The lipoprotein VirB7 interacts with VirB9 in the membranes of Agrobacterium tumefaciens

Protein-Protein Interactions: Yeast Two Hybrid

The yeast two-hybrid system is a powerful and commonly used genetic tool to investigate the interaction between artificial fusion proteins inside the nucleus of yeast. Here, we describe how to use the Matchmaker GAL4-based yeast two-hybrid system to detect the interaction of the Agrobacterium type VI secretion system (T6SS) sheath components TssB and TssC41. The bait and prey gene are expressed as a fusion to the GAL4 DNA-binding domain (DNA-BD) and GAL4 activation domain (AD, prey/library fusion protein), respectively. When bait and prey fusion proteins interact in yeast nucleus, the DNA-BD and AD are brought into proximity, thus activating transcription of reporter genes. This technology can be widely used to identify interacting partners, confirm suspected interactions, and define interacting domains.

The VirB System Plays a Crucial Role in Brucella Intracellular Infection

Brucellosis is a highly prevalent zoonotic disease caused by Brucella. Brucella spp. are gram-negative facultative intracellular parasitic bacteria. Its intracellular survival and replication depend on a functional virB system, an operon encoded by VirB1–VirB12. Type IV secretion system (T4SS) encoded by the virB operon is an important virulence factor of Brucella. It can subvert cellular pathway and induce host immune response by secreting effectors, which promotes Brucella replication in host cells and induce persistent infection. Therefore, this paper summarizes the function and significance of the VirB system, focusing on the structure of the VirB system where VirB T4SS mediates biogenesis of the endoplasmic reticulum (ER)-derived replicative Brucella-containing vacuole (rBCV), the effectors of T4SS and the cellular pathways it subverts, which will help better understand the pathogenic mechanism of Brucella and provide new ideas for clinical vaccine research and development.

Architecture of the outer-membrane core complex from a conjugative type IV secretion system

Conjugation is one of the most important processes that bacteria utilize to spread antibiotic resistance genes among bacterial populations. Interbacterial DNA transfer requires a large double membrane-spanning nanomachine called the type 4 secretion system (T4SS) made up of the inner-membrane complex (IMC), the outer-membrane core complex (OMCC) and the conjugative pilus. The iconic F plasmid-encoded T4SS has been central in understanding conjugation for several decades, however atomic details of its structure are not known. Here, we report the structure of a complete conjugative OMCC encoded by the pED208 plasmid from E. coli, solved by cryo-electron microscopy at 3.3 Å resolution. This 2.1 MDa complex has a unique arrangement with two radial concentric rings, each having a different symmetry eventually contributing to remarkable differences in protein stoichiometry and flexibility in comparison to other OMCCs. Our structure suggests that F-OMCC is a highly dynamic complex, with implications for pilus extension and retraction during conjugation.

Two pKM101-encoded proteins, the pilus-tip protein TraC and Pep, assemble on the Escherichia coli cell surface as adhesins required for efficient conjugative DNA transfer

Mobile genetic elements (MGEs) encode type IV secretion systems (T4SSs) known as conjugation machines for their transmission between bacterial cells. Conjugation machines are composed of an envelope-spanning translocation channel, and those functioning in Gram-negative species additionally elaborate an extracellular pilus to initiate donor-recipient cell contacts. We report that pKM101, a self-transmissible MGE functioning in the Enterobacteriaceae, has evolved a second target cell attachment mechanism. Two pKM101-encoded proteins, the pilus-tip adhesin TraC and a protein termed Pep, are exported to the cell surface where they interact and also form higher order complexes appearing as distinct foci or patches around the cell envelope. Surface-displayed TraC and Pep are required for an efficient conjugative transfer, 'extracellular complementation' potentially involving intercellular protein transfer, and activation of a Pseudomonas aeruginosa type VI secretion system. Both proteins are also required for bacteriophage PRD1 infection. TraC and Pep are exported across the outer membrane by a mechanism potentially involving the β-barrel assembly machinery. The pKM101 T4SS, thus, deploys alternative routing pathways for the delivery of TraC to the pilus tip or both TraC and Pep to the cell surface. We propose that T4SS-encoded, pilus-independent attachment mechanisms maximize the probability of MGE propagation and might be widespread among this translocation superfamily.

The Agrobacterium VirB/VirD4 T4SS: Mechanism and Architecture Defined Through In Vivo Mutagenesis and Chimeric Systems

The Agrobacterium tumefaciens VirB/VirD4 translocation machine is a member of a superfamily of translocators designated as type IV secretion systems (T4SSs) that function in many species of gram-negative and gram-positive bacteria. T4SSs evolved from ancestral conjugation systems for specialized purposes relating to bacterial colonization or infection. A. tumefaciens employs the VirB/VirD4 T4SS to deliver oncogenic DNA (T-DNA) and effector proteins to plant cells, causing the tumorous disease called crown gall. This T4SS elaborates both a cell-envelope-spanning channel and an extracellular pilus for establishing target cell contacts. Recent mechanistic and structural studies of the VirB/VirD4 T4SS and related conjugation systems in Escherichia coli have defined T4SS architectures, bases for substrate recruitment, the translocation route for DNA substrates, and steps in the pilus biogenesis pathway. In this review, we provide a brief history of A. tumefaciens VirB/VirD4 T4SS from its discovery in the 1980s to its current status as a paradigm for the T4SS superfamily. We discuss key advancements in defining VirB/VirD4 T4SS function and structure, and we highlight the power of in vivo mutational analyses and chimeric systems for identifying mechanistic themes and specialized adaptations of this fascinating nanomachine.

Type IV Secretion in Agrobacterium tumefaciens and Development of Specific Inhibitors

The Agrobacterium tumefaciens VirB/D4 type IV secretion system (T4SS) comprises 12 membrane-bound proteins, and it assembles a surface-exposed T-pilus. It is considered to be the archetypical system that is generally used to orient the nomenclature of other T4SS. Whereas the sequence similarities between T4SSs from different organisms are often limited, the general mechanism of action appears to be conserved, and the evolutionary relationship to bacterial conjugation systems and to T4SSs from animal pathogens is well established. Agrobacterium is a natural genetic engineer that is extensively used for the generation of transgenic plants for research and for agro-biotechnological applications. It also served as an early model for the understanding of pathogen-host interactions and for the transfer of macromolecular virulence factors into host cells. The knowledge on the mechanism of its T4SS inspired the search for small molecules that inhibit the virulence of bacterial pathogens and of bacterial conjugation. Inhibitors of bacterial virulence and of conjugation have interesting potential as alternatives to antibiotics and as inhibitors of antimicrobial resistance gene transfer. Mechanistic work on the Agrobacterium T4SS will continue to inspire the search for inhibitor target sites and drug design.

Protein-Protein Interactions: Yeast Two-Hybrid System

The yeast two-hybrid system is a powerful and commonly used genetic tool to investigate interactions between artificial fusion proteins inside the nucleus of yeast. Here we describe how to use the Matchmaker GAL4-based yeast two-hybrid system to detect the interaction of the Agrobacterium type VI secretion system (T6SS) sheath components TssB and TssC₄₁. The bait and prey gene are expressed as a fusion to the GAL4 DNA-binding domain (DNA-BD) and GAL4 activation domain (AD, prey/library fusion protein) respectively. When bait and prey fusion proteins interact in yeast nucleus, the DNA-BD and AD are brought into proximity, thereby activating the transcription of reporter genes. This technology can be widely used to identify interacting partners, confirm suspected interactions, and define interacting domains.

Structural Biology of Bacterial Type IV Secretion Systems

Type IV secretion systems (T4SSs) are large multisubunit translocons, found in both gram-negative and gram-positive bacteria and in some archaea. These systems transport a diverse array of substrates from DNA and protein-DNA complexes to proteins, and play fundamental roles in both bacterial pathogenesis and bacterial adaptation to the cellular milieu in which bacteria live. This review describes the various biochemical and structural advances made toward understanding the biogenesis, architecture, and function of T4SSs.

