The actin-based motility defect of a Shigella flexneri rmlD rough LPS mutant is not due to loss of IcsA polarity

Detection of a disulphide bond and conformational changes in Shigella flexneri Wzy, and the role of cysteine residues in polymerase activity

Shigella flexneri utilises the Wzy-dependent pathway for the production of a plethora of complex polysaccharides, including the lipopolysaccharide O-antigen (Oag) component. The inner membrane protein Wzy_SF polymerises Oag repeat units, whilst two co-polymerase proteins, Wzz_SF and Wzz_pHS-2, together interact with Wzy_SF to regulate production of short- (S-Oag) and very long- (VL-Oag) Oag modal lengths, respectively. The 2D arrangement of Wzy_SF transmembrane and soluble regions has been previously deciphered, however, attaining information on the 3D structural and conformational arrangement of Wzy_SF, or any homologue, has proven difficult. For the first time, the current study detected insights into the in situ Wzy_SF arrangement. In vitro assays using thiol-reactive PEG-maleimide were used to probe Wzy_SF conformation, which additionally detected novel, unique conformational changes in response to interaction with intrinsic factors, including Wzz_SF and Wzz_pHS-2, and extrinsic factors, such as temperature. Site-directed mutagenesis of Wzy_SF cysteine residues revealed the presence of a putative intramolecular disulphide bond, between cysteine moieties 13 and 60. Subsequent analyses highlighted both the structural and functional importance of Wzy_SF cysteines. Substitution of Wzy_SF cysteine residues significantly decreased biosynthesis of the VL-Oag modal length, without disruption to S-Oag production. This phenotype was corroborated in the absence of co-polymerase competition for Wzy_SF interaction. These data suggest Wzy_SF cysteine substitutions directly impair the interaction between Wzy/Wzz_pHS-2, without altering the Wzy/Wzz_SF interplay, and in combination with structural data, we propose that the N- and C-termini of Wzy_SF are arranged in close proximity, and together may form the unique Wzz_pHS-2 interaction site.

Analysis of the Localization of NLRs upon Shigella flexneri Infection Exemplified by NOD1

NOD-like receptors (NLRs) are a family of pattern recognition receptors, able to respond to conserved microbial structures and endogenous danger signals. The NLR NOD1 responds to bacterial peptidoglycan, leading to recruitment of RIPK2, following activation of NFκB and MAPK pathways. In this chapter, we describe a fluorescent light microscopic approach to analyze the subcellular distribution of NOD1 upon infection with the invasive, Gram-negative bacterial pathogen Shigella flexneri. This method is based on exogenously expressed EGFP-tagged NOD1 and describes a protocol to obtain inducible cell lines with functional NOD1 signaling. The described protocol is useful to study NOD1 function, also in living cells, using live cell imaging and can be adopted for the study of other NLR proteins.

Human guanylate binding proteins: nanomachines orchestrating host defense

Disease-causing microorganisms not only breach anatomical barriers and invade tissues but also frequently enter host cells, nutrient-enriched environments amenable to support parasitic microbial growth. Protection from many infectious diseases is therefore reliant on the ability of individual host cells to combat intracellular infections through the execution of cell-autonomous defense programs. Central players in human cell-autonomous immunity are members of the family of dynamin-related guanylate binding proteins (GBPs). The importance of these interferon-inducible GTPases in host defense to viral, bacterial, and protozoan pathogens has been established for some time; only recently, cell biological and biochemical studies that largely focused on the prenylated paralogs GBP1, GBP2, and GBP5 have provided us with robust molecular frameworks for GBP-mediated immunity. Specifically, the recent characterization of GBP1 as a bona fide pattern recognition receptor for bacterial lipopolysaccharide (LPS) disrupting the integrity of bacterial outer membranes through LPS aggregation, the discovery of a link between hydrolysis-induced GMP production by GBP1 and inflammasome activation, and the classification of GBP2 and GBP5 as inhibitors of viral envelope glycoprotein processing via suppression of the host endoprotease furin have paved the way for a vastly improved conceptual understanding of the molecular mechanisms by which GBP nanomachines execute cell-autonomous immunity. The herein discussed models incorporate our current knowledge of the antimicrobial, proinflammatory, and biochemical properties of human GBPs and thereby provide testable hypotheses that will guide future studies into the intricacies of GBP-controlled host defense and their role in human disease.

