The secretion apparatus of Pseudomonas aeruginosa: identification of a fifth pseudopilin, XcpX (GspK family)

Mechanism of assembly of type 4 filaments: everything you always wanted to know (but were afraid to ask)

Type 4 filaments (T4F) are a superfamily of filamentous nanomachines - virtually ubiquitous in prokaryotes and functionally versatile - of which type 4 pili (T4P) are the defining member. T4F are polymers of type 4 pilins, assembled by conserved multi-protein machineries. They have long been an important topic for research because they are key virulence factors in numerous bacterial pathogens. Our poor understanding of the molecular mechanisms of T4F assembly is a serious hindrance to the design of anti-T4F therapeutics. This review attempts to shed light on the fundamental mechanistic principles at play in T4F assembly by focusing on similarities rather than differences between several (mostly bacterial) T4F. This holistic approach, complemented by the revolutionary ability of artificial intelligence to predict protein structures, led to an intriguing mechanistic model of T4F assembly.

Structural interactions define assembly adapter function of a type II secretion system pseudopilin

The type IV filament superfamily comprises widespread membrane-associated polymers in prokaryotes. The type II secretion system (T2SS), a virulence pathway in many pathogens, belongs to this superfamily. A knowledge gap in understanding of the T2SS is the molecular role of a small "pseudopilin" protein. Using multiple biophysical techniques, we have deciphered how this missing component of the Xcp T2SS architecture is structurally integrated, and thereby unlocked its function. We demonstrate that low-abundance XcpH is the adapter that bridges a trimeric initiating tip complex, XcpIJK, with a periplasmic filament of XcpG subunits. Each pseudopilin protein caps an XcpG protofilament in an overall pseudopilus compatible with dimensions of the periplasm and the outer membrane-spanning secretin through which substrates pass. Unexpectedly, to fulfill its adapter function, the XcpH N-terminal helix must be unwound, a property shared with XcpG subunits. We provide an experimentally validated three-dimensional structural model of a complete type IV filament.

A Putative Type II Secretion System Is Involved in Cellulose Utilization in Cytophaga hutchisonii

Cytophaga hutchinsonii is a gliding cellulolytic bacterium that degrades cellulose in a substrate contact-dependent manner. Specific proteins are speculated to be translocated to its extracellular milieu or outer membrane surface to participate in adhesion to cellulose and further digestion. In this study, we show that three orthologous genes encoding the major components (T2S-D, -F, and -G) of type II secretion system (T2SS) are involved in cellulose degradation but not in cell motility. The individual disruption of the three t2s genes results in a significantly retarded growth on cellobiose, regenerated amorphous cellulose, and Avicel cellulose. Enzymatic analyses demonstrate that, whereas the endoglucanase activity of the t2s mutant cells is increased, the β-glucosidase activity is remarkably reduced compared to that of WT cells. Further analyses reveal that the t2s mutant cells not only exhibit a different profile of cellulose-bound outer membrane proteins from that of wild-type cells, but also display a significant decrease in their capability to adhere to cellulose. These results indicate that a functional link exits between the putative T2SS and cellulose utilization in C. hutchinsonii, and thus provide a conceptual framework to understand the unique strategy deployed by C. hutchinsonii to assimilate cellulose.

The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin-Coregulated Pilus

Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The Vibrio cholerae toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a V. cholerae tcpB Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system.

Bacterial pathogens utilize a number of highly complex and sophisticated molecular systems to colonize their hosts and alter them, creating customized niches in which to reproduce. One such system is the Type IV pilus system, made up of dozens of proteins that form a macromolecular machine to polymerize small pilin proteins into long thin filaments that are displayed on the bacterial surface. These pili have a remarkable array of functions that rely on their ability to (i) adhere to many substrates, including host cell surfaces, pili from nearby bacteria, DNA and bacterial viruses (bacteriophage), and (ii) to depolymerize or retract, which pulls the bacteria along mucosal surfaces, pulls them close together in protective aggregates, and can even draw in substrates like DNA and bacteriophage for nutrition and genetic variation. For most Type IV pilus systems, retraction is an energy-driven process facilitated by a retraction ATPase. We show here that in the simplest of the Type IV pilus systems, the Vibrio cholerae toxin-coregulated pilus, a pilin-like protein initiates pilus retraction by what appears to be mechanical rather than enzymatic means. Our results provide a framework for understanding more complex Type IV pili and the related Type II secretion systems, which represent targets for novel highly specific antibiotics.

Crystal Structure of the Minor Pilin CofB, the Initiator of CFA/III Pilus Assembly in Enterotoxigenic Escherichia coli

Type IV pili are extracellular polymers of the major pilin subunit. These subunits are held together in the pilus filament by hydrophobic interactions among their N-terminal α-helices, which also anchor the pilin subunits in the inner membrane prior to pilus assembly. Type IV pilus assembly involves a conserved group of proteins that span the envelope of Gram-negative bacteria. Among these is a set of minor pilins, so named because they share their hydrophobic N-terminal polymerization/membrane anchor segment with the major pilins but are much less abundant. Minor pilins influence pilus assembly and retraction, but their precise functions are not well defined. The Type IV pilus systems of enterotoxigenic Escherichia coli and Vibrio cholerae are among the simplest of Type IV pilus systems and possess only a single minor pilin. Here we show that the enterotoxigenic E. coli minor pilins CofB and LngB are required for assembly of their respective Type IV pili, CFA/III and Longus. Low levels of the minor pilins are optimal for pilus assembly, and CofB can be detected in the pilus fraction. We solved the 2.0 Å crystal structure of N-terminally truncated CofB, revealing a pilin-like protein with an extended C-terminal region composed of two discrete domains connected by flexible linkers. The C-terminal region is required for CofB to initiate pilus assembly. We propose a model for CofB-initiated pilus assembly with implications for understanding filament growth in more complex Type IV pilus systems as well as the related Type II secretion system.

Pseudomonas aeruginosa minor pilins prime type IVa pilus assembly and promote surface display of the PilY1 adhesin

Type IV pili (T4P) contain hundreds of major subunits, but minor subunits are also required for assembly and function. Here we show that Pseudomonas aeruginosa minor pilins prime pilus assembly and traffic the pilus-associated adhesin and anti-retraction protein, PilY1, to the cell surface. PilV, PilW, and PilX require PilY1 for inclusion in surface pili and vice versa, suggestive of complex formation. PilE requires PilVWXY1 for inclusion, suggesting that it binds a novel interface created by two or more components. FimU is incorporated independently of the others and is proposed to couple the putative minor pilin-PilY1 complex to the major subunit. The production of small amounts of T4P by a mutant lacking the minor pilin operon was traced to expression of minor pseudopilins from the P. aeruginosa type II secretion (T2S) system, showing that under retraction-deficient conditions, T2S minor subunits can prime T4P assembly. Deletion of all minor subunits abrogated pilus assembly. In a strain lacking the minor pseudopilins, PilVWXY1 and either FimU or PilE comprised the minimal set of components required for pilus assembly. Supporting functional conservation of T2S and T4P minor components, our 1.4 Å crystal structure of FimU revealed striking architectural similarity to its T2S ortholog GspH, despite minimal sequence identity. We propose that PilVWXY1 form a priming complex for assembly and that PilE and FimU together stably couple the complex to the major subunit. Trafficking of the anti-retraction factor PilY1 to the cell surface allows for production of pili of sufficient length to support adherence and motility.