C-terminal domain of CagX is responsible for its interaction with CagT protein of Helicobacter pylori type IV secretion system

Helicobacter pylori are the well known human pathogen associated with gastric cancer and peptic ulcer. Pathogenesis is mainly due to the presence of 40 kb cagPAI (cag Pathogenicity Island) region that encodes the type IV secretion system (TFSS) consisting of a cytoplasmic part, a middle part/core complex (spans from inner membrane to outer membrane), and an outer membrane associated part. CagX and CagT are two important proteins of TFSS that have homology with virB9 and virB7 of Agrobacterium tumefaciens TFSS. In this study, we have shown that the CagX and CagT interact directly by using co-immunoprecipitation of endogenous CagX and CagT and MBP pull down assay. We further authenticate this observation using yeast two-hybrid assay and co-expression of both the protein coding gene in Escherichia coli. We also observed that the C-terminal region of CagX is important for CagT interaction. We reconfirm that CagT depends on CagX for its stabilization. These observations could contribute in overall visualization of assembly and architecture of TFSS because protein-protein interactions among Cag proteins are likely to have an important role in assembly. Thorough understanding about architecture and mechanism of action of cag-TFSS may lead to design controlled drug delivery system.

TraK and TraB are conserved outer membrane proteins of the Neisseria gonorrhoeae Type IV secretion system and are expressed at low levels in wild-type cells

Neisseria gonorrhoeae uses a type IV secretion system (T4SS) to secrete chromosomal DNA into the medium, and this DNA is effective in transforming other gonococci via natural transformation. In addition, the T4SS is important in the initial stages of biofilm development and mediates intracellular iron uptake in the absence of TonB. To better understand the mechanism of type IV secretion in N. gonorrhoeae, we examined the expression levels and localization of two predicted T4SS outer membrane proteins, TraK and TraB, in the wild-type strain as well as in overexpression strains and in a strain lacking all of the T4SS proteins. Despite very low sequence similarity to known homologues, TraB (VirB10 homolog) and TraK (VirB9 homolog) localized similarly to related proteins in other systems. Additionally, we found that TraV (a VirB7 homolog) interacts with TraK, as in other T4SSs. However, unlike in other systems, neither TraK nor TraB required the presence of other T4SS components for proper localization. Unlike other gonococcal T4SS proteins we have investigated, protein levels of the outer membrane proteins TraK and TraB were extremely low in wild-type cells and were undetectable by Western blotting unless overexpressed or tagged with a FLAG3 triple-epitope tag. Localization of TraK-FLAG3 in otherwise wild-type cells using immunogold electron microscopy of thin sections revealed a single gold particle on some cells. These results suggest that the gonococcal T4SS may be present in single copy per cell and that small amounts of T4SS proteins TraK and TraB are sufficient for DNA secretion.

Structural independence of conjugative coupling protein TrwB from its Type IV secretion machinery

The stability of components of multiprotein complexes often relies on the presence of the functional complex. To assess structural dependence among the components of the R388 Type IV secretion system (T4SS), the steady-state level of several Trw proteins was determined in the absence of other Trw components. While several Trw proteins were affected by the lack of others, we found that the coupling protein TrwB is not affected by the absence of other T4SS components, nor did its absence alter significantly the levels of integral components of the complex, underscoring the independent role of the coupling protein on the T4SS architecture. The cytoplasmic ATPases TrwK (VirB4) and TrwD (VirB11) were affected by the absence of several core complex components, while the pilus component TrwJ (VirB5) required the presence of all other Trw proteins (except for TrwB) to be detectable. Overall, the results delineate a possible assembly pathway for the T4SS of R388. We have also tested structural complementation of TrwD (VirB11) and TrwJ (VirB5) by their homologues in the highly related Trw system of Bartonella tribocorum (Bt). The results reveal a correlation with the functional complementation data previously reported.

Type IV secretion machinery: molecular architecture and function

Bacteria have evolved several secretion machineries to bring about transport of various virulence factors, nutrients, nucleic acids and cell-surface appendages that are essential for their pathogenesis. T4S (Type IV secretion) systems are versatile secretion systems found in various Gram-negative and Gram-positive bacteria and in few archaea. They are large multisubunit translocons secreting a diverse array of substrates varying in size and nature from monomeric proteins to nucleoprotein complexes. T4S systems have evolved from conjugation machineries and are implicated in antibiotic resistance gene transfer and transport of virulence factors in Legionella pneumophila causing Legionnaires’ disease, Brucella suis causing brucellosis and Helicobacter pylori causing gastroduodenal diseases. The best-studied are the Agrobacterium tumefaciens VirB/D4 and the Escherichia coli plasmid pKM101 T4S systems. Recent structural advances revealing the cryo-EM (electron microscopy) structure of the core translocation assembly and high-resolution structure of the outer-membrane pore of T4S systems have made paradigm shifts in the understanding of T4S systems. The present paper reviews the advances made in biochemical and structural studies and summarizes our current understanding of the molecular architecture of this mega-assembly.

Surface organelles assembled by secretion systems of Gram-negative bacteria: diversity in structure and function

Gram-negative bacteria express a wide variety of organelles on their cell surface. These surface structures may be the end products of secretion systems, such as the hair-like fibers assembled by the chaperone/usher (CU) and type IV pilus pathways, which generally function in adhesion to surfaces and bacterial-bacterial and bacterial-host interactions. Alternatively, the surface organelles may be integral components of the secretion machinery itself, such as the needle complex and pilus extensions formed by the type III and type IV secretion systems, which function in the delivery of bacterial effectors inside host cells. Bacterial surface structures perform functions critical for pathogenesis and have evolved to withstand forces exerted by the external environment and cope with defenses mounted by the host immune system. Given their essential roles in pathogenesis and exposed nature, bacterial surface structures also make attractive targets for therapeutic intervention. This review will describe the structure and function of surface organelles assembled by four different Gram-negative bacterial secretion systems: the CU pathway, the type IV pilus pathway, and the type III and type IV secretion systems.

Brucellosis and type IV secretion

Brucellosis is a global disease of domestic and wild mammals that is caused by intracellular bacteria of the genus Brucella. Although humans are not a natural reservoir for Brucella, infection in the human population is common in many countries, and brucellosis is one of the most common zoonotic infections. Brucella species have evolved to avoid the host's immune system and infection is usually characterized by long-term persistence of the bacteria. One important Brucella virulence factor for intracellular survival and persistence in the host is the type IV secretion system. This review will discuss the Brucella type IV secretion system in detail, including current knowledge of architecture and regulation, as well as the newly identified effector substrates that this system transports into host cells.

The Gonococcal Genetic Island and Type IV Secretion in the Pathogenic Neisseria

Eighty percent of Neisseria gonorrhoeae strains and some Neisseria meningitidis strains encode a 57-kb gonococcal genetic island (GGI). The GGI was horizontally acquired and is inserted in the chromosome at the replication terminus. The GGI is flanked by direct repeats, and site-specific recombination at these sites results in excision of the GGI and may be responsible for its original acquisition. Although the role of the GGI in N. meningitidis is unclear, the GGI in N. gonorrhoeae encodes a type IV secretion system (T4SS). T4SS are versatile multi-protein complexes and include both conjugation systems as well as effector systems that translocate either proteins or DNA-protein complexes. In N. gonorrhoeae, the T4SS secretes single-stranded chromosomal DNA into the extracellular milieu in a contact-independent manner. Importantly, the DNA secreted through the T4SS is effective in natural transformation and therefore contributes to the spread of genetic information through Neisseria populations. Mutagenesis experiments have identified genes for DNA secretion including those encoding putative structural components of the apparatus, peptidoglycanases which may act in assembly, and relaxosome components for processing the DNA and delivering it to the apparatus. The T4SS may also play a role in infection by N. gonorrhoeae. During intracellular infection, N. gonorrhoeae requires the Ton complex for iron acquisition and survival. However, N. gonorrhoeae strains that do not express the Ton complex can survive intracellularly if they express structural components of the T4SS. These data provide evidence that the T4SS is expressed during intracellular infection and suggest that the T4SS may provide an advantage for intracellular survival. Here we review our current understanding of how the GGI and type IV secretion affect natural transformation and pathogenesis in N. gonorrhoeae and N. meningitidis.

