Implication of proteins containing tetratricopeptide repeats in conditional virulence phenotypes of Legionella pneumophila

Legionella pneumophila, the causative agent of Legionnaires' disease, is a ubiquitous freshwater bacterium whose virulence phenotypes require a type IV secretion system (T4SS). L. pneumophila strain JR32 contains two virulence-associated T4SSs, the Dot/Icm and Lvh T4SSs. Defective entry and phagosome acidification phenotypes of dot/icm mutants are conditional and reversed by incubating broth-grown stationary-phase cultures in water (WS treatment) prior to infection, as a mimic of the aquatic environment of Legionella. Reversal of dot/icm virulence defects requires the Lvh T4SS and is associated with a >10-fold induction of LpnE, a tetratricopeptide repeat (TPR)-containing protein. In the current study, we demonstrated that defective entry and phagosome acidification phenotypes of mutants with changes in LpnE and EnhC, another TPR-containing protein, were similarly reversed by WS treatment. In contrast to dot/icm mutants for which the Lvh T4SS was required, reversal for the ΔlpnE or the ΔenhC mutant required that the other TPR-containing protein be present. The single and double ΔlpnE and ΔenhC mutants showed a hypersensitivity to sodium ion, a phenotype associated with dysfunction of the Dot/Icm T4SS. The ΔlpnE single and the ΔlpnE ΔenhC double mutant showed 3- to 9-fold increases in translocation of Dot/Icm T4SS substrates, LegS2/SplY and LepB. Taken together, these data identify TPR-containing proteins in a second mechanism by which the WS mimic of a Legionella environmental niche can reverse virulence defects of broth-grown cultures and implicate LpnE and EnhC directly or indirectly in translocation of Dot/Icm T4SS protein substrates.     

The Legionella pneumophila IcmSW complex interacts with multiple Dot/Icm effectors to facilitate type IV translocation

Many gram-negative pathogens use a type IV secretion system (T4SS) to deliver effector proteins into eukaryotic host cells. The fidelity of protein translocation depends on the efficient recognition of effector proteins by the T4SS. Legionella pneumophila delivers a large number of effector proteins into eukaryotic cells using the Dot/Icm T4SS. How the Dot/Icm system is able to recognize and control the delivery of effectors is poorly understood. Recent studies suggest that the IcmS and IcmW proteins interact to form a stable complex that facilitates translocation of effector proteins by the Dot/Icm system by an unknown mechanism. Here we demonstrate that the IcmSW complex is necessary for the productive translocation of multiple Dot/Icm effector proteins. Effector proteins that were able to bind IcmSW in vitro required icmS and icmW for efficient translocation into eukaryotic cells during L. pneumophila infection. We identified regions in the effector protein SidG involved in icmSW-dependent translocation. Although the full-length SidG protein was translocated by an icmSW-dependent mechanism, deletion of amino terminal regions in the SidG protein resulted in icmSW-independent translocation, indicating that the IcmSW complex is not contributing directly to recognition of effector proteins by the Dot/Icm system. Biochemical and genetic studies showed that the IcmSW complex interacts with a central region of the SidG protein. The IcmSW interaction resulted in a conformational change in the SidG protein as determined by differences in protease sensitivity in vitro. These data suggest that IcmSW binding to effectors could enhance effector protein delivery by mediating a conformational change that facilitates T4SS recognition of a translocation domain located in the carboxyl region of the effector protein.     