Txc, a new type II secretion system of Pseudomonas aeruginosa strain PA7, is regulated by the TtsS/TtsR two-component system and directs specific secretion of the CbpE chitin-binding protein

We present here the functional characterization of a third complete type II secretion system (T2SS) found in newly sequenced Pseudomonas aeruginosa strain PA7. We call this system Txc (third Xcp homolog). This system is encoded by the RGP69 region of genome plasticity found uniquely in strain PA7. In addition to the 11 txc genes, RGP69 contains two additional genes encoding a possible T2SS substrate and a predicted unorthodox sensor protein, TtsS (type II secretion sensor). We also identified a gene encoding a two-component response regulator called TtsR (type II secretion regulator), which is located upstream of the ttsS gene and just outside RGP69. We show that TtsS and TtsR constitute a new and functional two-component system that controls the production and secretion of the RGP69-encoded T2SS substrate in a Txc-dependent manner. Finally, we demonstrate that this Txc-secreted substrate binds chitin, and we therefore name it CbpE (chitin-binding protein E).

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.

Molecular motors in bacterial secretion

Secretion of effectors across bacterial membranes is usually mediated by large multisubunit complexes. In most cases, the secreted effectors are virulent factors normally associated to pathogenic diseases. The biogenesis of these secretion systems and the transport of the effectors are processes that require energy. This energy could be directly obtained by using the proton motive force, but in most cases the energy associated to these processes is derived from ATP hydrolysis. Here, a description of the machineries involved in generating the energy required for system biogenesis and substrate transport by type II, III and IV secretion systems is provided, with special emphasis on highlighting the structural similarities and evolutionary relationships among the secretion ATPases.

Type II-dependent secretion of a Pseudomonas aeruginosa DING protein

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that uses a wide range of protein secretion systems to interact with its host. Genes encoding the PAO1 Hxc type II secretion system are linked to genes encoding phosphatases (LapA/LapB). Microarray genotyping suggested that Pseudomonas aeruginosa clinical isolates, including urinary tract (JJ692) and blood (X13273) isolates, lacked the lapA/lapB genes. Instead, we show that they carry a gene encoding a protein of the PstS family. This protein, which we call LapC, also has significant similarities with LapA/LapB. LapC belongs to the family of DING proteins and displays the canonical DINGGG motif within its N terminus. DING proteins are members of a prokaryotic phosphate binding protein superfamily. We show that LapC is secreted in an Hxc-dependent manner and is under the control of the PhoB response regulator. The genetic organization hxc-lapC found in JJ692 and X13273 is similar to PA14, which is the most frequent P. aeruginosa genotype. While the role of LapA, LapB and LapC proteins remains unclear in P. aeruginosa pathogenesis, they are likely to be part of a phosphate scavenging or sensing system needed to survive and thrive when low phosphate environments are encountered within the host.

On the path to uncover the bacterial type II secretion system

Gram-negative bacteria have evolved several secretory pathways to release enzymes or toxins into the surrounding environment or into the target cells. The type II secretion system (T2SS) is conserved in Gram-negative bacteria and involves a set of 12 to 16 different proteins. Components of the T2SS are located in both the inner and outer membranes where they assemble into a supramolecular complex spanning the bacterial envelope, also called the secreton. The T2SS substrates transiently go through the periplasm before they are translocated across the outer membrane and exposed to the extracellular milieu. The T2SS is unique in its ability to promote secretion of large and sometimes multimeric proteins that are folded in the periplasm. The present review describes recently identified protein-protein interactions together with structural and functional advances in the field that have contributed to improve our understanding on how the type II secretion apparatus assembles and on the role played by individual proteins of this highly sophisticated system.

The type II secretion system: biogenesis, molecular architecture and mechanism

Many gram-negative bacteria use the sophisticated type II secretion system (T2SS) to translocate a wide range of proteins from the periplasm across the outer membrane. The inner-membrane platform of the T2SS is the nexus of the system and orchestrates the secretion process through its interactions with the periplasmic filamentous pseudopilus, the dodecameric outer-membrane complex and a cytoplasmic secretion ATPase. Here, recent structural and biochemical information is reviewed to describe our current knowledge of the biogenesis and architecture of the T2SS and its mechanism of action.

Protein Secretion Systems in Pseudomonas aeruginosa: An Essay on Diversity, Evolution, and Function

Protein secretion systems are molecular nanomachines used by Gram-negative bacteria to thrive within their environment. They are used to release enzymes that hydrolyze complex carbon sources into usable compounds, or to release proteins that capture essential ions such as iron. They are also used to colonize and survive within eukaryotic hosts, causing acute or chronic infections, subverting the host cell response and escaping the immune system. In this article, the opportunistic human pathogen Pseudomonas aeruginosa is used as a model to review the diversity of secretion systems that bacteria have evolved to achieve these goals. This diversity may result from a progressive transformation of cell envelope complexes that initially may not have been dedicated to secretion. The striking similarities between secretion systems and type IV pili, flagella, bacteriophage tail, or efflux pumps is a nice illustration of this evolution. Differences are also needed since various secretion configurations call for diversity. For example, some proteins are released in the extracellular medium while others are directly injected into the cytosol of eukaryotic cells. Some proteins are folded before being released and transit into the periplasm. Other proteins cross the whole cell envelope at once in an unfolded state. However, the secretion system requires conserved basic elements or features. For example, there is a need for an energy source or for an outer membrane channel. The structure of this review is thus quite unconventional. Instead of listing secretion types one after each other, it presents a melting pot of concepts indicating that secretion types are in constant evolution and use basic principles. In other words, emergence of new secretion systems could be predicted the way Mendeleïev had anticipated characteristics of yet unknown elements.

