Microbiol. 62:227-237, 2006). Using site-directed mutagenesis, we demonstrated that the secretion of TcpF is dependent on the presence of selected amino acid R groups at position five. We were unable to find other secretion determinants, suggesting that Y5 is the major secretion determinant within TcpF. We also report that proteins secreted in a type IV pilus biogenesis apparatus-dependent manner have a YXS motif within the first 15 amino acids following the Sec cleavage site. The YXS motif is not present in proteins secreted by type II secretion systems, indicating that this is unique to type IV pilus-mediated secretion. Moreover, we show that TcpF interacts with the pilin TcpA, suggesting that these proteins are secreted by the type IV pilus biogenesis system. These data provide a starting point for understanding how type IV pili can mediate secretion of virulence factors important for bacterial pathogenesis.

Assembly of the type II secretion system

The type II secretion system is utilized by many Gram-negative bacteria to export folded proteins to the surface and/or the extracellular environment of the cell. Although the function of the system is to move proteins from the periplasm to the outside of the cell, it is a large trans-envelope structure composed of more than a dozen different proteins present in multiple copies, including peripheral, integral inner membrane and integral outer membrane proteins plus a pseudopilus stretching between them. The establishment of this structure as an integral component of the entire envelope including the peptidoglycan layer between the two membranes requires assembly. Many of the participants and processes involved in this assembly have now been established, while other aspects remain to be discovered or more fully understood.

Function-related positioning of the type II secretion ATPase of Xanthomonas campestris pv. campestris

Gram-negative bacteria use the type II secretion (T2S) system to secrete exoproteins for attacking animal or plant cells or to obtain nutrients from the environment. The system is unique in helping folded proteins traverse the outer membrane. The secretion machine comprises multiple proteins spanning the cell envelope and a cytoplasmic ATPase. Activity of the ATPase, when copurified with the cytoplasmic domain of an interactive ATPase partner, is stimulated by an acidic phospholipid, suggesting the membrane-associated ATPase is actively engaged in secretion. How the stimulated ATPase activity is terminated when secretion is complete is unclear. We fused the T2S ATPase of Xanthomonas campestris pv. campestris, the causal agent of black rot in the crucifers, with fluorescent protein and found that the ATPase in secretion-proficient cells was mainly diffused in cytoplasm. Focal spots at the cell periphery were detectable only in a few cells. The discrete foci were augmented in abundance and intensity when the secretion channel was depleted and the exoprotein overproduced. The foci abundance was inversely related to secretion efficiency of the secretion channel. Restored function of the secretion channel paralleled reduced ATPase foci abundance. The ATPase foci colocalized with the secretion channel. The ATPase may be transiently associated with the T2S machine by alternating between a cytoplasmic and a machine-associated state in a secretion-dependent manner. This provides a logical means for terminating the ATPase activity when secretion is completed. Function-related dynamic assembly may be the essence of the T2S machine.

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.

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.

Structural insights into the Type II secretion nanomachine

The Type II secretion nanomachine transports folded proteins across the outer membrane of Gram-negative bacteria. Recent X-ray crystallography, electron microscopy, and molecular modeling studies provide structural insights into three functionally and spatially connected units of this nanomachine: the cytoplasmic and inner membrane energy-harvesting complex, the periplasmic helical pseudopilus, and the outer membrane secretin. Key advances include cryo-EM reconstruction of the secretin and demonstration that it interacts with both secreted substrates and a crucial transmembrane clamp protein, plus a biochemical and structural explanation of the role of low-abundance pseudopilins in initiating pseudopilus growth. Combining structures and protein interactions, we synthesize a 3D view of the complete complex consistent with a stepwise pathway in which secretin oligomerization defines sites of nanomachine biogenesis.

Outer membrane targeting, ultrastructure, and single molecule localization of the enteropathogenic Escherichia coli type IV pilus secretin BfpB

