The Legionella pneumophila LidA protein: a translocated substrate of the Dot/Icm system associated with maintenance of bacterial integrity

[Type IV secretion system and their effectors: an update]

Subversion of eukaryotic hosts by bacterial pathogens requires specialized macromolecules secretion systems delivering virulence factors either into the environment or directly into host cells. Transport of molecules across bacterial and eukaryotic membranes is a process requiring multi-component machineries called secretion systems. This review focuses on the Type IV secretion system. This complex is required for genetic exchange (DNA transport) and secretion of effectors (proteins, macromolecules, DNA-proteins complex) into target cells. They transport a wide variety of substrates including large DNA/protein complexes, multi protein toxins, or individual proteins. We describe recent advances on the structure and the function of this secretion system, their effectors and their effects on the functions of eukaryotic cell.

Dividing oceans into pools: strategies for the global analysis of bacterial genes

In bacterial pathogenesis, it is often easy to accept the results of large-scale screens without independent verification of the results. How can one critically read this literature? Here we review issues inherent in genome-wide screens in bacteria, focusing on experiments that attempt to comprehensively identify genes required for bacterial growth under specific conditions. Our analysis suggests that the methodologies employed undoubtedly shape the results. It is clear, however, that the question is not which method is better but which provides the data most suited to a given question.

Adaptation of Legionella pneumophila to the host environment: role of protein secretion, effectors and eukaryotic-like proteins

The intracellular pathogen Legionella pneumophila has evolved sophisticated mechanisms that enable it to subvert host functions, enter, survive and replicate in amoebae or alveolar macrophages, and to finally evade these hosts. Protozoa are essential for the growth of Legionella and the interaction with amoeba seems to be the driving force in the evolution of its pathogenicity. This is reflected in the genome of this pathogen, which encodes a high number and variety of eukaryotic-like proteins that are able to interfere in the various steps of the infectious cycle by mimicking functions of eukaryotic proteins. Central to the pathogenicity of L. pneumophila are the many secretion systems delivering these and other effectors to the host cell. Recent studies have highlighted the multi-functional role of these factors secreted by L. pneumophila, in host-pathogen interactions.

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.

Activated Macrophages Infected withLegionellaInhibit T Cells by Means of MyD88-Dependent Production of Prostaglandins

To understand how macrophages (Mphi) activated with IFN-gamma modulate the adaptive immune response to intracellular pathogens, the interaction of IFN-gamma-treated bone marrow-derived murine Mphi (BMphi) with Legionella pneumophila was investigated. Although Legionella was able to evade phagosome lysosome fusion initially, and was capable of de novo protein synthesis within IFN-gamma-treated BMphi, intracellular growth of Legionella was restricted. It was determined that activated BMphi infected with Legionella suppressed IFN-gamma production by Ag-specific CD4 and CD8 T cells. A factor sufficient for suppression of T cell responses was present in culture supernatants isolated from activated BMphi following Legionella infection. Signaling pathways requiring MyD88 and TLR2 were important for production of a factor produced by IFN-gamma-treated BMphi that interfered with effector T cell functions. Cyclooxygenase-2-dependent production of PGs by IFN-gamma-treated BMphi infected with Legionella was required for inhibition of effector T cell responses. From these data we conclude that activated Mphi can down-modulate Ag-specific T cell responses after they encounter bacterial pathogens through production of PGs, which may be important in preventing unnecessary immune-mediated damage to host tissues.

Additive effect on intracellular growth by Legionella pneumophila Icm/Dot proteins containing a lipobox motif

Legionella pneumophila, the causative agent of Legionnaires' disease, utilizes a type IVB secretion system to subvert its host cells and grow intracellularly. This type IV secretion system is composed of 25 icm (or dot) genes that probably constitute parts of a secretion complex as well as more than 30 proteins that are translocated via this system into the host cells. Three of the Icm/Dot proteins (DotD, DotC, and IcmN) contain a lipobox motif at their N terminals and are predicted to be lipoproteins. Two of these lipoproteins (DotD and DotC) were found to be essential for intracellular growth in both HL-60-derived human macrophages and in the protozoan host Acanthamoeba castellanii, while the third lipoprotein (IcmN) was found to be partially required for intracellular growth only in A. castellanii. Mutation analysis of the lipobox cysteine residue, which was shown previously to be indispensable for the lipobox function, indicated that both DotC and DotD are partially functional without this conserved residue. Cysteine mutations in both DotC and DotD or in DotC together with an icmN deletion or in DotD together with an icmN deletion were found to be additive, indicating that each of these lipoproteins performs its function independently from the others. Analysis of the transcriptional regulation of both the dotDC operon and the icmN gene revealed that both had higher levels of expression at stationary phase which were partially dependent on the LetA regulator. Our results indicate that the lipoproteins of the L. pneumophila icm (or dot) system are essential components of the secretion system and that they perform their functions independently.