The assembly mode of the pseudopilus: a hallmark to distinguish a novel secretion system subtype

In gram-negative bacteria, type II secretion systems assemble a piston-like structure, called pseudopilus, which expels exoproteins out of the cell. The pseudopilus is constituted by a major pseudopilin that when overproduced multimerizes into a long cell surface structure named hyper-pseudopilus. Pseudomonas aeruginosa possesses two type II secretion systems, Xcp and Hxc. Although major pseudopilins are exchangeable among type II secretion systems, we show that XcpT and HxcT are not. We demonstrate that HxcT does not form a hyper-pseudopilus and is different in amino acid sequence and multimerization properties. Using structure-based mutagenesis, we observe that five mutations are sufficient to revert HxcT into a functional XcpT-like protein, which also becomes capable of forming a hyper-pseudopilus. Phylogenetic and experimental analysis showed that the whole Hxc system was acquired by P. aeruginosa PAO1 and other Pseudomonas species through horizontal gene transfer. We thus identified a new type II secretion subfamily, of which the P. aeruginosa Hxc system is the archetype. This finding demonstrates how similar bacterial machineries evolve toward distinct mechanisms that may contribute specific functions.

Pseudomonas aeruginosa minor pilins are incorporated into type IV pili

Type IV pili are long filamentous appendages required for both adhesion and a unique form of motility known as twitching. Twitching motility involves the extension and retraction of the pilus and requires a number of gene products, including five conserved pilin-like proteins of unknown function (FimU, PilV, PilW, PilX, and PilE in Pseudomonas aeruginosa), termed 'minor' pilins. Maintenance of a specific stoichiometric ratio among the minor pilins was important for function, as loss or overexpression of any component impaired motility. Disruption of individual minor pilin genes, or of the AlgR positive regulator of minor pilin operon expression in a strain where pilus retraction was blocked by inactivation of the PilT retraction ATPase, revealed that pili were produced, although levels of piliation were reduced relative to pilT positive control. Differences in the levels of piliation of complemented strains pointed to specific roles for each protein in the assembly process, with FimU and PilX being implicated as key promoters of pilus assembly on the cell surface. Using specific antibodies for each protein, we showed that the minor pilins FimU, PilV, PilW, PilX, and PilE were processed by the pre-pilin peptidase PilD and incorporated throughout the growing pilus filament. This is the first study to demonstrate that the minor pilins, conserved among bacteria expressing type IVa pili, are incorporated into the fiber and support a role for them in the initiation, but not termination, of pilus assembly.

The XcpV/GspI pseudopilin has a central role in the assembly of a quaternary complex within the T2SS pseudopilus

Gram-negative bacteria use the sophisticated type II secretion system (T2SS) to secrete a large number of exoproteins into the extracellular environment. Five proteins of the T2SS, the pseudopilins GspG-H-I-J-K, are proposed to assemble into a pseudopilus involved in the extrusion of the substrate through the outer membrane channel. Recent structural data have suggested that the three pseudopilins GspI-J-K are organized in a trimeric complex located at the tip of the GspG-containing pseudopilus. In the present work we combined two biochemical techniques to investigate the protein-protein interaction network between the five Pseudomonas aeruginosa Xcp pseudopilins. The soluble domains of XcpT-U-V-W-X (respectively homologous to GspG-H-I-J-K) were purified, and the interactions were tested by surface plasmon resonance and affinity co-purification in all possible combinations. We found an XcpV(I)-W(J)-X(K) complex, which demonstrates that the crystallized trimeric complex also exists in the P. aeruginosa T2SS. Interestingly, our systematic approach revealed an additional and yet uncharacterized interaction between XcpU(H) and XcpW(J). This observation suggested the existence of a quaternary, rather than ternary, complex (XcpU(H)-V(I)-W(J)-X(K)) at the tip of the pseudopilus. The assembly of this quaternary complex was further demonstrated by co-purification using affinity chromatography. Moreover, by testing various combinations of pseudopilins by surface plasmon resonance and affinity chromatography, we were able to dissect the different possible successive steps occurring during the formation of the quaternary complex. We propose a model in which XcpV(I) is the nucleator that first binds XcpX(K) and XcpW(J) at different sites. Then the ternary complex recruits XcpU(H) through a direct interaction with XcpW(J).

Diversity of archaeal type IV pilin-like structures

Bacterial type IV pili perform important functions in such disparate biological processes as surface adhesion, cell-cell interactions, autoaggregation, conjugation, and twitching motility. Unlike bacteria, archaea use a type IV pilus related structure to drive swimming motility. While this unique flagellum is the best-studied example of an archaeal IV pilus-like structure, recent in silico, in vivo and structural analyses have revealed a highly diverse set of archaeal non-flagellar type IV pilus-like structures. Accumulating evidence suggests that these structures play important diverse roles in archaea.

Polar secretion of proteins via the Xcp type II secretion system in Pseudomonas aeruginosa

The subcellular localization of the major type II secretion system of Pseudomonas aeruginosa, the Xcp system, was studied microscopically using a biarsenical ligand that becomes fluorescent upon binding to a tetracysteine motif (Lumio tag), which was fused to several Xcp components. Fusion of the Lumio tag to the C termini of the XcpR and XcpS proteins did not affect the functionality of these proteins. Fluorescence microscopy showed that they were predominantly localized to the poles of P. aeruginosa cells, when produced at levels comparable to chromosomally encoded XcpR and XcpS. In most labelled cells, the proteins were found at one of the poles, although bipolar localization was also observed. When produced in the absence of other Xcp components, labelled XcpS was still found to locate at the poles, whereas XcpR was evenly distributed in the cell. These data suggest that XcpS, but not XcpR, contains information required for polar localization. The polar location of the Xcp machinery was further confirmed by the visualization of protease secretion with an intramolecularly quenched casein conjugate.

Structure of the GspK-GspI-GspJ complex from the enterotoxigenic Escherichia coli type 2 secretion system

Gram-negative bacteria translocate various proteins including virulence factors across their outer membrane via type 2 secretion systems (T2SSs). T2SSs are thought to contain a pseudopilus, a subcomplex formed by one major and several minor pseudopilins. We report the crystal structure of the complex formed by three minor pseudopilins from enterotoxigenic Escherichia coli. The GspK-GspI-GspJ complex has quasihelical characteristics and an architecture consistent with a localization at the pseudopilus tip. The alpha-domain of GspK has a previously unobserved fold with an unexpected dinuclear metal binding site. The area surrounding its disulfide bridge is conserved and might interact with other T2SS components or with secreted proteins.

