Structure-function relationship of type-IV prepilin peptidase of Pseudomonas aeruginosa--a review

Type IV pili of Enterobacteriaceae species

Type IV pili (T4Ps) are surface filaments widely distributed among bacteria and archaea. T4Ps are involved in many cellular functions and contribute to virulence in some species of bacteria. Due to the diversity of T4Ps, different properties have been observed for homologous proteins that make up T4Ps in various organisms. In this review, we highlight the essential components of T4Ps, their functions, and similarities to related systems. We emphasize the unique T4Ps of enteric pathogens within the Enterobacteriaceae family, which includes pathogenic strains of Escherichia coli and Salmonella. These include the bundle-forming pilus (BFP) of enteropathogenic E. coli (EPEC), longus (Lng) and colonization factor III (CFA/III) of enterotoxigenic E. coli (ETEC), T4P of Salmonella enterica serovar Typhi, Colonization Factor Citrobacter (CFC) of Citrobacter rodentium, T4P of Yersinia pseudotuberculosis, a ubiquitous T4P that was characterized in enterohemorrhagic E. coli (EHEC), and the R64 plasmid thin pilus. Finally, we highlight areas for further study.

High throughput and targeted screens for prepilin peptidase inhibitors do not identify common inhibitors of eukaryotic gamma-secretase

INTRODUCTION

Prepilin peptidases (PPP) are essential enzymes for the biogenesis of important virulence factors, such as type IV pili (T4P), type II secretion systems, and other T4P-related systems of bacteria and archaea. PPP inhibitors could be valuable pharmaceuticals, but only a few have been reported. Interestingly, PPP share similarities with presenilin enzymes from the gamma-secretase protease complex, which are linked to Alzheimer's disease. Numerous gamma-secretase inhibitors have been reported, and some have entered clinical trials, but none has been tested against PPP.

OBJECTIVE

The objective of this study is to develop a high-throughput screening (HTS) method to search for inhibitors of PPP from various chemical libraries and reported gamma-secretase inhibitors.

METHOD

More than 15,000 diverse compounds, including 13 reported gamma-secretase inhibitors and other reported peptidase inhibitors, were screened to identify potential PPP inhibitors.

RESULTS

The authors developed a novel screening method and screened 15,869 compounds. However, the screening did not identify a PPP inhibitor. Nevertheless, the study suggests that gamma-secretase is sufficiently different from PPP that specific inhibitors may exist in a larger chemical space.

CONCLUSION

The authors believe that the HTS method that they describe has numerous advantages and encourage others to consider its application in the search for PPP inhibitors.

Bacterial Type II Secretion System and Its Mitochondrial Counterpart

Over the billions of years that bacteria have been around, they have evolved several sophisticated protein secretion nanomachines to deliver toxins, hydrolytic enzymes, and effector proteins into their environments. Of these, the type II secretion system (T2SS) is used by Gram-negative bacteria to export a wide range of folded proteins from the periplasm across the outer membrane.

Cftr deletion in mouse epithelial and immune cells differentially influence the intestinal microbiota

Cystic fibrosis (CF) is a life-threatening genetic disorder, caused by mutations in the CF transmembrane-conductance regulator gene (cftr) that encodes CFTR, a cAMP-activated chloride and bicarbonate channel. Clinically, CF lung disease dominates the adult patient population. However, its gastrointestinal illness claims the early morbidity and mortality, manifesting as intestinal dysbiosis, inflammation and obstruction. As CF is widely accepted as a disease of epithelial dysfunction, it is unknown whether CFTR loss-of-function in immune cells contributes to these clinical outcomes. Using cftr genetic knockout and bone marrow transplantation mouse models, we performed 16S rRNA gene sequencing of the intestinal microbes. Here we show that cftr deletion in both epithelial and immune cells collectively influence the intestinal microbiota. However, the immune defect is a major factor determining the dysbiosis in the small intestine, while the epithelial defect largely influences that in the large intestine. This finding revises the current concept by suggesting that CF epithelial defect and immune defect play differential roles in CF intestinal disease.

Computational and biochemical analysis of type IV pilus dynamics and stability

Type IV pili (T4P) are distinctive dynamic filaments at the surface of many bacteria that can rapidly extend and retract and withstand strong forces. T4P are important virulence factors in many human pathogens, including Enterohemorrhagic Escherichia coli (EHEC). The structure of the EHEC T4P has been determined by integrating nuclear magnetic resonance (NMR) and cryo-electron microscopy data. To better understand pilus assembly, stability, and function, we performed a total of 108 ms all-atom molecular dynamics simulations of wild-type and mutant T4P. Extensive characterization of the conformational landscape of T4P in different conditions of temperature, pH, and ionic strength is complemented with targeted mutagenesis and biochemical analyses. Our simulations and NMR experiments reveal a conserved set of residues defining a calcium-binding site at the interface between three pilin subunits. Calcium binding enhances T4P stability ex vivo and in vitro, supporting the role of this binding site as a potential pocket for drug design.

Cell envelope proteases and peptidases of Pseudomonas aeruginosa: multiple roles, multiple mechanisms

Pseudomonas aeruginosa is a Gram-negative bacterium that is commonly isolated from damp environments. It is also a major opportunistic pathogen, causing a wide range of problematic infections. The cell envelope of P. aeruginosa, comprising the cytoplasmic membrane, periplasmic space, peptidoglycan layer and outer membrane, is critical to the bacteria's ability to adapt and thrive in a wide range of environments. Over 40 proteases and peptidases are located in the P. aeruginosa cell envelope. These enzymes play many crucial roles. They are required for protein secretion out of the cytoplasm to the periplasm, outer membrane, cell surface or the environment; for protein quality control and removal of misfolded proteins; for controlling gene expression, allowing adaptation to environmental changes; for modification and remodelling of peptidoglycan; and for metabolism of small molecules. The key roles of cell envelope proteases in ensuring normal cell functioning have prompted the development of inhibitors targeting some of these enzymes as potential new anti-Pseudomonas therapies. In this review, we summarise the current state of knowledge across the breadth of P. aeruginosa cell envelope proteases and peptidases, with an emphasis on recent findings, and highlight likely future directions in their study.

