Structural mimicry in bacterial virulence

An important mechanism underlying the strategies used by microbial pathogens to manipulate cellular functions is that of functional mimicry of host activities. In some cases, mimicry is achieved through virulence factors that are direct homologues of host proteins. In others, convergent evolution has produced new effectors that, although having no obvious amino-acid sequence similarity to host factors, are revealed by structural studies to display mimicry at the molecular level.

CdtA, CdtB, and CdtC form a tripartite complex that is required for cytolethal distending toxin activity

Campylobacter jejuni encodes a cytolethal distending toxin (CDT) that causes cells to arrest in the G(2)/M transition phase of the cell cycle. Highly related toxins are also produced by other important bacterial pathogens. CDT activity requires the function of three genes: cdtA, cdtB, and cdtC. Recent studies have established that CdtB is the active subunit of CDT, exerting its effect as a nuclease that damages the DNA and triggers cell cycle arrest. Microinjection of CdtB into target cells led to G(2)/M arrest and cytoplasmic distention, in a manner indistinguishable from that caused by CDT treatment. Despite this progress, nothing is known about the composition of the CDT holotoxin or the function of CdtA and CdtC. We show here that, when applied individually, purified CdtA, CdtB, or CdtC does not exhibit toxic activity. In contrast, CdtA, CdtB, and CdtC when combined, interact with one another to form an active tripartite holotoxin that exhibits full cellular toxicity. CdtA has a domain that shares similarity with the B chain of ricin-related toxins. We therefore proposed that CDT is a tripartite toxin composed of CdtB as the enzymatically active subunit and of CdtA and CdtC as the heterodimeric B subunit required for the delivery of CdtB.

Characterization of the mutS-proximal region of the Salmonella typhimurium SPI-1 identifies a group of pathogenicity island-associated genes

The virulence properties of Salmonella enterica are largely encoded within a set of horizontally acquired gene blocks termed pathogenicity islands. One such pathogenicity island, SPI-1, located at centisome 63 of the Salmonella chromosome between the mutS and fhlA genes, encodes a type III protein secretion system and an iron uptake system. We have characterized the mutS-proximal border of this pathogenicity island and have identified two sets of genes, pigAB and pigCD. All four genes have homologs of unknown function in several bacteria that share the ability to establish an intimate association with higher eukaryotic hosts. The expression of at least two of these genes, pigA and pigB, is controlled by SprA, a transcription factor encoded within SPI-1 that controls the expression of genes associated with the type III secretion system of this island. In addition, we found that homologs of the pig genes are also found at different locations of the S. enterica chromosome in association with segments of DNA that exhibit features of pathogenicity islands. The presence of several apparently functional copies of these genes argues for an important role in the biology of this bacterial pathogen. Furthermore, they constitute a valuable tool to identify potential pathogenicity islands.

In vitro transposition system for efficient generation of random mutants of Campylobacter jejuni

Campylobacter jejuni is the most common cause of food-borne illnesses in the United States. Despite the fact that the entire nucleotide sequence of its genome has recently become available, its mechanisms of pathogenicity are poorly understood. This is in part due to the lack of an efficient mutagenesis system. Here we describe an in vitro transposon mutagenesis system based on the Staphylococcus aureus transposable element Tn552 that allows the efficient generation of insertion mutants of C. jejuni. Insertions occur randomly and throughout the entire bacterial genome. We have tested this system in the isolation of nonmotile mutants of C. jejuni. Demonstrating the utility of the system, six nonmotile mutants from a total of nine exhibited insertions in genes known to be associated with motility. An additional mutant had an inactivating insertion in sigma 54, implicating this transcription factor in flagellum regulation. The availability of this efficient system will greatly facilitate the study of the mechanisms of pathogenesis of this important pathogen.