Xylanase attachment to the cell wall of the hyperthermophilic bacterium Thermotoga maritima

The cellular localization and processing of the endo-xylanases (1,4-beta-D-xylan-xylanohydrolase; EC 3.2.1.8) of the hyperthermophile Thermotoga maritima were investigated, in particular with respect to the unusual outer membrane ("toga") of this gram-negative bacterium. XynB (40 kDa) was detected in the periplasmic fraction of T. maritima cells and in the culture supernatant. XynA (120 kDa) was partially released to the surrounding medium, but most XynA remained cell associated. Immunogold labeling of thin sections revealed that cell-bound XynA was localized mainly in the outer membranes of T. maritima cells. Amino-terminal sequencing of purified membrane-bound XynA revealed processing of the signal peptide after the eighth residue, thereby leaving the hydrophobic core of the signal peptide attached to the enzyme. This mode of processing is reminiscent of type IV prepilin signal peptide cleavage. Removal of the entire XynA signal peptide was necessary for release from the cell because enzyme purified from the culture supernatant lacked 44 residues at the N terminus, including the hydrophobic part of the signal peptide. We conclude that toga association of XynA is mediated by residues 9 to 44 of the signal peptide. The biochemical and electron microscopic localization studies together with the amino-terminal processing data indicate that XynA is held at the cell surface of T. maritima via a hydrophobic peptide anchor, which is highly unusual for an outer membrane protein.

The tad locus: postcards from the widespread colonization island

The Tad (tight adherence) macromolecular transport system, which is present in many bacterial and archaeal species, represents an ancient and major new subtype of type II secretion. The tad genes are present on a genomic island named the widespread colonization island (WCI), and encode the machinery that is required for the assembly of adhesive Flp (fimbrial low-molecular-weight protein) pili. The tad genes are essential for biofilm formation, colonization and pathogenesis in the genera Aggregatibacter (Actinobacillus), Haemophilus, Pasteurella, Pseudomonas, Yersinia, Caulobacter and perhaps others. Here we review the structure, function and evolution of the Tad secretion system.

Type IV fimbrial biogenesis is required for protease secretion and natural transformation in Dichelobacter nodosus

The objective of this study was to develop an understanding of the molecular mechanisms by which type IV fimbrial biogenesis, natural transformation, and protease secretion are linked in the ovine foot rot pathogen, Dichelobacter nodosus. We have shown that like the D. nodosus fimbrial subunit FimA, the pilin-like protein PilE and the FimN, FimO, and FimP proteins, which are homologs of PilB, PilC, and PilD from Pseudomonas aeruginosa, are essential for fimbrial biogenesis and natural transformation, indicating that transformation requires an intact type IV fimbrial apparatus. The results also showed that extracellular protease secretion in the fimN, fimO, fimP, and pilE mutants was significantly reduced, which represents the first time that PilB, PilC, and PilE homologs have been shown to be required for the secretion of unrelated extracellular proteins in a type IV fimbriate bacterium. Quantitative real-time PCR analysis of the three extracellular protease genes aprV2, aprV5, and bprV showed that the effects on protease secretion were not mediated at the transcriptional level. Bioinformatic analysis did not identify a classical type II secretion system, and the putative fimbrial biogenesis gene pilQ was the only outer membrane secretin gene identified. Based on these results, it is postulated that in D. nodosus, protease secretion occurs by a type II secretion-related process that directly involves components of the type IV fimbrial biogenesis machinery, which represents the only type II secretion system encoded by the small genome of this highly evolved pathogen.

Interaction domains in the Pseudomonas aeruginosa type II secretory apparatus component XcpS (GspF)

Pseudomonas aeruginosa is an opportunistic pathogen, which secretes a wide variety of enzymes and toxins into the extracellular medium. Most exoproteins are exported by the type II secretion machinery, the Xcp system, which encompasses 12 different proteins. One of the core components of the Xcp system is the inner-membrane protein XcpS (GspF), homologues of which can be identified in type II secretion machineries as well as in type IV piliation systems. In this study, XcpS was shown to be stabilized by co-expression of the XcpR (GspE) and XcpY (GspL) components of the machinery, demonstrating an interaction between these three proteins. By replacing segments of P. aeruginosa XcpS with the corresponding parts of its Pseudomonas putida counterpart, XcpS domains were identified that are important for species-specific functioning and thus represent putative interaction domains. The cytoplasmic loop of XcpS was found to be involved in the stabilization by XcpR and XcpY.

The TadV protein of Actinobacillus actinomycetemcomitans is a novel aspartic acid prepilin peptidase required for maturation of the Flp1 pilin and TadE and TadF pseudopilins

The tad locus of Actinobacillus actinomycetemcomitans encodes genes for the biogenesis of Flp pili, which allow the bacterium to adhere tenaciously to surfaces and form strong biofilms. Although tad (tight adherence) loci are widespread among bacterial and archaeal species, very little is known about the functions of the individual components of the Tad secretion apparatus. Here we characterize the mechanism by which the pre-Flp1 prepilin is processed to the mature pilus subunit. We demonstrate that the tadV gene encodes a prepilin peptidase that is both necessary and sufficient for proteolytic maturation of Flp1. TadV was also found to be required for maturation of the TadE and TadF pilin-like proteins, which we term pseudopilins. Using site-directed mutagenesis, we show that processing of pre-Flp1, pre-TadE, and pre-TadF is required for biofilm formation. Mutation of a highly conserved glutamic acid residue at position +5 of Flp1, relative to the cleavage site, resulted in a processed pilin that was blocked in assembly. In contrast, identical mutations in TadE or TadF had no effect on biofilm formation, indicating that the mechanisms by which Flp1 pilin and the pseudopilins function are distinct. We also determined that two conserved aspartic acid residues in TadV are critical for function of the prepilin peptidase. Together, our results indicate that the A. actinomycetemcomitans TadV protein is a member of a novel subclass of nonmethylating aspartic acid prepilin peptidases.

Interactions between peptidoglycan and the ExeAB complex during assembly of the type II secretin of Aeromonas hydrophila

Aeromonas hydrophila transports extracellular protein toxins via the type II secretion system, an export mechanism comprised of numerous proteins that spans both the inner and outer membranes. Two components of this secretion system, ExeA and ExeB, form a complex in the inner membrane that functions to locate and/or assemble the ExeD secretin in the outer membrane. In the studies reported here, two-codon insertion mutagenesis of exeA revealed that an insertion at amino acid 495 in the C-terminal region of ExeA did not alter ExeAB complex formation yet completely abrogated its involvement in ExeD secretin assembly and thus rendered the bacteria secretion negative. In silico analysis of protein motifs with similar amino acid profiles revealed that this amino acid is located within a putative peptidoglycan (PG) binding motif in the periplasmic domain of ExeA. Substitution mutations of three highly conserved amino acids in the motif were constructed. In cells expressing each of these mutants, the ability to assemble the ExeD secretin or secrete aerolysin was lost, while ExeA retained the ability to form a complex with ExeB. In in vivo cross-linking experiments, wild-type ExeA could be cross-linked to PG, whereas the three substitution mutants of ExeA could not. These data indicate that PG binding and/or remodelling plays a role in the function of the ExeAB complex during assembly of the ExeD secretin.