Interaction of variable bacterial outer membrane lipoproteins with brain endothelium

Background

Previously we reported that the variable outer membrane lipoprotein Vsp1 from the relapsing fever spirochete Borrelia turicatae disseminates from blood to brain better than the closely related Vsp2 [1]. Here we studied the interaction between Vsp1 and Vsp2 with brain endothelium in more detail.

Methodology/Principal Findings

We compared Vsp1 to Vsp2 using human brain microvascular endothelial cell (HBMEC) association assays with aminoacid radiolabeled Vsp-expressing clones of recombinant Borrelia burgdorferi and lanthanide-labeled purified lipidated Vsp1 (LVsp1) and Vsp2 (LVsp2) and inoculations of the lanthanide-labeled proteins into mice. The results showed that heterologous expression of LVsp1 or LVsp2 in B. burgdorferi increased its association with HBMEC to a similar degree. Purified lanthanide-labeled lipidated Vsp1 (LVsp1) and LVsp2 by themselves were capable of associating with HBMEC. The association of LVsp1 with brain endothelium was time-dependent, saturable, and required the lipidation. The association of Vsp1 with HBMEC was inhibited by incubation at lower temperature or with excess unlabeled LVsp1 or LVsp2 but not with excess rVsp1 or mouse albumin or an anti Vsp1 monoclonal antibody. The association of LVsp2 with HBMEC and its movement from blood to brain parenchyma significantly increased in the presence of LVsp1.

Conclusions/Significance

Variable bacterial outer membrane lipoproteins interact with brain endothelium differently; the lipidation and variable features at the protein dome region are key modulators of this interaction.

Quantitative analysis of VirB8-VirB9-VirB10 interactions provides a dynamic model of type IV secretion system core complex assembly

Type IV secretion systems are multiprotein complexes that translocate macromolecules across the bacterial cell envelope. The type IV secretion system in Brucella species encodes 12 VirB proteins that permit this pathogen to translocate effectors into mammalian cells, where they contribute to its survival inside the host. The "core" complex proteins are conserved in all type IV secretion systems, and they are believed to form the channel for substrate translocation. We have investigated the in vitro interactions between the soluble periplasmic domains of three of these VirB components, VirB8, VirB9, and VirB10, using enzyme-linked immunosorbent assays, circular dichroism, and surface plasmon resonance techniques. The in vitro experiments helped in the quantification of the self-association and binary interactions of VirB8, VirB9, and VirB10. Individually, distinct binding properties were revealed that may explain their biological functions, and collectively, we provide direct evidence of the in vitro formation of the VirB8-VirB9-VirB10 ternary complex. To assess the dynamics of these interactions in a simplified in vivo model of complex assembly, we applied the bacterial two-hybrid system in studying interactions between the full-length proteins. This approach demonstrated that VirB9 stimulates the self-association of VirB8 but inhibits VirB10-VirB10 and VirB8-VirB10 interaction. Analysis of a dimerization site variant of VirB8 (VirB8(M102R)) suggested that the interactions with VirB9 and VirB10 are independent of its self-association, which stabilizes VirB8 in this model assay. We propose a dynamic model for secretion system assembly in which VirB8 plays a role as an assembly factor that is not closely associated with the functional core complex comprising VirB9 and VirB10.

Phylogenomics reveals a diverse Rickettsiales type IV secretion system

With an obligate intracellular lifestyle, Alphaproteobacteria of the order Rickettsiales have inextricably coevolved with their various eukaryotic hosts, resulting in small, reductive genomes and strict dependency on host resources. Unsurprisingly, large portions of Rickettsiales genomes encode proteins involved in transport and secretion. One particular transporter that has garnered recent attention from researchers is the type IV secretion system (T4SS). Homologous to the well-studied archetypal vir T4SS of Agrobacterium tumefaciens, the Rickettsiales vir homolog (rvh) T4SS is characterized primarily by duplication of several of its genes and scattered genomic distribution of all components in several conserved islets. Phylogeny estimation suggests a single event of ancestral acquirement of the rvh T4SS, likely from a nonalphaproteobacterial origin. Bioinformatics analysis of over 30 Rickettsiales genome sequences illustrates a conserved core rvh scaffold (lacking only a virB5 homolog), with lineage-specific diversification of several components (rvhB1, rvhB2, and rvhB9b), likely a result of modifications to cell envelope structure. This coevolution of the rvh T4SS and cell envelope morphology is probably driven by adaptations to various host cells, identifying the transporter as an important target for vaccine development. Despite the genetic intractability of Rickettsiales, recent advancements have been made in the characterization of several components of the rvh T4SS, as well as its putative regulators and substrates. While current data favor a role in effector translocation, functions in DNA uptake and release and/or conjugation cannot at present be ruled out, especially considering that a mechanism for plasmid transfer in Rickettsia spp. has yet to be proposed.

Two-step and one-step secretion mechanisms in Gram-negative bacteria: contrasting the type IV secretion system and the chaperone-usher pathway of pilus biogenesis

Gram-negative bacteria have evolved diverse secretion systems/machineries to translocate substrates across the cell envelope. These various machineries fulfil a wide variety of functions but are also essential for pathogenic bacteria to infect human or plant cells. Secretion systems, of which there are seven, utilize one of two secretion mechanisms: (i) the one-step mechanism, whereby substrates are translocated directly from the bacterial cytoplasm to the extracellular medium or into the eukaryotic target cell; (ii) the two-step mechanism, whereby substrates are first translocated across the bacterial inner membrane; once in the periplasm, substrates are targeted to one of the secretion systems that mediate transport across the outer membrane and released outside the bacterial cell. The present review provides an example for each of these two classes of secretion systems and contrasts the various solutions evolved to secrete substrates.

The structural biology of type IV secretion systems

Type IV secretion systems (T4SSs) are versatile secretion systems that are found in both Gram-negative and Gram-positive bacteria and secrete a wide range of substrates, from single proteins to protein-protein and protein-DNA complexes. They usually consist of 12 components that are organized into ATP-powered, double-membrane-spanning complexes. The structures of single soluble components or domains have been solved, but an understanding of how these structures come together has only recently begun to emerge. This Review focuses on the structural advances that have been made over the past 10 years and how the corresponding structural insights have helped to elucidate many of the details of the mechanism of type IV secretion.

An anomalous type IV secretion system in Rickettsia is evolutionarily conserved

BACKGROUND

Bacterial type IV secretion systems (T4SSs) comprise a diverse transporter family functioning in conjugation, competence, and effector molecule (DNA and/or protein) translocation. Thirteen genome sequences from Rickettsia, obligate intracellular symbionts/pathogens of a wide range of eukaryotes, have revealed a reduced T4SS relative to the Agrobacterium tumefaciens archetype (vir). However, the Rickettsia T4SS has not been functionally characterized for its role in symbiosis/virulence, and none of its substrates are known.

