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

Biological and Structural Diversity of Type IV Secretion Systems

The bacterial type IV secretion systems (T4SSs) are a functionally diverse superfamily of secretion systems found in many species of bacteria. Collectively, the T4SSs translocate DNA and monomeric and multimeric protein substrates to bacterial and eukaryotic cell types. T4SSs are composed of two large subfamilies, the conjugation machines and the effector translocators that transmit their cargoes through establishment of direct donor-target cell contacts, and a third small subfamily capable of importing or exporting substrates from or to the milieu. This review summarizes recent mechanistic and structural findings that are shedding new light on how T4SSs have evolved such functional diversity. Translocation signals are now known to be located C terminally or embedded internally in structural folds; these signals in combination with substrate-associated adaptor proteins mediate the docking of specific substrate repertoires to cognate VirD4-like receptors. For the Legionella pneumophila Dot/Icm system, recent work has elucidated the structural basis for adaptor-dependent substrate loading onto the VirD4-like DotL receptor. Advances in definition of T4SS machine structures now allow for detailed comparisons of nanomachines closely related to the Agrobacterium tumefaciens VirB/VirD4 T4SS with those more distantly related, e.g., the Dot/Icm and Helicobacter pylori Cag T4SSs. Finally, it is increasingly evident that T4SSs have evolved a variety of mechanisms dependent on elaboration of conjugative pili, membrane tubes, or surface adhesins to establish productive contacts with target cells. T4SSs thus have evolved extreme functional diversity through a plethora of adaptations impacting substrate selection, machine architecture, and target cell binding.

Structural and Molecular Biology of Type IV Secretion Systems

Type IV secretion systems (T4SSs) are nanomachines that Gram-negative, Gram-positive bacteria, and some archaea use to transport macromolecules across their membranes into bacterial or eukaryotic host targets or into the extracellular milieu. They are the most versatile secretion systems, being able to deliver both proteins and nucleoprotein complexes into targeted cells. By mediating conjugation and/or competence, T4SSs play important roles in determining bacterial genome plasticity and diversity; they also play a pivotal role in the spread of antibiotic resistance within bacterial populations. T4SSs are also used by human pathogens such as Legionella pneumophila, Bordetella pertussis, Brucella sp., or Helicobacter pylori to sustain infection. Since they are essential virulence factors for these important pathogens, T4SSs might represent attractive targets for vaccines and therapeutics. The best-characterized conjugative T4SSs of Gram-negative bacteria are composed of twelve components that are conserved across many T4SSs. In this chapter, we will review our current structural knowledge on the T4SSs by describing the structures of the individual components and how they assemble into large macromolecular assemblies. With the combined efforts of X-ray crystallography, nuclear magnetic resonance (NMR), and more recently electron microscopy, structural biology of the T4SS has made spectacular progress during the past fifteen years and has unraveled the properties of unique proteins and complexes that assemble dynamically in a highly sophisticated manner.

Using Cryo-EM to Investigate Bacterial Secretion Systems

Bacterial secretion systems are responsible for releasing macromolecules to the extracellular milieu or directly into other cells. These membrane complexes are associated with pathogenicity and bacterial fitness. Understanding of these large assemblies has exponentially increased in the last few years thanks to electron microscopy. In fact, a revolution in this field has led to breakthroughs in characterizing the structures of secretion systems and other macromolecular machineries so as to obtain high-resolution images of complexes that could not be crystallized. In this review, we give a brief overview of structural advancements in the understanding of secretion systems, focusing in particular on cryo-electron microscopy, whether tomography or single-particle analysis. We describe how such techniques have contributed to knowledge of the mechanism of macromolecule secretion in bacteria and the impact they will have in the future.

Crystal structure of the type IV secretion system component CagX from Helicobacter pylori

Helicobacter pylori, a Gram-negative bacterial pathogen prevalent in the human population, is the causative agent of severe gastric diseases. An H. pylori type IV secretion (T4S) system encoded by the cytotoxin-associated gene pathogenicity island (cagPAI) is responsible for communication with host cells. As a component of the cagPAI T4S system core complex, CagX plays an important role in virulence-protein translocation into the host cells. In this work, the crystal structure of the C-terminal domain of CagX (CagXct), which is a homologue of the VirB9 protein from the VirB/D4 T4S system, is presented. CagXct is only the second three-dimensional structure to be elucidated of a VirB9-like protein. Another homologue, TraO, which is encoded on the Escherichia coli conjugative plasmid pKM101, shares only 19% sequence identity with CagXct; however, there is a remarkable similarity in tertiary structure between these two β-sandwich protein domains. Most of the residues that are conserved between CagXct and TraO are located within the protein core and appear to be responsible for the preservation of this domain fold. The studies presented here will contribute to our understanding of different bacterial T4S systems.