Characterization of Zoospore Type IV Pili in Actinoplanes missouriensis

The rare actinomycete Actinoplanes missouriensis produces terminal sporangia containing a few hundred flagellated spores. After release from the sporangia, the spores swim rapidly in aquatic environments as zoospores. The zoospores stop swimming and begin to germinate in niches for vegetative growth. Here, we report the characterization and functional analysis of zoospore type IV pili in A. missouriensis The pilus gene (pil) cluster, consisting of three apparently σ^FliA-dependent transcriptional units, is activated during sporangium formation similarly to the flagellar gene cluster, indicating that the zoospore has not only flagella but also pili. With a new method in which zoospores were fixed with glutaraldehyde to prevent pilus retraction, zoospore pili were observed relatively easily using transmission electron microscopy, showing 6 ± 3 pili per zoospore (n = 37 piliated zoospores) and a length of 0.62 ± 0.35 μm (n = 206), via observation of fliC-deleted, nonflagellated zoospores. No pili were observed in the zoospores of a prepilin-encoding pilA deletion (ΔpilA) mutant. In addition, the deletion of pilT, which encodes an ATPase predicted to be involved in pilus retraction, substantially reduced the frequency of pilus retraction. Several adhesion experiments using wild-type and ΔpilA zoospores indicated that the zoospore pili are required for the sufficient adhesion of zoospores to hydrophobic solid surfaces. Many zoospore-forming rare actinomycetes conserve the pil cluster, which indicates that the zoospore pili yield an evolutionary benefit in the adhesion of zoospores to hydrophobic materials as footholds for germination in their mycelial growth.IMPORTANCE Bacterial zoospores are interesting cells in that their physiological state changes dynamically: they are dormant in sporangia, show temporary mobility after awakening, and finally stop swimming to germinate in niches for vegetative growth. However, the cellular biology of a zoospore remains largely unknown. This study describes unprecedented zoospore type IV pili in the rare actinomycete Actinoplanes missouriensis Similar to the case for the usual bacterial type IV pili, zoospore pili appeared to be retractable. Our findings that the zoospore pili have a functional role in the adhesion of zoospores to hydrophobic solid surfaces and that the zoospores use both pili and flagella properly according to their different purposes provide an important insight into the cellular biology of the zoospore.

The Biosynthesis and Structures of Bacterial Pili

To interact with the external environments, bacteria often display long proteinaceous appendages on their cell surface, called pili or fimbriae. These non-flagellar thread-like structures are polymers composed of covalently or non-covalently interacting repeated pilin subunits. Distinct pilus classes can be identified on basis of their assembly pathways, including chaperone-usher pili, type V pili, type IV pili, curli and fap fibers, conjugative and type IV secretion pili, as well as sortase-mediated pili. Pili play versatile roles in bacterial physiology, and can be involved in adhesion and host cell invasion, DNA and protein secretion and uptake, biofilm formation, cell motility and more. Recent advances in structure determination of components involved in the various pilus systems has enabled a better molecular understanding of their mechanisms of assembly and function. In this chapter we describe the diversity in structure, biogenesis and function of the different pilus systems found in Gram-positive and Gram-negative bacteria, and review their potential as anti-microbial targets.

A comprehensive guide to pilus biogenesis in Gram-negative bacteria

Pili are crucial virulence factors for many Gram-negative pathogens. These surface structures provide bacteria with a link to their external environments by enabling them to interact with, and attach to, host cells, other surfaces or each other, or by providing a conduit for secretion. Recent high-resolution structures of pilus filaments and the machineries that produce them, namely chaperone-usher pili, type IV pili, conjugative type IV secretion pili and type V pili, are beginning to explain some of the intriguing biological properties that pili exhibit, such as the ability of chaperone-usher pili and type IV pili to stretch in response to external forces. By contrast, conjugative pili provide a conduit for the exchange of genetic information, and recent high-resolution structures have revealed an integral association between the pilin subunit and a phospholipid molecule, which may facilitate DNA transport. In addition, progress in the area of cryo-electron tomography has provided a glimpse of the overall architecture of the type IV pilus machinery. In this Review, we examine recent advances in our structural understanding of various Gram-negative pilus systems and discuss their functional implications.

Natural Selection in Virulence Genes of Francisella tularensis

A fundamental tenet of evolution is that alleles that are under negative selection are often deleterious and confer no evolutionary advantage. Negatively selected alleles are removed from the gene pool and are eventually extinguished from the population. Conversely, alleles under positive selection do confer an evolutionary advantage and lead to an increase in the overall fitness of the organism. These alleles increase in frequency until they eventually become fixed in the population. Francisella tularensis is a zoonotic pathogen and a potential biothreat agent. The most virulent type of F. tularensis, Type A, is distributed across North America with Type A.I occurring mainly in the east and Type A.II appearing mainly in the west. F. tularensis is thought to be a genome in decay (losing genes) because of the relatively large number of pseudogenes present in its genome. We hypothesized that the observed frequency of gene loss/pseudogenes may be an artifact of evolution in response to a changing environment, and that genes involved in virulence should be under strong positive selection. To test this hypothesis, we sequenced and compared whole genomes of Type A.I and A.II isolates. We analyzed a subset of virulence and housekeeping genes from several F. tularensis subspecies genomes to ascertain the presence and extent of positive selection. Eleven previously identified virulence genes were screened for positive selection along with 10 housekeeping genes. Analyses of selection yielded one housekeeping gene and 7 virulence genes which showed significant evidence of positive selection at loci implicated in cell surface structures and membrane proteins, metabolism and biosynthesis, transcription, translation and cell separation, and substrate binding and transport. Our results suggest that while the loss of functional genes through disuse could be accelerated by negative selection, the genome decay in Francisella could also be the byproduct of adaptive evolution driven by complex interactions between host, pathogen, and thier environment, as evidenced by several of its virulence genes which are undergoing strong, positive selection.

Identification of inhibitors of the transmembrane protease FlaK of Methanococcus maripaludis

GxGD‐type intramembrane cleaving proteases (I‐CLiPs) form a family of proteolytic enzymes that feature an aspartate‐based catalytic mechanism. Yet, they structurally and functionally largely differ from the classical pepsin‐like aspartic proteases. Among them are the archaeal enzyme FlaK, processing its substrate FlaB2 during the formation of flagella and γ‐secretase, which is centrally involved in the etiology of the neurodegenerative Alzheimer's disease. We developed an optimized activity assay for FlaK and based on screening of a small in‐house library and chemical synthesis, we identified compound 9 as the first inhibitor of this enzyme. Our results show that this intramembrane protease differs from classical pepsin‐like aspartic proteases and give insights into the substrate recognition of this enzyme. By providing the needed tools to further study the enzymatic cycle of FlaK, our results also enable further studies towards a functional understanding of other GxGD‐type I‐CLiPs.

The archaellum: how Archaea swim

Recent studies on archaeal motility have shown that the archaeal motility structure is unique in several aspects. Although it fulfills the same swimming function as the bacterial flagellum, it is evolutionarily and structurally related to the type IV pilus. This was the basis for the recent proposal to term the archaeal motility structure the "archaellum." This review illustrates the key findings that led to the realization that the archaellum was a novel motility structure and presents the current knowledge about the structural composition, mechanism of assembly and regulation, and the posttranslational modifications of archaella.