RESULTS

Superimposition of T4SS structural/functional information over previously identified Rickettsia components implicate a functional Rickettsia T4SS. virB4, virB8 and virB9 are duplicated, yet only one copy of each has the conserved features of similar genes in other T4SSs. An extraordinarily duplicated VirB6 gene encodes five hydrophobic proteins conserved only in a short region known to be involved in DNA transfer in A. tumefaciens. virB1, virB2 and virB7 are newly identified, revealing a Rickettsia T4SS lacking only virB5 relative to the vir archetype. Phylogeny estimation suggests vertical inheritance of all components, despite gene rearrangements into an archipelago of five islets. Similarities of Rickettsia VirB7/VirB9 to ComB7/ComB9 proteins of epsilon-proteobacteria, as well as phylogenetic affinities to the Legionella lvh T4SS, imply the Rickettsiales ancestor acquired a vir-like locus from distantly related bacteria, perhaps while residing in a protozoan host. Modern modifications of these systems likely reflect diversification with various eukaryotic host cells.

CONCLUSION

We present the rvh (Rickettsiales vir homolog) T4SS, an evolutionary conserved transporter with an unknown role in rickettsial biology. This work lays the foundation for future laboratory characterization of this system, and also identifies the Legionella lvh T4SS as a suitable genetic model.

Structure of a type IV secretion system core complex

Type IV secretion systems (T4SSs) are important virulence factors used by Gram-negative bacterial pathogens to inject effectors into host cells or to spread plasmids harboring antibiotic resistance genes. We report the 15 angstrom resolution cryo-electron microscopy structure of the core complex of a T4SS. The core complex is composed of three proteins, each present in 14 copies and forming a approximately 1.1-megadalton two-chambered, double membrane-spanning channel. The structure is double-walled, with each component apparently spanning a large part of the channel. The complex is open on the cytoplasmic side and constricted on the extracellular side. Overall, the T4SS core complex structure is different in both architecture and composition from the other known double membrane-spanning secretion system that has been structurally characterized.

Agrobacterium VirB10 domain requirements for type IV secretion and T pilus biogenesis

Agrobacterium tumefaciens VirB10 couples inner membrane (IM) ATP energy consumption to substrate transfer through the VirB/D4 type IV secretion (T4S) channel and also mediates biogenesis of the virB-encoded T pilus. Here, we determined the functional importance of VirB10 domains denoted as the: (i) N-terminal cytoplasmic region, (ii) transmembrane (TM) alpha-helix, (iii) proline-rich region (PRR) and (iv) C-terminal beta-barrel domain. Mutations conferring a transfer- and pilus-minus (Tra(-), Pil(-)) phenotype included PRR deletion and beta-barrel substitution mutations that prevented VirB10 interaction with the outer membrane (OM) VirB7-VirB9 channel complex. Mutations permissive for substrate transfer but blocking pilus production (Tra(+), Pil(-)) included a cytoplasmic domain deletion and TM domain insertion mutations. Another class of Tra(+) mutations also selectively disrupted pilus biogenesis but caused release of pilin monomers to the milieu; these mutations included deletions of alpha-helical projections extending from the beta-barrel domain. Our findings, together with results of Cys accessibility studies, indicate that VirB10 stably integrates into the IM, extends via its PRR across the periplasm, and interacts via its beta-barrel domain with the VirB7-VirB9 channel complex. The data further support a model that distinct domains of VirB10 regulate formation of the secretion channel or the T pilus.

Bioinformatic and partial functional analysis of pEspA and pEspB, two plasmids from Exiguobacterium arabatum sp. nov. RFL1109

The complete nucleotide sequences of two plasmids from Exiguobacterium arabatum sp. nov. RFL1109, pEspA (4563bp) and pEspB (38,945bp), have been determined. Five ORFs were identified in the pEspA plasmid, and putative functions were assigned to two of them. Using deletion mapping approach, the Rep-independent replication region of pEspA, which functions in Bacillus subtilis, was localized within a 0.6kb DNA region. Analysis of the pEspB sequence revealed 42 ORFs. From these, function of two genes encoding enzymes of the Lsp1109I restriction-modification system was confirmed experimentally, while putative functions of another 18 ORFs were suggested based on comparative analysis. Three functional regions have been proposed for the pEspB plasmid: the putative conjugative transfer region, the region involved in plasmid replication and maintenance, and the region responsible for transposition of the IS21 family-like transposable elements.

SciN is an outer membrane lipoprotein required for type VI secretion in enteroaggregative Escherichia coli

Enteroaggregative Escherichia coli (EAEC) is a pathogen implicated in several infant diarrhea or diarrheal outbreaks in areas of endemicity. Although multiple genes involved in EAEC pathogenesis have been identified, the overall mechanism of virulence is not well understood. Recently, a novel secretion system, called type VI secretion (T6S) system (T6SS), has been identified in EAEC and most animal or plant gram-negative pathogens. T6SSs are multicomponent cell envelope machines responsible for the secretion of at least two putative substrates, Hcp and VgrG. In EAEC, two copies of T6S gene clusters, called sci-1 and sci-2, are present on the pheU pathogenicity island. In this study, we focused our work on the sci-1 gene cluster. The Sci-1 apparatus is probably composed of all, or a subset of, the 21 gene products encoded on the cluster. Among these subunits, some are shared by all T6SSs identified to date, including a ClpV-type AAA(+) ATPase (SciG) and an IcmF (SciS) and an IcmH (SciP) homologue, as well as a putative lipoprotein (SciN). In this study, we demonstrate that sciN is a critical gene necessary for T6S-dependent secretion of the Hcp-like SciD protein and for biofilm formation. We further show that SciN is a lipoprotein, as shown by the inhibition of its processing by globomycin and in vivo labeling with [(3)H]palmitic acid. SciN is tethered to the outer membrane and exposed in the periplasm. Sequestration of SciN at the inner membrane by targeting the +2 residue responsible for lipoprotein localization (Gly2Asp) fails to complement an sciN mutant for SciD secretion and biofilm formation. Together, these results support a model in which SciN is an outer membrane lipoprotein exposed in the periplasm and essential for the Sci-1 apparatus function.

The type IV secretion system component VirB5 binds to the trans-zeatin biosynthetic enzyme Tzs and enables its translocation to the cell surface of Agrobacterium tumefaciens

VirB5 is a minor component of the extracellular T pilus determined by the Agrobacterium tumefaciens type IV secretion system. To identify proteins that interact with VirB5 during the pilus assembly process, we purified VirB5 as a recombinant fusion protein and, by using a gel overlay assay, we detected a 26-kDa interacting protein in Agrobacterium cell lysates. The VirB5-binding protein was purified from A. tumefaciens and identified as the cytokinin biosynthetic enzyme Tzs. The VirB5-Tzs interaction was confirmed using pulldown assays with purified proteins and the yeast two-hybrid system. An analysis of the subcellular localization in A. tumefaciens showed that Tzs was present in the soluble as well as the membrane fraction. Tzs was extracted from the membranes with the mild detergent dodecyl-beta-D-maltoside in complexes of different molecular masses, and this association was strongly reduced in the absence of VirB5. Using immunoelectron microscopy, we also detected Tzs on the Agrobacterium cell surface. A functional type IV secretion system was required for efficient translocation to the surface, but Tzs was not secreted into the cell supernatant. The fact that Tzs localizes on the cell surface suggests that it may contribute to the interaction of Agrobacterium with plants.