Genetic analysis of assembly of the Salmonella enterica serovar Typhimurium type III secretion-associated needle complex

Several pathogenic bacteria have evolved a specialized protein secretion system termed type III to secrete and deliver effector proteins into eukaryotic host cells. Salmonella enterica serovar Typhimurium uses one such system to mediate entry into nonphagocytic cells. This system is composed of more than 20 proteins which are encoded within a pathogenicity island (SPI-1) located at centisome 63 of its chromosome. A subset of these components form a supramolecular structure, termed the needle complex, that resembles the flagellar hook-basal body complex. The needle complex is composed of a multiple-ring cylindrical base that spans the bacterial envelope and a needle-like extension that protrudes from the bacterial outer surface. Although the components of this structure have been identified, little is known about its assembly. In this study we examined the effect of loss-of-function mutations in each of the type III secretion-associated genes encoded within SPI-1 on the assembly of the needle complex. This analysis indicates that the assembly of this organelle occurs in discrete, genetically separable steps. A model for the assembly pathway of this important organelle is proposed that involves a sec-dependent step leading to the assembly of the base substructure followed by a sec-independent process resulting in the assembly of the needle portion.

A Salmonella inositol polyphosphatase acts in conjunction with other bacterial effectors to promote host cell actin cytoskeleton rearrangements and bacterial internalization

A central feature of Salmonella pathogenicity is the bacterium's ability to enter into non-phagocytic cells. Bacterial internalization is the consequence of cellular responses characterized by Cdc42- and Rac-dependent actin cytoskeleton rearrangements. These responses are triggered by the co-ordinated function of bacterial proteins delivered into the host cell by a specialized protein secretion system termed type III. We report here that SopB, a Salmonella inositol polyphosphatase delivered to the host cell by this secretion system, mediates actin cytoskeleton rearrangements and bacterial entry in a Cdc42-dependent manner. SopB exhibits overlapping functions with two other effectors of bacterial entry, the Rho family GTPase exchange factors SopE and SopE2. Thus, Salmonella strains deficient in any one of these proteins can enter into cells at high efficiency, whereas a strain lacking all three effectors is completely defective for entry. Consistent with an important role for inositol phosphate metabolism in Salmonella-induced cellular responses, a catalytically defective mutant of SopB failed to stimulate actin cytoskeleton rearrangements and bacterial entry. Furthermore, bacterial infection of intestinal cells resulted in a marked increase in Ins(1,4,5,6)P4, a consumption of InsP5 and the activation of phospholipase C. In agreement with the in vivo findings, purified SopB specifically dephosphorylated InsP5 to Ins(1,4,5,6)P4 in vitro. Surprisingly, the inositol phosphate fluxes induced by Salmonella were not caused exclusively by SopB. We show that the SopB-independent inositol phosphate fluxes are the consequence of the SopE-dependent activation of an endogenous inositol phosphatase. The ability of Salmonella to stimulate Rho GTPases signalling and inositol phosphate metabolism through alternative mechanisms is an example of the remarkable ability of this bacterial pathogen to manipulate host cellular functions.

Modulation of host signaling by a bacterial mimic: structure of the Salmonella effector SptP bound to Rac1

Salmonella spp. utilize a specialized protein secretion system to deliver a battery of effector proteins into host cells. Several of these effectors stimulate Cdc42- and Rac1-dependent cytoskeletal changes that promote bacterial internalization. These potentially cytotoxic alterations are rapidly reversed by the effector SptP, a tyrosine phosphatase and GTPase activating protein (GAP) that targets Cdc42 and Rac1. The 2.3 A resolution crystal structure of an SptP-Rac1 transition state complex reveals an unusual GAP architecture that mimics host functional homologs. The phosphatase domain possesses a conserved active site but distinct surface properties. Binding to Rac1 induces a dramatic stabilization in SptP of a four-helix bundle that makes extensive contacts with the Switch I and Switch II regions of the GTPase.

