Identification of substrates and chaperone from the Yersinia enterocolitica 1B Ysa type III secretion system

Delivering the pain: an overview of the type III secretion system with special consideration for aquatic pathogens

Gram-negative bacteria are known to subvert eukaryotic cell physiological mechanisms using a wide array of virulence factors, among which the type three-secretion system (T3SS) is often one of the most important. The T3SS constitutes a needle-like apparatus that the bacterium uses to inject a diverse set of effector proteins directly into the cytoplasm of the host cells where they can hamper the host cellular machinery for a variety of purposes. While the structure of the T3SS is somewhat conserved and well described, effector proteins are much more diverse and specific for each pathogen. The T3SS can remodel the cytoskeleton integrity to promote intracellular invasion, as well as silence specific eukaryotic cell signals, notably to hinder or elude the immune response and cause apoptosis. This is also the case in aquatic bacterial pathogens where the T3SS can often play a central role in the establishment of disease, although it remains understudied in several species of important fish pathogens, notably in Yersinia ruckeri. In the present review, we summarise what is known of the T3SS, with a special focus on aquatic pathogens and suggest some possible avenues for research including the potential to target the T3SS for the development of new anti-virulence drugs.

Functional relatedness in the Inv/Mxi-Spa type III secretion system family

Type III Secretion Systems (T3SSs) are structurally conserved nanomachines that span the inner and outer bacterial membranes, and via a protruding needle complex contact host cell membranes and deliver type III effector proteins. T3SS are phylogenetically divided into several families based on structural basal body components. Here we have studied the evolutionary and functional conservation of four T3SS proteins from the Inv/Mxi-Spa family: a cytosolic chaperone, two hydrophobic translocators that form a plasma membrane-integral pore, and the hydrophilic 'tip complex' translocator that connects the T3SS needle to the translocon pore. Salmonella enterica serovar Typhimurium (S. Typhimurium), a common cause of food-borne gastroenteritis, possesses two T3SSs, one belonging to the Inv/Mxi-Spa family. We used invasion-deficient S. Typhimurium mutants as surrogates for expression of translocator orthologs identified from an extensive phylogenetic analysis, and type III effector translocation and host cell invasion as a readout for complementation efficiency, and identified several Inv/Mxi-Spa orthologs that can functionally substitute for the S. Typhimurium chaperone and translocator proteins. Functional complementation correlates with amino acid sequence identity between orthologs, but varies considerably between the four proteins. This is the first in-depth survey of the functional interchangeability of Inv/Mxi-Spa T3SS proteins acting directly at the host-pathogen interface.

Identification and molecular characterization of YsaL (Ye3555): a novel negative regulator of YsaN ATPase in type three secretion system of enteropathogenic bacteria Yersinia enterocolitica

Type Three Secretion (T3S) ATPases are involved in delivery of virulent factors from bacteria to their hosts (through injectisome) in an energy (ATP) dependent manner during pathogenesis. The activities of these ATPases are tightly controlled by their specific regulators. In Yersinia enterocolitica, YsaN was predicted as a putative ATPase of the Ysa-Ysp Type Three Secretion System (T3SS) based on sequence similarity with other T3S ATPases. However detailed study and characterization of YsaN and its regulation remains largely obscure. Here, in this study, we have successfully cloned, over-expressed, purified and characterized the molecular properties of YsaN from Yersinia enterocolitica. YsaN acts as a Mg(2+) dependent ATPase and exists in solution as higher order oligomer (dodecamer). The ATPase activity of oligomeric YsaN is several fold higher than the monomeric form. Furthermore, by employing in silico studies we have identified the existence of a negative regulator of YsaN--a hypothetical protein YE3555 (termed 'YsaL'). To verify the functionality of YsaL, we have evaluated the biochemical and biophysical properties of YsaL. Purified YsaL is dimeric in solution and strongly associates with YsaN to form a stable heterotrimeric YsaL-YsaN complex (stoichiometry--2∶1). The N terminal 6-20 residues of YsaN are invariably required for stable YsaL-YsaN complex formation. YsaL inhibited the ATPase activity of YsaN with a maximum inhibition at the molar ratio 2∶1 (YsaL: YsaN). In short, our studies provide an insight into the presence of YsaN ATPase in Yersinia enterocolitica and its regulator YsaL. Our studies also correlate the functionality of one of the existing protein interaction networks that possibly is indispensable for the energy dependent process of Ysa-Ysp T3SS in pathogenic Yersinia enterocolitica.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