Evidence for acquisition of Legionella type IV secretion substrates via interdomain horizontal gene transfer

Intracellular pathogens exploit host cell functions to create a replication niche inside eukaryotic cells. The causative agent of Legionnaires' disease, the gamma-proteobacterium Legionella pneumophila, resides and replicates within a modified vacuole of protozoan and mammalian cells. L. pneumophila translocates effector proteins into host cells through the Icm-Dot complex, a specialized type IVB secretion system that is required for intracellular growth. To find out if some effector proteins may have been acquired through interdomain horizontal gene transfer (HGT), we performed a bioinformatic screen that searched for eukaryotic motifs in all open reading frames of the L. pneumophila Philadelphia-1 genome. We found 44 uncharacterized genes with many distinct eukaryotic motifs. Most of these genes contain G+C biases compared to other L. pneumophila genes, supporting the theory that they were acquired through HGT. Furthermore, we found that several of them are expressed and up-regulated in stationary phase in an RpoS-dependent manner. In addition, at least seven of these gene products are translocated into host cells via the Icm-Dot complex, confirming their role in the intracellular environment. Reminiscent of the case with most Icm-Dot substrates, most of the strains containing mutations in these genes grew comparably to the parent strain intracellularly. Our findings suggest that in L. pneumophila, interdomain HGT may have been a major mechanism for the acquisition of determinants of infection.

Characterization of Legionella pneumophila pmiA, a gene essential for infectivity of protozoa and macrophages

The ability of Legionella pneumophila to cause pneumonia is dependent on intracellular replication within alveolar macrophages. The Icm/Dot secretion apparatus is essential for the ability of L. pneumophila to evade endocytic fusion, to remodel the phagosome by the endoplasmic reticulum (ER), and to replicate intracellularly. Protozoan and macrophage infectivity (pmi) mutants of L. pneumophila, which include 11 dot/icm mutants, exhibit defects in intracellular growth and replication within both protozoa and macrophages. In this study we characterized one of the pmi loci, pmiA. In contrast to the parental strain, the pmiA mutant is defective in cytopathogenicity for protozoa and macrophages. This is a novel mutant that exhibits a partial defect in survival within U937 human macrophage-like cells but exhibits a severe growth defect within Acanthamoeba polyphaga, which results in elimination from this host. The intracellular defects of this mutant are complemented by the wild-type pmiA gene on a plasmid. In contrast to phagosomes harboring the wild-type strain, which exclude endosomal-lysosomal markers, the pmiA mutant-containing phagosomes acquire the late endosomal-lysosomal markers LAMP-1 and LAMP-2. In contrast to the parental strain-containing phagosomes that are remodeled by the ER, there was a decrease in the number of ER-remodeled phagosomes harboring the pmiA mutant. Among several Legionella species examined, the pmiA gene is specific for L. pneumophila. The predicted amino acid sequence of the PmiA protein suggests that it is a transmembrane protein with three membrane-spanning regions. PmiA is similar to several hypothetical proteins produced by bacteria with a type IV secretion apparatus. Importantly, the defect in pmiA abolishes the pore-forming activity, which has been attributed to the Icm/Dot type IV secretion system. However, the mutant is sensitive to NaCl, and this sensitivity is abrogated in the icm/dot mutants. These results suggest that PmiA is a novel virulence factor that is involved in intracellular survival and replication of L. pneumophila in macrophages and protozoan cells.

Presence of multiple sites containing polar material in spherical Escherichia coli cells that lack MreB

In rod-shaped bacteria, certain proteins are specifically localized to the cell poles. The nature of the positional information that leads to the proper localization of these proteins is unclear. In a screen for factors required for the localization of the Shigella sp. actin assembly protein IcsA to the bacterial pole, a mutant carrying a transposon insertion in mreB displayed altered targeting of IcsA. The phenotype of cells containing a transposon insertion in mreB was indistinguishable from that of cells containing a nonpolar mutation in mreB or that of wild-type cells treated with the MreB inhibitor A22. In cells lacking MreB, a green fluorescent protein (GFP) fusion to a cytoplasmic derivative of IcsA localized to multiple sites. Secreted full-length native IcsA was present in multiple faint patches on the surfaces of these cells in a pattern similar to that seen for the cytoplasmic IcsA-GFP fusion. EpsM, the polar Vibrio cholerae inner membrane protein, also localized to multiple sites in mreB cells and colocalized with IcsA, indicating that localization to multiple sites is not unique to IcsA. Our results are consistent with the requirement, either direct or indirect, for MreB in the restriction of certain polar material to defined sites within the cell and, in the absence of MreB, with the formation of ectopic sites containing polar material.