A bacterial toxin that controls cell cycle progression as a deoxyribonuclease I-like protein

Many bacterial pathogens encode a multisubunit toxin, termed cytolethal distending toxin (CDT), that induces cell cycle arrest, cytoplasm distention, and, eventually, chromatin fragmentation and cell death. In one such pathogen, Campylobacter jejuni, one of the subunits of this toxin, CdtB, was shown to exhibit features of type I deoxyribonucleases. Transient expression of this subunit in cultured cells caused marked chromatin disruption. Microinjection of low amounts of CdtB induced cytoplasmic distention and cell cycle arrest. CdtB mutants with substitutions in residues equivalent to those required for catalysis or magnesium binding in type I deoxyribonucleases did not cause chromatin disruption. CDT holotoxin containing these mutant forms of CdtB did not induce morphological changes or cell cycle arrest.

Biophysical characterization of SipA, an actin-binding protein from Salmonella enterica

An essential step in the pathogenesis of Salmonella enterica infections is bacterial entry into non-phagocytic cells of the intestinal epithelium. Proteins injected by Salmonella into host cells stimulate cellular responses that lead to extensive actin cytoskeleton reorganization and subsequent bacterial uptake. One of these proteins, SipA, modulates actin dynamics by directly binding to F-actin. We have biophysically characterized a C-terminal fragment, SipA(446-684), which has previously been shown to retain activity. Our results show that SipA(446-684) exhibits an elongated shape with a predominantly helical conformation and predict the existence of a coiled-coil domain. We suggest that the protein is able to span two adjacent actin monomers in a filament and propose a model that is consistent with the observed effects of SipA(446-684) on actin dynamics and F-actin stability and morphology.

Molecular characterization and assembly of the needle complex of the Salmonella typhimurium type III protein secretion system

Many bacterial pathogens of plants and animals have evolved a specialized protein-secretion system termed type III to deliver bacterial proteins into host cells. These proteins stimulate or interfere with host cellular functions for the pathogen's benefit. The Salmonella typhimurium pathogenicity island 1 encodes one of these systems that mediates this bacterium's ability to enter nonphagocytic cells. Several components of this type III secretion system are organized in a supramolecular structure termed the needle complex. This structure is made of discrete substructures including a base that spans both membranes and a needle-like projection that extends outward from the bacterial surface. We demonstrate here that the type III secretion export apparatus is required for the assembly of the needle substructure but is dispensable for the assembly of the base. We show that the length of the needle segment is determined by the type III secretion associated protein InvJ. We report that InvG, PrgH, and PrgK constitute the base and that PrgI is the main component of the needle of the type III secretion complex. PrgI homologs are present in type III secretion systems from bacteria pathogenic for animals but are absent from bacteria pathogenic for plants. We hypothesize that the needle component may establish the specificity of type III secretion systems in delivering proteins into either plant or animal cells.

Developmental control of endocytosis in dendritic cells by Cdc42

Dendritic cells (DCs) developmentally regulate antigen uptake by controlling their endocytic capacity. Immature DCs actively internalize antigen. However, mature DCs are poorly endocytic, functioning instead to present antigens to T cells. We have found that endocytic downregulation reflects a decrease in endocytic activity controlled by Rho family GTPases, especially Cdc42. Blocking Cdc42 function by Toxin B treatment or injection of dominant-negative inhibitors of Cdc42 abrogates endocytosis in immature DCs. In mature DCs, injection of constitutively active Cdc42 or microbial delivery of a Cdc42 nucleotide exchange factor reactivates endocytosis. DCs regulate endogenous levels of Cdc42-GTP with activated Cdc42 detectable only in immature cells. We conclude that DCs developmentally regulate endocytosis at least in part by controlling levels of activated Cdc42.

