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

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.

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.

Recombination, diversity and allele sharing of infectivity proteins between Bartonella species from rodents

The alpha-Proteobacterium Bartonella is a common parasite of voles and mice, giving rise to short-lived (4 weeks to 2 months) infections. Here, we report high sequence diversity in genes of the VirB/VirD type IV secretion system (T4SS), amongst Bartonella from natural rodent populations in NE Poland. The VirB5 protein is predicted to consist of three conserved alpha helices separated by loops of variable length which include numerous indels. The C-terminal domain includes repeat stretches of KEK residues, reflecting underlying homopolymeric stretches of adenine residues. A total of 16 variants of VirB5, associated with host identity, but not bacterial taxon, were identified from 22 Bartonella isolates. One was clearly a recombinant from two others, another included an insertion of two KEK repeats. The virB5 gene appears to evolve via both mutation and recombination, as well as slippage mediated insertion/deletion events. The recombinational units are thought to be relatively short, as there was no evidence of linkage disequilibrium between virB5 and the bepA locus only 5.5 kb distant. The diversity of virB5 is assumed to be related to immunological role of this protein in Bartonella infections; diversity of virB5 may assist persistence of Bartonella in the rodent population, despite the relatively short (3-4 weeks) duration of individual infections. It is clear from the distribution of virB5 and bepA alleles that recombination within and between clades is widespread, and frequently crosses the boundaries of conventionally recognised Bartonella species.

Persistence of Bartonella spp. stealth pathogens: from subclinical infections to vasoproliferative tumor formation

Bartonella spp. are facultative intracellular bacteria that typically cause a long-lasting intraerythrocytic bacteremia in their mammalian reservoir hosts, thereby favoring transmission by blood-sucking arthropods. In most cases, natural reservoir host infections are subclinical and the relapsing intraerythrocytic bacteremia may last weeks, months, or even years. In this review, we will follow the infection cycle of Bartonella spp. in a reservoir host, which typically starts with an intradermal inoculation of bacteria that are superficially scratched into the skin from arthropod feces and terminates with the pathogen exit by the blood-sucking arthropod. The current knowledge of bacterial countermeasures against mammalian immune response will be presented for each critical step of the pathogenesis. The prevailing models of the still-enigmatic primary niche and the anatomical location where bacteria reside, persist, and are periodically seeded into the bloodstream to cause the typical relapsing Bartonella spp. bacteremia will also be critically discussed. The review will end up with a discussion of the ability of Bartonella spp., namely Bartonella henselae, Bartonella quintana, and Bartonella bacilliformis, to induce tumor-like vascular deformations in humans having compromised immune response such as in patients with AIDS.

Evidence of transfer by conjugation of type IV secretion system genes between Bartonella species and Rhizobium radiobacter in amoeba

Background

Bartonella species cospeciate with mammals and live within erythrocytes. Even in these specific niches, it has been recently suggested by bioinformatic analysis of full genome sequences that Lateral Gene Transfer (LGT) may occur but this has never been demonstrated biologically. Here we describe the sequence of the B. rattaustraliani (AUST/NH4^T) circular plasmid (pNH4) that encodes the tra cluster of the Type IV secretion system (T4SS) and we eventually provide evidence that Bartonella species may conjugate and exchange this plasmid inside amoeba.

Principal Findings

The T4SS of pNH4 is critical for intracellular viability of bacterial pathogens, exhibits bioinformatic evidence of LGT among bacteria living in phagocytic protists. For instance, 3 out of 4 T4SS encoding genes from pNH4 appear to be closely related to Rhizobiales, suggesting that gene exchange occurs between intracellular bacteria from mammals (bartonellae) and plants (Rhizobiales). We show that B. rattaustraliani and Rhizobium radiobacter both survived within the amoeba Acanthamoeba polyphaga and can conjugate together. Our findings further support the hypothesis that tra genes might also move into and out of bacterial communities by conjugation, which might be the primary means of genomic evolution for intracellular adaptation by cross-talk of interchangeable genes between Bartonella species and plant pathogens.

Conclusions

Based on this, we speculate that amoeba favor the transfer of genes as phagocytic protists, which allows for intraphagocytic survival and, as a consequence, promotes the creation of potential pathogenic organisms.

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.

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.