VirB1* promotes T-pilus formation in the vir-Type IV secretion system of Agrobacterium tumefaciens

The vir-type IV secretion system of Agrobacterium is assembled from 12 proteins encoded by the virB operon and virD4. VirB1 is one of the least-studied proteins encoded by the virB operon. Its N terminus is a lytic transglycosylase. The C-terminal third of the protein, VirB1*, is cleaved from VirB1 and secreted to the outside of the bacterial cell, suggesting an additional function. We show that both nopaline and octopine strains produce abundant amounts of VirB1* and perform detailed studies on nopaline VirB1*. Both domains are required for wild-type virulence. We show here that the nopaline type VirB1* is essential for the formation of the T pilus, a subassembly of the vir-T4SS composed of processed and cyclized VirB2 (major subunit) and VirB5 (minor subunit). A nopaline virB1 deletion strain does not produce T pili. Complementation with full-length VirB1 or C-terminal VirB1*, but not the N-terminal lytic transglycosylase domain, restores T pili containing VirB2 and VirB5. T-pilus preparations also contain extracellular VirB1*. Protein-protein interactions between VirB1* and VirB2 and VirB5 were detected in the yeast two-hybrid assay. We propose that VirB1 is a bifunctional protein required for virT4SS assembly. The N-terminal lytic transglycosylase domain provides localized lysis of the peptidoglycan cell wall to allow insertion of the T4SS. The C-terminal VirB1* promotes T-pilus assembly through protein-protein interactions with T-pilus subunits.

VirB8: a conserved type IV secretion system assembly factor and drug target

Type IV secretion systems are used by many gram-negative bacteria for the translocation of macromolecules (proteins, DNA, or DNA-protein complexes) across the cell envelope. Among them are many pathogens for which type IV secretion systems are essential virulence factors. Type IV secretion systems comprise 8-12 conserved proteins, which assemble into a complex spanning the inner and the outer membrane, and many assemble extracellular appendages, such as pili, which initiate contact with host and recipient cells followed by substrate translocation. VirB8 is an essential assembly factor for all type IV secretion systems. Biochemical, cell biological, genetic, and yeast two-hybrid analyses showed that VirB8 undergoes multiple interactions with other type IV secretion system components and that it directs polar assembly of the membrane-spanning complex in the model organism Agrobacterium tumefaciens. The availability of the VirB8 X-ray structure has enabled a detailed structure-function analysis, which identified sites for the binding of VirB4 and VirB10 and for self-interaction. Due to its multiple interactions, VirB8 is an excellent model for the analysis of assembly factors of multiprotein complexes. In addition, VirB8 is a possible target for drugs that target its protein-protein interactions, which would disarm bacteria by depriving them of their essential virulence functions.

NMR structure of a complex between the VirB9/VirB7 interaction domains of the pKM101 type IV secretion system

Type IV secretion (T4S) systems translocate DNA and protein effectors through the double membrane of Gram-negative bacteria. The paradigmatic T4S system in Agrobacterium tumefaciens is assembled from 11 VirB subunits and VirD4. Two subunits, VirB9 and VirB7, form an important stabilizing complex in the outer membrane. We describe here the NMR structure of a complex between the C-terminal domain of the VirB9 homolog TraO (TraO(CT)), bound to VirB7-like TraN from plasmid pKM101. TraO(CT) forms a beta-sandwich around which TraN winds. Structure-based mutations in VirB7 and VirB9 of A. tumefaciens show that the heterodimer interface is conserved. Opposite this interface, the TraO structure shows a protruding three-stranded beta-appendage, and here, we supply evidence that the corresponding region of VirB9 of A. tumefaciens inserts in the membrane and protrudes extracellularly. This complex structure elucidates the molecular basis for the interaction between two essential components of a T4S system.

Detection of conjugation related type four secretion machinery in Aeromonas culicicola

Background

Aeromonas sp. can now be considered relatively common enteropathogens due to the increase of diseases in humans. Aeromonas culicicola is a gram negative rod-shaped bacterium isolated for the first time from the mosquito mid-gut, but subsequently detected in other insects and waters also. Our previous study discovered that A. culicicola harbors three plasmids, which we designated as pAc3249A, pAc3249B and pAc3249C. We investigated and report here the existence and genetic organization of a Conjugal Type IV Secretion System (TFSS) in pAc3249A.

Methodology/Principle Finding

The complete operon is 11,061 bp in length and has G+C content of 47.20% code for 12 ORFs. The gene order and orientation were similar to those found in other bacteria with some differences. We have designated this system as AcTra for Aeromonas culicicola transfer system. BLAST results of ORFs and phylogenetic analysis showed significant similarity towards the respective proteins of the IncI2 plasmid R721 of E. coli. Other bioinformatics studies have been performed to predict conserved motifs/domains, signal peptides, transmembrane helices, etc. of the ORFs.

Conclusions/Significance

BLAST results of ORFs and phylogenetic analysis showed significant similarity towards the respective proteins of the IncI2 plasmid R721 of E. coli.

Identification of the core transmembrane complex of the Legionella Dot/Icm type IV secretion system

Type IV secretion systems (T4SS) are utilized by a wide range of Gram negative bacteria to deliver protein and DNA substrates to recipient cells. The best characterized T4SS are the type IVA systems, which exhibit extensive similarity to the Agrobacterium VirB T4SS. In contrast, type IVB secretion systems share almost no sequence homology to the type IVA systems, are composed of approximately twice as many proteins, and remain largely uncharacterized. Type IVB systems include the Dot/Icm systems found in the pathogens Legionella and Coxiella and the conjugative apparatus of IncI plasmids. Here we report the first extensive characterization of a type IVB system, the Legionella Dot/Icm secretion apparatus. Based on biochemical and genetic analysis, we discerned the existence of a critical five-protein subassembly that spans both bacterial membranes and comprises the core of the secretion complex. This transmembrane connection is mediated by protein dimer pairs consisting of two inner membrane proteins, DotF and DotG, which are able to independently associate with DotH/DotC/DotD in the outer membrane. The Legionella core subcomplex appears to be functionally analogous to the Agrobacterium VirB7-10 subcomplex, suggesting a remarkable conservation of the core subassembly in these evolutionarily distant type IV secretion machines.

Protein-protein interactions among Helicobacter pylori cag proteins

Many Helicobacter pylori isolates contain a 40-kb region of chromosomal DNA known as the cag pathogenicity island (PAI). The risk for development of gastric cancer or peptic ulcer disease is higher among humans infected with cag PAI-positive H. pylori strains than among those infected with cag PAI-negative strains. The cag PAI encodes a type IV secretion system that translocates CagA into gastric epithelial cells. To identify Cag proteins that are expressed by H. pylori during growth in vitro, we compared the proteomes of a wild-type H. pylori strain and an isogenic cag PAI deletion mutant using two-dimensional difference gel electrophoresis (2D-DIGE) in multiple pH ranges. Seven Cag proteins were identified by this approach. We then used a yeast two-hybrid system to detect potential protein-protein interactions among 14 Cag proteins. One heterotypic interaction (CagY/7 with CagX/8) and two homotypic interactions (involving H. pylori VirB11/ATPase and Cag5) were similar to interactions previously reported to occur among homologous components of the Agrobacterium tumefaciens type IV secretion system. Other interactions involved Cag proteins that do not have known homologues in other bacterial species. Biochemical analysis confirmed selected interactions involving five of the proteins that were identified by 2D-DIGE. Protein-protein interactions among Cag proteins are likely to have an important role in the assembly of the H. pylori type IV secretion apparatus.

The Brucella suis type IV secretion system assembles in the cell envelope of the heterologous host Agrobacterium tumefaciens and increases IncQ plasmid pLS1 recipient competence

Pathogenic Brucella species replicate within mammalian cells, and their type IV secretion system is essential for intracellular survival and replication. The options for biochemical studies on the Brucella secretion system are limited due to the rigidity of the cells and biosafety concerns, which preclude large-scale cell culture and fractionation. To overcome these problems, we heterologously expressed the Brucella suis virB operon in the closely related alpha(2)-proteobacterium Agrobacterium tumefaciens and showed that the VirB proteins assembled into a complex. Eight of the twelve VirB proteins were detected in the membranes of the heterologous host with specific antisera. Cross-linking indicated protein-protein interactions similar to those in other type IV secretion systems, and the results of immunofluorescence analysis supported the formation of VirB protein complexes in the cell envelope. Production of a subset of the B. suis VirB proteins (VirB3-VirB12) in A. tumefaciens strongly increased its ability to receive IncQ plasmid pLS1 in conjugation experiments, and production of VirB1 further enhanced the conjugation efficiency. Plasmid recipient competence correlated with periplasmic leakage and the detergent sensitivity of A. tumefaciens, suggesting a weakening of the cell envelope. Heterologous expression thus permits biochemical characterization of B. suis type IV secretion system assembly.