Not just an antibiotic target: Exploring the role of type I signal peptidase in bacterial virulence

The looming antibiotic crisis has prompted the development of new strategies towards fighting infection. Traditional antibiotics target bacterial processes essential for viability, whereas proposed antivirulence approaches rely on the inhibition of factors that are required only for the initiation and propagation of infection within a host. Although antivirulence compounds have yet to prove their efficacy in the clinic, bacterial signal peptidase I (SPase) represents an attractive target in that SPase inhibitors exhibit broad-spectrum antibiotic activity, but even at sub-MIC doses also impair the secretion of essential virulence factors. The potential consequences of SPase inhibition on bacterial virulence have not been thoroughly examined, and are explored within this review. In addition, we review growing evidence that SPase has relevant biological functions outside of mediating secretion, and discuss how the inhibition of these functions may be clinically significant.

VirB7 and VirB9 Interactions Are Required for the Assembly and Antibacterial Activity of a Type IV Secretion System

The type IV secretion system (T4SS) from the phytopathogen Xanthomonas citri (Xac) is a bactericidal nanomachine. The T4SS core complex is a ring composed of multiple copies of VirB7-VirB9-VirB10 subunits. Xac-VirB7 contains a disordered N-terminal tail (VirB7^NT) that recognizes VirB9, and a C-terminal domain (VirB7^CT) involved in VirB7 self-association. Here, we show that VirB7^NT forms a short β strand upon binding to VirB9 and stabilizes it. A tight interaction between them is essential for T4SS assembly and antibacterial activity. Abolishing VirB7 self-association or deletion of the VirB7 C-terminal domain impairs this antibacterial activity without disturbing T4SS assembly. These findings reveal protein interactions within the core complex that are critical for the stability and activity of a T4SS.

The Rickettsia type IV secretion system: unrealized complexity mired by gene family expansion

Many prokaryotes utilize type IV secretion systems (T4SSs) to translocate substrates (e.g. nucleoprotein, DNA, protein) across the cell envelope, and/or to elaborate surface structures (i.e. pili or adhesins). Among eight distinct T4SS classes, P-T4SSs are typified by the Agrobacterium tumefaciens vir T4SS, which is comprised of 12 scaffold components (VirB1-VirB11, VirD4). While most P-T4SSs include all 12 Vir proteins, some differ from the vir archetype by either containing additional scaffold components not analogous to Vir proteins or lacking one or more of the Vir proteins. In a special case, the Rickettsiales vir homolog (rvh) P-T4SS comprises unprecedented gene family expansion. rvh contains three families of gene duplications (rvhB9, rvhB8, rvhB4): RvhB9,8,4-I are conserved relative to equivalents in other P-T4SSs, while RvhB9,8,4-II have evolved atypical features that deviate substantially from other homologs. Furthermore, rvh contains five VirB6-like genes (rvhB6a-e), which are tandemly arrayed and contain large N- and C-terminal extensions. Our work herein focuses on the complexity underpinned by rvh gene family expansion. Furthermore, we describe an RvhB10 insertion, which occurs in a region that forms the T4SS pore. The significance of these curious properties to rvh structure and function is evaluated, shedding light on a highly complex T4SS.

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 Insight into How Bacteria Prevent Interference between Multiple Divergent Type IV Secretion Systems

Prokaryotes use type IV secretion systems (T4SSs) to translocate substrates (e.g., nucleoprotein, DNA, and protein) and/or elaborate surface structures (i.e., pili or adhesins). Bacterial genomes may encode multiple T4SSs, e.g., there are three functionally divergent T4SSs in some Bartonella species (vir, vbh, and trw). In a unique case, most rickettsial species encode a T4SS (rvh) enriched with gene duplication. Within single genomes, the evolutionary and functional implications of cross-system interchangeability of analogous T4SS protein components remains poorly understood. To lend insight into cross-system interchangeability, we analyzed the VirB8 family of T4SS channel proteins. Crystal structures of three VirB8 and two TrwG Bartonella proteins revealed highly conserved C-terminal periplasmic domain folds and dimerization interfaces, despite tremendous sequence divergence. This implies remarkable structural constraints for VirB8 components in the assembly of a functional T4SS. VirB8/TrwG heterodimers, determined via bacterial two-hybrid assays and molecular modeling, indicate that differential expression of trw and vir systems is the likely barrier to VirB8-TrwG interchangeability. We also determined the crystal structure of Rickettsia typhi RvhB8-II and modeled its coexpressed divergent paralog RvhB8-I. Remarkably, while RvhB8-I dimerizes and is structurally similar to other VirB8 proteins, the RvhB8-II dimer interface deviates substantially from other VirB8 structures, potentially preventing RvhB8-I/RvhB8-II heterodimerization. For the rvh T4SS, the evolution of divergent VirB8 paralogs implies a functional diversification that is unknown in other T4SSs. Collectively, our data identify two different constraints (spatiotemporal for Bartonellatrw and vir T4SSs and structural for rvh T4SSs) that mediate the functionality of multiple divergent T4SSs within a single bacterium.