Legionella pneumophila exploits PI(4)P to anchor secreted effector proteins to the replicative vacuole

The causative agent of Legionnaires' disease, Legionella pneumophila, employs the intracellular multiplication (Icm)/defective organelle trafficking (Dot) type IV secretion system (T4SS) to upregulate phagocytosis and to establish a replicative vacuole in amoebae and macrophages. Legionella-containing vacuoles (LCVs) do not fuse with endosomes but recruit early secretory vesicles. Here we analyze the role of host cell phosphoinositide (PI) metabolism during uptake and intracellular replication of L. pneumophila. Genetic and pharmacological evidence suggests that class I phosphatidylinositol(3) kinases (PI3Ks) are dispensable for phagocytosis of wild-type L. pneumophila but inhibit intracellular replication of the bacteria and participate in the modulation of the LCV. Uptake and degradation of an icmT mutant strain lacking a functional Icm/Dot transporter was promoted by PI3Ks. We identified Icm/Dot-secreted proteins which specifically bind to phosphatidylinositol(4) phosphate (PI(4)P) in vitro and preferentially localize to LCVs in the absence of functional PI3Ks. PI(4)P was found to be present on LCVs using as a probe either an antibody against PI(4)P or the PH domain of the PI(4)P-binding protein FAPP1 (phosphatidylinositol(4) phosphate adaptor protein-1). Moreover, the presence of PI(4)P on LCVs required a functional Icm/Dot T4SS. Our results indicate that L. pneumophila modulates host cell PI metabolism and exploits the Golgi lipid second messenger PI(4)P to anchor secreted effector proteins to the LCV.     

The Legionella pneumophila IcmS-LvgA protein complex is important for Dot/Icm-dependent intracellular growth

Many bacterial pathogens require a functional type IV secretion system (T4SS) for virulence. Legionella pneumophila, the causative agent of Legionnaires' disease, employs the Dot/Icm T4SS to inject a large number of protein substrates into its host, thereby altering phagosome trafficking. The L. pneumophila T4SS substrate SdeA has been shown to require the accessory factor IcmS for its export. IcmS, defined as a type IV adaptor, exists as a heterodimer with IcmW and this complex functions in a manner similar to a type III secretion chaperone. Here we report an interaction between IcmS and the previously identified virulence factor LvgA. Similar to the icmS mutant, the lvgA mutant appears to assemble a fully functional Dot/Icm complex. Both LvgA and IcmS are small, acidic proteins localized to the cytoplasm and are not exported by the Dot/Icm system, suggesting they form a novel type IV adaptor complex. Inactivation of lvgA causes a minimal defect in growth in the human monocytic cell line U937 and the environmental host Acanthamoeba castellanii. However, the lvgA mutant was severely attenuated for intracellular growth of L. pneumophila in mouse macrophages, suggesting LvgA may be a critical factor that confers host specificity.     

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.     

The Coxiella burnetii cryptic plasmid is enriched in genes encoding type IV secretion system substrates

The intracellular bacterial pathogen Coxiella burnetii directs biogenesis of a phagolysosome-like parasitophorous vacuole (PV), in which it replicates. The organism encodes a Dot/Icm type IV secretion system (T4SS) predicted to deliver to the host cytosol effector proteins that mediate PV formation and other cellular events. All C. burnetii isolates carry a large, autonomously replicating plasmid or have chromosomally integrated plasmid-like sequences (IPS), suggesting that plasmid and IPS genes are critical for infection. Bioinformatic analyses revealed two candidate Dot/Icm substrates with eukaryotic-like motifs uniquely encoded by the QpH1 plasmid from the Nine Mile reference isolate. CpeC, containing an F-box domain, and CpeD, possessing kinesin-related and coiled-coil regions, were secreted by the closely related Legionella pneumophila Dot/Icm T4SS. An additional QpH1-specific gene, cpeE, situated in a predicted operon with cpeD, also encoded a secreted effector. Further screening revealed that three hypothetical proteins (CpeA, CpeB, and CpeF) encoded by all C. burnetii plasmids and IPS are Dot/Icm substrates. By use of new genetic tools, secretion of plasmid effectors by C. burnetii during host cell infection was confirmed using β-lactamase and adenylate cyclase translocation assays, and a C-terminal secretion signal was identified. When ectopically expressed in HeLa cells, plasmid effectors trafficked to different subcellular sites, including autophagosomes (CpeB), ubiquitin-rich compartments (CpeC), and the endoplasmic reticulum (CpeD). Collectively, these results suggest that C. burnetii plasmid-encoded T4SS substrates play important roles in subversion of host cell functions, providing a plausible explanation for the absolute maintenance of plasmid genes by this pathogen.     