Biogenesis, architecture, and function of bacterial type IV secretion systems

Type IV secretion (T4S) systems are ancestrally related to bacterial conjugation machines. These systems assemble as a translocation channel, and often also as a surface filament or protein adhesin, at the envelopes of Gram-negative and Gram-positive bacteria. These organelles mediate the transfer of DNA and protein substrates to phylogenetically diverse prokaryotic and eukaryotic target cells. Many basic features of T4S are known, including structures of machine subunits, steps of machine assembly, substrates and substrate recognition mechanisms, and cellular consequences of substrate translocation. A recent advancement also has enabled definition of the translocation route for a DNA substrate through a T4S system of a Gram-negative bacterium. This review emphasizes the dynamics of assembly and function of model conjugation systems and the Agrobacterium tumefaciens VirB/D4 T4S system. We also summarize salient features of the increasingly studied effector translocator systems of mammalian pathogens.

Functional interactions between type IV secretion systems involved in DNA transfer and virulence

This paper reports an analysis of the functional interactions between type IV secretion systems (T4SS) that are part of the conjugative machinery for horizontal DNA transfer (cT4SS), and T4SS involved in bacterial pathogenicity (pT4SS). The authors' previous work showed that a conjugative coupling protein (T4CP) interacts with the VirB10-type component of the T4SS in order to recruit the protein-DNA complex to the transporter for conjugative DNA transfer. This study now shows by two-hybrid analysis that conjugative T4CPs also interact with the VirB10 element of the pT4SS of Agrobacterium tumefaciens (At), Bartonella tribocorum (Bt) and Brucella suis (Bs). Moreover, the VirB10 component of a cT4SS (protein TrwE of plasmid R388) could be partially substituted by that of a pT4SS (protein TrwE of Bt) for conjugation. This result opens the way for the construction of hybrid T4SS that deliver DNA into animal cells. Interestingly, in the presence of part of the Bs T4SS the R388 T4SS protein levels were decreased and R388 conjugation was strongly inhibited. Complementation assays between the Trw systems of R388 and Bt showed that only individual components from the so-called 'core complex' could be exchanged, supporting the concept that this core is the common scaffold for the transport apparatus while the other 'peripheral components' are largely system-specific.

The mating pair formation system of conjugative plasmids-A versatile secretion machinery for transfer of proteins and DNA

The mating pair formation (Mpf) system functions as a secretion machinery for intercellular DNA transfer during bacterial conjugation. The components of the Mpf system, comprising a minimal set of 10 conserved proteins, form a membrane-spanning protein complex and a surface-exposed sex pilus, which both serve to establish intimate physical contacts with a recipient bacterium. To function as a DNA secretion apparatus the Mpf complex additionally requires the coupling protein (CP). The CP interacts with the DNA substrate and couples it to the secretion pore formed by the Mpf system. Mpf/CP conjugation systems belong to the family of type IV secretion systems (T4SS), which also includes DNA-uptake and -release systems, as well as effector protein translocation systems of bacterial pathogens such as Agrobacterium tumefaciens (VirB/VirD4) and Helicobacter pylori (Cag). The increased efforts to unravel the molecular mechanisms of type IV secretion have largely advanced our current understanding of the Mpf/CP system of bacterial conjugation systems. It has become apparent that proteins coupled to DNA rather than DNA itself are the actively transported substrates during bacterial conjugation. We here present a unified and updated view of the functioning and the molecular architecture of the Mpf/CP machinery.

Agrobacterium tumefaciens VirB9, an outer-membrane-associated component of a type IV secretion system, regulates substrate selection and T-pilus biogenesis

Agrobacterium tumefaciens translocates DNA and protein substrates between cells via a type IV secretion system (T4SS) whose channel subunits include the VirD4 coupling protein, VirB11 ATPase, VirB6, VirB8, VirB2, and VirB9. In this study, we used linker insertion mutagenesis to characterize the contribution of the outer-membrane-associated VirB9 to assembly and function of the VirB/D4 T4SS. Twenty-five dipeptide insertion mutations were classified as permissive for intercellular substrate transfer (Tra+), completely transfer defective (Tra-), or substrate discriminating, e.g., selectively permissive for transfer only of the oncogenic transfer DNA and the VirE2 protein substrates or of a mobilizable IncQ plasmid substrate. Mutations inhibiting transfer of DNA substrates did not affect formation of close contacts of the substrate with inner membrane channel subunits but blocked formation of contacts with the VirB2 and VirB9 channel subunits, which is indicative of a defect in assembly or function of the distal portion of the secretion channel. Several mutations in the N- and C-terminal regions disrupted VirB9 complex formation with the outer-membrane-associated lipoprotein VirB7 or the inner membrane energy sensor VirB10. Several VirB9.i2-producing Tra+ strains failed to elaborate T pilus at detectable levels (Pil-), and three such Tra+ Pil- mutant strains were rendered Tra- upon deletion of virB2, indicating that the cellular form of pilin protein is essential for substrate translocation. Our findings, together with computer-based analyses, support a model in which distinct domains of VirB9 contribute to substrate selection and translocation, establishment of channel subunit contacts, and T-pilus biogenesis.

Identification of new protein-protein interactions involving the products of the chromosome- and plasmid-encoded type IV secretion loci of the phytopathogen Xanthomonas axonopodis pv. citri

The recently sequenced genome of the bacterial plant pathogen Xanthomonas axonopodis pv. citri contains two virB gene clusters, one on the chromosome and one on a 64-kb plasmid, each of which codes for a previously uncharacterized type IV secretion system (T4SS). Here we used a yeast two-hybrid assay to identify protein-protein interactions in these two systems. Our results revealed interactions between known T4SS components as well as previously uncharacterized interactions involving hypothetical proteins coded by open reading frames in the two X. axonopodis pv. citri virB loci. Our results indicate that both loci may code for previously unidentified VirB7 proteins, which we show interact with either VirB6 or VirB9 or with a hypothetical protein coded by the same locus. Furthermore, a set of previously uncharacterized Xanthomonas proteins have been found to interact with VirD4, whose gene is adjacent to the chromosomal virB locus. The gene for one member of this family is found within the chromosomal virB locus. All these uncharacterized proteins possess a conserved 120-amino-acid domain in their C termini and may represent a family of cofactors or substrates of the Xanthomonas T4SS.

Type IV secretion: the Agrobacterium VirB/D4 and related conjugation systems

The translocation of DNA across biological membranes is an essential process for many living organisms. In bacteria, type IV secretion systems (T4SS) are used to deliver DNA as well as protein substrates from donor to target cells. The T4SS are structurally complex machines assembled from a dozen or more membrane proteins in response to environmental signals. In Gram-negative bacteria, the conjugation machines are composed of a cell envelope-spanning secretion channel and an extracellular pilus. These dynamic structures (i) direct formation of stable contacts-the mating junction-between donor and recipient cell membranes, (ii) transmit single-stranded DNA as a nucleoprotein particle, as well as protein substrates, across donor and recipient cell membranes, and (iii) mediate disassembly of the mating junction following substrate transfer. This review summarizes recent progress in our understanding of the mechanistic details of DNA trafficking with a focus on the paradigmatic Agrobacterium tumefaciens VirB/D4 T4SS and related conjugation systems.