Bartonella henselae: subversion of vascular endothelial cell functions by translocated bacterial effector proteins

Bartonella henselae (Bh) is a worldwide distributed zoonotic pathogen. Depending on the immune status of the infected individual this bacterium can cause a wide spectrum of clinical manifestations, ranging from cat scratch disease (CSD) to bacillary angiomatosis (BA) and bacillary peliosis (BP). BA and BP are characterized by tumor-like lesions at the skin or in the inner organs, respectively. These structures display pathological sprouting of capillaries with enlarged and hyperproliferated vascular endothelial cells (ECs) that are frequently found in close association with bacteria. Here we review the cellular changes observed upon Bh infection of ECs in vitro and outline the role of the VirB type IV secretion system (T4SS) and its translocated effector proteins in the modulation of EC signalling cascades. The current model how this virulence system could contribute to the vasoproliferative activity of Bh is described.

The VirB5 protein localizes to the T-pilus tips in Agrobacterium tumefaciens

The Agrobacterium tumefaciens VirB/D4 type IV secretion system (T4SS) mediates the transfer of single-stranded DNA and protein virulence factors into plant cells, and also determines the assembly of the T-pilus, which is believed to play a role in host recognition. The T-pilus is composed of the major component VirB2 and the minor component VirB5. Using immuno-electron microscopy we detected the major component VirB2 along the entire length of detached T-pili, but not on cell-bound T-pili or on the cell surface. In contrast, the minor T-pilus component VirB5 was detected on the tips of cell-bound T-pili as well as on the ends of detached T-pili and on the cell surface. To gain further insights into the role of VirB5 we introduced changes at its C terminus. C-terminal deletions of up to four amino acids and alanine replacements did not abolish T-pilus formation and incorporation of the VirB5 variants at the tip, although they did impact the length of T-pili. Also, these changes differentially affected the ability of the T4SS to transfer DNA into plant and bacterial recipients, suggesting differential effects on host-cell specificity. The data presented here suggest that VirB5 localizes at the T-pilus tip, and provide novel insights into its role during the type IV secretion process.

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.

A type IV-secretion-like system is required for conjugative DNA transport of broad-host-range plasmid pIP501 in gram-positive bacteria

Plasmid pIP501 has a very broad host range for conjugative transfer among a wide variety of gram-positive bacteria and gram-negative Escherichia coli. Functionality of the pIP501 transfer (tra) genes in E. coli was proven by pIP501 retrotransfer to Enterococcus faecalis (B. Kurenbach, C. Bohn, J. Prabhu, M. Abudukerim, U. Szewzyk, and E. Grohmann, Plasmid 50:86-93, 2003). The 15 pIP501 tra genes are organized in a single operon (B. Kurenbach, J. Kopeć, M. Mägdefrau, K. Andreas, W. Keller, C. Bohn, M. Y. Abajy, and E. Grohmann, Microbiology 152:637-645, 2006). The pIP501 tra operon is negatively autoregulated at the transcriptional level by the conjugative DNA relaxase TraA. Three of the 15 pIP501-encoded Tra proteins show significant sequence similarity to the Agrobacterium type IV secretion system proteins VirB1, VirB4, and VirD4. Here we report a comprehensive protein-protein interaction map of all of the pIP501-encoded Tra proteins determined by the yeast two-hybrid assay. Most of the interactions were verified in vitro by isolation of the protein complexes with pull-down assays. In conjunction with known or postulated functions of the pIP501-encoded Tra proteins and computer-assisted prediction of their cellular location, we propose a model for the first type IV-secretion-like system encoded by a conjugative plasmid from gram-positive bacteria.

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.

Dynamics of the IncW genetic backbone imply general trends in conjugative plasmid evolution

Plasmids cannot be understood as mere tools for genetic exchange: they are themselves subject to the forces of evolution. Their genomic and phylogenetic features have been less studied in this respect. Focusing on the IncW incompatibility group, which includes the smallest known conjugative plasmids, we attempt to unveil some common trends in plasmid evolution. The functional modules of IncW genetic backbone are described, with emphasis on their architecture and relationships to other plasmid groups. Some plasmid regions exhibit strong phylogenetic mosaicism, in striking contrast to others of unusual synteny conservation. The presence of genes of unknown function that are widely distributed in plasmid genomes is also emphasized, exposing the existence of ill-defined yet conserved plasmid functions. Conjugation is an essential hallmark of IncW plasmid biology and special attention is given to the organization and evolution of its transfer modules. Genetic exchange between plasmids and their hosts is analysed by following the evolution of the type IV secretion system. Adaptation of the trw conjugative machinery to pathogenicity functions in Bartonella is discussed as an example of how plasmids can change their host modus vivendi. Starting from the phage paradigm, our analysis articulates novel concepts that apply to plasmid evolution.

