Secretome analysis uncovers an Hcp-family protein secreted via a type VI secretion system in Agrobacterium tumefaciens

Regulation and secretion of Xanthomonas virulence factors

Plant pathogenic bacteria of the genus Xanthomonas cause a variety of diseases in economically important monocotyledonous and dicotyledonous crop plants worldwide. Successful infection and bacterial multiplication in the host tissue often depend on the virulence factors secreted including adhesins, polysaccharides, LPS and degradative enzymes. One of the key pathogenicity factors is the type III secretion system, which injects effector proteins into the host cell cytosol to manipulate plant cellular processes such as basal defense to the benefit of the pathogen. The coordinated expression of bacterial virulence factors is orchestrated by quorum-sensing pathways, multiple two-component systems and transcriptional regulators such as Clp, Zur, FhrR, HrpX and HpaR. Furthermore, virulence gene expression is post-transcriptionally controlled by the RNA-binding protein RsmA. In this review, we summarize the current knowledge on the infection strategies and regulatory networks controlling secreted virulence factors from Xanthomonas species.

Evaluation of a Yersinia pestis mutant impaired in a thermoregulated type VI-like secretion system in flea, macrophage and murine models

Type VI secretion systems (T6SSs) have been identified recently in several Gram-negative organisms and have been shown to be associated with virulence in some bacterial pathogens. A T6SS of Yersinia pestis CO92 (locus YPO0499-YPO0516) was deleted followed by investigation of the phenotype of this mutation. We observed that this T6SS locus of Y. pestis was preferentially expressed at 26 degrees C in comparison to 37 degrees C suggesting a possible role in the flea cycle. However, we found that the deletion of T6SS locus YPO0499-YPO0516 in Y. pestis CO92 had no effect on the ability of this strain to infect the oriental rat flea, Xenopsylla cheopis. Nevertheless, this mutant displayed increased intracellular numbers in macrophage-like J774.A1 cells after 20 h post-infection for bacterial cells pre-grown at 26 degrees C indicating that expression of this T6SS locus limited intracellular replication in macrophages. In addition, deletion of the YPO0499-YPO0516 locus reduced the uptake by macrophages of the Y. pestis mutant pre-grown at 37 degrees C, suggesting that this T6SS locus has phagocytosis-promoting activity. Further study of the virulence of the T6SS mutant in murine bubonic and inhalation plague models revealed no attenuation in comparison with the parental CO92 strain.

Small heat-shock protein HspL is induced by VirB protein(s) and promotes VirB/D4-mediated DNA transfer in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a Gram-negative plant-pathogenic bacterium that causes crown gall disease by transferring and integrating its transferred DNA (T-DNA) into the host genome. We characterized the chromosomally encoded alpha-crystallin-type small heat-shock protein (alpha-Hsp) HspL, which was induced by the virulence (vir) gene inducer acetosyringone (AS). The transcription of hspL but not three other alpha-Hsp genes (hspC, hspAT1, hspAT2) was upregulated by AS. Further expression analysis in various vir mutants suggested that AS-induced hspL transcription is not directly activated by the VirG response regulator but rather depends on the expression of VirG-activated virB genes encoding components of the type IV secretion system (T4SS). Among the 11 virB genes encoded by the virB operon, HspL protein levels were reduced in strains with deletions of virB6, virB8 or virB11. VirB protein accumulation but not virB transcription levels were reduced in an hspL deletion mutant early after AS induction, implying that HspL may affect the stability of individual VirB proteins or of the T4S complex directly or indirectly. Tumorigenesis efficiency and the VirB/D4-mediated conjugal transfer of an IncQ plasmid RSF1010 derivative between A. tumefaciens strains were reduced in the absence of HspL. In conclusion, increased HspL abundance is triggered in response to certain VirB protein(s) and plays a role in optimal VirB protein accumulation, VirB/D4-mediated DNA transfer and tumorigenesis.