Complete nucleotide sequence of the conjugative tetracycline resistance plasmid pFBAOT6, a member of a group of IncU plasmids with global ubiquity

This study presents the first complete sequence of an IncU plasmid, pFBAOT6. This plasmid was originally isolated from a strain of Aeromonas caviae from hospital effluent (Westmorland General Hospital, Kendal, United Kingdom) in September 1997 (G. Rhodes, G. Huys, J. Swings, P. McGann, M. Hiney, P. Smith, and R. W. Pickup, Appl. Environ. Microbiol. 66:3883-3890, 2000) and belongs to a group of related plasmids with global ubiquity. pFBAOT6 is 84,748 bp long and has 94 predicted coding sequences, only 12 of which do not have a possible function that has been attributed. Putative replication, maintenance, and transfer functions have been identified and are located in a region in the first 31 kb of the plasmid. The replication region is poorly understood but exhibits some identity at the protein level with replication proteins from the gram-positive bacteria Bacillus and Clostridium. The mating pair formation system is a virB homologue, type IV secretory pathway that is similar in its structural organization to the mating pair formation systems of the related broad-host-range (BHR) environmental plasmids pIPO2, pXF51, and pSB102 from plant-associated bacteria. Partitioning and maintenance genes are homologues of genes in IncP plasmids. The DNA transfer genes and the putative oriT site also exhibit high levels of similarity with those of plasmids pIPO2, pXF51, and pSB102. The genetic load region encompasses 54 kb, comprises the resistance genes, and includes a class I integron, an IS630 relative, and other transposable elements in a 43-kb region that may be a novel Tn1721-flanked composite transposon. This region also contains 24 genes that exhibit the highest levels of identity to chromosomal genes of several plant-associated bacteria. The features of the backbone of pFBAOT6 that are shared with this newly defined group of environmental BHR plasmids suggest that pFBAOT6 may be a relative of this group, but a relative that was isolated from a clinical bacterial environment rather than a plant-associated bacterial environment.

Role of Cal, the colicin A lysis protein, in two steps of colicin A release and in the interaction with colicin A–porin complexes

Release of colicin A was studied inEscherichia colicells that differed in expressing thecolicinAlysis protein (Cal). Pools of released and unreleased colicin A were harvested throughout colicin A induction. The amount of colicin A in each pool varied with the time of induction, allowing the definition of two sequential steps in colicin A release, one of which was dependent on Cal. Each step of colicin A release was differently affected in cells containing Cal mutants in which the N-terminal cysteine residue was substituted by either proline or threonine, preventing them from being acylated and matured. These Cal mutants were only observed indegPcells, indicating that the DegP protease cleaved the unacylated precursor of Cal. Cal was found in the insoluble fraction of the pools of released and unreleased colicin A together with the hetero-oligomers of colicin A and porins (colicins Au). The biogenesis of colicins Au was studied in temperature-sensitivesecAandsecYstrains and found to be Sec-independent, indicating that they are formed by newly synthesized colicin A binding to mature porins already incorporated in the outer membrane. Cal is a lipoprotein similar to VirB7, a constituent of the type IV secretion system. It would interact with colicins Au to constitute the colicin A export machinery.

Plant proteins that interact with VirB2, the Agrobacterium tumefaciens pilin protein, mediate plant transformation

Agrobacterium tumefaciens uses a type IV secretion system (T4SS) to transfer T-DNA and virulence proteins to plants. The T4SS is composed of two major structural components: the T-pilus and a membrane-associated complex that is responsible for translocating substrates across both bacterial membranes. VirB2 protein is the major component of the T-pilus. We used the C-terminal-processed portion of VirB2 protein as a bait to screen an Arabidopsis thaliana cDNA library for proteins that interact with VirB2 in yeast. We identified three related plant proteins, VirB2-interacting protein (BTI) 1 (BTI1), BTI2, and BTI3 with unknown functions, and a membrane-associated GTPase, AtRAB8. The three BTI proteins also interacted with VirB2 in vitro. Preincubation of Agrobacterium with GST-BTI1 protein decreased the transformation efficiency of Arabidopsis suspension cells by Agrobacterium. Transgenic BTI and AtRAB8 antisense and RNA interference Arabidopsis plants are less susceptible to transformation by Agrobacterium than are wild-type plants. The level of BTI1 protein is transiently increased immediately after Agrobacterium infection. In addition, overexpression of BTI1 protein in transgenic Arabidopsis results in plants that are hypersusceptible to Agrobacterium-mediated transformation. Confocal microscopic data indicate that GFP-BTI proteins preferentially localize to the periphery of root cells in transgenic Arabidopsis plants, suggesting that BTI proteins may contact the Agrobacterium T-pilus. We propose that the three BTI proteins and AtRAB8 are involved in the initial interaction of Agrobacterium with plant cells.

Interaction between protein subunits of the type IV secretion system of Bartonella henselae

In this study we used the yeast two-hybrid system to identify interactions between protein subunits of the virB type IV secretion system of Bartonella henselae. We report interactions between inner membrane and periplasmic proteins, the pilus polypeptide, and the core complex and a novel interaction between VirB3 and VirB5.

VirB1 orthologs from Brucella suis and pKM101 complement defects of the lytic transglycosylase required for efficient type IV secretion from Agrobacterium tumefaciens

Type IV secretion systems mediate conjugative plasmid transfer as well as the translocation of virulence factors from various gram-negative pathogens to eukaryotic host cells. The translocation apparatus consists of 9 to 12 components, and the components from different organisms are believed to have similar functions. However, orthologs to proteins of the prototypical type IV system, VirB of Agrobacterium tumefaciens, typically share only 15 to 30% identical amino acids, and functional complementation between components of different type IV secretion systems has not been achieved. We here report a heterologous complementation in the case of A. tumefaciens virB1 defects with its orthologs from Brucella suis (VirB1s) and the IncN plasmid pKM101 (TraL). In contrast, expression of the genes encoding the VirB1 orthologs from the IncF plasmid (open reading frame 169) and from the Helicobacter pylori cag pathogenicity island (HP0523) did not complement VirB1 functions. The complementation of VirB1 activity was assessed by T-pilus formation, by tumor formation on wounded plants, by IncQ plasmid transfer, and by IncQ plasmid recipient assay. Replacement of the key active-site Glu residue by Ala abolished the complementation by VirB1 from B. suis and by TraL, demonstrating that heterologous complementation requires an intact lytic transglycosylase active site. In contrast, the VirB1 active-site mutant from A. tumefaciens retained considerable residual activity in various activity assays, implying that this protein exerts additional effects during the type IV secretion process.

F factor conjugation is a true type IV secretion system

The F sex factor of Escherichia coli is a paradigm for bacterial conjugation and its transfer (tra) region represents a subset of the type IV secretion system (T4SS) family. The F tra region encodes eight of the 10 highly conserved (core) gene products of T4SS including TraAF (pilin), the TraBF, -KF (secretin-like), -VF (lipoprotein) and TraCF (NTPase), -EF, -LF and TraGF (N-terminal region) which correspond to TrbCP, -IP, -GP, -HP, -EP, -JP, DP and TrbLP, respectively, of the P-type T4SS exemplified by the IncP plasmid RP4. F lacks homologs of TrbBP (NTPase) and TrbFP but contains a cluster of genes encoding proteins essential for F conjugation (TraFF, -HF, -UF, -WF, the C-terminal region of TraGF, and TrbCF) that are hallmarks of F-like T4SS. These extra genes have been implicated in phenotypes that are characteristic of F-like systems including pilus retraction and mating pair stabilization. F-like T4SS systems have been found on many conjugative plasmids and in genetic islands on bacterial chromosomes. Although few systems have been studied in detail, F-like T4SS appear to be involved in the transfer of DNA only whereas P- and I-type systems appear to transport protein or nucleoprotein complexes. This review examines the similarities and differences among the T4SS, especially F- and P-like systems, and summarizes the properties of the F transfer region gene products.