The bacterium Pantoea stewartii uses two different type III secretion systems to colonize its plant host and insect vector

Plant- and animal-pathogenic bacteria utilize phylogenetically distinct type III secretion systems (T3SS) that produce needle-like injectisomes or pili for the delivery of effector proteins into host cells. Pantoea stewartii subsp. stewartii (herein referred to as P. stewartii), the causative agent of Stewart's bacterial wilt and leaf blight of maize, carries phylogenetically distinct T3SSs. In addition to an Hrc-Hrp T3SS, known to be essential for maize pathogenesis, P. stewartii has a second T3SS (Pantoea secretion island 2 [PSI-2]) that is required for persistence in its flea beetle vector, Chaetocnema pulicaria (Melsh). PSI-2 belongs to the Inv-Mxi-Spa T3SS family, typically found in animal pathogens. Mutagenesis of the PSI-2 psaN gene, which encodes an ATPase essential for secretion of T3SS effectors by the injectisome, greatly reduces both the persistence of P. stewartii in flea beetle guts and the beetle's ability to transmit P. stewartii to maize. Ectopic expression of the psaN gene complements these phenotypes. In addition, the PSI-2 psaN gene is not required for P. stewartii pathogenesis of maize and is transcriptionally upregulated in insects compared to maize tissues. Thus, the Hrp and PSI-2 T3SSs play different roles in the life cycle of P. stewartii as it alternates between its insect vector and plant host.

Translocated effectors of Yersinia

Currently, all known translocated effectors of Yersinia are delivered into host cells by type III secretion systems (T3SSs). Pathogenic Yersinia maintain the plasmid-encoded Ysc T3SS for the specific delivery of the well-studied Yop effectors. New horizons for effector biology have opened with the discovery of the Ysps of Y. enterocolitica Biovar 1B, which are translocated into host cells by the chromosome-endoded Ysa T3SS. The reported arsenal of effectors is likely to expand since genomic analysis has revealed gene-clusters in some Yersinia that code for other T3SSs. These efforts also revealed possible type VI secretion (T6S) systems, which may indicate that translocation of effectors occurs by multiple mechanisms.

Synchronous gene expression of the Yersinia enterocolitica Ysa type III secretion system and its effectors

Type III secretion systems (T3SSs) are complex units that consist of many proteins. Often the proteins are encoded as a cohesive unit on virulence plasmids, but several systems have their various components dispersed around the chromosome. The Yersinia enterocolitica Ysa T3SS is such a system, where the apparatus genes, some regulatory genes, and four genes encoding secreted proteins (ysp genes) are contained in a single locus. The remaining ysp genes and at least one additional regulator are found elsewhere on the chromosome. Expression of ysa genes requires conditions of high ionic strength, neutral/basic pH, and low temperatures (26 degrees C) and is stimulated by exposure to solid surfaces. The AraC-like regulator YsaE and the dual-function chaperone/regulator SycB are required to stimulate the sycB promoter, which transcribes sycB and probably yspBCDA as well. The putative phosphorelay proteins YsrRS (located at the distal end of the ysa locus) and RcsB, the response regulator of the RcsBCD phosphorelay system, are required to initiate transcription at the ysaE promoter, which drives transcription of many apparatus genes. In this work, we sought to determine which ysp genes were coordinately regulated with the genes within the ysa locus. We found that six unlinked ysp genes responded to NaCl and required YsaE/SycB, YsrRS, and RcsB for expression. Three ysp genes had unique patterns, one of which was unaffected by all elements tested except NaCl. Thus, while the ysp genes were likely to have been acquired independently, most have acquired a synchronous regulatory pattern.