Xanthomonas citri subsp. citri type IV Pilus is required for twitching motility, biofilm development, and adherence

Bacterial type IV pili (T4P) are long, flexible surface filaments that consist of helical polymers of mostly pilin subunits. Cycles of polymerization, attachment, and depolymerization mediate several pilus-dependent bacterial behaviors, including twitching motility, surface adhesion, pathogenicity, natural transformation, escape from immune system defense mechanisms, and biofilm formation. The Xanthomonas citri subsp. citri strain 306 genome codes for a large set of genes involved in T4P biogenesis and regulation and includes several pilin homologs. We show that X. citri subsp. citri can exhibit twitching motility in a manner similar to that observed in other bacteria such as Pseudomonas aeruginosa and Xylella fastidiosa and that this motility is abolished in Xanthomonas citri subsp. citri knockout strains in the genes coding for the major pilin subunit PilAXAC3241, the ATPases PilBXAC3239 and PilTXAC2924, and the T4P biogenesis regulators PilZXAC1133 and FimXXAC2398. Microscopy analyses were performed to compare patterns of bacterial migration in the wild-type and knockout strains and we observed that the formation of mushroom-like structures in X. citri subsp. citri biofilm requires a functional T4P. Finally, infection of X. citri subsp. citri cells by the bacteriophage (ΦXacm4-11 is T4P dependent. The results of this study improve our understanding of how T4P influence Xanthomonas motility, biofilm formation, and susceptibility to phage infection.

The number of Neisseria meningitidis type IV pili determines host cell interaction

As mediators of adhesion, autoaggregation and bacteria-induced plasma membrane reorganization, type IV pili are at the heart of Neisseria meningitidis infection. Previous studies have proposed that two minor pilins, PilV and PilX, are displayed along the pilus structure and play a direct role in mediating these effects. In contrast with this hypothesis, combining imaging and biochemical approaches we found that PilV and PilX are located in the bacterial periplasm rather than along pilus fibers. Furthermore, preventing exit of these proteins from the periplasm by fusing them to the mCherry protein did not alter their function. Deletion of the pilV and pilX genes led to a decrease in the number, but not length, of pili displayed on the bacterial surface indicating a role in the initiation of pilus biogenesis. By finely regulating the expression of a central component of the piliation machinery, we show that the modest reductions in the number of pili are sufficient to recapitulate the phenotypes of the pilV and pilX mutants. We further show that specific type IV pili-dependent functions require different ranges of pili numbers.

Type IV pili in Gram-positive bacteria

Type IV pili (T4P) are surface-exposed fibers that mediate many functions in bacteria, including locomotion, adherence to host cells, DNA uptake (competence), and protein secretion and that can act as nanowires carrying electric current. T4P are composed of a polymerized protein, pilin, and their assembly apparatuses share protein homologs with type II secretion systems in eubacteria and the flagella of archaea. T4P are found throughout Gram-negative bacterial families and have been studied most extensively in certain model Gram-negative species. Recently, it was discovered that T4P systems are also widespread among Gram-positive species, in particular the clostridia. Since Gram-positive and Gram-negative bacteria have many differences in cell wall architecture and other features, it is remarkable how similar the T4P core proteins are between these organisms, yet there are many key and interesting differences to be found as well. In this review, we compare the two T4P systems and identify and discuss the features they have in common and where they differ to provide a very broad-based view of T4P systems across all eubacterial species.

Pathogenesis of meningococcemia

Neisseria meningitidis is responsible for two major diseases: cerebrospinal meningitis and/or septicemia. The latter can lead to a purpura fulminans, an often-fatal condition owing to the associated septic shock. These two clinical aspects of the meningococcal infection are consequences of a tight interaction of meningococci with host endothelial cells. This interaction, mediated by the type IV pili, is responsible for the formation of microcolonies on the apical surface of the cells. This interaction is followed by the activation of signaling pathways in the host cells leading to the formation of a microbiological synapse. A low level of bacteremia is likely to favor the colonization of brain vessels, leading to bacterial meningitis, whereas the colonization of a large number of vessels by a high number of bacteria is responsible for one of the most severe forms of septic shock observed.

Structure of the cytoplasmic domain of TcpE, the inner membrane core protein required for assembly of the Vibrio cholerae toxin-coregulated pilus

Type IV pili are long thin surface-displayed polymers of the pilin subunit that are present in a diverse group of bacteria. These multifunctional filaments are critical to virulence for pathogens such as Vibrio cholerae, which use them to form microcolonies and to secrete the colonization factor TcpF. The type IV pili are assembled from pilin subunits by a complex inner membrane machinery. The core component of the type IV pilus-assembly platform is an integral inner membrane protein belonging to the GspF superfamily of secretion proteins. These proteins somehow convert chemical energy from ATP hydrolysis by an assembly ATPase on the cytoplasmic side of the inner membrane to mechanical energy for extrusion of the growing pilus filament out of the inner membrane. Most GspF-family inner membrane core proteins are predicted to have N-terminal and central cytoplasmic domains, cyto1 and cyto2, and three transmembrane segments, TM1, TM2 and TM3. Cyto2 and TM3 represent an internal repeat of cyto1 and TM1. Here, the 1.88 Å resolution crystal structure of the cyto1 domain of V. cholerae TcpE, which is required for assembly of the toxin-coregulated pilus, is reported. This domain folds as a monomeric six-helix bundle with a positively charged membrane-interaction face at one end and a hydrophobic groove at the other end that may serve as a binding site for partner proteins in the pilus-assembly complex.

Mechanism of meningeal invasion by Neisseria meningitidis

The blood-cerebrospinal fluid barrier physiologically protects the meningeal spaces from blood-borne bacterial pathogens, due to the existence of specialized junctional interendothelial complexes. Few bacterial pathogens are able to reach the subarachnoidal space and among those, Neisseria meningitidis is the one that achieves this task the most constantly when present in the bloodstream. Meningeal invasion is a consequence of a tight interaction of meningococci with brain endothelial cells. This interaction, mediated by the type IV pili, is responsible for the formation of microcolonies on the apical surface of the cells. This interaction is followed by the activation of signaling pathways in the host cells leading to the formation of endothelial docking structures resembling those elicited by the interaction of leukocytes with endothelial cells during extravasation. The consequence of these bacterial-induced signaling events is the recruitment of intercellular junction components in the docking structure and the subsequent opening of the intercellular junctions.

Molecular evolution of key genes for type II secretion in Legionella pneumophila

Given the role of type II protein secretion system (T2S) in the ecology and pathogenesis of Legionella pneumophila, it is possible that this system is a target for adaptive evolution. The population genetic structure of L.pneumophila was inferred from the partial sequences of rpoB and from the complete sequence of three T2S structural components (lspD, lspE and pilD) and from two T2S effectors critical for intracellular infection of protozoa (proA and srnA) of 37 strains isolated from natural and man-made environments and disease-related from worldwide sources. A phylogenetic analysis was obtained for the concatenated alignment and for each individual locus. Seven main groups were identified containing the same L.pneumophila strains, suggesting an ancient divergence for each cluster and indicating that the operating selective pressures have equally affected the evolution of the five genes. Although linkage disequilibrium analysis indicate a clonal nature for population structure in this sample, our results indicate that recombination is a common phenomenon among T2S-related genes on this species, as 24 of the 37 L.pneumophila isolates contained at least one locus in which recombination was identified. Furthermore, neutral selection acting on the analysed T2S-related genes emerged as a clear result, namely on T2S effectors, ProA and SrnA, indicating that they are probably implicated in conserved virulence mechanisms through legionellae hosts.