Type IV pili (T4P) are filamentous surface appendages required for tissue adherence, motility, aggregation, and transformation in a wide array of bacteria and archaea. The bundle-forming pilus (BFP) of enteropathogenic Escherichia coli (EPEC) is a prototypical T4P and confirmed virulence factor. T4P fibers are assembled by a complex biogenesis machine that extrudes pili through an outer membrane (OM) pore formed by the secretin protein. Secretins constitute a superfamily of proteins that assemble into multimers and support the transport of macromolecules by four evolutionarily ancient secretion systems: T4P, type II secretion, type III secretion, and phage assembly. Here, we determine that the lipoprotein transport pathway is not required for targeting the BfpB secretin protein of the EPEC T4P to the OM and describe the ultrastructure of the single particle averaged structures of the assembled complex by transmission electron microscopy. Furthermore, we use photoactivated localization microscopy to determine the distribution of single BfpB molecules fused to photoactivated mCherry. In contrast to findings in other T4P systems, we found that BFP components predominantly have an uneven distribution through the cell envelope and are only found at one or both poles in a minority of cells. In addition, we report that concurrent mutation of both the T4bP secretin and the retraction ATPase can result in viable cells and found that these cells display paradoxically low levels of cell envelope stress response activity. These results imply that secretins can direct their own targeting, have complex distributions and provide feedback information on the state of pilus biogenesis.

Characterization of the PilN, PilO and PilP type IVa pilus subcomplex

Type IVa pili are bacterial nanomachines required for colonization of surfaces, but little is known about the organization of proteins in this system. The Pseudomonas aeruginosa pilMNOPQ operon encodes five key members of the transenvelope complex facilitating pilus function. While PilQ forms the outer membrane secretin pore, the functions of the inner membrane-associated proteins PilM/N/O/P are less well defined. Structural characterization of a stable C-terminal fragment of PilP (PilP(Δ71)) by NMR revealed a modified β-sandwich fold, similar to that of Neisseria meningitidis PilP, although complementation experiments showed that the two proteins are not interchangeable likely due to divergent surface properties. PilP is an inner membrane putative lipoprotein, but mutagenesis of the putative lipobox had no effect on the localization and function of PilP. A larger fragment, PilP(Δ18-6His), co-purified with a PilN(Δ44)/PilO(Δ51) heterodimer as a stable complex that eluted from a size exclusion chromatography column as a single peak with a molecular weight equivalent to two heterotrimers with 1:1:1 stoichiometry. Although PilO forms both homodimers and PilN-PilO heterodimers, PilP(Δ18-6His) did not interact stably with PilO(Δ51) alone. Together these data demonstrate that PilN/PilO/PilP interact directly to form a stable heterotrimeric complex, explaining the dispensability of PilP's lipid anchor for localization and function.

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.

Secretins: dynamic channels for protein transport across membranes

Secretins form megadalton bacterial-membrane channels in at least four sophisticated multiprotein systems that are crucial for translocation of proteins and assembled fibers across the outer membrane of many species of bacteria. Secretin subunits contain multiple domains, which interact with numerous other proteins, including pilotins, secretion-system partner proteins, and exoproteins. Our understanding of the structure of secretins is rapidly progressing, and it is now recognized that features common to all secretins include a cylindrical arrangement of 12-15 subunits, a large periplasmic vestibule with a wide opening at one end and a periplasmic gate at the other. Secretins might also play a key role in the biogenesis of their cognate secretion systems.

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.

Involvement of the GspAB complex in assembly of the type II secretion system secretin of Aeromonas and Vibrio species

The type II secretion system (T2SS) functions as a transport mechanism to translocate proteins from the periplasm to the extracellular environment. The ExeA homologue in Aeromonas hydrophila, GspA(Ah), is an ATPase that interacts with peptidoglycan and forms an inner membrane complex with the ExeB homologue (GspB(Ah)). The complex may be required to generate space in the peptidoglycan mesh that is necessary for the transport and assembly of the megadalton-sized ExeD homologue (GspD(Ah)) secretin multimer in the outer membrane. In this study, the requirement for GspAB in the assembly of the T2SS secretin in Aeromonas and Vibrio species was investigated. We have demonstrated a requirement for GspAB in T2SS assembly in Aeromonas salmonicida, similar to that previously observed in A. hydrophila. In the Vibrionaceae species Vibrio cholerae, Vibrio vulnificus, and Vibrio parahaemolyticus, gspA mutations significantly decreased assembly of the secretin multimer but had minimal effects on the secretion of T2SS substrates. The lack of effect on secretion of the mutant of gspA of V. cholerae (gspA(Vc)) was explained by the finding that native secretin expression greatly exceeds the level needed for efficient secretion in V. cholerae. In cross-complementation experiments, secretin assembly and secretion in an A. hydrophila gspA mutant were partially restored by the expression of GspAB from V. cholerae in trans, further suggesting that GspAB(Vc) performs the same role in Vibrio species as GspAB(Ah) does in the aeromonads. These results indicate that the GspAB complex is functional in the assembly of the secretin in Vibrio species but that a redundancy of GspAB function may exist in this genus.

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.