The GBP1 microcapsule interferes with IcsA-dependent septin cage assembly around Shigella flexneri

Many cytosolic bacterial pathogens hijack the host actin polymerization machinery to form actin tails that promote direct cell-to-cell spread, enabling these pathogens to avoid extracellular immune defenses. However, these pathogens are still susceptible to intracellular cell-autonomous immune responses that restrict bacterial actin-based motility. Two classes of cytosolic antimotility factors, septins and guanylate-binding proteins (GBPs), have recently been established to block actin tail formation by the human-adapted bacterial pathogen Shigella flexneri. Both septin cages and GBP1 microcapsules restrict S. flexneri cell-to-cell spread by blocking S. flexneri actin-based motility. While septins assemble into cage-like structures around immobile S. flexneri, GBP1 forms microcapsules around both motile and immobile bacteria. The interplay between these two defense programs remains elusive. Here, we demonstrate that GBP1 microcapsules block septin cage assembly, likely by interfering with the function of S. flexneri IcsA, the outer membrane protein that promotes actin-based motility, as this protein is required for septin cage formation. However, S. flexneri that escape from GBP1 microcapsules via the activity of IpaH9.8, a type III secreted effector that promotes the degradation of GBPs, are often captured within septin cages. Thus, our studies reveal how septin cages and GBP1 microcapsules represent complementary host cell antimotility strategies.

Conserved transmembrane glycine residues in the Shigella flexneri polysaccharide co-polymerase protein WzzB influence protein-protein interactions

The O antigen (Oag) component of lipopolysaccharides (LPS) is crucial for virulence and Oag chain-length regulation is controlled by the polysaccharide co-polymerase class 1 (PCP1) proteins. Crystal structure analyses indicate that structural conservation among PCP1 proteins is highly maintained, however the mechanism of Oag modal-chain-length control remains to be fully elucidated. Shigella flexneri PCP1 protein WzzBSF confers a modal-chain length of 10-17 Oag repeat units (RUs), whereas the Salmonella enterica Typhimurium PCP1 protein WzzBST confers a modal-chain length of ~16-28 Oag RUs. Both proteins share >70 % overall sequence identity and contain two transmembrane (TM1 and TM2) regions, whereby a conserved proline-glycine-rich motif overlapping the TM2 region is identical in both proteins. Conserved glycine residues within TM2 are functionally important, as glycine to alanine substitutions at positions 305 and 311 confer very short Oag modal-chain length (~2-6 Oag RUs). In this study, WzzBSF was co-expressed with WzzBST in S. flexneri and a single intermediate modal-chain length of ~11-21 Oag RUs was observed, suggesting the presence of Wzz:Wzz interactions. Interestingly, co-expression of WzzBSF with WzzBG305A/G311A conferred a bimodal LPS Oag chain length (despite over 99 % protein sequence identity), and we hypothesized that the proteins fail to interact. Co-purification assays detected His6-WzzBSF co-purifying with FLAG-tagged WzzBST but not with FLAG-tagged WzzBG305A/G311A, supporting our hypothesis. These data indicate that the conserved glycine residues in TM2 are involved in Wzz:Wzz interactions, and provide insight into key interactions that drive Oag modal length control.

Structural and Biochemical Analysis of a Single Amino-Acid Mutant of WzzBSF That Alters Lipopolysaccharide O-Antigen Chain Length in Shigella flexneri

Lipopolysaccharide (LPS), a surface polymer of Gram-negative bacteria, helps bacteria survive in different environments and acts as a virulence determinant of host infection. The O-antigen (Oag) component of LPS exhibits a modal chain-length distribution that is controlled by polysaccharide co-polymerases (PCPs). The molecular basis of the regulation of Oag chain-lengths remains unclear, despite extensive mutagenesis and structural studies of PCPs from Escherichia coli and Shigella. Here, we identified a single mutation (A107P) of the Shigella flexneri WzzBSF, by a random mutagenesis approach, that causes a shortened Oag chain-length distribution in bacteria. We determined the crystal structures of the periplasmic domains of wild-type WzzBSF and the A107P mutant. Both structures form a highly similar open trimeric assembly in the crystals, and show a similar tendency to self-associate in solution. Binding studies by bio-layer interferometry reveal cooperative binding of very short (VS)-core-plus-O-antigen polysaccharide (COPS) to the periplasmic domains of both proteins, but with decreased affinity for the A107P mutant. Our studies reveal that subtle and localized structural differences in PCPs can have dramatic effects on LPS chain-length distribution in bacteria, for example by altering the affinity for the substrate, which supports the role of the structure of the growing Oag polymer in this process.