Involvement of Type IV Pili in Pathogenicity of Plant Pathogenic Bacteria

Type IV pili (T4P) are hair-like appendages found on the surface of a wide range of bacteria belonging to the β-, γ-, and δ-Proteobacteria, Cyanobacteria and Firmicutes. They constitute an efficient device for a particular type of bacterial surface motility, named twitching, and are involved in several other bacterial activities and functions, including surface adherence, colonization, biofilm formation, genetic material uptake and virulence. Tens of genes are involved in T4P synthesis and regulation, with the majority of them being generally named pil/fim genes. Despite the multiple functionality of T4P and their well-established role in pathogenicity of animal pathogenic bacteria, relatively little attention has been given to the role of T4P in plant pathogenic bacteria. Only in recent years studies have begun to examine with more attention the relevance of these surface appendages for virulence of plant bacterial pathogens. The aim of this review is to summarize the current knowledge about T4P genetic machinery and its role in the interactions between phytopathogenic bacteria and their plant hosts.

The crystal structure of GXGD membrane protease FlaK

The GXGD proteases are polytopic membrane proteins with catalytic activities against membrane-spanning substrates that require a pair of aspartyl residues. Representative members of the family include preflagellin peptidase, type 4 prepilin peptidase, presenilin and signal peptide peptidase. Many GXGD proteases are important in medicine. For example, type 4 prepilin peptidase may contribute to bacterial pathogenesis, and mutations in presenilin are associated with Alzheimer's disease. As yet, there is no atomic-resolution structure in this protease family. Here we report the crystal structure of FlaK, a preflagellin peptidase from Methanococcus maripaludis, solved at 3.6 Å resolution. The structure contains six transmembrane helices. The GXGD motif and a short transmembrane helix, helix 4, are positioned at the centre, surrounded by other transmembrane helices. The crystal structure indicates that the protease must undergo conformational changes to bring the GXGD motif and a second essential aspartyl residue from transmembrane helix 1 into close proximity for catalysis. A comparison of the crystal structure with models of presenilin derived from biochemical analysis reveals three common transmembrane segments that are similarly arranged around the active site. This observation reinforces the idea that the prokaryotic and human proteases are evolutionarily related. The crystal structure presented here provides a framework for understanding the mechanism of the GXGD proteases, and may facilitate the rational design of inhibitors that target specific members of the family.

Identification of protein N-terminal methyltransferases in yeast and humans

Protein modification by methylation is important in cellular function. We show here that the Saccharomyces cerevisiae YBR261C/TAE1 gene encodes an N-terminal protein methyltransferase catalyzing the modification of two ribosomal protein substrates, Rpl12ab and Rps25a/Rps25b. The YBR261C/Tae1 protein is conserved across eukaryotes; all of these proteins share sequence similarity with known seven beta-strand class I methyltransferases. Wild-type yeast cytosol and mouse heart cytosol catalyze the methylation of a synthetic peptide (PPKQQLSKY) that contains the first eight amino acids of the processed N-terminus of Rps25a/Rps25b. However, no methylation of this peptide is seen in yeast cytosol from a DeltaYBR261C/tae1 deletion strain. Yeast YBR261C/TAE1 and the human orthologue METTL11A genes were expressed as fusion proteins in Escherichia coli and were shown to be capable of stoichiometrically dimethylating the N-terminus of the synthetic peptide. Furthermore, the YBR261C/Tae1 and METTL11A recombinant proteins methylate variants of the synthetic peptide containing N-terminal alanine and serine residues. However, methyltransferase activity is largely abolished when the proline residue in position 2 or the lysine residue in position 3 is substituted. Thus, the methyltransferases described here specifically recognize the N-terminal X-Pro-Lys sequence motif, and we suggest designating the yeast enzyme Ntm1 and the human enzyme NTMT1. These enzymes may account for nearly all previously described eukaryotic protein N-terminal methylation reactions. A number of other yeast and human proteins also share the recognition motif and may be similarly modified. We conclude that protein X-Pro-Lys N-terminal methylation reactions catalyzed by the enzymes described here may be widespread in nature.

Comparative evolutionary analysis of the major structural subunit of Vibrio vulnificus type IV pili

Type IV pili contribute to virulence in Vibrio vulnificus, the bacterium responsible for the majority of fatal seafood-related infections. Here, we performed within- and between-species evolutionary analysis of the gene that encodes the major structural subunit of the pilus, pilA, by comparing it with pilD and gyrB, the genes encoding the type IV prepilin peptidase and beta subunit of DNA gyrase, respectively. Although the diversity in pilD and gyrB is similar to each other and likely to have accumulated after speciation of V. vulnificus, pilA is several times more diverse at both nonsynonymous and synonymous levels. Also, in contrast to pilD and gyrB, there are virtually unrestricted and highly localized horizontal movements of pilA alleles between the major phylogenetic groups of V. vulnificus. The frequent movement of pilA involves homologous recombination of the entire gene with no evidence for intragenic recombination between the alleles. We propose that pilA allelic diversity and horizontal movement is maintained in the population by both diversifying and frequency-dependent selection most likely to escape shellfish innate immunity defense or lytic phages. Other possibilities leading to such selection dynamics of V. vulnificus pilA could involve adaptation to diverse host populations or within-host compartments, or natural DNA uptake and transformation. We show that the history of nucleotide diversification in pilA predates V. vulnificus speciation and this diversification started at or before the time of the last common ancestor for V. vulnificus, Vibrio parahaemolyticus, and Vibrio cholerae. At the same time, it appears that within the various pilA groups of V. vulnificus, there is no positive selection for structural mutations and consequently no evidence for source-sink selection. In contrast, pilD has accumulated a number of apparently adaptive mutations in the regions encoding the membrane-spanning portions of the prepilin peptidase, possibly affecting fimbrial expression and/or function, and is being subjected to source-sink selection dynamics.