XpsE oligomerization triggered by ATP binding, not hydrolysis, leads to its association with XpsL

GspE belongs to a secretion NTPase superfamily, members of which are involved in type II/IV secretion, type IV pilus biogenesis and DNA transport in conjugation or natural transformation. Predicted to be a cytoplasmic protein, GspE has nonetheless been shown to be membrane-associated by interacting with the N-terminal cytoplasmic domain of GspL. By taking biochemical and genetic approaches, we observed that ATP binding triggers oligomerization of Xanthomonas campestris XpsE (a GspE homolog) as well as its association with the N-terminal domain of XpsL (a GspL homolog). While isolated XpsE exhibits very low intrinsic ATPase activity, association with XpsL appears to stimulate ATP hydrolysis. Mutation at a conserved lysine residue in the XpsE Walker A motif causes reduction in its ATPase activity without significantly influencing its interaction with XpsL, congruent with the notion that XpsE-XpsL association precedes ATP hydrolysis. For the first time, functional significance of ATP binding to GspE in type II secretion system is clearly demonstrated. The implications may also be applicable to type IV pilus biogenesis.

Roles of the minor pseudopilins, XpsH, XpsI and XpsJ, in the formation of XpsG-containing pseudopilus in Xanthomonas campestris pv. campestris

Due to their similarity to type IV pilus (Tfp) subunits, the pseudopilins, XpsG, -H, -I, -J and -K, have been predicted to form a pilus-like structure in the type II secretion (T2S) pathway. While overexpression of GspG can result in the formation of bundle structures, the functions of other pseudopilin are not known yet. In this study, we investigate the mutual interaction among the pseudopilins and characterize the specialized minor pseudopilin, XpsJ. By using gel filtration and Ni-NTA affinity chromatography, a linearly ordered interactive relationship is revealed among the four pseudopilins, XpsG-XpsI-XpsH-XpsJ. Notably, unlike the mutant XpsJ194 staying in the inner membrane, wild type XpsJ stayed in the outer membrane and blocked the extension of overexpressed XpsG to outside of the cell. By analogy with the Type I pilus structures, we hypothesize that the XpsH and XpsI might act as an adaptor to connect XpsJ with the major pseudopilin XpsG, and XpsJ might act as a tip to restrict the out-growth of XpsG in the pilus-like structure of the T2S pathway.

Structure and function of the XpsE N-terminal domain, an essential component of the Xanthomonas campestris type II secretion system

Secretion of fully folded extracellular proteins across the outer membrane of Gram-negative bacteria is mainly assisted by the ATP-dependent type II secretion system (T2SS). Depending on species, 12-15 proteins are usually required for the function of T2SS by forming a trans-envelope multiprotein secretion complex. Here we report crystal structures of an essential component of the Xanthomonas campestris T2SS, the 21-kDa N-terminal domain of cytosolic secretion ATPase XpsE (XpsEN), in two conformational states. By mediating interaction between XpsE and the cytoplasmic membrane protein XpsL, XpsEN anchors XpsE to the membrane-associated secretion complex to allow the coupling between ATP utilization and exoprotein secretion. The structure of XpsEN observed in crystal form P4(3)2(1)2 is composed of a 90-residue alpha/beta sandwich core domain capped by a 62-residue N-terminal helical region. The core domain exhibits structural similarity with the NifU-like domain, suggesting that XpsE(N) may be involved in the regulation of XpsE ATPase activity. Surprisingly, although a similar core domain structure was observed in crystal form I4(1)22, the N-terminal 36 residues of the helical region undergo a large structural rearrangement. Deletion analysis indicates that these residues are required for exoprotein secretion by mediating the XpsE/XpsL interaction. Site-directed mutagenesis study further suggests the more compact conformation observed in the P4(3)2(1)2 crystal likely represents the XpsL binding-competent state. Based on these findings, we speculate that XpsE might function in T2SS by cycling between two conformational states. As a closely related protein to XpsE, secretion ATPase PilB may function similarly in the type IV pilus assembly.

XcpX controls biogenesis of the Pseudomonas aeruginosa XcpT-containing pseudopilus

Pseudomonas aeruginosa is an opportunistic gram-negative pathogen equipped with multiple secretion systems. The type II secretion machinery (Xcp secreton) is involved in the release of toxins and enzymes. The Xcp secreton is a multiprotein complex, and most of its components share homology with proteins involved in type IV pili biogenesis. Among them, the XcpT-X pseudopilins possess characteristics of the major constituent of the type IV pili, the pilin PilA. We have shown previously that XcpT can be assembled in a multifibrillar structure that was called the pseudopilus. By using two different microscopic approaches, we show here that the pseudopili are preferentially isolated fibers rather than tight bundles. Moreover, none of the other four pseudopilins are able to form a pseudopilus, suggesting that the assembly of such a structure is a unique property of XcpT. Moreover, we show that 5 of the 12 Xcp proteins are not required for pseudopilus biogenesis, whereas they are for type II secretion. Most interestingly, we showed that one pseudopilin, XcpX, controls the assembly of XcpT into a pseudopilus. Indeed, when the number of XcpX subunits increases, the length of the pseudopilus decreases. Conversely, in the absence of XcpX, the pseudopilus length is abnormally long. Our results indicate that XcpT and XcpX directly interact with each other. Furthermore, this interaction induces a clear destabilization of XcpT. The interaction between XcpT and XcpX could be part of the molecular mechanism underlying the dynamic control of pseudopilus elongation, which could be crucial for type II-dependent protein secretion.

Genome-wide identification of Pseudomonas aeruginosa exported proteins using a consensus computational strategy combined with a laboratory-based PhoA fusion screen

The Gram-negative pathogen Pseudomonas aeruginosa encodes multiple protein export systems, the substrates of which contain export signals such as N-terminal signal peptides. Here we report the first genome-wide computational and laboratory screen for N-terminal signal peptides in this important opportunistic pathogen. The computational identification of signal peptides was based on a consensus between multiple predictive tools and showed that 38% of the P. aeruginosa PAO1 proteome was predicted to encode exported proteins, most of which utilize cleavable type I signal peptides or uncleavable transmembrane helices. In addition, known and novel lipoproteins (type II), twin arginine transporter (TAT), and prepilin peptidase substrates (type IV) were also identified. A laboratory-based screen using the alkaline phosphatase (PhoA) fusion method was then used to test our predictions. In total, 310 nonredundant PhoA fusions were successfully identified, 296 of which possess a predicted export signal. Analysis of the PhoA fusion proteins lacking an export signal revealed that three proteins have alternate translation start sites that encode signal peptides, two proteins may use an unknown export signal, and the remaining nine proteins are likely cytoplasmic proteins and represent false positives associated with the PhoA screen. Our approach to identify exported proteins illustrates how computational and laboratory-based methods are complementary, where computational analyses provide a large number of accurate predictions while laboratory methods both confirm predictions and reveal unique cases meriting further analysis.