A small conserved motif supports polarity augmentation of Shigella flexneri IcsA

The rod-shaped enteric intracellular pathogen Shigella flexneri and other Shigella species are the causative agents of bacillary dysentery. S. flexneri are able to spread within the epithelial lining of the gut, resulting in lesion formation, cramps and bloody stools. The outer membrane protein IcsA is essential for this spreading process. IcsA is the initiator of an actin-based form of motility whereby it allows the formation of a filamentous actin 'tail' at the bacterial pole. Importantly, IcsA is specifically positioned at the bacterial pole such that this process occurs asymmetrically. The mechanism of IcsA polarity is not completely understood, but it appears to be a multifactorial process involving factors intrinsic to IcsA and other regulating factors. In this study, we further investigated IcsA polarization by its intramolecular N-terminal and central polar-targeting (PT) regions (nPT and cPT regions, respectively). The results obtained support a role in polar localization for the cPT region and contend the role of the nPT region. We identified single IcsA residues that have measurable impacts on IcsA polarity augmentation, resulting in decreased S. flexneri sprading efficiency. Intriguingly, regions and residues involved in PT clustered around a highly conserved motif which may provide a functional scaffold for polarity-augmenting residues. How these results fit with the current model of IcsA polarity determination is discussed.

Detection of Wzy/Wzz interaction in Shigella flexneri

The O antigen (Oag) component of Shigella flexneri lipopolysaccharide (LPS) is important for virulence and a protective antigen. It is synthesized by the Wzy-dependent mechanism. S. flexneri Wzy has 12 transmembrane segments and two large periplasmic loops. The modal chain length of the Oag is determined by Wzz. Experimental evidence supports multi-protein interactions in the Wzy-dependent pathway. However, evidence for direct interaction of Wzy with the other proteins of the Wzy-dependent pathway is limited. Initially, we purified Wzy-GFP-His8 and detected the presence of a dimeric form. In vivo cross-linking was then performed in an S. flexneri wzy mutant strain carrying plasmids encoding Wzy-GFP-His8 and untagged Wzz. Following solubilization with n-dodecyl-β-D-maltopyranoside (DDM) and affinity purification of Wzy-GFP-His8, Western immunoblotting with Wzz antibody detected co-purification of Wzz; this was supported by MS analysis. To the best of our knowledge, this is the first reported isolation of a complex between Wzy and Wzz. Wzy mutants (WzyR164A, WzyV92M, WzyY137H, and WzyR250K) whose properties are affected by Wzz were able to form complexes with Wzz. Their mutational alterations did not affect the interaction of Wzy with Wzz. Thus, the interaction may involve many regions of Wzy.

Shigella flexneri cell-to-cell spread, and growth and inflammation in mice, is limited by the outer membrane protease IcsP

The Shigella flexneri autotransporter protein IcsA is essential for intra- and intercellular spread, and icsA mutants are attenuated in several models. However, the pathogenic significance of the outer membrane protease IcsP, which orchestrates the polar distribution of IcsA on the bacterial surface, remains unclear. To further examine this point, we constructed icsP mutants in the two most commonly studied S. flexneri strains and evaluated their in vitro and in vivo performance relative to wild type. Both icsP mutants showed aberrant surface distribution of IcsA, but the in vitro consequences depended upon the cell line being used to assess bacterial motility and plaque formation. Evaluating the behaviour of the mutants in two mouse models suggested functional expression of icsP might limit bacterial persistence and the associated inflammation in host tissues, consistent with the findings in one of the three cell lines used.