Packaging of live Legionella pneumophila into pellets expelled by Tetrahymena spp. does not require bacterial replication and depends on a Dot/Icm-mediated survival mechanism

The freshwater ciliate Tetrahymena sp. efficiently ingested, but poorly digested, virulent strains of the gram-negative intracellular pathogen Legionella pneumophila. Ciliates expelled live legionellae packaged in free spherical pellets. The ingested legionellae showed no ultrastructural indicators of cell division either within intracellular food vacuoles or in the expelled pellets, while the number of CFU consistently decreased as a function of time postinoculation, suggesting a lack of L. pneumophila replication inside Tetrahymena. Pulse-chase feeding experiments with fluorescent L. pneumophila and Escherichia coli indicated that actively feeding ciliates maintain a rapid and steady turnover of food vacuoles, so that the intravacuolar residence of the ingested bacteria was as short as 1 to 2 h. L. pneumophila mutants with a defective Dot/Icm virulence system were efficiently digested by Tetrahymena sp. In contrast to pellets of virulent L. pneumophila, the pellets produced by ciliates feeding on dot mutants contained very few bacterial cells but abundant membrane whorls. The whorls became labeled with a specific antibody against L. pneumophila OmpS, indicating that they were outer membrane remnants of digested legionellae. Ciliates that fed on genetically complemented dot mutants produced numerous pellets containing live legionellae, establishing the importance of the Dot/Icm system to resist digestion. We thus concluded that production of pellets containing live virulent L. pneumophila depends on bacterial survival (mediated by the Dot/Icm system) and occurs in the absence of bacterial replication. Pellets of virulent L. pneumophila may contribute to the transmission of Legionnaires' disease, an issue currently under investigation.     

Molecular architecture of bacterial type IV secretion systems

Bacterial type IV secretion systems (T4SSs) are a versatile group of nanomachines that can horizontally transfer DNA through conjugation and deliver effector proteins into a wide range of target cells. The components of T4SSs in gram-negative bacteria are organized into several large subassemblies: an inner membrane complex, an outer membrane core complex, and, in some species, an extracellular pilus. Cryo-electron tomography has been used to define the structures of T4SSs in intact bacteria, and high-resolution structural models are now available for isolated core complexes from conjugation systems, the Xanthomonas citri T4SS, the Helicobacter pylori Cag T4SS, and the Legionella pneumophila Dot/Icm T4SS. In this review, we compare the molecular architectures of these T4SSs, focusing especially on the structures of core complexes. We discuss structural features that are shared by multiple T4SSs as well as evolutionary strategies used for T4SS diversification. Finally, we discuss how structural variations among T4SSs may confer specialized functional properties.     

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

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

Deciphering Legionella effector delivery by Icm/Dot secretion system reveals a new role for c-di-GMP signaling

Secretion of bacterial effector proteins into host cells plays a key role in bacterial virulence. Yet, the dynamics of the secretion systems activity remains poorly understood, especially when machineries deal with the export of numerous effectors. We address the question of multi-effector secretion by focusing on the Legionella pneumophila Icm/Dot T4SS that translocates a record number of 300 effectors. We set up a kinetic translocation assay, based on the β-lactamase translocation reporter system combined with the effect of the protonophore CCCP. When used for translocation analysis of Icm/Dot substrates constitutively produced by L. pneumophila, this assay allows a fine monitoring of the secretion activity of the T4SS, independently of the expression control of the effectors. We observed that effectors are translocated with a specific timing, suggesting a control of their docking/translocation by the T4SS. Their delivery is accurately organized to allow effective manipulation of the host cell, as exemplified by the sequential translocation of effectors targeting Rab1, namely SidM/DrrA, LidA, LepB. Remarkably, the timed delivery of effectors does not depend only on their interaction with chaperone proteins but implies cyclic-di-GMP signaling, as the diguanylate cyclase Lpl0780/Lpp0809, contributes to the timing of translocation.     