Comparative proteome analysis of secretory proteins from pathogenic and nonpathogenicListeriaspecies

Extracellular proteins of bacterial pathogens play a crucial role in the infection of the host. Here we present the first comprehensive validation of the secretory subproteome of the Gram positive pathogen Listeria monocytogenes using predictive bioinformatic and experimental proteomic approaches. The previous original signal peptide (SP) prediction (Glaser et al., Science 2001, 294, 849-852) has been greatly improved by an in-depth analysis using seven different bioinformatic tools. Subsequent careful classification of the resulting data gives a probability dependent annotation of 121 putatively secreted proteins of which 45 are novel. Complementary proteomic analysis using both two-dimensional gel electrophoresis/matrix assisted laser desorption/ionization mass spectrometry and high performance liquid chromatography/electrospray ionization-mass spectrometry has identified 105 proteins in the culture supernatant of L. monocytogenes. Among these, we were able to detect all the currently known virulence factors with an SP showing the importance of this subproteome and demonstrating the reliability of the techniques used. The comparison between the L. monocytogenes wildtype and the nonpathogenic species Listeria innocua was performed to reveal proteins probably involved in pathogenicity and/or the adaptation to their respective lifestyles. In addition to the eight known virulence factors, all of which have no orthologous genes in L. innocua, eight additional proteins have been identified that exhibit the typical key feature defining the known listerial virulence factors. Further significant differences between the two species are evident in the group of cell wall and secretory proteins that warrant further study. Our investigation clearly demonstrates that the major difference between the pathogenic and nonpathogenic species, noted in the comparative genome analysis, manifests itself strongest in the secretome.

A conserved set of pilin-like molecules controls type IV pilus dynamics and organelle-associated functions in Neisseria gonorrhoeae

Type IV pili (Tfp) play central roles in prokaryotic cell biology and disease pathogenesis. As dynamic filamentous polymers, they undergo rounds of extension and retraction modelled as pilin subunit polymerization and depolymerization events. Currently, the molecular mechanisms and components influencing Tfp dynamics remain poorly understood. Using Neisseria gonorrhoeae as a model system, we show that mutants lacking any one of a set of five proteins sharing structural similarity to the pilus subunit are dramatically reduced in Tfp expression and that these defects are suppressed in the absence of the PilT pilus retraction protein. Thus, these molecules are not canonical assembly factors but rather act as effectors of pilus homeostasis by promoting extension/polymerization events in the presence of PilT. Furthermore, localization studies support the conclusion that these molecules form a Tfp-associated complex and influence levels of PilC, the epithelial cell adhesin, in Tfp-enriched shear fractions. This is the first time that the step at which individual pilin-like proteins impact on Tfp expression has been defined. The findings have important implications for understanding Tfp dynamics and fundamental Tfp structure/function relationships.

Associations of the major pseudopilin XpsG with XpsN (GspC) and secretin XpsD of Xanthomonas campestris pv. campestris type II secretion apparatus revealed by cross-linking analysis

The major pseudopilin XpsG is an essential component of type II secretion apparatus of Xanthomonas campestris pv. campestris. Along with other ancillary pseudopilins, it forms a pilus-like structure spanning between cytoplasmic and outer membranes. Associations of pseudopilins with non-pseudopilin members of type II secretion apparatus were not well documented, probably due to their dynamic or unstable nature. In this study, by treating intact cells with a cleavable cross-linker dithiobis(succinimidylpropionate) (DSP), followed by metal chelating chromatography and immunoblotting on secretion-positive strains of X. campestris pv. campestris, we discovered associations of XpsGh with XpsN (GspC), as well as XpsD. These associations were detectable in a strain missing all components, but XpsO, of the type II secretion apparatus. However, chromosomal non-polar mutation in each gene exerted different effects upon the association between the other two. The XpsGh/XpsD association is undetectable in xpsN mutant; however, it was restored to a limited extent by overproducing XpsD protein. The XpsGh/XpsN association is unaltered by a lack of XpsD protein or an elevation of its abundance. Co-immune precipitation between XpsN and XpsD, while being independent of XpsG, was nonetheless enhanced by raising XpsG protein level. These observations agree with the proposition that the type II secretion apparatus in a cell may exist as an integrated multiprotein complex with all components working in concert. Moreover, in functional machinery, the association of the major pseudopilin XpsG with secretin XpsD appears strongly dependent on the existence of XpsN, the GspC protein.

Type V protein secretion pathway: the autotransporter story

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

The type II protein secretion system of Legionella pneumophila promotes growth at low temperatures

The gram-negative bacterium Legionella pneumophila grows in both natural and man-made water systems and in the mammalian lung as a facultative intracellular parasite. The PilD prepilin peptidase of L. pneumophila promotes type IV pilus biogenesis and type II protein secretion. Whereas pili enhance adherence, Legionella type II secretion is critical for intracellular growth and virulence. Previously, we observed that pilD transcript levels are greater in legionellae grown at 30 versus 37 degrees C. Using a new pilD::lacZ fusion strain, we now show that pilD transcriptional initiation increases progressively as L. pneumophila is grown at 30, 25, and 17 degrees C. Legionella pilD mutants also had a dramatically reduced ability to grow in broth and to form colonies on agar at the lower temperatures. Whereas strains specifically lacking type IV pili were not defective for low-temperature growth, mutations in type II secretion (lsp) genes greatly impaired the capacity of L. pneumophila to form colonies at 25, 17, and 12 degrees C. Indeed, the lsp mutants were completely unable to grow at 12 degrees C. The growth defect of the pilD and lsp mutants was complemented by reintroduction of the corresponding intact gene. Interestingly, the lsp mutants displayed improved growth at 25 degrees C when plated next to a streak of wild-type but not mutant bacteria, implying that a secreted, diffusible factor promotes low-temperature growth. Mutants lacking either the known secreted acid phosphatases, lipases, phospholipase C, lysophospholipase A, or protease grew normally at 25 degrees C, suggesting the existence of a critical, yet-to-be-defined exoprotein(s). In summary, these data document, for the first time, that L. pneumophila replicates at temperatures below 20 degrees C and that a bacterial type II protein secretion system facilitates growth at low temperatures.