The passenger-associated transport repeat promotes virulence factor secretion efficiency and delineates a distinct autotransporter subtype

Autotransporters are a superfamily of virulence factors secreted by Gram negative bacteria. They are comprised of an N-terminal passenger domain that is translocated across the outer membrane and a C-terminal domain that inserts into the outer membrane forming a β-barrel anchor. It is still poorly understood how the passenger is efficiently translocated in the absence of external energy inputs. Several mechanisms have been proposed in solution of this problem, yet due to the vast diversity of size, sequence and function of the passenger, it is not clear how widely these mechanisms are employed. In this study we functionally characterize a conserved repeat found in many passengers that we designate the Passenger-associated Transport Repeat (PATR). Using the autotransporter IcsA from the enteropathogen Shigella flexneri, we identified conserved PATR residues that are required for efficient export of the passenger during growth and infection. Furthermore, PATR-containing autotransporters are significantly larger than non-PATR autotransporters, with PATR copy number correlating with passenger size. We also show that PATR-containing autotransporters delineate a subgroup that associates with specific virulence traits and architectures. These results advance our understanding of autotransporter composition and indicate that an additional transport mechanism is important for thousands of these proteins.

Myosin IIA is essential for Shigella flexneri cell-to-cell spread

A key feature of Shigella pathogenesis is the ability to spread from cell-to-cell post-invasion. This is dependent on the bacteria's ability to initiate de novo F-actin tail polymerisation, followed by protrusion formation, uptake of bacteria-containing protrusion and finally, lysis of the double membrane vacuole in the adjacent cell. In epithelial cells, cytoskeletal tension is maintained by the actin-myosin II networks. In this study, the role of myosin II and its specific kinase, myosin light chain kinase (MLCK), during Shigella intercellular spreading was investigated in HeLa cells. Inhibition of MLCK and myosin II, as well as myosin IIA knockdown, significantly reduced Shigella plaque and infectious focus formation. Protrusion formation and intracellular bacterial growth was not affected. Low levels of myosin II were localised to the Shigella F-actin tail. HeLa cells were also infected with Shigella strains defective in cell-to-cell spreading. Unexpectedly loss of myosin IIA labelling was observed in HeLa cells infected with these mutant strains. This phenomenon was not observed with WT Shigella or with the less abundant myosin IIB isoform, suggesting a critical role for myosin IIA.

Relationship between O-antigen chain length and resistance to colicin E2 in Shigella flexneri

The Shigella flexneri polysaccharide co-polymerase class 1a (PCP1a) protein, WzzBSF, regulates LPS O-antigen (Oag) chain length to confer short (S)-type Oag chains of ~10-17 Oag repeat units (RUs). The S-type Oag chains affect Shigella flexneri virulence as they influence IcsA-mediated actin-based motility. However, they do not confer resistance to complement; this is conferred by the very-long (VL)-type Oag chains determined by WzzB(pHS2). Colicins are bacterial proteins produced by some Escherichia coli strains to kill related strains. While the presence of Oag chains has been shown to shield outer-membrane proteins from colicins, the impact of Oag chain length against colicins is unknown. In this study, initial testing indicated that a Shigella flexneri Y wzz : : kan(r) mutant was more sensitive to colicin E2 compared with the WT strain. Plasmids encoding Wzz mutant and WT PCP1a proteins conferring different Oag modal chain lengths were then expressed in the mutant background, and tested against purified colicin E2. Analysis of swab and spot sensitivity assays showed that strains expressing either S-type or long (L)-type Oag chains (16-28 Oag RUs) conferred greater resistance to colicin E2 compared with strains having very-short-type (2-8 Oag RUs), intermediate-short-type (8-14 Oag RUs) or VL-type (>80 Oag RUs) Oag chains. These results suggest a novel role for LPS Oag chain length control that may have evolved due to selection pressure from colicins in the environment.

Residues located inside the Escherichia coli FepE protein oligomer are essential for lipopolysaccharide O-antigen modal chain length regulation

The Escherichia coli O157 : H7 FepE protein regulates lipopolysaccharide (LPS) O-antigen (Oag) chain length to confer a very long modal chain length of >80 Oag repeat units (RUs). The mechanism by which FepE regulates Oag modal chain length and the regions within it that are important for its function remain unclear. Studies on the structure of FepE show that the protein oligomerizes. However, the exact size of the oligomer is in dispute, further hampering our understanding of its mechanism. Guided by information previously obtained for regions known to be important for Oag modal chain length determination in the homologous Shigella flexneri WzzBSF protein, a set of FepE mutant constructs with single amino acid substitutions was created. Analysis of the resulting LPS conferred by these mutant His6-FepE proteins showed that amino acid substitutions of leucine 168 (L168) and aspartic acid 268 (D268) resulted in LPS with consistently shortened Oag chain lengths of <80 Oag RUs. Substitution of FepE's transmembrane cysteine residues did not affect function. Chemical cross-linking experiments on mutant FepE proteins showed no consistent correlation between oligomer size and functional activity, and MS analysis of FepE oligomers indicated that the in vivo size of FepE is consistent with a maximum size of a hexamer. Our findings suggest that different FepE residues, mainly located within the internal cavity of the oligomer, contribute to Oag modal chain length determination but not the oligomeric state of the protein.