Quorum sensing differentially regulates Pseudomonas aeruginosa type VI secretion locus I and homologous loci II and III, which are required for pathogenesis

Pseudomonas aeruginosa harbours three type VI secretion (T6S) loci. Although HSI-I has been partially studied, limited knowledge is available on the homologous loci HSI-II and HSI-III. We show that quorum sensing (QS) differentially regulates the expression of genes at all three loci. HSI-I-associated gene expression is suppressed by both the homoserine lactone transcription factor LasR and the 4-hydroxy-2-alkylquinoline (HAQ) transcriptional regulator MvfR. Conversely, both HSI-II and HSI-III loci are positively controlled by LasR and MvfR. PqsE, a key component of the MvfR regulon, is required for the expression of part of HSI-III but not HSI-II, and previously identified inhibitors of HAQ biosynthesis significantly downregulate HSI-II and -III gene expression. Animal and plant infection studies reveal that both HSI-II and -III play important roles in pathogenesis. Furthermore, analysis of a double DeltaHSI-II : : III mutant suggests that these loci functionally compensate for one another in virulence. This study illustrates the contribution of the QS systems to T6S gene regulation and reveals the importance of HSI-II and -III in mediating P. aeruginosa pathogenesis. Moreover, this work provides new insights into the design and development of selective compounds that may restrict human P. aeruginosa and possibly other clinical infections.

Translocation of a Vibrio cholerae type VI secretion effector requires bacterial endocytosis by host cells

The type VI secretion system (T6SS) is a virulence mechanism common to several Gram-negative pathogens. In Vibrio cholerae, VgrG-1 is required for T6SS-dependent secretion. VgrG-1 is also secreted by T6SS and displays a C-terminal actin crosslinking domain (ACD). Using a heterologous reporter enzyme in place of the ACD, we show that the effector and secretion functions of VgrG-1 are genetically dissociable with the ACD being dispensable for secretion but required for T6SS-dependent phenotypes. Furthermore, internalization of bacteria is required for ACD translocation into phagocytic target cells. Inhibiting bacterial uptake abolishes actin crosslinking, while improving intracellular survival enhances it. Otherwise resistant nonphagocytic cells become susceptible to T6SS-mediated actin crosslinking when engineered to take up bacteria. Our results support a model for translocation of VgrG C-terminal effector domains into target cell cytosol by a process that requires trafficking of bacterial cells into an endocytic compartment where translocation is triggered by an unknown signal.

An IcmF family protein, ImpLM, is an integral inner membrane protein interacting with ImpKL, and its walker a motif is required for type VI secretion system-mediated Hcp secretion in Agrobacterium tumefaciens

An intracellular multiplication F (IcmF) family protein is a conserved component of a newly identified type VI secretion system (T6SS) encoded in many animal and plant-associated Proteobacteria. We have previously identified ImpL(M), an IcmF family protein that is required for the secretion of the T6SS substrate hemolysin-coregulated protein (Hcp) from the plant-pathogenic bacterium Agrobacterium tumefaciens. In this study, we characterized the topology of ImpL(M) and the importance of its nucleotide-binding Walker A motif involved in Hcp secretion from A. tumefaciens. A combination of beta-lactamase-green fluorescent protein fusion and biochemical fractionation analyses revealed that ImpL(M) is an integral polytopic inner membrane protein comprising three transmembrane domains bordered by an N-terminal domain facing the cytoplasm and a C-terminal domain exposed to the periplasm. impL(M) mutants with substitutions or deletions in the Walker A motif failed to complement the impL(M) deletion mutant for Hcp secretion, which provided evidence that ImpL(M) may bind and/or hydrolyze nucleoside triphosphates to mediate T6SS machine assembly and/or substrate secretion. Protein-protein interaction and protein stability analyses indicated that there is a physical interaction between ImpL(M) and another essential T6SS component, ImpK(L). Topology and biochemical fractionation analyses suggested that ImpK(L) is an integral bitopic inner membrane protein with an N-terminal domain facing the cytoplasm and a C-terminal OmpA-like domain exposed to the periplasm. Further comprehensive yeast two-hybrid assays dissecting ImpL(M)-ImpK(L) interaction domains suggested that ImpL(M) interacts with ImpK(L) via the N-terminal cytoplasmic domains of the proteins. In conclusion, ImpL(M) interacts with ImpK(L), and its Walker A motif is required for its function in mediation of Hcp secretion from A. tumefaciens.