Dimerization and interactions of Brucella suis VirB8 with VirB4 and VirB10 are required for its biological activity

VirB8-like proteins are essential components of type IV secretion systems, bacterial virulence factors that mediate the translocation of effector molecules from many bacterial pathogens into eukaryotic cells. Based on cell biological, genetic, and x-ray crystallographic data, VirB8 was proposed to undergo multiple protein-protein interactions to mediate assembly of the translocation machinery. Here we report the results of a structure-function analysis of the periplasmic domain of VirB8 from the mammalian pathogen Brucella suis, which identifies amino acid residues required for three protein-protein interactions. VirB8 variants changed at residues proposed to be involved in dimerization, and protein-protein interactions were purified and characterized in vitro and in vivo. Changes at M102, Y105, and E214 affected the self-association as measured by analytical ultracentrifugation and gel filtration. The interaction with B. suis VirB10 was reduced by changes at T201, and change at R230 inhibited the interaction with VirB4 in vitro. The in vivo functionality of VirB8 variants was determined by complementation of growth in macrophages by a B. suis virB8 mutant and by using a heterologous assay of type IV secretion system assembly in Agrobacterium tumefaciens. Changes at Y105, T201, R230, and at several other residues impaired the in vivo function of VirB8, suggesting that we have identified interaction sites of relevance in the natural biological context.

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.

The putative lytic transglycosylase VirB1 from Brucella suis interacts with the type IV secretion system core components VirB8, VirB9 and VirB11

VirB1-like proteins are believed to act as lytic transglycosylases, which facilitate the assembly of type IV secretion systems via localized lysis of the peptidoglycan. This paper presents the biochemical analysis of interactions of purified Brucella suis VirB1 with core components of the type IV secretion system. Genes encoding VirB1, VirB8, VirB9, VirB10 and VirB11 were cloned into expression vectors; the affinity-tagged proteins were purified from Escherichia coli, and analyses by gel filtration chromatography showed that they form monomers or homo-multimers. Analysis of protein-protein interactions by affinity precipitation revealed that VirB1 bound to VirB9 and VirB11. The results of bicistron expression experiments followed by gel filtration further supported the VirB1-VirB9 interaction. Peptide array mapping identified regions of VirB1 that interact with VirB8, VirB9 and VirB11 and underscored the importance of the C-terminus, especially for the VirB1-VirB9 interaction. The binding sites were localized on a structure model of VirB1, suggesting that different portions of VirB1 may interact with other VirB proteins during assembly of the type IV secretion machinery.

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.

From bioremediation to biowarfare: On the impact and mechanism of type IV secretion systems

Type IV secretion systems are employed by a wide variety of Gram-negative microorganisms for the translocation of macromolecules across the cell envelope. The translocated substrates (proteins, protein-DNA complexes and DNA) are as diverse as the organisms on the donor and recipient side of the translocation process. Over the course of evolution, these macromolecular transporters were adapted to many different purposes, but their basic mechanism was conserved. They impact human life in various ways, as there are driving forces of horizontal gene transfer, which spreads biodegradative capabilities of environmental bacteria as well as antibiotic resistance of pathogens in hospitals. Also, they translocate toxins and other effectors, which have an effect on host cell metabolism and are essential for the virulence of bacterial pathogens. We here present recent developments of research on the mechanism of type IV secretion focusing on the energetization of transport and assembly processes, formation of the translocation channel and of surface-exposed pili, which initiate host cell interactions.

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.

Structural and dynamic properties of bacterial type IV secretion systems (review)

The type IV secretion systems (T4SS) are widely distributed among the gram-negative and -positive bacteria. These systems mediate the transfer of DNA and protein substrates across the cell envelope to bacterial or eukaryotic cells generally through a process requiring direct cell-to-cell contact. Bacteria have evolved T4SS for survival during establishment of pathogenic or symbiotic relationships with eukaryotic hosts. The Agrobacterium tumefaciens VirB/D4 T4SS and related conjugation machines serve as models for detailed mechanistic studies aimed at elucidating the nature of translocation signals, machine assembly pathways and architectures, and the dynamics of substrate translocation. The A. tumefaciens VirB/D4 T4SS are polar-localized organelles composed of a secretion channel and an extracellular T pilus. These T4SS are assembled from 11 or more subunits. whose membrane topologies, intersubunit contacts and, in some cases, 3-dimensional structures are known. Recently, powerful in vivo assays have identified C-terminal translocation signals, defined for the first time the translocation route for a DNA substrate through a type IV secretion channel, and supplied evidence that ATP energy consumption contributes to a late stage of machine morphogenesis. Together, these recent findings describe the mechanics of type IV secretion in unprecedented detail.