Refining the Plasmid-Encoded Type IV Secretion System Substrate Repertoire of Coxiella burnetii

The intracellular bacterial agent of Q fever, Coxiella burnetii, translocates effector proteins into its host cell cytosol via a Dot/Icm type IV secretion system (T4SS). The T4SS is essential for parasitophorous vacuole formation, intracellular replication, and inhibition of host cell death, but the effectors mediating these events remain largely undefined. Six Dot/Icm substrate-encoding genes were recently discovered on the C. burnetii cryptic QpH1 plasmid, three of which are conserved among all C. burnetii isolates, suggesting that they are critical for conserved pathogen functions. However, the remaining hypothetical proteins encoded by plasmid genes have not been assessed for their potential as T4SS substrates. In the current study, we further defined the T4SS effector repertoire encoded by the C. burnetii QpH1, QpRS, and QpDG plasmids that were originally isolated from acute-disease, chronic-disease, and severely attenuated isolates, respectively. Hypothetical proteins, including those specific to QpRS or QpDG, were screened for translocation using the well-established Legionella pneumophila T4SS secretion model. In total, six novel plasmid-encoded proteins were translocated into macrophage-like cells by the Dot/Icm T4SS. Four newly identified effectors are encoded by genes present only on the QpDG plasmid from severely attenuated Dugway isolates, suggesting that the presence of specific effectors correlates with decreased virulence. These results further support the idea of a critical role for extrachromosomal elements in C. burnetii pathogenesis.     

The DotL protein, a member of the TraG-coupling protein family, is essential for Viability of Legionella pneumophila strain Lp02

Legionella pneumophila is able to survive inside phagocytic cells by an internalization route that bypasses fusion of the nascent phagosome with the endocytic pathway to allow formation of a replicative phagosome. The dot/icm genes, a major virulence system of L. pneumophila, encode a type IVB secretion system that is required for intracellular growth. One Dot protein, DotL, has sequence similarity to type IV secretion system coupling proteins (T4CPs). In other systems, coupling proteins are not required for viability of the organism. Here we report the first example of a strain, L. pneumophila Lp02, in which a putative T4CP is essential for viability of the organism on bacteriological media. This result is particularly surprising since the majority of the dot/icm genes in Lp02 are dispensable for growth outside of a host cell, a condition that does not require a functional Dot/Icm secretion complex. We were able to isolate suppressors of the Delta dotL lethality and found that many contained mutations in other components of the Dot/Icm secretion system. A systematic analysis of dot/icm deletion mutants revealed that the majority of them (20 of 26) suppressed the lethality phenotype, indicating a partially assembled secretion system may be the source of Delta dotL toxicity in the wild-type strain. These results are consistent with a model in which the DotL protein plays a role in regulating the activity of the L. pneumophila type IV secretion apparatus.     

The disulphide isomerase DsbC cooperates with the oxidase DsbA in a DsbD-independent manner

In Escherichia coli, DsbA introduces disulphide bonds into secreted proteins. DsbA is recycled by DsbB, which generates disulphides from quinone reduction. DsbA is not known to have any proofreading activity and can form incorrect disulphides in proteins with multiple cysteines. These incorrect disulphides are thought to be corrected by a protein disulphide isomerase, DsbC, which is kept in the reduced and active configuration by DsbD. The DsbC/DsbD isomerization pathway is considered to be isolated from the DsbA/DsbB pathway. We show that the DsbC and DsbA pathways are more intimately connected than previously thought. dsbA(-)dsbC(-) mutants have a number of phenotypes not exhibited by either dsbA(-), dsbC(-) or dsbA(-)dsbD(-) mutations: they exhibit an increased permeability of the outer membrane, are resistant to the lambdoid phage Phi80, and are unable to assemble the maltoporin LamB. Using differential two-dimensional liquid chromatographic tandem mass spectrometry/mass spectrometry analysis, we estimated the abundance of about 130 secreted proteins in various dsb(-) strains. dsbA(-)dsbC(-) mutants exhibit unique changes at the protein level that are not exhibited by dsbA(-)dsbD(-) mutants. Our data indicate that DsbC can assist DsbA in a DsbD-independent manner to oxidatively fold envelope proteins. The view that DsbC's function is limited to the disulphide isomerization pathway should therefore be reinterpreted.     