TagR promotes PpkA-catalysed type VI secretion activation in Pseudomonas aeruginosa

Type VI secretion systems (T6SSs) contribute to interactions of bacterial pathogens and symbionts with their hosts. Previously, we showed that Pseudomonas aeruginosa T6S is posttranslationally activated upon phosphorylation of Fha1, an FHA domain protein, by PpkA, a membrane-spanning threonine kinase. Herein, additional structural, enzymatic and genetic requirements for PpkA-catalysed T6SS activation are identified. We found that PpkA plays an essential structural role in the T6SS, and that this role is intimately linked to its ability to promote secretion and phosphorylate Fha1. Protein localization and protein-protein interaction studies show that a complex containing Fha1 and the T6S ATPase, ClpV1 is recruited to the T6S apparatus in a phosphorylation-dependent manner. The mechanism of PpkA activation was also investigated. We identified critical PpkA autophosphorylation sites and showed that small molecule-induced dimerization of the extracellular domains of PpkA is sufficient to activate the T6SS. Finally, we discovered TagR, a component of the T6S posttranslational regulatory pathway that functions upstream of PpkA to promote kinase activity. We present a model whereby an unknown cue causes dimerization of the extracellular domains of PpkA, leading to its autophosphorylation, recruitment of the Fha1-ClpV1 complex, phosphorylation of Fha1, and T6SS activation. Our findings should facilitate approaches for identifying physiological activators of T6S.

Characterization of the pathogenicity island protein PdpA and its role in the virulence of Francisella novicida

Francisella tularensis is a highly virulent, intracellular pathogen that causes the disease tularaemia. A research surrogate for F. tularensis is Francisella novicida, which causes a tularaemia-like disease in mice, grows similarly in macrophages, and yet is unable to cause disease in humans. Both Francisella species contain a cluster of genes referred to as the Francisella pathogenicity island (FPI). Pathogenicity determinant protein A (PdpA), encoded by the pdpA gene, is located within the FPI and has been associated with the virulence of Francisella species. In this work we examined the properties of PdpA protein expression and localization as well as the phenotype of a F. novicida pdpA deletion mutant. Monoclonal antibody detection of PdpA showed that it is a soluble protein that is upregulated in iron-limiting conditions and undetectable in an mglA or mglB mutant background. Deletion of pdpA resulted in a strain that was highly attenuated for virulence in chicken embryos and mice.

Protein secretion systems in bacterial-host associations, and their description in the Gene Ontology

Protein secretion plays a central role in modulating the interactions of bacteria with their environments. This is particularly the case when symbiotic bacteria (whether pathogenic, commensal or mutualistic) are interacting with larger host organisms. In the case of Gram-negative bacteria, secretion requires translocation across the outer as well as the inner membrane, and a diversity of molecular machines have been elaborated for this purpose. A number of secreted proteins are destined to enter the host cell (effectors and toxins), and thus several secretion systems include apparatus to translocate proteins across the plasma membrane of the host also. The Plant-Associated Microbe Gene Ontology (PAMGO) Consortium has been developing standardized terms for describing biological processes and cellular components that play important roles in the interactions of microbes with plant and animal hosts, including the processes of bacterial secretion. Here we survey bacterial secretion systems known to modulate interactions with host organisms and describe Gene Ontology terms useful for describing the components and functions of these systems, and for capturing the similarities among the diverse systems.

The type VI secretion system: translocation of effectors and effector-domains

A number of prominent Gram-negative bacteria use the type VI secretion system (T6SS) to transport proteins across the bacterial envelope. Rapid progress is being made in elucidating the structural components of the T6SS apparatus, and a few effectors have been reported to pass through it. However, this is not the complete story: a family of T6SS proteins, the VgrGs, share structural features with the cell-puncturing device of the T4 bacteriophage, and may be used in a similar fashion by bacteria to puncture host cell membranes and insert the T6SS apparatus into the host cytosol. Interestingly, a number of VgrGs contain C-terminal extensions with effector-domains. Thus, the T6SS may translocate soluble effectors, as well as VgrG effector-domains.