Characterization of the role of the receptors PEX5 and PEX7 in the import of proteins into glycosomes of Trypanosoma brucei

Peroxins 5 and 7 are receptors for protein import into the peroxisomal matrix. We studied the involvement of these peroxins in the biogenesis of glycosomes in the protozoan parasite Trypanosoma brucei. Glycosomes are peroxisome-like organelles in which a major part of the glycolytic pathway is sequestered. We here report the characterization of the T. brucei homologue of PEX7 and provide several data strongly suggesting that it can bind to PEX5. Depletion of PEX5 or PEX7 by RNA interference had a severe effect on the growth of both the bloodstream-form of the parasite, that relies entirely on glycolysis for its ATP supply, and the procyclic form representative of the parasite living in the tsetse-fly midgut and in which also other metabolic pathways play a prominent role. The role of the two receptors in import of glycosomal matrix proteins with different types of peroxisome/glycosome-targeting signals (PTS) was analyzed by immunofluorescence and subcellular fractionation studies. Knocking down the expression of either receptor gene resulted, in procyclic cells, in the mislocalization of proteins with both a type 1 or 2 targeting motif (PTS1, PTS2) located at the C- and N-termini, respectively, and proteins with a sequence-internal signal (I-PTS) to the cytosol. Electron microscopy confirmed the apparent integrity of glycosomes in these procyclic cells. In bloodstream-form trypanosomes, PEX7 depletion seemed to affect only the subcellular distribution of PTS2-proteins. Western blot analysis suggested that, in both life-cycle stages of the trypanosome, the levels of both receptors are controlled in a coordinated fashion, by a mechanism that remains to be determined. The observation that both PEX5 and PEX7 are essential for the viability of the parasite indicates that the respective branches of the glycosome-import pathway in which each receptor acts might be interesting drug targets.

The Ysa type 3 secretion system of Yersinia enterocolitica biovar 1B

Yersinia enterocolitica biovar 1B maintains two distinct and independently operating type 3 secretion (T3S) systems with the capacity to translocate toxic effector proteins into mammalian cells. Each of these T3S systems plays a role in the outcome of an infection by influencing different stages of infection. Recent investigations of the Ysa T3S system have revealed it is important for Y. enterocolitica survival during the gastrointestinal phase of infection. This sets this system apart from the Ysc T3S system which is important for systemic infections. Identification of the effector proteins has provided insight on how the Ysa T3S system modulates Y. enterocolitica interactions with the host. In part, the Ysa T3S system targets the innate immune response to suppress the ability of the host to rapidly clear an infection.

Application of comparative phylogenomics to study the evolution of Yersinia enterocolitica and to identify genetic differences relating to pathogenicity

Yersinia enterocolitica, an important cause of human gastroenteritis generally caused by the consumption of livestock, has traditionally been categorized into three groups with respect to pathogenicity, i.e., nonpathogenic (biotype 1A), low pathogenicity (biotypes 2 to 5), and highly pathogenic (biotype 1B). However, genetic differences that explain variation in pathogenesis and whether different biotypes are associated with specific nonhuman hosts are largely unknown. In this study, we applied comparative phylogenomics (whole-genome comparisons of microbes with DNA microarrays combined with Bayesian phylogenies) to investigate a diverse collection of 94 strains of Y. enterocolitica consisting of 35 human, 35 pig, 15 sheep, and 9 cattle isolates from nonpathogenic, low-pathogenicity, and highly pathogenic biotypes. Analysis confirmed three distinct statistically supported clusters composed of a nonpathogenic clade, a low-pathogenicity clade, and a highly pathogenic clade. Genetic differences revealed 125 predicted coding sequences (CDSs) present in all highly pathogenic strains but absent from the other clades. These included several previously uncharacterized CDSs that may encode novel virulence determinants including a hemolysin, a metalloprotease, and a type III secretion effector protein. Additionally, 27 CDSs were identified which were present in all 47 low-pathogenicity strains and Y. enterocolitica 8081 but absent from all nonpathogenic 1A isolates. Analysis of the core gene set for Y. enterocolitica revealed that 20.8% of the genes were shared by all of the strains, confirming this species as highly heterogeneous, adding to the case for the existence of three subspecies of Y. enterocolitica. Further analysis revealed that Y. enterocolitica does not cluster according to source (host).