Assembly of multiprotein complexes at the right time and at the right cellular location is a fundamentally important task for any organism. In this respect, bacteria that express multiple analogous type IV secretion systems (T4SSs), each composed of around 12 different components, face an overwhelming complexity. Our work here presents the first structural investigation on factors regulating the maintenance of multiple T4SSs within a single bacterium. The structural data imply that the T4SS-expressing bacteria rely on two strategies to prevent cross-system interchangeability: (i) tight temporal regulation of expression or (ii) rapid diversification of the T4SS components. T4SSs are ideal drug targets provided that no analogous counterparts are known from eukaryotes. Drugs targeting the barriers to cross-system interchangeability (i.e., regulators) could dysregulate the structural and functional independence of discrete systems, potentially creating interference that prevents their efficient coordination throughout bacterial infection.

Structural biology of the Gram-negative bacterial conjugation systems

Conjugation, the process by which plasmid DNA is transferred from one bacterium to another, is mediated by type IV secretion systems (T4SSs). T4SSs are versatile systems that can transport not only DNA, but also toxins and effector proteins. Conjugative T4SSs comprise 12 proteins named VirB1-11 and VirD4 that assemble into a large membrane-spanning exporting machine. Before being transported, the DNA substrate is first processed on the cytoplasmic side by a complex called the relaxosome. The substrate is then targeted to the T4SS for export into a recipient cell. In this review, we describe the recent progress made in the structural biology of both the relaxosome and the T4SS.

From crystal structure to in silico epitope discovery in the Burkholderia pseudomallei flagellar hook-associated protein FlgK

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is a potentially fatal infection that is endemic in Southeast Asia and Northern Australia that is poorly controlled by antibiotics. Research efforts to identify antigenic components for a melioidosis vaccine have led to the identification of several proteins, including subunits forming the flagella that mediate bacterial motility, host colonization, and virulence. This study focuses on the B. pseudomallei flagellar hook-associated protein (FlgK(Bp)), and provides the first insights into the 3D structure of FlgK proteins as targets for structure-based antigen engineering. The FlgK(Bp) crystal structure (presented here at 1.8-Å resolution) reveals a multidomain fold, comprising two small β-domains protruding from a large elongated α-helical bundle core. The evident structural similarity to flagellin, the flagellar filament subunit protein, suggests that, depending on the bacterial species, flagellar hook-associated proteins are likely to show a conserved, elongated α-helical bundle scaffold coupled to a variable number of smaller domains. Furthermore, we present immune serum recognition data confirming, in agreement with previous findings, that recovered melioidosis patients produce elevated levels of antibodies against FlgK(Bp), in comparison with seronegative and seropositive healthy subjects. Moreover, we show that FlgK(Bp) has cytotoxic effects on cultured murine macrophages, suggesting an important role in bacterial pathogenesis. Finally, computational epitope prediction methods applied to the FlgK(Bp) crystal structure, coupled with in vitro mapping, allowed us to predict three antigenic regions that locate to discrete protein domains. Taken together, our results point to FlgK(Bp) as a candidate for the design and production of epitope-containing subunits/domains as potential vaccine components.

Towards an integrated model of bacterial conjugation

Bacterial conjugation is one of the main mechanisms for horizontal gene transfer. It constitutes a key element in the dissemination of antibiotic resistance and virulence genes to human pathogenic bacteria. DNA transfer is mediated by a membrane-associated macromolecular machinery called Type IV secretion system (T4SS). T4SSs are involved not only in bacterial conjugation but also in the transport of virulence factors by pathogenic bacteria. Thus, the search for specific inhibitors of different T4SS components opens a novel approach to restrict plasmid dissemination. This review highlights recent biochemical and structural findings that shed new light on the molecular mechanisms of DNA and protein transport by T4SS. Based on these data, a model for pilus biogenesis and substrate transfer in conjugative systems is proposed. This model provides a renewed view of the mechanism that might help to envisage new strategies to curb the threating expansion of antibiotic resistance.