PILZ protein structure and interactions with PILB and the FIMX EAL domain: implications for control of type IV pilus biogenesis

The PilZ protein was originally identified as necessary for type IV pilus (T4P) biogenesis. Since then, a large and diverse family of bacterial PilZ homology domains have been identified, some of which have been implicated in signaling pathways that control important processes, including motility, virulence and biofilm formation. Furthermore, many PilZ homology domains, though not PilZ itself, have been shown to bind the important bacterial second messenger bis(3'-->5')cyclic diGMP (c-diGMP). The crystal structures of the PilZ orthologs from Xanthomonas axonopodis pv citri (PilZ(XAC1133), this work) and from Xanthomonas campestris pv campestris (XC1028) present significant structural differences to other PilZ homologs that explain its failure to bind c-diGMP. NMR analysis of PilZ(XAC1133) shows that these structural differences are maintained in solution. In spite of their emerging importance in bacterial signaling, the means by which PilZ proteins regulate specific processes is not clear. In this study, we show that PilZ(XAC1133) binds to PilB, an ATPase required for T4P polymerization, and to the EAL domain of FimX(XAC2398), which regulates T4P biogenesis and localization in other bacterial species. These interactions were confirmed in NMR, two-hybrid and far-Western blot assays and are the first interactions observed between any PilZ domain and a target protein. While we were unable to detect phosphodiesterase activity for FimX(XAC2398)in vitro, we show that it binds c-diGMP both in the presence and in the absence of PilZ(XAC1133). Site-directed mutagenesis studies for conserved and exposed residues suggest that PilZ(XAC1133) interactions with FimX(XAC2398) and PilB(XAC3239) are mediated through a hydrophobic surface and an unstructured C-terminal extension conserved only in PilZ orthologs. The FimX-PilZ-PilB interactions involve a full set of "degenerate" GGDEF, EAL and PilZ domains and provide the first evidence of the means by which PilZ orthologs and FimX interact directly with the TP4 machinery.

Meningococcal interactions with the host

Neisseria meningitidis interacts with host tissues through hierarchical, concerted and co-ordinated actions of a number of adhesins; many of which undergo antigenic and phase variation, a strategy that helps immune evasion. Three major structures, pili, Opa and Opc predominantly influence bacterial adhesion to host cells. Pili and Opa proteins also determine host and tissue specificity while Opa and Opc facilitate efficient cellular invasion. Recent studies have also implied a role of certain adhesin-receptor pairs in determining increased host susceptibility to infection. This chapter examines our current knowledge of meningococcal adhesion and invasion mechanisms particularly related to human epithelial and endothelial cells which are of primary importance in the disease process.

Structure and mechanism of intramembrane protease

Many functionally important membrane proteins are cleaved within their transmembrane helices to become activated. This unusual reaction is catalyzed by a group of highly specialized and membrane-bound proteases. Here I briefly summarize current knowledge about their structure and mechanism, with a focus on the rhomboid family. It has now become clear that rhomboid protease can cleave substrates not only within transmembrane domains, but also in the solvent-exposed juxtamembrane region. This dual specificity seems possible because the protease active site is positioned in a shallow pocket that can directly open to aqueous solution through the movement of a flexible capping loop. The narrow membrane-spanning region of the protease suggests a possible mechanism for accessing scissile bonds that are located near the end of substrate transmembrane helices. Similar principles may apply to the metalloprotease family, where a crystal structure has also become available. Although how the GxGD proteases work is still less clear, recent results indicate that presenilin also appears to clip substrate from the end of transmembrane helices.

Towards a systems biology approach to study type II/IV secretion systems

Many gram-negative bacteria produce thin protein filaments, named pili, which extend beyond the confines of the outer membrane. The importance of these pili is illustrated by the fact that highly complex, multi-protein pilus-assembly machines have evolved, not once, but several times. Their many functions include motility, adhesion, secretion, and DNA transfer, all of which can contribute to the virulence of bacterial pathogens or to the spread of virulence factors by horizontal gene transfer. The medical importance has stimulated extensive biochemical and genetic studies but the assembly and function of pili remains an enigma. It is clear that progress in this field requires a more holistic approach where the entire molecular apparatus that forms the pilus is studied as a system. In recent years systems biology approaches have started to complement classical studies of pili and their assembly. Moreover, continued progress in structural biology is building a picture of the components that make up the assembly machine. However, the complexity and multiple-membrane spanning nature of these secretion systems pose formidable technical challenges, and it will require a concerted effort before we can create comprehensive and predictive models of these remarkable molecular machines.

Post-transcriptional cross-talk between pro- and anti-colonization pili biosynthesis systems in Vibrio cholerae

The pathogen Vibrio cholerae modulates the expression of many genes in order to transition from its environmental reservoir to its niche in the human host. Among these are genes encoding two related Type IV pili, the mannose-sensitive haemagglutinin (MSHA) pilus, which aids V. cholerae persistence in aquatic environments but causes clearance of bacteria by host immune defences, and the toxin co-regulated pilus (TCP) required for colonization. These antagonistic effects are resolved transcriptionally by the regulator ToxT, which represses msh genes while activating tcp genes during infection. We show that these two pili systems are also intertwined post-transcriptionally through the ToxT-regulated pre-pilin peptidase TcpJ. We found that the major MSHA pilin, MshA, was degraded in V. cholerae in a TcpJ-dependent fashion. In a heterologous Escherichia coli system, TcpJ can recognize both MshA and its cognate substrate, the TCP subunit TcpA, but that processing by TcpJ causes the degradation of MshA. Through site-directed mutagenesis and chimeric pilin analysis, we show that this process targets a combination of MshA N-terminal motifs and depends on the proteolytic activity of TcpJ. Moreover, overexpression of tcpJ partially restored the ability of bacteria unable to transcriptionally downregulate msh genes to colonize infant mice. These findings describe co-ordinated proteolysis as a regulatory mechanism in V. cholerae and illustrate this organism's adaptability in the face of dramatic environmental changes.

3D structure/function analysis of PilX reveals how minor pilins can modulate the virulence properties of type IV pili

Type IV pili (Tfp) are widespread filamentous bacterial organelles that mediate multiple virulence-related phenotypes. They are composed mainly of pilin subunits, which are processed before filament assembly by dedicated prepilin peptidases. Other proteins processed by these peptidases, whose molecular nature and mode of action remain enigmatic, play critical roles in Tfp biology. We have performed a detailed structure/function analysis of one such protein, PilX from Neisseria meningitidis, which is crucial for formation of bacterial aggregates and adhesion to human cells. The x-ray crystal structure of PilX reveals the alpha/beta roll fold shared by all pilins, and we show that this protein colocalizes with Tfp. These observations suggest that PilX is a minor, or low abundance, pilin that assembles within the filaments in a similar way to pilin. Deletion of a PilX distinctive structural element, which is predicted to be exposed on the filament surface, abolishes aggregation and adhesion. Our results support a model in which surface-exposed motifs in PilX subunits stabilize bacterial aggregates against the disruptive force of pilus retraction and illustrate how a minor pilus component can enhance the functional properties of pili of rather simple composition and structure.

Molecular analysis of genes in Nostoc punctiforme involved in pilus biogenesis and plant infection

Hormogonia are the infective agents in many cyanobacterium-plant symbioses. Pilus-like appendages are expressed on the hormogonium surface, and mutations in pil-like genes altered surface piliation and reduced symbiotic competency. This is the first molecular evidence that pilus biogenesis in a filamentous cyanobacterium requires a type IV pilus system.