Proteomic and functional analysis of the suite of Ysp proteins exported by the Ysa type III secretion system of Yersinia enterocolitica Biovar 1B

Full virulence of Yersinia enterocolitica Biovar 1B requires two distinct and distantly related contact-dependent type III secretion (T3S) systems. The plasmid-encoded Ysc T3S system is essential for systemic stages of infection and the Yop effector proteins it translocates have been extensively studied. The chromosome-encoded Ysa T3S system contributes to gastrointestinal stages of infection, but the suite of Ysp effectors proteins it translocates into host cells remains obscure. Using a proteomics-based approach, the Ysa T3S system was analysed revealing a complex set of 15 secreted Ysp proteins. Seven of these proteins were previously described (YspA, YspB, YspC, YspD, YopE, YopN and YopP). Eight of these Ysps (YspK, YspI, YspE, YspF, YspP, YspY, YspN and YspL) had not previously been characterized. Several of the new Ysps are homologous to other virulence factors, including YspP with similarity to the Yersinia protein tyrosine phosphatase YopH and YspK with similarity to the Shigella serine/threonine kinase OspG. Biochemical analysis of purified hexa-histidine tagged YspK and YspP established that these proteins have kinase and phosphatase activity respectively. Infection of eukaryotic cells with Y. enterocolitica strains expressing a Ysp-CyaA chimeric protein resulted in Ysa T3S system-dependent increases in cytosolic levels of cAMP for six Ysps (YspK, YspI, YspE, YspF, YspP and YspL), but not two others (YspY and YspN). YspN, however, was required for translocation of effector proteins into eukaryotic cells by the Ysa T3S system. Competition assays in BALB/c mice revealed that mutants defective for the production of an individual Ysp are affected for colonization of gastrointestinal tissues. Collectively, the results of this study support the hypothesis that the Ysa T3S system targets a complex suite of effector proteins into host cells to affect the outcome of an infection. Identification of the suite of effectors delivered by the Ysa T3S system reveals that host cell signalling pathways are the probable targets of several Ysp effectors.

Expression of and secretion through the Aeromonas salmonicida type III secretion system

Aeromonas salmonicida subsp. salmonicida is the aetiological agent of furunculosis, a disease of farmed and wild salmonids. The type III secretion system (TTSS) is one of the primary virulence factors in A. salmonicida. Using a combination of differential proteomic analysis and reverse transcriptase (RT)-PCR, it is shown that A. salmonicida A449 induces the expression of TTSS proteins at 28 °C, but not at its more natural growth temperature of 17 °C. More modest increases in expression occur at 24 °C. This temperature-induced up-regulation of the TTSS in A. salmonicida A449 occurs within 30 min of a growth temperature increase from 16 to 28 °C. Growth conditions such as low-iron, low pH, low calcium, growth within the peritoneal cavity of salmon and growth to high cell densities do not induce the expression of the TTSS in A. salmonicida A449. The only other known growth condition that induces expression of the TTSS is growth of the bacterium at 16 °C in salt concentrations ranging from 0·19 to 0·38 M NaCl. It is also shown that growth at 28 °C followed by exposure to low calcium results in the secretion of one of the TTSS effector proteins. This study presents a simple in vitro model for the expression of TTSS proteins in A. salmonicida.

Yersinia outer proteins: role in modulation of host cell signaling responses and pathogenesis

A type III secretion system (TTSS) is encoded on a virulence plasmid that is common to three pathogenic Yersinia species: Y. enterocolitica, Y. pseudotuberculosis, and Y. pestis. Pathogenic Yersinia species require this TTSS to survive and replicate within lymphoid tissues of their animal or human hosts. A set of pathogenicity factors, including those known as Yersinia outer proteins (Yops), is exported by this system upon bacterial infection of host cells. Two translocator Yops (YopB and YopD) insert into the host plasma membrane and function to transport six effector Yops (YopO, YopH, YopM, YopT, YopJ, and YopE) into the cytosol of the host cell. Effector Yops function to counteract multiple signaling responses in the infected host cell. The signaling responses counteracted by Yops are initiated by phagocytic receptors, Toll-like receptors, translocator Yops, and additional mechanisms. Innate and adaptive immune responses are thwarted as a consequence of Yop activities. A biochemical function for each effector Yop has been established, and the importance of these proteins for the pathogenesis process is being elucidated. This review focuses on the biochemical functions of Yops, the signaling pathways they modulate, and the role of these proteins in Yersinia virulence.

Type III secretion: more systems than you think

The type III secretion (T3S) pathway allows bacteria to inject effector proteins into the cytosol of target animal or plant cells. T3S systems evolved into seven families that were distributed among Gram-negative bacteria by horizontal gene transfer. There are probably a few hundred effectors interfering with control and signaling in eukaryotic cells and offering a wealth of new tools to cell biologists.