The flagellar sigma factor FliA (sigma(28)) regulates the expression of Salmonella genes associated with the centisome 63 type III secretion system

One of the essential features of all pathogenic strains of Salmonella enterica is the ability to enter into nonphagocytic cells. This pathogenic property is mediated by the Salmonella pathogenicity island 1 (SPI-1)-encoded type III secretion system. Expression of components and substrates of this system is subject to complex regulatory mechanisms. These mechanisms include a number of specific and global transcriptional regulatory proteins. In this study we have compared in S. enterica serovars Typhimurium and Typhi the effect of mutations in flagellar genes on the phenotypes associated with the SPI-1 type III protein secretion system. We found that serovar Typhi strains carrying a null mutation in either of the flagellar regulatory genes flhDC or fliA were severely deficient in entry into cultured epithelial cells and macrophage cytotoxicity. This defect could not be reversed by applying a mild centrifugal force, suggesting that the effects of the mutations were not due to the absence of motility. In contrast, the same mutations had no significant effect on the ability of serovar Typhimurium to enter into cultured Henle-407 cells or to induce macrophage cell death. Consistent with these observations, we found that the mutations in the flagellar regulatory proteins significantly reduced the expression of components of the SPI-1-encoded type III system in serovar Typhi but had a marginal effect in serovar Typhimurium. Our results therefore indicate that there is an overlap between regulatory mechanisms that control flagellar and type III secretion gene expression in Salmonella serovar Typhi.

Complex function for SicA, a Salmonella enterica serovar typhimurium type III secretion-associated chaperone

Salmonella enterica encodes a type III secretion system within a pathogenicity island located at centisome 63 that is essential for virulence. All type III secretion systems require the function of a family of low-molecular-weight proteins that aid the secretion process by acting as partitioning factors and/or secretion pilots. One such protein is SicA, which is encoded immediately upstream of the type III secreted proteins SipB and SipC. We found that the absence of SicA results in the degradation of both SipB and SipC. Interestingly, in the absence of SipC, SipB was not only stable but also secreted at wild-type levels in a sicA mutant background, indicating that SicA is not required for SipB secretion. We also found that SicA is capable of binding both SipB and SipC. These results are consistent with a SicA role as a partitioning factor for SipB and SipC, thereby preventing their premature association and degradation. We also found that introduction of a sicA null mutation results in the lack of expression of SopE, another type III-secreted protein. Such an effect was shown to be transcriptional. Introduction of a loss-of-function sipC mutation into the sicA mutant background rescued sopE expression. These results indicate that the effect of sicA on sopE expression is indirect and most likely exerted through a regulatory factor(s) partitioned by SicA from SipC. These studies therefore describe a surprisingly complex function for the Salmonella enterica type III secretion-associated chaperone SicA.

A salmonella protein antagonizes Rac-1 and Cdc42 to mediate host-cell recovery after bacterial invasion

An essential feature of the bacterial pathogen Salmonella spp. is its ability to enter cells that are normally non-phagocytic, such as those of the intestinal epithelium. The bacterium achieves entry by delivering effector proteins into the host-cell cytosol by means of a specialized protein-secretion system (termed type III), which causes reorganization of the cell's actin cytoskeleton and ruffling of its membrane. One of the bacterial effectors that stimulates these cellular responses is SopE, which acts as a guanyl-nucleotide-exchange factor on Rho GTPase proteins such as Cdc42 and Rac. As the actin-cytoskeleton reorganization induced by Salmonella is reversible and short-lived, infected cells regain their normal architecture after bacterial internalization. We show here that the S. Typhimurium effector protein SptP, which is delivered to the host-cell cytosol by the type-III secretion system, is directly responsible for the reversal of the actin cytoskeletal changes induced by the bacterium. SptP exerts this function by acting as a GTPase-activating protein (GAP) for Rac-1 and Cdc42.

An invasion-associated Salmonella protein modulates the actin-bundling activity of plastin

The entry of Salmonella typhimurium into nonphagocytic cells requires a panel of bacterial effector proteins that are delivered to the host cell via a type III secretion system. These proteins modulate host-cell signal-transduction pathways and the actin cytoskeleton to induce membrane ruffling and bacterial internalization. One of these bacterial effectors, termed SipA, is an actin-binding protein that is required for efficient Salmonella entry into host cells. We report here that SipA forms a complex with T-plastin on bacterial infection. Formation of such a complex, which requires the presence of F-actin, results in a marked increase in the actin-bundling activity of T-plastin. We also report that T-plastin is recruited to S. typhimurium-induced membrane ruffles by a CDC42-dependent signaling process and is required for bacterial entry. We propose that modulation of the actin-bundling activity of T-plastin by SipA results in the stabilization of the actin filaments at the point of bacterial-host cell contact, which leads to more efficient Salmonella internalization.