Subcellular location of the coupling protein TrwB and the role of its transmembrane domain

Conjugation is the most important mechanism for horizontal gene transfer and it is the main responsible for the successful adaptation of bacteria to the environment. Conjugative plasmids are the DNA molecules transferred and a multiprotein system encoded by the conjugative plasmid itself is necessary. The high number of proteins involved in the process suggests that they should have a defined location in the cell and therefore, they should be recruited to that specific point. One of these proteins is the coupling protein that plays an essential role in bacterial conjugation. TrwB is the coupling protein of R388 plasmid that is divided in two domains: i) The N-terminal domain referred as transmembrane domain and ii) a large cytosolic domain that contains a nucleotide-binding motif similar to other ATPases. To investigate the role of these domains in the subcellular location of TrwB, we constructed two mutant proteins that comprised the transmembrane (TrwBTM) or the cytoplasmic (TrwBΔN70) domain of TrwB. By immunofluorescence and GFP-fusion proteins we demonstrate that TrwB and TrwBTM mutant protein were localized to the cell pole independently of the remaining R388 proteins. On the contrary, a soluble mutant protein (TrwBΔN70) was localized to the cytoplasm in the absence of R388 proteins. However, in the presence of other R388-encoded proteins, TrwBΔN70 localizes uniformly to the cell membrane, suggesting that interactions between the cytosolic domain of TrwB and other membrane proteins of R388 plasmid may happen. Our results suggest that the transmembrane domain of TrwB leads the protein to the cell pole.

Molecular motors in bacterial secretion

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

Isolation of bacterial type IV machine subassemblies

The bacterial type IV secretion systems (T4SSs) deliver DNA and protein substrates to bacterial and eukaryotic target cells generally by a mechanism requiring direct contact between donor and target cells. Recent advances in defining the architectures of T4SSs have been made through isolation of machine subassemblies for further biochemical and ultrastructural analysis. Here, we describe a protocol for isolation and characterization of VirB protein complexes from the paradigmatic VirB/VirD4 T4SS of Agrobacterium tumefaciens. This protocol can be adapted for isolation of T4SS subassemblies from other gram-negative bacteria as well as gram-positive bacteria. The biological importance of isolated T4SS subcomplexes can be assessed by assaying for copurification of trapped or cross-linked substrates. This can be achieved with a modified form of the chromatin immunoprecipitation (ChIP) assay termed transfer DNA immunoprecipitation (TrIP). Here, a TrIP protocol is described for recovery of formaldehyde-cross-linked DNA substrate-channel subunit complexes from cells employing T4SSs for conjugative DNA transfer.

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.

Structure of the type IV secretion system in different strains of Anaplasma phagocytophilum

BACKGROUND

Anaplasma phagocytophilum is an intracellular organism in the Order Rickettsiales that infects diverse animal species and is causing an emerging disease in humans, dogs and horses. Different strains have very different cell tropisms and virulence. For example, in the U.S., strains have been described that infect ruminants but not dogs or rodents. An intriguing question is how the strains of A. phagocytophilum differ and what different genome loci are involved in cell tropisms and/or virulence. Type IV secretion systems (T4SS) are responsible for translocation of substrates across the cell membrane by mechanisms that require contact with the recipient cell. They are especially important in organisms such as the Rickettsiales which require T4SS to aid colonization and survival within both mammalian and tick vector cells. We determined the structure of the T4SS in 7 strains from the U.S. and Europe and revised the sequence of the repetitive virB6 locus of the human HZ strain.

RESULTS

Although in all strains the T4SS conforms to the previously described split loci for vir genes, there is great diversity within these loci among strains. This is particularly evident in the virB2 and virB6 which are postulated to encode the secretion channel and proteins exposed on the bacterial surface. VirB6-4 has an unusual highly repetitive structure and can have a molecular weight greater than 500,000. For many of the virs, phylogenetic trees position A. phagocytophilum strains infecting ruminants in the U.S. and Europe distant from strains infecting humans and dogs in the U.S.

CONCLUSIONS

Our study reveals evidence of gene duplication and considerable diversity of T4SS components in strains infecting different animals. The diversity in virB2 is in both the total number of copies, which varied from 8 to 15 in the herein characterized strains, and in the sequence of each copy. The diversity in virB6 is in the sequence of each of the 4 copies in the single locus and the presence of varying numbers of repetitive units in virB6-3 and virB6-4. These data suggest that the T4SS should be investigated further for a potential role in strain virulence of A. phagocytophilum.