Identification of the sequence encoding N-acetylneuraminate-9-phosphate phosphatase

The synthesis of N-acetylneuraminate (Neu5Ac), the main form of sialic acid, proceeds in vertebrates through the condensation of N-acetylmannosamine 6-phosphate and phosphoenolpyruvate to Neu5Ac-9-phosphate, followed by the dephosphorylation of the latter by a specific phosphatase. The sequence encoding Neu5Ac-9-phosphate phosphatase (Neu5Ac-9-Pase; E.C. 3.1.3.29) has not been determined until now. In this work, we have purified Neu5Ac-9-Pase more than 1000-fold from rat liver. Its dependency on Mg2+ and the fact that it was inhibited by vanadate and Ca2+ suggested that it belonged to the haloacid dehalogenase family of phosphatases. Trypsin digestion and mass spectrometry analysis of a polypeptide of about 30 kDa that co-eluted with the activity in the last purification step indicated the presence of a protein designated "haloacid dehalogenase-like hydrolase domain containing 4." The human ortholog of this protein is encoded by a 2-exon gene present on chromosome 20p11. The human protein was overexpressed in Escherichia coli as a fusion protein with a polyHis tag and purified to homogeneity. The recombinant enzyme displayed a >230-fold higher catalytic efficiency on Neu5Ac-9-phosphate than on its second best substrate. Its properties were similar to those of the enzyme purified from rat liver. Neu5Ac inhibited the enzymatic activity by 50% at 15 mM, indicating that no significant inhibition is exerted at physiological concentrations of Neu5Ac. The identification of the gene encoding Neu5Ac-9-Pase will facilitate studies aimed at testing its potential implication in unexplained forms of glycosylation deficiency.

Environmental regulation and virulence attributes of the Ysa type III secretion system of Yersinia enterocolitica biovar 1B

Pathogenic biovars of Yersinia enterocolitica maintain the well-studied plasmid-encoded Ysc type III secretion (TTS) system, which has a definitive role in virulence. Y. enterocolitica biovar 1B additionally has a distinct chromosomal locus, the Yersinia secretion apparatus pathogenicity island (YSA PI) that encodes the Ysa TTS system. The signals to which the Ysa TTS system responds and its role in virulence remain obscure. This exploratory study was conducted to define environmental cues that promote the expression of Ysa TTS genes and to define how the Ysa TTS system influences bacterium-host interactions. Using a genetic approach, a collection of Y. enterocolitica Ysa TTS mutants was generated by mutagenesis with a transposon carrying promoterless lacZYA. This approach identified genes both within and outside of the YSA PI that contribute to Ysa TTS. Expression of these genes was regulated in response to growth phase, temperature, NaCl, and pH. Additional genetic analysis demonstrated that two regulatory genes encoding components of the YsrR-YsrS (ysrS) and RcsC-YojN-RcsB (rcsB) phosphorelay systems affect the expression of YSA PI genes and each other. The collection of Ysa TTS-defective transposon mutants, along with other strains carrying defined mutations that block Ysa and Ysc TTS, was examined for changes in virulence properties by using the BALB/c mouse model of infection. This analysis revealed that the Ysa TTS system impacts the ability of Y. enterocolitica to colonize gastrointestinal tissues. These results reveal facets of how Y. enterocolitica controls the function of the Ysa TTS system and uncovers a role for the Ysa TTS during the gastrointestinal phase of infection.

Bioinformatics, genomics and evolution of non-flagellar type-III secretion systems: a Darwinian perpective

We review the biology of non-flagellar type-III secretion systems from a Darwinian perspective, highlighting the themes of evolution, conservation, variation and decay. The presence of these systems in environmental organisms such as Myxococcus, Desulfovibrio and Verrucomicrobium hints at roles beyond virulence. We review newly discovered sequence homologies (e.g., YopN/TyeA and SepL). We discuss synapomorphies that might be useful in formulating a taxonomy of type-III secretion. The problem of information overload is likely to be ameliorated by launch of a web site devoted to the comparative biology of type-III secretion ().