Two periplasmic disulfide oxidoreductases, DsbA and SrgA, target outer membrane protein SpiA, a component of the Salmonella pathogenicity island 2 type III secretion system

The formation of disulfide is essential for the folding, activity, and stability of many proteins secreted by Gram-negative bacteria. The disulfide oxidoreductase, DsbA, introduces disulfide bonds into proteins exported from the cytoplasm to periplasm. In pathogenic bacteria, DsbA is required to process virulence determinants for their folding and assembly. In this study, we examined the role of the Dsb enzymes in Salmonella pathogenesis, and we demonstrated that DsbA, but not DsbC, is required for the full expression of virulence in a mouse infection model of Salmonella enterica serovar Typhimurium. Salmonella strains carrying a dsbA mutation showed reduced function mediated by type III secretion systems (TTSSs) encoded on Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2). To obtain a more detailed understanding of the contribution of DsbA to both SPI-1 and SPI-2 TTSS function, we identified a protein component of the SPI-2 TTSS apparatus affected by DsbA. Although we found no substrate protein for DsbA in the SPI-1 TTSS apparatus, we identified SpiA (SsaC), an outer membrane protein of SPI-2 TTSS, as a DsbA substrate. Site-directed mutagenesis of the two cysteine residues present in the SpiA protein resulted in the loss of SPI-2 function in vitro and in vivo. Furthermore, we provided evidence that a second disulfide oxidoreductase, SrgA, also oxidizes SpiA. Analysis of in vivo mixed infections demonstrated that a Salmonella dsbA srgA double mutant strain was more attenuated than either single mutant, suggesting that DsbA acts in concert with SrgA in vivo.     

Modulation of ubiquitin dynamics and suppression of DALIS formation by the Legionella pneumophila Dot/Icm system

Legionella pneumophila is an intracellular pathogen that uses effector proteins translocated by the Dot/Icm type IV secretion system to modulate host cellular processes. Here we investigate the dynamics of subcellular structures containing ubiquitin during L. pneumophila infection of phagocytic host cells. The Dot/Icm system mediated the formation of K48 and K63 poly-ubiquitin conjugates to proteins associated with L. pneumophila -containing vacuoles in macrophages and dendritic cells, suggesting that regulatory events and degradative events involving ubiquitin are regulated by bacterial effectors during infection. Stimulation of TLR2 on the surface of macrophages and dendritic cells by L. pneumophila- derived molecules resulted in the production of ubiquitin-rich dendritic cell aggresome-like structures (DALIS). Cells infected by L. pneumophila with a functional Dot/Icm system, however, failed to produce DALIS. Suppression of DALIS formation did not affect the accumulation of ubiquitinated proteins on vacuoles containing L. pneumophila. Examining other species of Legionella revealed that Legionella jordanis was unable to suppress DALIS formation after creating a ubiquitin-decorated vacuole. Thus, the L. pneumophila Dot/Icm system has the ability to modulate host processes to promote K48 and K63 ubiquitin conjugates on proteins at the vacuole membrane, and independently suppress cellular events required for the formation of DALIS.     