Identification of Shigella flexneri IcsA residues affecting interaction with N-WASP, and evidence for IcsA-IcsA co-operative interaction

The Shigella flexneri IcsA (VirG) protein is a polarly distributed outer membrane protein that is a fundamental virulence factor which interacts with neural Wiskott-Aldrich syndrome protein (N-WASP). The activated N-WASP then activates the Arp2/3 complex which initiates de novo actin nucleation and polymerisation to form F-actin comet tails and allows bacterial cell-to-cell spreading. In a previous study, IcsA was found to have three N-WASP interacting regions (IRs): IR I (aa 185-312), IR II (aa 330-382) and IR III (aa 508-730). The aim of this study was to more clearly define N-WASP interacting regions II and III by site-directed mutagenesis of specific amino acids. Mutant IcsA proteins were expressed in both smooth lipopolysaccharide (S-LPS) and rough LPS (R-LPS) S. flexneri strains and characterised for IcsA production level, N-WASP recruitment and F-actin comet tail formation. We have successfully identified new amino acids involved in N-WASP recruitment within different N-WASP interacting regions, and report for the first time using co-expression of mutant IcsA proteins, that N-WASP activation involves interactions with different regions on different IcsA molecules as shown by Arp3 recruitment. In addition, our findings suggest that autochaperone (AC) mutant protein production was not rescued by another AC region provided in trans, differing to that reported for two other autotransporters, PrtS and BrkA autotransporters.

Outer membrane protein A (OmpA): a new player in shigella flexneri protrusion formation and inter-cellular spreading

Outer membrane protein A (OmpA) is a multifaceted predominant outer membrane protein of Escherichia coli and other Enterobacteriaceae whose role in the pathogenesis of various bacterial infections has recently been recognized. Here, the role of OmpA on the virulence of Shigella flexneri has been investigated. An ompA mutant of wild-type S. flexneri 5a strain M90T was constructed (strain HND92) and it was shown to be severely impaired in cell-to-cell spreading since it failed to plaque on HeLa cell monolayers. The lack of OmpA significantly reduced the levels of IcsA while the levels of cell associated and released IcsP-cleaved 95 kDa amino-terminal portion of the mature protein were similar. Nevertheless, the ompA mutant displayed IcsA exposed across the entire bacterial surface. Surprisingly, the ompA mutant produced proper F-actin comet tails, indicating that the aberrant IcsA exposition at bacterial lateral surface did not affect proper activation of actin-nucleating proteins, suggesting that the absence of OmpA likely unmasks mature or cell associated IcsA at bacterial lateral surface. Moreover, the ompA mutant was able to invade and to multiply within HeLa cell monolayers, although internalized bacteria were found to be entrapped within the host cell cytoplasm. We found that the ompA mutant produced significantly less protrusions than the wild-type strain, indicating that this defect could be responsible of its inability to plaque. Although we could not definitely rule out that the ompA mutation might exert pleiotropic effects on other S. flexneri genes, complementation of the ompA mutation with a recombinant plasmid carrying the S. flexneri ompA gene clearly indicated that a functional OmpA protein is required and sufficient for proper IcsA exposition, plaque and protrusion formation. Moreover, an independent ompA mutant was generated. Since we found that both mutants displayed identical virulence profile, these results further supported the findings presented in this study.

Absence of O antigen suppresses Shigella flexneri IcsA autochaperone region mutations

