Insertion mutagenesis of XpsD, an outer-membrane protein involved in extracellular protein secretion in Xanthomonas campestris pv. campestris

Identification of an Extracellular Endoglucanase That Is Required for Full Virulence in Xanthomonas citri subsp. citri

Xanthomonas citri subsp. citri causes citrus canker disease, which is characterized by the formation of water-soaked lesions, white or yellow spongy pustules and brown corky canker. In this work, we report the contribution of extracellular endoglucanase to canker development during infection. The ectopic expression of nine putative cellulases in Escherichia coli indicated that two endoglucanases, BglC3 and EngXCA, show carboxymethyl cellulase activity. Both bglC3 and engXCA genes were transcribed in X. citri subsp. citri, however, only BglC3 protein was detected outside the cell in western blot analysis. The deletion of bglC3 gene resulted in complete loss of extracellular carboxymethyl cellulase activity and delayed the onset of canker symptoms in both infiltration- and wound-inoculation assays. When growing in plant tissue, the cell density of bglC3 mutant was lower than that of the wild type. Our data demonstrated that BglC3 is an extracellular endoglucanase required for the full virulence of X. citri subsp. citri.

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.

Xylella fastidiosa gene expression analysis by DNA microarrays

Xylella fastidiosa genome sequencing has generated valuable data by identifying genes acting either on metabolic pathways or in associated pathogenicity and virulence. Based on available information on these genes, new strategies for studying their expression patterns, such as microarray technology, were employed. A total of 2,600 primer pairs were synthesized and then used to generate fragments using the PCR technique. The arrays were hybridized against cDNAs labeled during reverse transcription reactions and which were obtained from bacteria grown under two different conditions (liquid XDM(2) and liquid BCYE). All data were statistically analyzed to verify which genes were differentially expressed. In addition to exploring conditions for X. fastidiosa genome-wide transcriptome analysis, the present work observed the differential expression of several classes of genes (energy, protein, amino acid and nucleotide metabolism, transport, degradation of substances, toxins and hypothetical proteins, among others). The understanding of expressed genes in these two different media will be useful in comprehending the metabolic characteristics of X. fastidiosa, and in evaluating how important certain genes are for the functioning and survival of these bacteria in plants.

Two Xanthomonas extracellular polygalacturonases, PghAxc and PghBxc, are regulated by type III secretion regulators HrpX and HrpG and are required for virulence

Xanthomonas campestris pv. campestris, the causal agent of black rot disease, produces a suite of extracellular cell-wall degrading enzymes (CWDE) that are involved in bacterial virulence. Polygalacturonase (PG) is an important CWDE and functions to degrade the pectic layers of plant cell walls. Although previous studies have documented the virulence functions of PG in Erwinia and Ralstonia species, the regulation of PG genes still needs to be elucidated. In this study, we identified two novel PG genes (pghAxc and pghBxc) encoding functional PG from X. campestris pv. campestris 8004. The expressions of these two PG genes are regulated by the type III secretion regulators HrpX and HrpG and the global regulator Clp. These PG genes could be efficiently induced in planta and were required for the full virulence of X. campestris pv. campestris to Arabidopsis. In addition, these PG were confirmed to be secreted via the type II secretion system in an Xps-dependent manner.

Subcomplexes from the Xcp secretion system of Pseudomonas aeruginosa

In Gram-negative bacteria, most of the sec-dependent exoproteins are secreted via the type II secretion system (T2SS or secreton). In Pseudomonas aeruginosa, T2SS consists of 12 Xcp proteins (XcpA and XcpP to XcpZ) organized as a multiproteic complex within the envelope. In this study, by a co-purification approach using a His-tagged XcpZ as a bait, XcpY and XcpZ were found associated together to constitute the most stable functional unit so far isolated from the P. aeruginosa secreton. This subcomplex was also found to interact with XcpR and XcpS to form a XcpRSYZ complex which was isolated under native conditions. Another component, XcpP was not found to be associated to the complex but the results suggest that it can transiently interact with the XcpYZ subcomplex in vivo.

Agrobacterium tumefaciens VirB9, an outer-membrane-associated component of a type IV secretion system, regulates substrate selection and T-pilus biogenesis

Agrobacterium tumefaciens translocates DNA and protein substrates between cells via a type IV secretion system (T4SS) whose channel subunits include the VirD4 coupling protein, VirB11 ATPase, VirB6, VirB8, VirB2, and VirB9. In this study, we used linker insertion mutagenesis to characterize the contribution of the outer-membrane-associated VirB9 to assembly and function of the VirB/D4 T4SS. Twenty-five dipeptide insertion mutations were classified as permissive for intercellular substrate transfer (Tra+), completely transfer defective (Tra-), or substrate discriminating, e.g., selectively permissive for transfer only of the oncogenic transfer DNA and the VirE2 protein substrates or of a mobilizable IncQ plasmid substrate. Mutations inhibiting transfer of DNA substrates did not affect formation of close contacts of the substrate with inner membrane channel subunits but blocked formation of contacts with the VirB2 and VirB9 channel subunits, which is indicative of a defect in assembly or function of the distal portion of the secretion channel. Several mutations in the N- and C-terminal regions disrupted VirB9 complex formation with the outer-membrane-associated lipoprotein VirB7 or the inner membrane energy sensor VirB10. Several VirB9.i2-producing Tra+ strains failed to elaborate T pilus at detectable levels (Pil-), and three such Tra+ Pil- mutant strains were rendered Tra- upon deletion of virB2, indicating that the cellular form of pilin protein is essential for substrate translocation. Our findings, together with computer-based analyses, support a model in which distinct domains of VirB9 contribute to substrate selection and translocation, establishment of channel subunit contacts, and T-pilus biogenesis.