Novel type IV secretion system involved in propagation of genomic islands

Type IV secretion systems (T4SSs) mediate horizontal gene transfer, thus contributing to genome plasticity, evolution of infectious pathogens, and dissemination of antibiotic resistance and other virulence traits. A gene cluster of the Haemophilus influenzae genomic island ICEHin1056 has been identified as a T4SS involved in the propagation of genomic islands. This T4SS is novel and evolutionarily distant from the previously described systems. Mutation analysis showed that inactivation of key genes of this system resulted in a loss of phenotypic traits provided by a T4SS. Seven of 10 mutants with a mutation in this T4SS did not express the type IV secretion pilus. Correspondingly, disruption of the genes resulted in up to 100,000-fold reductions in conjugation frequencies compared to those of the parent strain. Moreover, the expression of this T4SS was found to be positively regulated by one of its components, the tfc24 gene. We concluded that this gene cluster represents a novel family of T4SSs involved in propagation of genomic islands.

A systematic genetic analysis in Neisseria meningitidis defines the Pil proteins required for assembly, functionality, stabilization and export of type IV pili

Although type IV pili (Tfp) are among the commonest virulence factors in bacteria, their biogenesis by complex machineries of 12-15 proteins, and thereby their function remains poorly understood. Interestingly, some of these proteins were reported to merely antagonize the retraction of the fibres powered by PilT, because piliation could be restored in their absence by a mutation in the pilT gene. The recent identification of the 15 Pil proteins dedicated to Tfp biogenesis in Neisseria meningitidis offered us the unprecedented possibility to define their exact contribution in this process. We therefore systematically introduced a pilT mutation into the corresponding non-piliated mutants and characterized them for the rescue of Tfp and Tfp-mediated virulence phenotypes. We found that in addition to the pilin, the main constituent of Tfp, only six Pil proteins were required for the actual assembly of the fibres, because apparently normal fibres were restored in the remaining mutants. Restored fibres were surface-exposed, except in the pilQ/T mutant in which they were trapped in the periplasm, suggesting that the PilQ secretin was the sole Pil component necessary for their emergence on the surface. Importantly, although in most mutants the restored Tfp were not functional, the pilG/T and pilH/T mutants could form bacterial aggregates and adhere to human cells efficiently, suggesting that Tfp stabilization and functional maturation are two discrete steps. These findings have numerous implications for understanding Tfp biogenesis/function and provide a useful groundwork for the characterization of the precise function of each Pil protein in this process.

Active-site residues in the type IV prepilin peptidase homologue PibD from the archaeon Sulfolobus solfataricus

Archaeal preflagellin peptidases and bacterial type IV prepilin peptidases belong to a family of aspartic acid proteases that cleave the leader peptides of precursor proteins with type IV prepilin signal sequences. The substrate repertoire of PibD from the crenarchaeon Sulfolobus solfataricus is unusually diverse. In addition to flagellin, PibD cleaves three sugar-binding proteins unique to this species and a number of proteins with unknown function. Here we demonstrate that PibD contains two aspartic acid residues that are essential for cleavage activity. An additional pair of aspartic acids in a large cytoplasmic loop is also important for function and is possibly involved in leader peptide recognition. Combining the results of transmembrane segment predictions and cysteine-labeling experiments, we suggest a membrane topology model for PibD with the active-site aspartic acid residues exposed to the cytosol.

Crystal structure of PilF: functional implication in the type 4 pilus biogenesis in Pseudomonas aeruginosa

PilF is a requisite protein involved in the type 4 pilus biogenesis system from the Gram-negative human pathogenic bacteria, Pseudomonas aeruginosa. We determined the PilF structure at a 2.2A resolution; this includes six tandem tetratrico peptide repeat (TPR) units forming right-handed superhelix. PilF structure was similar to the heat shock protein organizing protein, which interacts with the C-terminal peptide of Hsp90 and Hsp70 via a concave Asn ladder in the inner groove of TPR superhelix. After simulated screening, the C-terminal pentapeptides of PilG, PilU, PilY, and PilZ proved to be a likely candidate binding to PilF, which are ones of 25 necessary components involved in the type 4 pilus biogenesis system. We proposed that PilF would be critical as a bridgehead in protein-protein interaction and thereby, PilF may bind a necessary molecule in type 4 pilus biogenesis system such as PilG, PilU, PilY, and PilZ.

Factors triggering type III secretion in Pseudomonas aeruginosa

The type III secretion system of Pseudomonas aeruginosa is tightly regulated by various environmental signals, such as low calcium and contact with the host cell. However, the exact signals triggering type III secretion are unknown. The present study describes the finding that secretion of P. aeruginosa type III effector molecules requires protein factors from serum and L broth, designated type III secretion factors (TSFs), in addition to the low-calcium environment. In the absence of TSF or calcium chelator EGTA, basal levels of type III effector molecules are accumulated intracellularly. Addition of TSF and EGTA together effectively triggers the secretion of pre-existing effector molecules in a short time, even before the active expression of type III genes; thus, active type III gene expression does not seem to be a prerequisite for type III secretion. A search for TSF molecules in serum and L broth resulted in the identification of albumin and casein as the functional TSF molecules. Although there is no clear sequence similarity between albumin and casein, both proteins are known to have a low-affinity, high-capacity calcium-binding property. Tests of well-studied calcium-binding proteins seemed to indicate that low-affinity calcium-binding proteins have TSF activity, although the requirement of low-affinity calcium-binding ability for the TSF activity is not clear. P. aeruginosa seems to have evolved a sensing mechanism to detect target cells for type III injection through host-derived proteins in combination with a low-calcium signal. Disruption of the bacterial ability to sense low calcium or TSF might be a valid avenue to the effective control of this bacterial pathogen.

Analysis of the pilU gene for the prepilin peptidase involved in the biogenesis of type IV pili encoded by plasmid R64

In many type IV pili, the N-terminal amino acid of the pilin subunit is N-methylated phenylalanine. A prepilin peptidase removes the leader peptide from the precursor and methylates the amino group of the newly formed phenylalanine. PilS, the precursor of the pilin encoded by plasmid R64, is processed by the prepilin peptidase PilU, but the N-terminal amino acid of the mature pilin is a non-methylated tryptophan that is otherwise modified. To study the relationship between the structure and function of PilU, 42 missense pilU mutations were constructed by PCR and site-directed mutagenesis, and the ability of these pilU mutants to complement a pilU null mutant for mating in liquid culture was analyzed. Although practically no conjugation was noted for 21 of the mutants, the remaining 21 supported varying levels of residual plasmid transfer activity. Two mutants with aspartic acid replacements in conserved motifs exhibited no PilU activity, suggesting that the product of the pilU gene is an aspartic acid peptidase, like TcpJ, the prepilin peptidare of Vibrio cholerae. No PilS processing was detected in 21 of the mutants, but the remaining 21 exhibited varying levels of residual PilS processing. A close correlation was noted between residual PilS processing activity and conjugative transfer, suggesting that the pilU gene product possesses prepilin peptidase activity, but is unable to methylate the N-terminal tryptophan. Based on the activity of pilU-phoA and pilU-lacZ fusion genes encoding different segments of PilU, a model for the membrane topology of the protein is also proposed. Furthermore, some amino acid substitutions in the pilU portion of the pilU-phoA and pilU-lacZ fusion genes were found to alter the membrane topology of the product.