Differential regulation of Salmonella typhimurium type III secreted proteins by pathogenicity island 1 (SPI-1)-encoded transcriptional activators InvF and hilA

Salmonella enterica encodes a type III protein secretion system within a pathogenicity island (SPI-1) that is located at centisome 63 of its chromosome. This system is required for the ability of these bacteria to stimulate cellular responses that are essential for their pathogenicity. Expression of components and substrates of this system is subject to complex regulatory mechanisms. These mechanisms involve the function of HilA and InvF, two transcriptional regulatory proteins encoded within SPI-1. In this study, we examined the functional relationship between these two regulatory proteins. We found that strains carrying loss-of-function mutations in either hilA or invF differ in their ability to stimulate cellular responses. An S. typhimurium hilA mutant strain retained considerable signaling capacity that resulted in significant levels of internalization into host cells. In contrast, introduction of a nonpolar loss-of-function mutation in invF rendered S. typhimurium significantly impaired in its ability to enter host cells. Consistent with these different phenotypes, we found that HilA and InvF control the expression of different genes. HilA regulates the expression of components of the type III secretion machinery, whereas InvF controls the expression of type III secreted proteins encoded outside of SPI-1. We also found that the expression of secreted proteins encoded within SPI-1 are under the control of both HilA and InvF. Our results therefore indicate that InvF and HilA differentially control the expression of components and substrates of the invasion-associated type III secretion system.

Characterization of SprA, an AraC-like transcriptional regulator encoded within the Salmonella typhimurium pathogenicity island 1

Pathogenicity island 1 (SPI-1) located at centisome 63 of the Salmonella chromosome encodes a type III protein secretion system that is essential for its pathogenicity. The translocation of effector proteins through this system results in the stimulation of signalling events, leading to actin cytoskeletal rearrangements and nuclear responses. These cellular responses ultimately lead to bacterial uptake, production of proinflammatory cytokines in non-phagocytic cells and the initiation of programmed cell death in macrophages. The regulation of expression of components and substrates of this type III secretion system is complex and involves the activity of several specific transcriptional regulatory proteins encoded within SPI-1. Here, we describe two additional regulatory proteins, SprA and SprB, which are encoded within SPI-1. SprA and SprB exhibit significant sequence similarity to the AraC/XylS and the LuxR/UhaP family of transcriptional regulatory proteins respectively. Insertion mutations in sprA and sprB did not significantly affect the transcription of invasion-associated genes and, consequently, did not affect the ability of Salmonella typhimurium to gain access into host cells. However, expression of sprA from an inducible heterologous promoter resulted in increased expression of genes associated with the centisome 63 type III secretion system and increased the ability of S. typhimurium to enter into host cells. Further analysis demonstrated that SprA acts either upstream or at the same level as HilA in the SPI-1 transcriptional regulatory cascade.

Type III secretion machines: bacterial devices for protein delivery into host cells

Several Gram-negative pathogenic bacteria have evolved a complex protein secretion system termed type III to deliver bacterial effector proteins into host cells that then modulate host cellular functions. These bacterial devices are present in both plant and animal pathogenic bacteria and are evolutionarily related to the flagellar apparatus. Although type III secretion systems are substantially conserved, the effector molecules they deliver are unique for each bacterial species. Understanding the biology of these devices may allow the development of novel prevention and therapeutic approaches for several infectious diseases.