The Shigella flexneri IcsA (VirG) protein is a polarly distributed autotransporter protein. IcsA functions as a virulence factor by interacting with the host actin regulatory protein N-WASP, which in turn activates the Arp2/3 complex, initiating actin polymerization. Formation of F-actin comet tails allows bacterial cell-to-cell spreading. Although various accessory proteins such as periplasmic chaperones and the β-barrel assembly machine (BAM) complex have been shown to be involved in the export of IcsA, the IcsA translocation mechanism remains to be fully elucidated. A putative autochaperone (AC) region (amino acids 634-735) located at the C-terminal end of the IcsA passenger domain, which forms part of the self-associating autotransporter (SAAT) domain, has been suggested to be required for IcsA biogenesis, as well as for N-WASP recruitment, based on mutagenesis studies. IcsA(i) proteins with linker insertion mutations within the AC region have a significant reduction in production and are defective in N-WASP recruitment when expressed in smooth LPS (S-LPS) S. flexneri. In this study, we have found that the LPS O antigen plays a role in IcsA(i) production based on the use of an rmlD (rfbD) mutant having rough LPS (R-LPS) and a novel assay in which O antigen is depleted using tunicamycin treatment and then regenerated. In addition, we have identified a new N-WASP binding/interaction site within the IcsA AC region.

Self-association of the Shigella flexneri IcsA autotransporter protein

The IcsA autotransporter protein is a major virulence factor of the human intracellular pathogen Shigella flexneri. IcsA is distributed at the poles in the outer membrane (OM) of S. flexneri and interacts with components of the host actin-polymerization machinery to facilitate intracellular actin-based motility and subsequent cell-to-cell spreading of the bacterium. We sought to characterize the biochemical properties of IcsA in the bacterial OM. Chemical cross-linking data suggested that IcsA exists in a complex in the OM. Furthermore, reciprocal co-immunoprecipitation of differentially epitope-tagged IcsA proteins indicated that IcsA is able to self-associate. The identification of IcsA linker-insertion mutants that were negatively dominant provided genetic evidence of IcsA-IcsA interactions. From these results, we propose a model whereby IcsA self-association facilitates efficient actin-based motility.

Mutagenesis and chemical cross-linking suggest that Wzz dimer stability and oligomerization affect lipopolysaccharide O-antigen modal chain length control

In Shigella flexneri, the polysaccharide copolymerase (PCP) protein Wzz(SF) confers a modal length of 10 to 17 repeat units (RUs) to the O-antigen (Oag) component of lipopolysaccharide (LPS). PCPs form oligomeric structures believed to be related to their function. To identify functionally important regions within Wzz(SF), random in-frame linker mutagenesis was used to create mutants with 5-amino-acid insertions (termed Wzz(i) proteins), and DNA sequencing was used to locate the insertions. Analysis of the resulting LPS conferred by Wzz(i) proteins identified five mutant classes. The class I mutants were inactive, resulting in nonregulated LPS Oag chains, while classes II and III conferred shorter LPS Oag chains of 2 to 10 and 8 to 14 RUs, respectively. Class IV mutants retained near-wild-type function, and class V mutants increased the LPS Oag chain length to 16 to 25 RUs. In vivo formaldehyde cross-linking indicated class V mutants readily formed high-molecular-mass oligomers; however, class II and III Wzz(i) mutants were not effectively cross-linked. Wzz dimer stability was also investigated by heating cross-linked oligomers at 100 degrees C in the presence of SDS. Unlike the Wzz(SF) wild type and class IV and V Wzz(i) mutants, the class II and III mutant dimers were not detectable. The location of each insertion was mapped onto available PCP three-dimensional (3D) structures, revealing that class V mutations were most likely located within the inner cavity of the PCP oligomer. These data suggest that the ability to produce stable dimers may be important in determining Oag modal chain length.

Mutagenesis of the Shigella flexneri autotransporter IcsA reveals novel functional regions involved in IcsA biogenesis and recruitment of host neural Wiscott-Aldrich syndrome protein

The IcsA (VirG) protein of Shigella flexneri is a polarly localized, outer membrane protein that is essential for virulence. Within host cells, IcsA activates the host actin regulatory protein, neural Wiskott-Aldrich syndrome protein (N-WASP), which in turn recruits the Arp2/3 complex, which nucleates host actin to form F-actin comet tails and initiate bacterial motility. Linker insertion mutagenesis was undertaken to randomly introduce 5-amino-acid in-frame insertions within IcsA. Forty-seven linker insertion mutants were isolated and expressed in S. flexneri Delta icsA strains. Mutants were characterized for IcsA protein production, cell surface expression and localization, intercellular spreading, F-actin comet tail formation, and N-WASP recruitment. Using this approach, we have identified a putative autochaperone region required for IcsA biogenesis, and our data suggest an additional region, not previously identified, is required for N-WASP recruitment.