Legionella pneumophila type II protein secretion promotes virulence in the A/J mouse model of Legionnaires' disease pneumonia

Legionella pneumophila, the gram-negative agent of Legionnaires' disease, possesses type IV pili and a type II protein secretion (Lsp) system, both of which are dependent upon the PilD prepilin peptidase. By analyzing multiple pilD mutants and various types of Lsp mutants as well as performing trans-complementation of these mutants, we have confirmed that PilD and type II secretion genes are required for L. pneumophila infection of both amoebae and human macrophages. Based upon a complete analysis of lspDE, lspF, and lspG mutants, we found that the type II system controls the secretion of protease, RNase, lipase, phospholipase A, phospholipase C, lysophospholipase A, and tartrate-sensitive and tartrate-resistant acid phosphatase activities and influences the appearance of colonies. Examination of the developing L. pneumophila genome database indicated that the organism has two other loci (lspC and lspLM) that are predicted to promote secretion and thus a set of genes that is comparable to the type II secretion genes in other gram-negative bacteria. In contrast to lsp mutants, L. pneumophila pilus mutants lacking either the PilQ secretin, the PspA pseudopilin, or pilin were not defective for colonial growth, secreted activities, or intracellular replication. L. pneumophila dot/icm mutants were also not impaired for type II-dependent exoenzymes. Upon intratracheal inoculation into A/J mice, lspDE, lspF, and pilD mutants, but not pilus mutants, exhibited a reduced ability to grow in the lung, as measured by competition assays. The lspF mutant was also defective in an in vivo kinetic assay. Examination of infected mouse sera revealed that type II secreted proteins are expressed in vivo. Thus, the L. pneumophila Lsp system is a virulence factor and the only type II secretion system linked to intracellular infection.

Type II protein secretion and its relationship to bacterial type IV pili and archaeal flagella

Homologues of the protein constituents of theKlebsiella pneumoniae(Klebsiella oxytoca) type II secreton (T2S), thePseudomonas aeruginosatype IV pilus/fimbrium biogenesis machinery (T4P) and theMethanococcus voltaeflagellum biogenesis machinery (Fla) have been identified. Known constituents of these systems include (1) a major prepilin (preflagellin), (2) several minor prepilins (preflagellins), (3) a prepilin (preflagellin) peptidase/methylase, (4) an ATPase, (5) a multispanning transmembrane (TM) protein, (6) an outer-membrane secretin (lacking in Fla) and (7) several functionally uncharacterized envelope proteins. Sequence and phylogenetic analyses led to the conclusion that, although many of the protein constituents are probably homologous, extensive sequence divergence during evolution clouds this homology so that a common ancestry can be established for all three types of systems for only two constituents, the ATPase and the TM protein. Sequence divergence of the individual T2S constituents has occurred at characteristic rates, apparently without shuffling of constituents between systems. The same is probably also true for the T4P and Fla systems. The family of ATPases is much larger than the family of TM proteins, and many ATPase homologues function in capacities unrelated to those considered here. Many phylogenetic clusters of the ATPases probably exhibit uniform function. Some of these have a corresponding TM protein homologue although others probably function without one. It is further shown that proteins that compose the different phylogenetic clusters in both the ATPase and the TM protein families exhibit unique structural characteristics that are of probable functional significance. The TM proteins are shown to have arisen by at least two dissimilar intragenic duplication events, one in the bacterial kingdom and one in the archaeal kingdom. The archaeal TM proteins are twice as large as the bacterial TM proteins, suggesting an oligomeric structure for the latter.

Type IV-like pili formed by the type II secreton: specificity, composition, bundling, polar localization, and surface presentation of peptides

The secreton or type II secretion machinery of gram-negative bacteria includes several type IV pilin-like proteins (the pseudopilins) that are absolutely required for secretion. We previously reported the presence of a bundled pilus composed of the pseudopilin PulG on the surface of agar-grown Escherichia coli K-12 cells expressing the Klebsiella oxytoca pullulanase (Pul) secreton genes at high levels (N. Sauvonnet, G. Vignon, A. P. Pugsley, and P. Gounon, EMBO J. 19:2221-2228, 2000). We show here that PulG is the only pseudopilin in purified pili and that the phenomenon is not restricted to the Pul secreton reconstituted in E. coli or to PulG. For example, high-level expression of the endogenous E. coli gsp secreton genes caused production of bundled pili composed of the pseudopilin GspG, and the Pul secreton was able to form pili composed of PulG-like proteins from secreton systems of other bacteria. PulG derivatives in which the C terminus was extended by the addition of eight different peptides were also assembled into pili and functioned in secretion. Three of the C-terminal peptides were shown to be exposed along the entire length of the assembled pili. Hence, the C terminus of PulG may represent a permissive site for the insertion of immunogenic epitopes or other peptide sequences. One of these PulG variants, with a six-histidine tag at its C terminus, formed nonpolar, nonbundled pili, suggesting that bundle formation and polar localization are not correlated with the ability of PulG to function in secretion. We propose that the PulG pilus is an artifactual manifestation of a periplasmic "pseudopilus" and that cycles of pseudopilus extension and retraction within the periplasm propel pullulanase through secretin channels in the outer membrane. Abnormally long pili that extend beyond the outer membrane are produced only when pilus length control and retraction are deregulated by overproduction of the major pseudopilus subunit (PulG).

Type II protein secretion in Pseudomonas aeruginosa: the pseudopilus is a multifibrillar and adhesive structure

The type II secretion pathway of Pseudomonas aeruginosa is involved in the extracellular release of various toxins and hydrolytic enzymes such as exotoxin A and elastase. This pathway requires the function of a macromolecular complex called the Xcp secreton. The Xcp secreton shares many features with the machinery involved in type IV pilus assembly. More specifically, it involves the function of five pilin-like proteins, the XcpT-X pseudopilins. We show that, upon overexpression, the XcpT pseudopilin can be assembled in a pilus, which we call a type II pseudopilus. Image analysis and filtering of electron micrographs indicated that these appendages are composed of individual fibrils assembled together in a bundle structure. Our observations thus revealed that XcpT has properties similar to those of type IV pilin subunits. Interestingly, the assembly of the type II pseudopilus is not exclusively dependent on the Xcp machinery but can be supported by other similar machineries, such as the Pil (type IV pilus) and Hxc (type II secretion) systems of P. aeruginosa. In addition, heterologous pseudopilins can be assembled by P. aeruginosa into a type II pseudopilus. Finally, we showed that assembly of the type II pseudopilus confers increased bacterial adhesive capabilities. These observations confirmed the ability of pseudopilins to form a pilus structure and raise questions with respect to their function in terms of secretion and adhesion, two crucial biological processes in the course of bacterial infections.