Spatial location and requirements for the assembly of the Agrobacterium tumefaciens type IV secretion apparatus

Type IV secretion is used by pathogenic microorganisms to transfer effector macromolecules to eukaryotic target cells. The VirB/D4 apparatus of Agrobacterium tumefaciens transfers DNA and proteins to plant cells. We postulated that the cell pole is the site of assembly of the A. tumefaciens type IV apparatus. Using immunofluorescence microscopy, we now demonstrate that 10 of the VirB proteins localized primarily to one cell pole and a macromolecular VirB complex is assembled at the pole. Neither the assembly of the complex nor polar localization of a VirB protein requires ATP utilization by the VirB ATPases. The requirement of other VirB proteins for the polar localization of at least six VirB proteins indicates an essential role of protein-protein interaction in polar targeting. Four proteins (VirB3, VirB4, VirB8, and VirB11) could target themselves to a cell pole independent of a VirB protein. We provide evidence that VirB6-VirB10 are the structural components of the type IV apparatus. Using strains that express defined subsets of the virB genes, we demonstrate that VirB7-VirB10 are the minimum components sufficient for the assembly of a polar VirB complex. VirB6 associates with this complex to form the type IV secretion apparatus. VirB8 functions as the assembly factor and targets the apparatus to the cell pole.

Bartonella–host-cell interactions and vascular tumour formation

Bartonellae are arthropod-borne bacterial pathogens that typically cause persistent infection of erythrocytes and endothelial cells in their mammalian hosts. In human infection, these host-cell interactions result in a broad range of clinical manifestations. Most remarkably, bartonellae can trigger massive proliferation of endothelial cells, leading to vascular tumour formation. The recent availability of infection models and bacterial molecular genetic techniques has fostered research on the pathogenesis of the bartonellae and has advanced our understanding of the virulence mechanisms that underlie the host-cell tropism, the subversion of host-cell functions during bacterial persistence, as well as the formation of vascular tumours by these intriguing pathogens.

Virulence-associated type IV secretion systems of Bartonella

Type IV secretion systems (T4SSs) are transport machineries of Gram-negative bacteria that mediate interbacterial DNA-transfer, and secretion of virulence factors into eukaryotic target cells. A growing number of human pathogenic bacteria use T4SSs for intercellular delivery of effector molecules that modify host cellular functions in favour of the pathogen. Recent advances in studying the molecular mechanisms of Bartonella pathogenesis have provided evidence for the central roles of two distinct T4SSs, VirB/VirD4 and Trw, in the ability of the bacteria to colonize, invade and persist within either vascular endothelial cells or erythrocytes, respectively. The identification of VirB/VirD4-transported substrates and the delineation of their secretion signal have paved the way towards understanding the molecular mechanisms underlying Bartonella-host cell interaction and modulation, as well as the exploitation of this system for engineered substrate delivery into mammalian target cells.

Identification of the VirB4-VirB8-VirB5-VirB2 pilus assembly sequence of type IV secretion systems

Type IV secretion systems mediate the translocation of virulence factors (proteins and/or DNA) from Gram-negative bacteria into eukaryotic cells. A complex of 11 conserved proteins (VirB1-VirB11) spans the inner and the outer membrane and assembles extracellular T-pili in Agrobacterium tumefaciens. Here we report a sequence of protein interactions required for the formation of complexes between VirB2 and VirB5, which precedes their incorporation into pili. The NTPase Walker A active site of the inner membrane protein VirB4 is required for virulence, but an active site VirB4 variant stabilized VirB3 and VirB8 and enabled T-pilus formation. Analysis of VirB protein complexes extracted from the membranes with mild detergent revealed that VirB2-VirB5 complex formation depended on VirB4, which identified a novel T-pilus assembly step. Bicistron expression demonstrated direct interaction of VirB4 with VirB8, and analyses with purified proteins showed that VirB5 bound to VirB8 and VirB10. VirB4 therefore localizes at the basis of a trans-envelope interaction sequence, and by stabilization of VirB8 it mediates the incorporation of VirB5 and VirB2 into extracellular pili.