An in vivo inducible gene of Pseudomonas aeruginosa encodes an anti-ExsA to suppress the type III secretion system

We have previously reported on the isolation of in vivo inducible genes of Pseudomonas aeruginosa using IVET system. One of such genes isolated from burn mouse infection model encodes a short open reading frame with unknown function. In this study, we demonstrate that this gene product specifically suppresses the expression of type III secretion genes in P. aeruginosa, thus named PtrA (Pseudomonas type III repressor A). A direct interaction between the PtrA and type III transcriptional activator ExsA was demonstrated, suggesting that its repressor function is probably realized through inhibition of the ExsA protein function. Indeed, an elevated expression of the exsA compensates the repressor effect of the PtrA. Interestingly, expression of the ptrA is highly and specifically induced by copper cation. A copper- responsive two-component regulatory system, copR-copS, has also been identified and shown to be essential for the copper resistance in P. aeruginosa as well as the activation of ptrA in response to the copper signal. Elevated expression of the ptrA during the infection of mouse burn wound suggests that P. aeruginosa has evolved tight regulatory systems to shut down energy-expensive type III secretion apparatus in response to specific environmental signals, such as copper stress.

Unique modifications with phosphocholine and phosphoethanolamine define alternate antigenic forms of Neisseria gonorrhoeae type IV pili

Several major bacterial pathogens and related commensal species colonizing the human mucosa express phosphocholine (PC) at their cell surfaces. PC appears to impact host-microbe biology by serving as a ligand for both C-reactive protein and the receptor for platelet-activating factor. Type IV pili of Neisseria gonorrhoeae (Ng) and Neisseria meningitidis, filamentous protein structures critical to the colonization of their human hosts, are known to react variably with monoclonal antibodies recognizing a PC epitope. However, the structural basis for this reactivity has remained elusive. To address this matter, we exploited the finding that the PilE pilin subunit in Ng mutants lacking the PilV protein acquired the PC epitope independent of changes in pilin primary structure. Specifically, we show by using mass spectrometry that PilE derived from the pilV background is composed of a mixture of subunits bearing O-linked forms of either phosphoethanolamine (PE) or PC at the same residue, whereas the wild-type background carries only PE at that same site. Therefore, PilV can influence pilin structure and antigenicity by modulating the incorporation of these alternative modifications. The disaccharide covalently linked to Ng pilin was also characterized because it is present on the same peptides bearing the PE and PC modifications and, contrary to previous reports, was found to be linked by means of 2,4-diacetamido-2,4,6-trideoxyhexose. Taken together, these findings provide new insights into Ng type IV pilus structure and antigenicity and resolve long-standing issues regarding the nature of both the PC epitope and the pilin glycan.

Presence of pili on the surface of Francisella tularensis

Francisella tularensis is a highly infectious gram-negative bacterium with potential for use as a bioweapon. Analysis of the F. tularensis live vaccine strain (LVS) ultrastructure by electron microscopy revealed the presence of long, thin fibers, similar in appearance to type 4 pili. The highly virulent F. tularensis Schu S4 strain was found to contain type 4 pilus genes, and we confirmed that these genes are present and expressed in the LVS.

Host cell contact-induced transcription of the type IV fimbria gene cluster of Actinobacillus pleuropneumoniae

Type IV pili (Tfp) of gram-negative species share many characteristics, including a common architecture and conserved biogenesis pathway. Much less is known about the regulation of Tfp expression in response to changing environmental conditions. We investigated the diversity of Tfp regulatory systems by searching for the molecular basis of the reported variable expression of the Tfp gene cluster of the pathogen Actinobacillus pleuropneumoniae. Despite the presence of an intact Tfp gene cluster consisting of four genes, apfABCD, no Tfp were formed under standard growth conditions. Sequence analysis of the predicted major subunit protein ApfA showed an atypical alanine residue at position -1 from the prepilin peptidase cleavage site in 42 strains. This alanine deviates from the consensus glycine at this position in Tfp from other species. Yet, cloning of the apfABCD genes under a constitutive promoter in A. pleuropneumoniae resulted in pilin and Tfp assembly. Tfp promoter-luxAB reporter gene fusions demonstrated that the Tfp promoter was intact but tightly regulated. Promoter activity varied with bacterial growth phase and was detected only when bacteria were grown in chemically defined medium. Infection experiments with cultured epithelial cells demonstrated that Tfp promoter activity was upregulated upon adherence of the pathogen to primary cultures of lung epithelial cells. Nonadherent bacteria in the culture supernatant exhibited virtually no promoter activity. A similar upregulation of Tfp promoter activity was observed in vivo during experimental infection of pigs. The host cell contact-induced and in vivo-upregulated Tfp promoter activity in A. pleuropneumoniae adds a new dimension to the diversity of Tfp regulation.

Archaeal homolog of bacterial type IV prepilin signal peptidases with broad substrate specificity

A large number of secretory proteins in the thermoacidophile Sulfolobus solfataricus are synthesized as a precursor with an unusual leader peptide that resembles bacterial type IV prepilin signal sequences. This set of proteins includes the flagellin subunit but also various solute binding proteins. Here we describe the identification of the S. solfataricus homolog of bacterial type IV prepilin peptidases, termed PibD. PibD is an integral membrane protein that is phylogenetically related to the bacterial enzymes. When heterologously expressed in Escherichia coli, PibD is capable of processing both the flagellin and glucose-binding protein (GlcS) precursors. Site-directed mutagenesis of the GlcS signal peptide shows that the substrate specificity of PibD is consistent with the variations found in proteins with type IV prepilin-like signal sequences of S. solfataricus. We conclude that PibD is responsible for the processing of these secretory proteins in S. solfataricus.

Type IV pili and twitching motility

Twitching motility is a flagella-independent form of bacterial translocation over moist surfaces. It occurs by the extension, tethering, and then retraction of polar type IV pili, which operate in a manner similar to a grappling hook. Twitching motility is equivalent to social gliding motility in Myxococcus xanthus and is important in host colonization by a wide range of plant and animal pathogens, as well as in the formation of biofilms and fruiting bodies. The biogenesis and function of type IV pili is controlled by a large number of genes, almost 40 of which have been identified in Pseudomonas aeruginosa. A number of genes required for pili assembly are homologous to genes involved in type II protein secretion and competence for DNA uptake, suggesting that these systems share a common architecture. Twitching motility is also controlled by a range of signal transduction systems, including two-component sensor-regulators and a complex chemosensory system.