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.

The ins and outs of pertussis toxin

Pertussis toxin, produced and secreted by the whooping cough agent Bordetella pertussis, is one of the most complex soluble bacterial proteins. It is actively secreted through the B. pertussis cell envelope by the Ptl secretion system, a member of the widespread type IV secretion systems. The toxin is composed of five subunits (named S1 to S5 according to their decreasing molecular weights) arranged in an A-B structure. The A protomer is composed of the enzymatically active S1 subunit, which catalyzes ADP-ribosylation of the α subunit of trimeric G proteins, thereby disturbing the metabolic functions of the target cells, leading to a variety of biological activities. The B oligomer is composed of 1S2:1S3:2S4:1S5 and is responsible for binding of the toxin to the target cell receptors and for intracellular trafficking via receptor-mediated endocytosis and retrograde transport. The toxin is one of the most important virulence factors of B. pertussis and is a component of all current vaccines against whooping cough.

Type IVB Secretion Systems of Legionella and Other Gram-Negative Bacteria

Type IV secretion systems (T4SSs) play a central role in the pathogenicity of many important pathogens, including Agrobacterium tumefaciens, Helicobacter pylori, and Legionella pneumophila. The T4SSs are related to bacterial conjugation systems, and are classified into two subgroups, type IVA (T4ASS) and type IVB (T4BSS). The T4BSS, which is closely related to conjugation systems of IncI plasmids, was originally found in human pathogen L. pneumophila; pathogenesis by L. pneumophila infection requires functional Dot/Icm T4BSS. A zoonotic pathogen, Coxiella burnetii, and an arthropod pathogen, Rickettsiella grylli – both of which carry T4BSSs highly similar to the Legionella Dot/Icm system – are evolutionarily closely related and comprise a monophyletic group. A growing body of bacterial genomic information now suggests that T4BSSs are not limited to Legionella and related bacteria and IncI plasmids. Here, we review the current knowledge on T4BSS apparatus and component proteins, gained mainly from studies on L. pneumophila Dot/Icm T4BSS. Recent structural studies, along with previous findings, suggest that the Dot/Icm T4BSS contains components with primary or higher-order structures similar to those in other types of secretion systems – types II, III, IVA, and VI, thus highlighting the mosaic nature of T4BSS architecture.

A component of the Xanthomonadaceae type IV secretion system combines a VirB7 motif with a N0 domain found in outer membrane transport proteins

Type IV secretion systems (T4SS) are used by Gram-negative bacteria to translocate protein and DNA substrates across the cell envelope and into target cells. Translocation across the outer membrane is achieved via a ringed tetradecameric outer membrane complex made up of a small VirB7 lipoprotein (normally 30 to 45 residues in the mature form) and the C-terminal domains of the VirB9 and VirB10 subunits. Several species from the genera of Xanthomonas phytopathogens possess an uncharacterized type IV secretion system with some distinguishing features, one of which is an unusually large VirB7 subunit (118 residues in the mature form). Here, we report the NMR and 1.0 Å X-ray structures of the VirB7 subunit from Xanthomonas citri subsp. citri (VirB7_XAC2622) and its interaction with VirB9. NMR solution studies show that residues 27–41 of the disordered flexible N-terminal region of VirB7_XAC2622 interact specifically with the VirB9 C-terminal domain, resulting in a significant reduction in the conformational freedom of both regions. VirB7_XAC2622 has a unique C-terminal domain whose topology is strikingly similar to that of N0 domains found in proteins from different systems involved in transport across the bacterial outer membrane. We show that VirB7_XAC2622 oligomerizes through interactions involving conserved residues in the N0 domain and residues 42–49 within the flexible N-terminal region and that these homotropic interactions can persist in the presence of heterotropic interactions with VirB9. Finally, we propose that VirB7_XAC2622 oligomerization is compatible with the core complex structure in a manner such that the N0 domains form an extra layer on the perimeter of the tetradecameric ring.