Topology and membrane interaction of Helicobacter pylori ComB proteins involved in natural transformation competence

The human gastric pathogen Helicobacter pylori is naturally competent for genetic transformation. The H. pylori comB gene duster encodes the VirB4-homologous ATPase ComB4 and the structural proteins ComB7-ComB10, which share significant sequence identity to the Agrobacterium tumefaciens virB-encoded type IV secretion system. To study the topology of the ComB7-10 proteins, we applied TnMax transposon mutagenesis by generating fusions of ComB proteins with mature beta-lactamase (BlaM) or alkaline phosphatase (PhoA). Our data show that the putative lipoprotein ComB7 is secreted and is found membrane-attached, probably by its lipid anchor. According to our topology mapping ComB8 is a bitopic membrane protein with a short N-terminal portion in the cytoplasm and the remainder of the protein expanding into the periplasmic space. ComB9 was verified as a periplasmic protein, tightly attached to the membrane. The N-terminus of ComB10 is anchored in the cytoplasmic membrane and the major portion of the protein, including a putative coiled-coil domain, is located in the periplasm. Limited protease digestion and protein extraction under different salt and pH conditions confirmed the periplasmic localization and the tight membrane association of the ComB protein complex. A hypothetical model of the ComB DNA transformation pore in H. pylori is presented.

Functional subsets of the virB type IV transport complex proteins involved in the capacity of Agrobacterium tumefaciens to serve as a recipient in virB-mediated conjugal transfer of plasmid RSF1010

The virB-encoded type IV transport complex of Agrobacterium tumefaciens mediates the transfer of DNA and proteins into plant cells, as well as the conjugal transfer of IncQ plasmids, such as RSF1010, between Agrobacterium strains. While several studies have indicated that there are physical interactions among the 11 VirB proteins, the functional significance of the interactions has been difficult to establish since all of the proteins are required for substrate transfer. Our previous studies, however, indicated that although all of the VirB proteins are required for the capacity of a strain to serve as an RSF1010 donor, only a subset of these proteins in the recipient is necessary to increase the conjugal frequency by 3 to 4 logs. The roles of particular groups of VirB proteins in this increased recipient activity were examined in the study reported here. Examination of the expression of subgroups of virB genes revealed that translation of virB6 is necessary for expression of downstream open reading frames. Expression of limited subsets of the VirB proteins in a recipient strain lacking the Ti plasmid revealed that the VirB7 to VirB10 proteins yield a subcomplex that is functional in the recipient assay but that the VirB1 to VirB4 proteins, as a group, dramatically increase this activity in strains expressing VirB7 to VirB10. Finally, the membrane distribution and cross-linking patterns of VirB10, but not of VirB8 or VirB9, in a strain expressing only VirB7 to VirB10 are significantly altered compared to the patterns of the wild type. These characteristics are, however, restored to the wild-type status by coexpression of VirB1 to VirB3. Taken together, these results define subsets of type IV transport complex proteins that are critical in allowing a strain to participate as a recipient in virB-mediated conjugal RSF1010 transfer.

Xanthomonas citri: breaking the surface

SUMMARY Taxonomy: Bacteria; Proteobacteria, Gammaproteobacteria; Xanthomonadales; Xanthomonadaceae, Xanthomonas. Microbiological properties: Gram-negative, obligately aerobic, straight rods, motile by a single polar flagellum, yellow pigment. Related species: X. campestris, X. axonopodis, X. oryzae, X. albilineans.

HOST RANGE

Affects Rutaceous plants, primarily Citrus spp., Fortunella spp., and Poncirus spp., world-wide. Quarantined pathogen in many countries. Economically important hosts are cultivated orange, grapefruit, lime, lemon, pomelo and citrus rootstock. Disease symptoms: On leaves, first appearance is as oily looking, 2-10 mm, similarly sized, circular spots, usually on the abaxial surface. On leaves, stems, thorns and fruit, circular lesions become raised and blister-like, growing into white or yellow spongy pustules. These pustules then darken and thicken into a light tan to brown corky canker, which is rough to the touch. On stems, pustules may coalesce to split the epidermis along the stem length, and occasionally girdling of young stems may occur. Older lesions on leaves and fruit tend to have more elevated margins and are at times surrounded by a yellow chlorotic halo (that may disappear) and a sunken centre. Sunken craters are especially noticeable on fruit, but the lesions do not penetrate far into the rind. Defoliation and premature abscission of affected fruit occurs on heavily infected trees.

USEFUL WEBSITES

<http://www.biotech.ufl.edu/PlantContainment/canker.htm>; <http://cancer.lbi.ic.unicamp.br/xanthomonas/>

Agrobacterium tumefaciens VirB6 protein participates in formation of VirB7 and VirB9 complexes required for type IV secretion

This study characterized the contribution of Agrobacterium tumefaciens VirB6, a polytopic inner membrane protein, to the formation of outer membrane VirB7 lipoprotein and VirB9 protein multimers required for type IV secretion. VirB7 assembles as a disulfide cross-linked homodimer that associates with the T pilus and a VirB7-VirB9 heterodimer that stabilizes other VirB proteins during biogenesis of the secretion machine. Two presumptive VirB protein complexes, composed of VirB6, VirB7, and VirB9 and of VirB7, VirB9, and VirB10, were isolated by immunoprecipitation or glutathione S-transferase pulldown assays from detergent-solubilized membrane extracts of wild-type A348 and a strain producing only VirB6 through VirB10 among the VirB proteins. To examine the biological importance of VirB6 complex formation for type IV secretion, we monitored the effects of nonstoichiometric VirB6 production and the synthesis of VirB6 derivatives with 4-residue insertions (VirB6.i4) on VirB7 and VirB9 multimerization, T-pilus assembly, and substrate transfer. A virB6 gene deletion mutant accumulated VirB7 dimers at diminished steady-state levels, whereas complementation with a plasmid bearing wild-type virB6 partially restored accumulation of the dimers. VirB6 overproduction was correlated with formation of higher-order VirB9 complexes or aggregates and also blocked substrate transfer without a detectable disruption of T-pilus production; these phenotypes were displayed by cells grown at 28 degrees C, a temperature that favors VirB protein turnover, but not by cells grown at 20 degrees C. Strains producing several VirB6.i4 mutant proteins assembled novel VirB7 and VirB9 complexes detectable by nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and two strains producing the D60.i4 and L191.i4 mutant proteins translocated IncQ plasmid and VirE2 effector protein substrates in the absence of a detectable T pilus. Our findings support a model that VirB6 mediates formation of VirB7 and VirB9 complexes required for biogenesis of the T pilus and the secretion channel.

Peptide linkage mapping of the Agrobacterium tumefaciens vir-encoded type IV secretion system reveals protein subassemblies

Numerous bacterial pathogens use type IV secretion systems (T4SS) to deliver virulence factors directly to the cytoplasm of plant, animal, and human host cells. Here, evidence for interactions among components of the Agrobacterium tumefaciens vir-encoded T4SS is presented. The results derive from a high-resolution yeast two-hybrid assay, in which a library of small peptide domains of T4SS components was screened for interactions. The use of small peptides overcomes problems associated with assaying for interactions involving membrane-associated proteins. We established interactions between VirB11 (an inner membrane pore-forming protein), VirB9 (a periplasmic protein), and VirB7 (an outer membrane-associated lipoprotein and putative pilus component). We provide evidence for an interaction pathway, among conserved members of a T4SS, spanning the A. tumefaciens envelope and including a potential pore protein. In addition, we have determined interactions between VirB1 (a lytic transglycosylase likely involved in the local remodeling of the peptidoglycan) and primarily VirB8, but also VirB4, VirB10, and VirB11 (proteins likely to assemble the core structure of the T4SS). VirB4 interacts with VirB8, VirB10, and VirB11, also establishing a connection to the core components. The identification of these interactions suggests a model for assembly of the T4SS.