Manipulation of host vesicular trafficking and innate immune defence by Legionella Dot/Icm effectors

Legionella pneumophila, the causative agent of Legionnaires' disease, infects and replicates in macrophages and amoebas. Following internalization, L. pneumophila resides in a vacuole structure called Legionella-containing vacuole (LCV). The LCV escapes from the endocytic maturation process and avoids fusion with the lysosome, a hallmark of Legionella pathogenesis. Interference with the secretory vesicle transport and avoiding lysosomal targeting render the LCV permissive for L. pneumophila intracellular replication. Central to L. pneumophila pathogenesis is a defect in the organelle trafficking/intracellular multiplication (Dot/Icm) type IV secretion system that translocates a large number of effector proteins into host cells. Many of the Dot/Icm effectors employ diverse and sophisticated biochemical strategies to manipulate the host vesicular transport system, playing an important role in LCV biogenesis and trafficking. Similar to other bacterial pathogens, L. pneumophila also delivers effector proteins to modulate or counteract host innate immune defence pathways such as the NF-κB and apoptotic signalling. This review summarizes the known functions and mechanisms of Dot/Icm effectors that target host membrane trafficking and innate immune defence pathways.     

Characterization of DsbC, a periplasmic protein of Erwinia chrysanthemi and Escherichia coli with disulfide isomerase activity.

We identified and characterized an Erwinia chrysanthemi gene able to complement an Escherichia coli dsbA mutation that prevents disulfide bond formation in periplasmic proteins. This gene, dsbC, codes for a 24 kDa periplasmic protein that contains a characteristic active site sequence of disulfide isomerases, Phe-X-X-X-X-Cys-X-X-Cys. Besides the active site, DsbC has no homology with DsbA, thioredoxin or eukaryotic protein disulfide isomerase and it could define a new subfamily of disulfide isomerases. Purified DsbC protein is able to catalyse insulin oxidation in a dithiothreitol dependent manner. The E.coli gene xprA codes for a protein functionally equivalent to DsbC. The in vivo function of DsbC seems to be the formation of disulfide bonds in proteins. The presence of XprA could explain the residual disulfide isomerase activity existing in dsbA mutants. Re-oxidation of XprA does not seem to occur through DsbB, the protein that probably re-oxidizes DsbA.     

Reassessing the Role of DotF in the Legionella pneumophila Type IV Secretion System

Legionella pneumophila, the causative agent of a severe pneumonia termed Legionnaires’ Disease, survives and replicates within both protozoan hosts and human alveolar macrophages. Intracellular survival is dependent upon secretion of a plethora of protein effectors that function to form a replicative vacuole, evade the endocytic pathway and subvert host immune defenses. Export of these factors requires a type IV secretion system (T4SS) called Dot/Icm that is composed of twenty-seven proteins. This report focuses on the DotF protein, which was previously postulated to have several different functions, one of which centered on binding Dot/Icm substrates. In this report, we examined if DotF functions as the T4SS inner membrane receptor for Dot/Icm substrates. Although we were able to recapitulate the previously published bacterial two-hybrid interaction between DotF and several substrates, the interaction was not dependent on the Dot/Icm substrates’ signal sequences as predicted for a substrate:receptor interaction. In addition, binding did not require the cytoplasmic domain of DotF, which was anticipated to be involved in recognizing substrates in the cytoplasm. Finally, inactivation of dotF did not abolish intracellular growth of L. pneumophila or translocation of substrates, two phenotypes dependent on the T4SS receptor. These data strongly suggest that DotF does not act as the major receptor for Dot/Icm substrates and therefore likely performs an accessory function within the core-transmembrane subcomplex of the L. pneumophila Dot/Icm type IV secretion system.     

The Icm/Dot type-IV secretion systems of Legionella pneumophila and Coxiella burnetii

Type-IV secretion systems are devices present in a wide range of bacteria (including bacterial pathogens) that deliver macromolecules (proteins and single-strand-DNA) across kingdom barriers (as well as between bacteria and into the surroundings). The type-IV secretion systems were divided into two subgroups and Legionella pneumophila and Coxiella burnetii are the only two bacteria known today to utilize a type-IVB secretion system for pathogenesis. In this review we summarized the available information concerning the icm/dot type-IVB secretion systems by comparing the two bacteria that possess this system, the proteins components of their systems as well as the homology of proteins from type-IVB secretion systems to proteins from type-IVA secretion systems. In addition, the phenotypes associated with mutants in the L. pneumophila icm/dot genes, their relations to properties of specific Icm/Dot proteins as well as the protein substrates delivered by this system are described.