Many aspects of bacterial life require that they translocate proteins to the cell exterior. To do this, different macromolecular secretion systems of varying complexity have evolved (Type I–VI secretion systems). These secretion systems are often at the front lines of pathogen-host interactions and are important for the development of disease. In this work, we have determined the structure and studied the interactions of an unusually large VirB7 subunit (VirB7_XAC2622) of the outer membrane pore of the Type IV secretion system found in the Xanthomonas genera of phytopathogens. Its mosaic structure combines a canonical VirB7 N-terminal region with a C-terminal globular domain whose topology is observed in a relatively limited set of proteins, all involved in molecular transport across outer membranes. Our results lead to the hypothesis that the VirB7_XAC2622 globular domains can form an extra ring around the perimeter of the outer membrane pore and reveal deeper structural and evolutionary relationships among bacterial macromolecular secretion systems that have evolved to adopt a variety of functions, including structural modules in outer membrane pores (secretins from Type II, III and IV secretion systems, Type IV pili and filamentous phages), signal-transduction modules in TonB-dependent receptors and membrane-penetrating devices in T6SS and long-tailed bacteriophages.

Architecture of the Helicobacter pylori Cag-type IV secretion system

Type IV secretion systems (T4SS) are macromolecular assemblies used by bacteria to transport material across their membranes. T4SS are generally composed of a set of twelve proteins (VirB1-11 and VirD4). This represents a dynamic machine powered by three ATPases. T4SS are widespread in pathogenic bacteria where they are often used to deliver effectors into host cells. For example, the human pathogen Helicobacter pylori encodes a T4SS, the Cag-T4SS, which mediates the injection of the toxin CagA. We review the progress made in the past decade in our understanding of T4SS architecture. We translate this new knowledge to derive an understanding of the structure of the H. pylori Cag system, and use recent protein-protein interaction data to refine this model.

An in vivo high-throughput screening approach targeting the type IV secretion system component VirB8 identified inhibitors of Brucella abortus 2308 proliferation

As bacterial pathogens develop resistance against most currently used antibiotics, novel alternatives for treatment of microbial infectious diseases are urgently needed. Targeting bacterial virulence functions in order to disarm pathogens represents a promising alternative to classical antibiotic therapy. Type IV secretion systems, which are multiprotein complexes in the cell envelope that translocate effectors into host cells, are critical bacterial virulence factors in many pathogens and excellent targets for such "antivirulence" drugs. The VirB8 protein from the mammalian pathogen Brucella was chosen as a specific target, since it is an essential type IV secretion system component, it participates in multiple protein-protein interactions, and it is essential for the assembly of this translocation machinery. The bacterial two-hybrid system was adapted to assay VirB8 interactions, and a high-throughput screen identified specific small-molecule inhibitors. VirB8 interaction inhibitors also reduced the levels of VirB8 and of other VirB proteins, and many of them inhibited virB gene transcription in Brucella abortus 2308, suggesting that targeting of the secretion system has complex regulatory effects in vivo. One compound strongly inhibited the intracellular proliferation of B. abortus 2308 in a J774 macrophage infection model. The results presented here show that in vivo screens with the bacterial two-hybrid assay are suited to the identification of inhibitors of Brucella type IV secretion system function.

Crystal structure of Legionella DotD: insights into the relationship between type IVB and type II/III secretion systems

The Dot/Icm type IVB secretion system (T4BSS) is a pivotal determinant of Legionella pneumophila pathogenesis. L. pneumophila translocate more than 100 effector proteins into host cytoplasm using Dot/Icm T4BSS, modulating host cellular functions to establish a replicative niche within host cells. The T4BSS core complex spanning the inner and outer membranes is thought to be made up of at least five proteins: DotC, DotD, DotF, DotG and DotH. DotH is the outer membrane protein; its targeting depends on lipoproteins DotC and DotD. However, the core complex structure and assembly mechanism are still unknown. Here, we report the crystal structure of DotD at 2.0 Å resolution. The structure of DotD is distinct from that of VirB7, the outer membrane lipoprotein of the type IVA secretion system. In contrast, the C-terminal domain of DotD is remarkably similar to the N-terminal subdomain of secretins, the integral outer membrane proteins that form substrate conduits for the type II and the type III secretion systems (T2SS and T3SS). A short β-segment in the otherwise disordered N-terminal region, located on the hydrophobic cleft of the C-terminal domain, is essential for outer membrane targeting of DotH and Dot/Icm T4BSS core complex formation. These findings uncover an intriguing link between T4BSS and T2SS/T3SS.