Requirement of p21-activated kinase (PAK) for Salmonella typhimurium-induced nuclear responses

Salmonella typhimurium has sustained a long-standing association with its host and therefore has evolved sophisticated strategies to multiply and survive within this environment. Central to Salmonella pathogenesis is the function of a dedicated type III secretion system that delivers bacterial effector proteins into the host cell cytoplasm. These effectors stimulate nuclear responses and actin cytoskeleton reorganization leading to the production of proinflammatory cytokines and bacterial internalization. The stimulation of these responses requires the function of Cdc42, a member of the Rho family of small molecular weight GTPases, and SopE, a bacterial effector protein that stimulates guanine nucleotide exchange on Rho GTPases. However, nothing is known about the role of Cdc42 effector proteins in S. typhimurium-induced responses. We showed here that S. typhimurium infection of cultured epithelial cells results in the activation of p21-activated kinase (PAK), a serine/threonine kinase that is an effector of Cdc42-dependent responses. Transient expression of a kinase-defective PAK blocked both S. typhimurium- and SopE-induced c-Jun NH2-terminal kinase (JNK) activation but did not interfere with bacteria-induced actin cytoskeleton rearrangements. Similarly, expression of SH3-binding mutants of PAK did not block actin-mediated S. typhimurium entry into cultured cells. However, expression of an effector loop mutant of Cdc42Hs (Cdc42HsC40) unable to bind PAK and other CRIB (for Cdc42/Rac interacting binding)-containing target proteins resulted in abrogation of both S. typhimurium-induced nuclear and cytoskeletal responses. These results show that PAK kinase activity is required for bacteria-induced nuclear responses but it is not required for cytoskeletal rearrangements, indicating that S. typhimurium stimulates cellular responses through different Cdc42 downstream effector activities. In addition, these results demonstrate that the effector loop of Cdc42 implicated in the binding of PAK and other CRIB-containing target proteins is required for both responses.

Salmonella typhimurium encodes a putative iron transport system within the centisome 63 pathogenicity island

Upon entry into the host, Salmonella enterica strains are presumed to encounter an iron-restricted environment. Consequently, these bacteria have evolved a variety of often-redundant high-affinity acquisition systems to obtain iron in this restricted environment. We have identified an iron transport system that is encoded within the centisome 63 pathogenicity island of Salmonella typhimurium. The nucleotide composition of this locus is significantly different from that of the rest of this pathogenicity island, suggesting a different ancestry and a mosaic structure for this region of the S. typhimurium chromosome. This locus, designated sit, consists of four open reading frames which encode polypeptides with extensive homology to the yfe ABC iron transport system of Yersinia pestis, as well as other ABC transporters. The sitA gene encodes a putative periplasmic binding protein, sitB encodes an ATP-binding protein, and sitC and sitD encode two putative permeases (integral membrane proteins). This operon is capable of complementing the growth defect of the enterobactin-deficient Escherichia coli strain SAB11 in iron-restricted minimal medium. Transcription of the sit operon is repressed under iron-rich growth conditions in a fur-dependent manner. Introduction of a sitBCD deletion into wild-type S. typhimurium resulted in no apparent growth defect in either nutrient-rich or minimal medium and no measurable virulence phenotype. These results further support the existence of redundant iron uptake systems in S. enterica.

Role of the S. typhimurium actin-binding protein SipA in bacterial internalization

Entry of the bacterium Salmonella typhimurium into host cells requires membrane ruffling and rearrangement of the actin cytoskeleton. Here, it is shown that the bacterial protein SipA plays a critical role in this process. SipA binds directly to actin, decreases its critical concentration, and inhibits depolymerization of actin filaments. These activities result in the spatial localization and more pronounced outward extension of the Salmonella-induced membrane ruffles, thereby facilitating bacterial uptake.

Interaction of Salmonella with host cells through the centisome 63 type III secretion system

Salmonella enterica engages host cells in a complex two-way biochemical interaction that results in a variety of responses from both the bacteria and the host cell. Central to this interaction is the function of a type III protein secretion system that delivers effector proteins into the host cell. During the past year we have seen major advances in our knowledge of both the bacterial determinants and the host-signal transduction pathways involved in these interactions. A coherent picture of the mechanisms by which Salmonella engages the host cell is now beginning to emerge.