Identification of a novel Gsp-related pathway required for secretion of the manganese-oxidizing factor of Pseudomonas putida strain GB-1

The manganese-oxidizing factor of Pseudomonas putida strain GB-1 is associated with the outer membrane. One of the systems of protein transport across the outer membrane is the general secretory pathway (Gsp). The gsp genes are called xcp in Pseudomonas species. In a previous study, it was shown that mutation of the prepilin peptidase XcpA and of a homologue of the pseudopilin XcpT inhibited transport of the factor. In the present study, we describe the genomic region flanking the xcpT homologue (designated xcmT1). We show that xcmT1 is part of a two-gene operon that includes an xcpS homologue (designated xcmS). No other xcp-like genes are present in the regions flanking the xcmT1/xcmS cluster. We also characterized the site of transposon insertion of another transport mutant of P. putida GB-1. This insertion appeared to be located in a gene (designated xcmX) possibly encoding another pseudopilin-related protein. This xcmX is clustered with two other xcpT-related genes (designated xcmT2 and xcmT3) on one side and homologues of three csg genes (designated csmE, csmF and csmG) on the other side. The csg genes are involved in production of aggregative fibres in Escherichia coli and Salmonella typhimurium. A search for XcmX homologues revealed that the recently published genome of Ralstonia solanacearum and the unannotated genome of P. putida KT2440 contain comparable gene clusters with xcmX and xcp homologues that are different from the well-described 'regular'xcp/gsp clusters. They do contain xcpR and xcpQ homologues but, for example, homologues of xcpP, Y and Z are lacking. The results suggest a novel Xcp-related system for the transport of manganese-oxidizing enzymes to the cell surface.

Identification of XcpZ domains required for assembly of the secreton of Pseudomonas aeruginosa

Most of the exoproteins secreted by Pseudomonas aeruginosa are transported via the type II secretion system. This machinery, which is widely conserved in gram-negative bacteria, consists of 12 Xcp proteins organized as a multiprotein complex, also called the secreton. We previously reported that the mutual stabilization of XcpZ and XcpY plays an important role in the assembly of the secreton. In this study, we engineered variant XcpZ proteins by using linker insertion mutagenesis. We identified three distinct regions of XcpZ required for both the stabilization of XcpY and the functionality of the secreton. Interestingly, we also demonstrated that another component of the machinery, XcpP, can modulate the stabilizing activity of XcpZ on XcpY.

A novel type II secretion system in Pseudomonas aeruginosa

The genome sequence of Pseudomonas aeruginosa strain PAO1 has been determined to facilitate postgenomic studies aimed at understanding the capacity of adaptation of this ubiquitous opportunistic pathogen. P. aeruginosa produces toxins and hydrolytic enzymes that are secreted via the type II secretory pathway using the Xcp machinery or 'secreton'. In this study, we characterized a novel gene cluster, called hxc for homologous to xcp. Characterization of an hxcR mutant, grown in phosphate-limiting medium, revealed the absence of a 40 kDa protein found in the culture supernatant of wild-type or xcp derivative mutant strains. The protein corresponded to the alkaline phosphatase L-AP, renamed LapA, which is secreted in an xcp-independent but hxc-dependent manner. Finally, we showed that expression of the hxc gene cluster is under phosphate regulation. This is the first report of the existence of two functional type II secretory pathways within the same organism, which could be related to the high adaptation potential of P. aeruginosa.

Biology of type II secretion

The type II secretion pathway or the main terminal branch of the general secretion pathway, as it has also been referred to, is widely distributed among Proteobacteria, in which it is responsible for the extracellular secretion of toxins and hydrolytic enzymes, many of which contribute to pathogenesis in both plants and animals. Secretion through this pathway differs from most other membrane transport systems, in that its substrates consist of folded proteins. The type II secretion apparatus is composed of at least 12 different gene products that are thought to form a multiprotein complex, which spans the periplasmic compartment and is specifically required for translocation of the secreted proteins across the outer membrane. This pathway shares many features with the type IV pilus biogenesis system, including the ability to assemble a pilus-like structure. This review discusses recent findings on the organization of the secretion apparatus and the role of its various components in secretion. Different models for pilus-mediated secretion through the gated pore in the outer membrane are also presented, as are the possible properties that determine whether a protein is recognized and secreted by the type II pathway.

Disulfide bond formation in secreton component PulK provides a possible explanation for the role of DsbA in pullulanase secretion

When expressed in Escherichia coli, the 15 Klebsiella oxytoca pul genes that encode the so-called Pul secreton or type II secretion machinery promote pullulanase secretion and the assembly of one of the secreton components, PulG, into pili. Besides these pul genes, efficient pullulanase secretion also requires the host dsbA gene, encoding a periplasmic disulfide oxidoreductase, independently of disulfide bond formation in pullulanase itself. Two secreton components, the secretin pilot protein PulS and the minor pseudopilin PulK, were each shown to posses an intramolecular disulfide bond whose formation was catalyzed by DsbA. PulS was apparently destabilized by the absence of its disulfide bond, whereas PulK stability was not dramatically affected either by a dsbA mutation or by the removal of one of its cysteines. The pullulanase secretion defect in a dsbA mutant was rectified by overproduction of PulK, indicating reduced disulfide bond formation in PulK as the major cause of the secretion defect under the conditions tested (in which PulS is probably present in considerable excess of requirements). PulG pilus formation was independent of DsbA, probably because PulK is not needed for piliation.

Exchange of Xcp (Gsp) secretion machineries between Pseudomonas aeruginosa and Pseudomonas alcaligenes: species specificity unrelated to substrate recognition

Pseudomonas aeruginosa and Pseudomonas alcaligenes are gram-negative bacteria that secrete proteins using the type II or general secretory pathway, which requires at least 12 xcp gene products (XcpA and XcpP to -Z). Despite strong conservation of this secretion pathway, gram-negative bacteria usually cannot secrete exoproteins from other species. Based on results obtained with Erwinia, it has been proposed that the XcpP and/or XcpQ homologs determine this secretion specificity (M. Linderberg, G. P. Salmond, and A. Collmer, Mol. Microbiol. 20:175-190, 1996). In the present study, we report that XcpP and XcpQ of P. alcaligenes could not substitute for their respective P. aeruginosa counterparts. However, these complementation failures could not be correlated to species-specific recognition of exoproteins, since these bacteria could secrete exoproteins of each other. Moreover, when P. alcaligenes xcpP and xcpQ were expressed simultaneously in a P. aeruginosa xcpPQ deletion mutant, complementation was observed, albeit only on agar plates and not in liquid cultures. After growth in liquid culture the heat-stable P. alcaligenes XcpQ multimers were not detected, whereas monomers were clearly visible. Together, our results indicate that the assembly of a functional Xcp machinery requires species-specific interactions between XcpP and XcpQ and between XcpP or XcpQ and another, as yet uncharacterized component(s).