Molecular analyses of the natural transformation machinery and identification of pilus structures in the extremely thermophilic bacterium Thermus thermophilus strain HB27

Thermus thermophilus HB27, an extremely thermophilic bacterium, exhibits high competence for natural transformation. To identify genes of the natural transformation machinery of T. thermophilus HB27, we performed homology searches in the partially completed T. thermophilus genomic sequence for conserved competence genes. These analyses resulted in the detection of 28 open reading frames (ORFs) exhibiting significant similarities to known competence proteins of gram-negative and gram-positive bacteria. Disruption of 15 selected potential competence genes led to the identification of 8 noncompetent mutants and one transformation-deficient mutant with a 100-fold reduced transformation frequency. One competence protein is similar to DprA of Haemophilus influenzae, seven are similar to type IV pilus proteins of Pseudomonas aeruginosa or Neisseria gonorrhoeae (PilM, PilN, PilO, PilQ, PilF, PilC, PilD), and another deduced protein (PilW) is similar to a protein of unknown function in Deinococcus radiodurans R1. Analysis of the piliation phenotype of T. thermophilus HB27 revealed the presence of single pilus structures on the surface of the wild-type cells, whereas the noncompetent pil mutants of Thermus, with the exception of the pilF mutant, were devoid of pilus structures. These results suggest that pili and natural transformation in T. thermophilus HB27 are functionally linked.

Twitching motility of Ralstonia solanacearum requires a type IV pilus system

Twitching motility is a form of bacterial translocation over firm surfaces that requires retractile type IV pili. Microscopic colonies of Ralstonia solanacearum strains AW1, K60 and GMI1000 growing on the surface of a rich medium solidified with 1.6% agar appeared to exhibit twitching motility, because early on they divided into motile 'rafts' of cells and later developed protruding 'spearheads' at their margins. Individual motile bacteria were observed only when they were embedded within masses of other cells. Varying degrees of motility were observed for 33 of 35 strains of R. solanacearum in a selected, diverse collection. Timing was more important than culture conditions for observing motility, because by the time wild-type colonies were easily visible by eye (about 48 h) this activity ceased and the spearheads were obscured by continued bacterial multiplication. In contrast, inactivation of PhcA, a transcriptional regulator that is essential for R. solanacearum to cause plant disease, resulted in colonies that continued to expand for at least several additional days. Multiple strains with mutations in regulatory genes important for virulence were tested, but all exhibited wild-type motility. Many of the genes required for production of functional type IV pili, and hence for twitching motility, are conserved among unrelated bacteria, and pilD, pilQ and pilT orthologues were identified in R. solanacearum. Colonies of R. solanacearum pilQ and pilT mutants did not develop spearheads or rafts, confirming that the movement of cells that had been observed was due to twitching motility. Compared to the wild-type parents, both pilQ and pilT mutants caused slower and less severe wilting on susceptible tomato plants. This is the first report of twitching motility by a phytopathogenic bacterium, and the first example where type IV pili appear to contribute significantly to plant pathogenesis.

Bacterial virulence gene regulation: an evolutionary perspective

Coevolution between bacteria and their plant or animal hosts determines characteristics of the interaction, the bacterial virulence genes involved, and the regulatory systems controlling expression of virulence genes. The long-standing association between Salmonellae and their animal hosts has resulted in the acquisition by Salmonella subspecies of a variety of virulence genes and the evolution of complex regulatory networks. The particular repertoire of virulence genes acquired by different Salmonella enterica subspecies and the regulatory systems that control them dictate subspecies-specific infection characteristics. Although the association between Vibrio cholerae and humans appears to be more recent, to reflect a simpler pathogenic strategy, and to involve fewer virulence genes than that of Salmonellae, complex virulence-regulatory networks have nonetheless evolved. In contrast, there is no evidence for acquisition of virulence genes by horizontal gene transfer in bordetellae, and their virulence regulon is less complex in overall structure than those of salmonellae and Vibrio cholerae. In Bordetellae, subspecies-specific differences in pathogenic strategy appear to result from differential gene expression within and across Bordetella subspecies.

Identification of secreted proteins of the cyanobacterium Synechocystis sp. strain PCC 6803

We investigated the spectrum of secreted proteins in the cyanobacterium Synechocystis, and identified these proteins by amino-terminal sequencing. In total, seven sequences have been determined that corresponded to the proteins Sll0044, Sll1694, Sll1891, Slr0924, Slr0841, Slr0168, and Slr1855. The protein Sll1694 of 18 kDa that formed one of two major bands on SDS-PAGE was identified as cyanobacterial pilin, PilA. The amino-terminal sequence of another protein that formed a second major band was blocked. The analysis of the data revealed that five of seven proteins had distinct putative leader sequences for secretion.

Components and dynamics of fiber formation define a ubiquitous biogenesis pathway for bacterial pili

Type IV pili (Tfp) are a unique class of multifunctional surface organelles in Gram-negative bacteria, which play important roles in prokaryotic cell biology. Although components of the Tfp biogenesis machinery have been characterized, it is not clear how they function or interact. Using Neisseria gonorrhoeae as a model system, we report here that organelle biogenesis can be resolved into two discrete steps: fiber formation and translocation of the fiber to the cell surface. This conclusion is based on the capturing of an intermediate state in which the organelle is retained within the cell owing to the simultaneous absence of the secretin family member and biogenesis component PilQ and the twitching motility/pilus retraction protein PilT. This finding is the first demonstration of a specific translocation defect associated with loss of secretin function, and additionally confirms the role of PilT as a conditional antagonist of stable pilus fiber formation. These findings have important implications for Tfp structure and function and are pertinent to other membrane translocation systems that utilize a highly related set of components.

ComP, a pilin-like protein essential for natural competence in Acinetobacter sp. Strain BD413: regulation, modification, and cellular localization

We recently identified a pilin-like competence factor, ComP, which is essential for natural transformation of the gram-negative soil bacterium Acinetobacter sp. strain BD413. Here we demonstrate that transcription and synthesis of the pilin-like competence factor ComP are maximal in the late stationary growth phase, whereas competence is induced immediately after inoculation of a stationary-phase culture into fresh medium. Western blot analyses revealed three forms of ComP, one with an apparent molecular mass of 15 kDa, which correlates with the molecular mass deduced from the DNA sequence, one 20-kDa form, which was found to be glycosylated, and one 23-kDa form. The glycosylation of ComP was not required for its function in DNA binding and uptake. The 20-kDa form was present in the cytoplasmic membrane, the periplasm, and the outer membrane, whereas the 23-kDa form was located in the outer membrane and might be due to a further modification. Immunological data suggest that ComP is not a subunit of the pilus structures. Possible functions of ComP in the DNA transformation machinery of Acinetobacter sp. strain BD413 are discussed.

Secretion of virulence determinants by the general secretory pathway in gram-negative pathogens: an evolving story

Secretion of proteins by the general secretory pathway (GSP) is a two-step process requiring the Sec translocase in the inner membrane and a separate substrate-specific secretion apparatus for translocation across the outer membrane. Gram-negative bacteria with pathogenic potential use the GSP to deliver virulence factors into the extracellular environment for interaction with the host. Well-studied examples of virulence determinants using the GSP for secretion include extracellular toxins, pili, curli, autotransporters, and crystaline S-layers. This article reviews our current understanding of the GSP and discusses examples of terminal branches of the GSP which are utilized by factors implicated in bacterial virulence.

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

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