Bacterial pathogens deliver virulence factors such as exotoxins and effector proteins to host cells. To accomplish this bacteria utilize specialized secretion systems such as type III and type IV secretion systems. The type IV secretion systems (T4SS) play a central role in pathogenesis by many important pathogens including Agrobacterium tumefaciens, Helicobacter pylori and Legionella pneumophila. T4SS is ancestrally related to the bacterial conjugation system and is divided into two subgroups, type IVA (T4ASS) and type IVB (T4BSS), which are derived from distinct conjugation systems. In spite of its pivotal role in bacterial pathogenesis, the structural bases and molecular mechanisms of the type IVB secretion still remain largely unknown. Here we show the crystal structure of DotD, one of the core components of Legionella T4BSS. Surprisingly, the structure of DotD is not related to those of T4ASS core components. In contrast, the structure of DotD is remarkably similar to that of a subdomain of secretin family proteins, which form substrate conduits for other types of secretion systems. This finding provides intriguing insights into the nature and the evolution of bacterial secretion systems essential for pathogenesis.

Molecular architecture of bacterial type IV secretion systems

In Gram-negative bacteria, type IV secretion (T4S) systems form ATP-powered complexes that span the entire cellular envelope and secrete a wide variety of substrates from single proteins to protein-protein and protein-DNA complexes. Recent structural data, namely the electron microscopy structure of the T4S core complex and the atomic-resolution structure of its outer-membrane pore, have profoundly altered our understanding of T4S architecture and mechanisms.

Membrane-associated DNA transport machines

DNA pumps play important roles in bacteria during cell division and during the transfer of genetic material by conjugation and transformation. The FtsK/SpoIIIE proteins carry out the translocation of double-stranded DNA to ensure complete chromosome segregation during cell division. In contrast, the complex molecular machines that mediate conjugation and genetic transformation drive the transport of single stranded DNA. The transformation machine also processes this internalized DNA and mediates its recombination with the resident chromosome during and after uptake, whereas the conjugation apparatus processes DNA before transfer. This article reviews these three types of DNA pumps, with attention to what is understood of their molecular mechanisms, their energetics and their cellular localizations.

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.

Anaplasma marginale type IV secretion system proteins VirB2, VirB7, VirB11, and VirD4 are immunogenic components of a protective bacterial membrane vaccine

Anaplasma and related Ehrlichia spp. are important tick-borne, Gram-negative bacterial pathogens of livestock and humans that cause acute infection and disease and can persist. Immunization of cattle with an Anaplasma marginale fraction enriched in outer membranes (OM) can provide complete protection against disease and persistent infection. Serological responses of OM vaccinees to the OM proteome previously identified over 20 antigenic proteins, including three type IV secretion system (T4SS) proteins, VirB9-1, VirB9-2, and VirB10. Subsequent studies showed that these three proteins also stimulated CD4(+) T-cell responses in OM vaccinees. The T4SS, composed of a complex of proteins spanning the inner and outer membranes of certain bacteria, is an important virulence factor but is relatively unexplored as a vaccine target. The goal of this study was to determine if additional T4SS proteins are immunogenic for animals immunized with the protective OM fraction of A. marginale. T4SS proteins expressed by in vitro transcription and translation were screened for stimulating proliferation of T cells from OM vaccinees, and immunogenic proteins were expressed as recombinant proteins in Escherichia coli and their immunogenicity was verified. VirB2, a putative VirB7, VirB11, and VirD4 were immunogenic for OM vaccinees expressing several common major histocompatibility complex (MHC) class II haplotypes. VirB2 is encoded by multiple genes that share a conserved central region, and epitope mapping revealed T-cell epitopes in this region. The discovery of novel immunogenic T4SS proteins recognized by outbred individuals with common MHC haplotypes further justifies evaluating the T4SS as a potential vaccine candidate for pathogenic bacteria.

Structure of the outer membrane complex of a type IV secretion system

Type IV secretion systems are secretion nanomachines spanning the two membranes of Gram-negative bacteria. Three proteins, VirB7, VirB9 and VirB10, assemble into a 1.05 megadalton (MDa) core spanning the inner and outer membranes. This core consists of 14 copies of each of the proteins and forms two layers, the I and O layers, inserting in the inner and outer membrane, respectively. Here we present the crystal structure of a approximately 0.6 MDa outer-membrane complex containing the entire O layer. This structure is the largest determined for an outer-membrane channel and is unprecedented in being composed of three proteins. Unexpectedly, this structure identifies VirB10 as the outer-membrane channel with a unique hydrophobic double-helical transmembrane region. This structure establishes VirB10 as the only known protein crossing both membranes of Gram-negative bacteria. Comparison of the cryo-electron microscopy (cryo-EM) and crystallographic structures points to conformational changes regulating channel opening and closing.

Biological diversity of prokaryotic type IV secretion systems

Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

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.