Type III protein secretion mechanism in mammalian and plant pathogens

The type III protein secretion system (TTSS) is a complex organelle in the envelope of many Gram-negative bacteria; it delivers potentially hundreds of structurally diverse bacterial virulence proteins into plant and animal cells to modulate host cellular functions. Recent studies have revealed several basic features of this secretion system, including assembly of needle/pilus-like secretion structures, formation of putative translocation pores in the host membrane, recognition of N-terminal/5' mRNA-based secretion signals, and requirement of small chaperone proteins for optimal delivery and/or expression of effector proteins. Although most of our knowledge about the TTSS is derived from studies of mammalian pathogenic bacteria, similar and unique features are learned from studies of plant pathogenic bacteria. Here, we summarize the most salient aspects of the TTSS, with special emphasis on recent findings.

Process of protein transport by the type III secretion system

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

Process of protein transport by the type III secretion system

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

Identification of a dehydrogenase acting on D-2-hydroxyglutarate

Extracts of frozen rat liver were found to catalyse the formation of 3H2O from DL-2-hydroxy[2-3H]glutarate. Three peaks of enzyme activities were observed on separation by chromatography on DEAE-Sepharose. The first and second peaks corresponded to an enzyme acting on L-2-hydroxyglutarate and the third peak corresponded to an enzyme acting on D-2-hydroxyglutarate, as indicated by competitive inhibition of the detritiation of the racemic radioactive compound by the unlabelled L- and D-isomers respectively. The enzyme acting on the D-form was further characterized. It was independent of NAD or NADP and it converted D-2-hydroxyglutarate into a-ketoglutarate, transferring electrons to artificial electron acceptors. It also oxidized D-lactate, D-malate and meso-tartrate and was stimulated by Zn2+, Co2+ and Mn2+, but not by Mg2+ or Ca2+. Subcellular fractionation indicated that it was present in the mitochondrial fraction. The enzyme was further purified by chromatography on Blue Trisacryl and phenyl-Sepharose, up to a stage where only a few bands were still visible by SDS/PAGE. Among the four candidate polypeptides that were identified by MS, one corresponded to a predicted mitochondrial protein homologous with FAD-dependent D-lactate dehydrogenase. The corresponding human protein was expressed in HEK-293 cells and it was shown to catalyse the detritiation of DL-2-hydroxy[2-3H]glutarate with similar properties as the purified rat enzyme.

Yersinia enterocolitica type III secretion depends on the proton motive force but not on the flagellar motor components MotA and MotB

The flagellum is believed to be the common ancestor of all type III secretion systems (TTSSs). In Yersinia enterocolitica, expression of the flagellar TTSS and the Ysc (Yop secretion) TTSS are inversely regulated. We therefore hypothesized that the Ysc TTSS may adopt flagellar motor components in order to use the pathogenicity-related translocon in a drill-like manner. As a prerequisite for this hypothesis, we first tested a requirement for the proton motive force by both systems using the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). Motility as well as type III-dependent secretion of Yop proteins was inhibited by CCCP. We deleted motAB, which resulted in an immotile phenotype. This mutant, however, secreted amounts of Yops to the supernatant comparable to those of the wild type. Translocation of Yops into host cells was also not affected by the motAB deletion. Virulence of the mutant was comparable to that of the wild type in the mouse oral infection model. Thus, the hypothesis that the Ysc TTSS might adopt flagellar motor components was not confirmed. The finding that, in addition to consumption of ATP, Ysc TTSS requires the proton motive force is discussed.

Regulation of the Ysa type III secretion system of Yersinia enterocolitica by YsaE/SycB and YsrS/YsrR