Remodelling of VipA/VipB tubules by ClpV-mediated threading is crucial for type VI protein secretion

The recently identified type VI secretion systems (T6SS) have a crucial function in the virulence of various proteobacteria, including the human pathogen Vibrio cholerae. T6SS are encoded by a conserved gene cluster comprising approximately 15 open reading frames, mediating the appearance of Hcp and VgrG proteins in cell culture supernatants. Here, we analysed the function of the V. cholerae T6SS member ClpV, a specialized AAA+ protein. ClpV is crucial for a functional T6SS and interacts through its N-terminal domain with the VipA/VipB complex that is composed of two conserved and essential members of T6SS. Transferring ClpV substrate specificity to a distinct AAA+ protein involved in proteolysis caused degradation of VipA but not Hcp or VgrG2, suggesting that VipA rather than Hcp/VgrG2 functions as a primary ClpV substrate. Strikingly, VipA/VipB form tubular, cogwheel-like structures that are converted by a threading activity of ClpV into small complexes. ClpV-mediated remodelling of VipA/VipB tubules represents a crucial step in T6S, illuminating an unexpected role of an ATPase component in protein secretion.

Microarray profiling of host-extract-induced genes and characterization of the type VI secretion cluster in the potato pathogen Pectobacterium atrosepticum

Pectobacterium atrosepticum is a Gram-negative plant-pathogenic bacterium that rots potato stems and tubers. Microarray analysis was used to identify genes that were differentially expressed when host extracts were added to the growth medium. Potato extracts downregulated the expression of ribosomal genes and genes related to uptake and metabolism of nutrients, and upregulated genes needed for nitrate or phosphonate use. Some of the observed changes in gene expression in host-extract-induced cultures are similar to those during attachment of the bacterium to host tissues. Other responses indicated defence against toxic metabolites in the extract. Tuber extract induced a large gene cluster having homology to type VI secretion genes shown to be virulence determinants in many, but not all, animal and human pathogens. Two of the genes in the type VI cluster were found to be expressed during infection in potato tubers and stems, and mutants with knockouts of the corresponding genes had increased virulence on potato. One of the type VI secretion mutants was further characterized and found to grow to higher cell density in culture in the presence of host extract and to produce slightly more extracellular tissue-macerating enzymes than the wild-type strain. Analysis of secreted proteins showed that this type VI mutant was affected in the production of haemolysin-coregulated proteins (Hcps), which have been suggested to be secreted by the type VI pathway in other bacteria. The results suggest that the type VI secretion system of P. atrosepticum is needed for secretion of Hcps but not for virulence on its host plant, potato.

A citrate-inducible gene, encoding a putative tricarboxylate transporter, is downregulated by the organic solvent DMSO in Agrobacterium tumefaciens

AIMS

To investigate the effects of the organic solvent dimethyl sulfoxide (DMSO) on the expression of a citrate-inducible gene, encoding a putative tricarboxylate transporter, in Agrobacterium tumefaciens.

METHODS AND RESULTS

By two-dimensional gel electrophoresis, we discovered a putative tricarboxylate transporter named ActC, whose expression was downregulated by DMSO. The expression of actC is also induced by tricarboxylates but not affected by other organic acids of the TCA cycle. Intriguingly, transcriptional activation of actC by citrate is compromised in the presence of DMSO. Furthermore, expression of actC is abolished by deletion of actDE, encoding a putative two-component regulatory system upstream of the actCBA gene cluster.

CONCLUSIONS

actC is a citrate-inducible gene that is repressed by DMSO and whose expression is likely regulated by a two-component system.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides useful information as to a potential DMSO-regulatory system of A. tumefaciens or other soil bacteria when encountering DMSO in nature. In addition, DMSO-regulated genes should be taken into account for studies in which bacterial cultures were treated with compounds dissolved in DMSO.

The type VI secretion toolkit

Bacterial secretion systems are macromolecular complexes that release virulence factors into the medium or translocate them into the target host cell. These systems are widespread in bacteria allowing them to infect eukaryotic cells and survive or replicate within them. A new secretion system, the type VI secretion system (T6SS), was recently described and characterized in several pathogens. Genomic data suggest that T6SS exist in most bacteria that come into close contact with eukaryotic cells, including plant and animal pathogens. Many research groups are now investigating the underlying mechanisms and the way in which the effector proteins translocated through this machine subvert host defences. This review provides an overview of our current knowledge about type VI secretion, focusing on gene regulation, components of the secretion machine, substrate secretion and the cellular consequences for the host cell.