Bacterial type IV secretion systems in human disease

Type IV secretion (T4S) systems are versatile machines involved in many processes relevant to bacterial virulence, such as horizontal DNA transfer and effector translocation into human cells. A recent workshop organized by the International University of Andalousia in Baeza, Spain, covered most aspects of bacterial T4S relevant to human disease, ranging from the structural and mechanistic analysis of the T4S systems to the physiological roles of the translocated effector proteins in subverting cellular functions in infected humans. This review reports the highlights from this workshop, which include the first visualization of a T4S system core complex spanning both membranes of Gram-negative bacteria, the identification of the first host receptors for T4S systems, the identification and characterization of novel T4S effector proteins, the analysis of the molecular function of effector proteins in subverting human cellular functions and an analysis of the role of T4S systems in the evolution of pathogenic bacteria. Our increasing knowledge of the biology of T4S systems improves our ability to exploit them as biotechnological tools or to use them as novel targets for a new generation of antimicrobials.

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.

Protein oligomerization in the bacterial outer membrane (Review)

The formation of homo-oligomeric assemblies is a well-established characteristic of many soluble proteins and enzymes. Oligomerization has been shown to increase protein stability, allow allosteric cooperativity, shape reaction compartments and provide multivalent interaction sites in soluble proteins. In comparison, our understanding of the prevalence and reasons behind protein oligomerization in membrane proteins is relatively sparse. Recent progress in structural biology of bacterial outer membrane proteins has suggested that oligomerization may be as common and versatile as in soluble proteins. Here we review the current understanding of oligomerization in the bacterial outer membrane from a structural and functional point of view.

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.

Four VirB6 paralogs and VirB9 are expressed and interact in Ehrlichia chaffeensis-containing vacuoles

The type IV secretion system is an important virulence factor in several host cell-associated pathogens, as it delivers various bacterial macromolecules to target eukaryotic cells. Genes homologous to several virB genes and virD4 of Agrobacterium tumefaciens are found in an intravacuolar pathogen Ehrlichia chaffeensis, the tick-borne causative agent of human monocytic ehrlichiosis. In particular, despite its small genome size, E. chaffeensis has four tandem virB6 paralogs (virB6-1, -2, -3, and -4) that are 3- to 10-fold larger than A. tumefaciens virB6. The present study for the first time illustrates the relevance of the larger quadruple VirB6 paralogs by demonstrating the protein expression and interaction in E. chaffeensis. All four virB6 paralogs were cotranscribed in THP-1 human leukemia and ISE6 tick cell cultures. The four VirB6 proteins and VirB9 were expressed by E. chaffeensis in THP-1 cells, and amounts of these five proteins were similar in isolated E. chaffeensis-containing vacuoles and vacuole-free E. chaffeensis. In addition, an 80-kDa fragment of VirB6-2 was detected, which was strikingly more prevalent in E. chaffeensis-containing vacuoles than in vacuole-free E. chaffeensis. Coimmunoprecipitation analysis revealed VirB9 interaction with VirB6-1 and VirB6-2; VirB6-4 interaction with VirB6-1, VirB6-2, and VirB6-3; and VirB6-2 80-kDa fragment interaction with VirB6-3 and VirB6-4. The interaction of VirB9 and VirB6-2 was confirmed by far-Western blotting. The results suggest that E. chaffeensis VirB9, the quadruple VirB6 proteins, and the VirB6-2 80-kDa fragment form a unique molecular subassembly to cooperate in type IV secretion.

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.

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

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

Protein subassemblies of the Helicobacter pylori Cag type IV secretion system revealed by localization and interaction studies

Type IV secretion systems are possibly the most versatile protein transport systems in gram-negative bacteria, with substrates ranging from small proteins to large nucleoprotein complexes. In many cases, such as the cag pathogenicity island of Helicobacter pylori, genes encoding components of a type IV secretion system have been identified due to their sequence similarities to prototypical systems such as the VirB system of Agrobacterium tumefaciens. The Cag type IV secretion system contains at least 14 essential apparatus components and several substrate translocation and auxiliary factors, but the functions of most components cannot be inferred from their sequences due to the lack of similarities. In this study, we have performed a comprehensive sequence analysis of all essential or auxiliary Cag components, and we have used antisera raised against a subset of components to determine their subcellular localization. The results suggest that the Cag system contains functional analogues to all VirB components except VirB5. Moreover, we have characterized mutual stabilization effects and performed a comprehensive yeast two-hybrid screening for potential protein-protein interactions. Immunoprecipitation studies resulted in identification of a secretion apparatus subassembly at the outer membrane. Combining these data, we provide a first low-resolution model of the Cag type IV secretion apparatus.