Yersinia enterocolitica biovar 1B contains two type III secretion systems (TTSSs), the plasmid-encoded Ysc-Yop system and the chromosomally encoded Ysa-Ysp system. Proteins secreted from the Ysa TTSS (Ysps) have only been detected in vitro when cells are cultured at 26 degrees C in a high-NaCl medium. However, the exact role of the Ysa TTSS is unclear. Thus, investigations into the regulation of this system may help elucidate the role of the Ysps during the life cycle of Y. enterocolitica. Here we present evidence that the AraC-like regulator YsaE acts together with the chaperone SycB to regulate transcription of the sycByspBCDA operon, a phenomenon similar to that seen in the closely related Salmonella SPI-1 and Shigella flexneri Mxi-Spa-Ipa TTSSs. Deletion of either sycB or ysaE results in a twofold reduction in the activity of a sycB-lacZ fusion compared to the wild type. In a reconstituted Escherichia coli system, transcription of sycB was activated sixfold only when both YsaE and SycB were present, demonstrating that they are necessary for activation. ysrR and ysrS are located near the ysa genes and encode a putative two-component regulatory system. Mutations in either gene indicated that both YsrR and YsrS were required for secretion of Ysps. In addition, transcription from sycB-lacZ and ysaE-lacZ fusions was decreased 6.5- and 25-fold, respectively, in the ysrS mutant compared to the wild type. Furthermore, in the absence of NaCl, the activity of ysaE-lacZ was reduced 25-fold in the wild-type and DeltaysrS strains, indicating that YsrS is probably required for the salt-dependent expression of the ysa locus. These results suggest that the putative two-component system YsrRS may be a key element in the regulatory cascade for the Ysa TTSS.

Genomic analysis of secretion systems

Secretion of proteins into the extracellular environment is important to almost all bacteria, and in particular mediates interactions between pathogenic or symbiotic bacteria with their eukaryotic hosts. The accumulation of bacterial genome sequence data in the past few years has provided great insights into the distribution and function of these secretion systems. Three systems are responsible for secretion of proteins across the bacterial cytoplasmic membrane: Sec, SRP and Tat. Many novel examples of systems for transport across the Gram-negative bacterial cell envelope have been discovered through genome sequencing and surveys, including many novel type III secretion systems and autotransporters. Similarly, genomic data mining has revealed many new potential secretion substrates and identified unsuspected domains in secretion-associated proteins. Interestingly, genomic analyses have also hinted at the existence of a dedicated protein secretion system in Gram-positive bacteria, targeting members of the WXG100/ESAT-6 family of proteins, and have revealed an unexpectedly wide distribution of sortase-driven protein-targeting systems.

A Cluster of Mutations in the UMOD Gene Causes Familial Juvenile Hyperuricemic Nephropathy with Abnormal Expression of Uromodulin

ABSTRACT. Familial juvenile hyperuricemic nephropathy (FJHN [MIM 162000]) is an autosomal-dominant disorder characterized by abnormal tubular handling of urate and late development of chronic interstitial nephritis leading to progressive renal failure. A locus for FJHN was previously identified on chromosome 16p12 close to the MCKD2 locus, which is responsible for a variety of autosomal-dominant medullary cystic kidney disease (MCKD2). UMOD, the gene encoding the Tamm-Horsfall/uromodulin protein, maps within the FJHN/MCKD2 critical region. Mutations in UMOD were recently reported in nine families with FJHN/MCKD2 disease. A mutation in UMOD has been identified in 11 FJHN families (10 missense and one in-frame deletion)—10 of which are novel—clustering in the highly conserved exon 4. The consequences of UMOD mutations on uromodulin expression were investigated in urine samples and renal biopsies from nine patients in four families. There was a markedly increased expression of uromodulin in a cluster of tubule profiles, suggesting an accumulation of the protein in tubular cells. Consistent with this observation, urinary excretion of wild-type uromodulin was significantly decreased. The latter findings were not observed in patients with FJHN without UMOD mutations. In conclusion, this study points to a mutation clustering in exon 4 of UMOD as a major genetic defect in FJHN. Mutations in UMOD may critically affect the function of uromodulin, resulting in abnormal accumulation within tubular cells and reduced urinary excretion. E-mail: Dahan@gmed.ucl.ac.be

Targeting bacterial virulence: inhibitors of type III secretion in Yersinia

Agents that target bacterial virulence without detrimental effect on bacterial growth are useful chemical probes for studies of virulence and potential candidates for drug development. Several gram-negative pathogens employ type III secretion to evade the innate immune response of the host. Screening of a chemical library with a luciferase reporter gene assay in viable Yersinia pseudotuberculosis furnished several compounds that inhibit the reporter gene signal expressed from the yopE promoter and effector protein secretion at concentrations with no or modest effect on bacterial growth. The selectivity patterns observed for inhibition of various reporter gene strains indicate that the compounds target the type III secretion machinery at different levels. Identification of this set of inhibitors illustrates the approach of utilizing cell-based assays to identify compounds that affect complex bacterial virulence systems.