O-antigen modal chain length in Shigella flexneri 2a is growth-regulated through RfaH-mediated transcriptional control of the wzy gene

Shigella flexneri 2a 2457T produces lipopolysaccharide (LPS) with two O-antigen (OAg) chain lengths: a short (S-OAg) controlled by WzzB and a very long (VL-OAg) determined by Wzz(pHS-2). This study demonstrates that the synthesis and length distribution of the S. flexneri OAg are under growth-phase-dependent regulation. Quantitative electrophoretic analysis showed that the VL-OAg increased during growth while the S-OAg distribution remained constant. Increased production of VL-OAg correlated with the growth-phase-regulated expression of the transcription elongation factor RfaH, and was severely impaired in a DeltarfaH mutant, which synthesized only low-molecular-mass OAg molecules and a small amount of S-OAg. Real-time RT-PCR revealed a drastic reduction of wzy polymerase gene expression in the DeltarfaH mutant. Complementation of this mutant with the wzy gene cloned into a high-copy-number plasmid restored the bimodal OAg distribution, suggesting that cellular levels of Wzy influence not only OAg polymerization but also chain-length distribution. Accordingly, overexpression of wzy in the wild-type strain resulted in production of a large amount of high-molecular-mass OAg molecules. An increased dosage of either wzzB or wzz(pHS-2) also altered OAg chain-length distribution. Transcription of wzzB and wzz(pHS-2) genes was regulated during bacterial growth but in an RfaH-independent manner. Overall, these findings indicate that expression of the wzy, wzzB and wzz(pHS-2) genes is finely regulated to determine an appropriate balance between the proteins responsible for polymerization and chain-length distribution of S. flexneri OAg.

A group of Escherichia coli and Salmonella enterica O antigens sharing a common backbone structure

The O-antigen moiety of the LPS is one of the most variable cell surface components of the Gram-negative bacterial outer membrane. Variation is due to the presence of different sugars and sugar linkages. Here, it is reported that a group of Escherichia coli O serogroups (O17, O44, O73, O77 and O106), and the Salmonella enterica serogroup O : 6,14 (H), share a common four-sugar backbone O-subunit structure, and possess almost identical O-antigen gene clusters. Whereas the E. coli O77 antigen does not have any substitutions, the other O antigens in this group differ by the addition of one or two glucose side branches at various positions of the backbone. The O-antigen gene clusters for all members of the group encode only the proteins required for biosynthesis of the common four-sugar backbone. The identification of three genes within a putative prophage in the E. coli O44 genome is also reported; these genes are presumably involved in the glucosylation of the basic tetrasaccharide unit. This was confirmed by deletion of one of the genes, which encodes a putative glucosyltransferase. Structural analysis of the O antigen produced by the mutant strain demonstrated the absence of glucosylation. An O-antigen structure shared by five E. coli and one S. enterica serogroups, all of which have a long evolutionary history, suggests that the common backbone may be important for the survival of E. coli strains in the environment, or for their pathogenicity.

Inactivation of Endotoxin by Human Plasma Gelsolin

Septic shock from bacterial endotoxin, triggered by the release of lipopolysaccharide (LPS) molecules from the outer wall of Gram-negative bacteria, is a major cause of human death for which there is no effective treatment once the complex inflammatory pathways stimulated by these small amphipathic molecules are activated. Here we report that plasma gelsolin, a highly conserved human protein, binds LPS from various bacteria with high affinity. Solid-phase binding assays, fluorescence measurements, and functional assays of actin depolymerizing effects show that gelsolin binds more tightly to LPS than it does to its other known lipid ligands, phosphatidylinositol 4,5-bisphosphate and lysophosphatidic acid. Gelsolin also competes with LPS-binding protein (LBP), a high-affinity carrier for LPS. One result of gelsolin-LPS binding is inhibition of the actin binding activity of gelsolin as well as the actin depolymerizing activity of blood serum. Simultaneously, effects of LPS on cellular functions, including cytoskeletal actin remodeling, and collagen-induced platelet activation by pathways independent of toll-like receptors (TLRs) are neutralized by gelsolin and by a peptide based on gelsolin residues 160-169 (GSN160-169) which comprise part of gelsolin's phosphoinositide binding site. Additionally, TLR-dependent NF-kappaB translocation in astrocytes appears to be blocked by gelsolin. These results show a strong effect of LPS on plasma gelsolin function and suggest that some effects of endotoxin in vivo may be mediated or inhibited by plasma gelsolin.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.