Identification of a new membrane-associated protein that influences transport/maturation of gingipains and adhesins of Porphyromonas gingivalis

The dual membrane envelopes of Gram-negative bacteria provide two barriers of unlike nature that regulate the transport of molecules into and out of organisms. Organisms have developed several systems for transport across the inner and outer membranes. The Gram-negative periodontopathogenic bacterium Porphyromonas gingivalis produces proteinase and adhesin complexes, gingipains/adhesins, on the cell surface and in the extracellular milieu as one of the major virulence factors. Gingipains and/or adhesins are encoded by kgp, rgpA, rgpB, and hagA on the chromosome. In this study, we isolated a P. gingivalis mutant (porT), which showed very weak activities of gingipains in the cell lysates and culture supernatants. Subcellular fractionation and immunoblot analysis demonstrated that precursor forms of gingipains and adhesins were accumulated in the periplasmic space of the porT mutant cells. Peptide mass fingerprinting and N-terminal amino acid sequencing of the precursor proteins and the kgp'-'rgpB chimera gene product in the porT mutant indicated that these proteins lacked the signal peptide regions, consistent with their accumulation in the periplasm. The PorT protein seemed to be membrane-associated and exposed to the periplasmic space, as revealed by subcellular fractionation and immunoblot analysis using anti-PorT antiserum. These results suggest that the membrane-associated protein PorT is essential for transport of the kgp, rgpA, rgpB, and hagA gene products across the outer membrane from the periplasm to the cell surface, where they are processed and matured.

Protein-translocating outer membrane porins of Gram-negative bacteria

Five families of outer membrane porins that function in protein secretion in Gram-negative bacteria are currently recognized. In this report, these five porin families are analyzed from structural and phylogenetic standpoints. They are the fimbrial usher protein (FUP), outer membrane factor (OMF), autotransporter (AT), two-partner secretion (TPS) and outer membrane secretin (Secretin) families. All members of these families in the current databases were identified, and all full-length homologues were multiply aligned for structural and phylogenetic analyses. The organismal distribution of homologues in each family proved to be unique with some families being restricted to proteobacteria and others being widespread in other bacterial kingdoms as well as eukaryotes. The compositions of and size differences between subfamilies provide evidence for specific orthologous relationships, which agree with available functional information and intra-subfamily phylogeny. The results reveal that horizontal transfer of genes encoding these proteins between phylogenetically distant organisms has been exceptionally rare although transfer within select bacterial kingdoms may have occurred. The resultant in silico analyses are correlated with available experimental evidence to formulate models relevant to the structures and evolutionary origins of these proteins.

Secreted enzymatic activities of wild-type and pilD-deficient Legionella pneumophila

Legionella pneumophila, the agent of Legionnaires' disease, is an intracellular pathogen of protozoa and macrophages. Previously, we had determined that the Legionella pilD gene is involved in type IV pilus biogenesis, type II protein secretion, intracellular infection, and virulence. Since the loss of pili and a protease do not account for the infection defect exhibited by a pilD-deficient strain, we sought to define other secreted proteins absent in the mutant. Based upon the release of p-nitrophenol (pNP) from p-nitrophenyl phosphate, acid phosphatase activity was detected in wild-type but not in pilD mutant supernatants. Mutant supernatants also did not release either pNP from p-nitrophenyl caprylate and palmitate or free fatty acid from 1-monopalmitoylglycerol, suggesting that they lack a lipase-like activity. However, since wild-type samples failed to release free fatty acids from 1,2-dipalmitoylglycerol or to cleave a triglyceride derivative, this secreted activity should be viewed as an esterase-monoacylglycerol lipase. The mutant supernatants were defective for both release of free fatty acids from phosphatidylcholine and degradation of RNA, indicating that PilD-negative bacteria lack a secreted phospholipase A (PLA) and nuclease. Finally, wild-type but not mutant supernatants liberated pNP from p-nitrophenylphosphorylcholine (pNPPC). Characterization of a new set of mutants defective for pNPPC-hydrolysis indicated that this wild-type activity is due to a novel enzyme, as opposed to a PLC or another known enzyme. Some, but not all, of these mutants were greatly impaired for intracellular infection, suggesting that a second regulator or processor of the pNPPC hydrolase is critical for L. pneumophila virulence.

Genetic dissection of the outer membrane secretin PulD: are there distinct domains for multimerization and secretion specificity?

Linker and deletion mutagenesis and gene fusions were used to probe the possible domain structure of the dodecameric outer membrane secretin PulD from the pullulanase secretion pathway of Klebsiella oxytoca. Insertions of 24 amino acids close to or within strongly predicted and highly conserved amphipathic beta strands in the C-terminal half of the polypeptide (the beta domain) abolished sodium dodecyl sulfate (SDS)-resistant multimer formation that is characteristic of this protein, whereas insertions elsewhere generally had less dramatic effects on multimer formation. However, the beta domain alone did not form SDS-resistant multimers unless part of the N-terminal region of the protein (the N domain) was produced in trans. All of the insertions except one, close to the C terminus of the protein, abolished function. The N domain alone was highly unstable and did not form SDS-resistant multimers even when the beta domain was present in trans. We conclude that the beta domain is a major determinant of multimer stability and that the N domain contributes to multimer formation. The entire or part of the N domain of PulD could be replaced by the corresponding region of the OutD secretin from the pectate lyase secretion pathway of Erwinia chrysanthemi without abolishing pullulanase secretion. This suggests that the N domain of PulD is not involved in substrate recognition, contrary to the role proposed for the N domain of OutD, which binds specifically to pectate lyase secreted by E. chrysanthemi (V. E. Shevchik, J. Robert-Badouy, and G. Condemine, EMBO J. 16:3007-3016, 1997).

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

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