Incomplete activation of macrophage apoptosis during intracellular replication of Legionella pneumophila

The ability of the intracellular bacterium Legionella pneumophila to cause disease is totally dependent on its ability to modulate the biogenesis of its phagosome and to replicate within alveolar cells. Upon invasion, L. pneumophila activates caspase-3 in macrophages, monocytes, and alveolar epithelial cells in a Dot/Icm-dependent manner that is independent of the extrinsic or intrinsic pathway of apoptosis, suggesting a novel mechanism of caspase-3 activation by this intracellular pathogen. We have shown that the inhibition of caspase-3 prior to infection results in altered biogenesis of the L. pneumophila-containing phagosome and in an inhibition of intracellular replication. In this report, we show that the preactivation of caspase-3 prior to infection does not rescue the intracellular replication of L. pneumophila icmS, icmR, and icmQ mutant strains. Interestingly, preactivation of caspase-3 through the intrinsic and extrinsic pathways of apoptosis in both human and mouse macrophages inhibits intracellular replication of the parental stain of L. pneumophila. Using single-cell analysis, we show that intracellular L. pneumophila induces a robust activation of caspase-3 during exponential replication. Surprisingly, despite this robust activation of caspase-3 in the infected cell, the host cell does not undergo apoptosis until late stages of infection. In sharp contrast, the activation of caspase-3 by apoptosis-inducing agents occurs concomitantly with the apoptotic death of all cells that exhibit caspase-3 activation. It is only at a later stage of infection, and concomitant with the termination of intracellular replication, that the L. pneumophila-infected cells undergo apoptotic death. We conclude that although a robust activation of caspase-3 is exhibited throughout the exponential intracellular replication of L. pneumophila, apoptotic cell death is not executed until late stages of the infection, concomitant with the termination of intracellular replication.

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.

Dictyostelium discoideum strains lacking the RtoA protein are defective for maturation of the Legionella pneumophila replication vacuole

To identify host proteins involved in Legionella pneumophila intracellular replication, the soil amoeba Dictyostelium discoideum was analysed. The absence of the amoebal RtoA protein is demonstrated here to depress L. pneumophila intracellular growth. Uptake of L. pneumophila into a D. discoideum rtoA(-) strain was marginally defective, but this effect was not sufficient to account for the defective intracellular growth of L. pneumophila. The rtoA mutant was also more resistant to high-multiplicity killing by the bacterium. A targeting assay testing the colocalization of L. pneumophila-containing vacuole with an endoplasmic reticulum/pre-Golgi intermediate compartment marker protein, GFP-HDEL, was used to analyse these defects. In parental D. discoideum, the L. pneumophila vacuole showed recruitment of GFP-HDEL within 40 min after introduction of bacteria to the amoebae. By 6 h after infection it was clear that the rtoA mutant acquired and retained the GFP-HDEL less efficiently than the parental strain, and that the mutant was defective for promoting the physical expansion of the membranous compartment surrounding the bacteria. Depressed intracellular growth of L. pneumophila in a D. discoideum rtoA(-) mutant therefore appeared to result from a lowered efficiency of vesicle trafficking events that are essential for the modification and expansion of the L. pneumophila-containing compartment.

Genetic analysis of the Legionella pneumophila DotB ATPase reveals a role in type IV secretion system protein export

The pulmonary pathogen Legionella pneumophila uses the Dot/Icm type IV secretion system (T4SS) to replicate inside host cells. This apparatus translocates proteins into macrophages to alter their endocytic pathway and enable bacterial growth. Although the secretion ATPase DotB is critical for T4SS function, its specific role in type IV secretion remains undefined. Due to similarity to the VirB11 and PilT ATPases, DotB has been proposed to play a role in assembly of the T4SS, retraction of pili and/or export of substrates. With the goal of understanding the protein's function(s), we isolated and characterized 30 dotB alleles using a variety of phenotypic and biochemical assays. Twenty-four of these alleles possess several dot/icm mutant phenotypes, including a complete lack of intracellular replication, plasmid mobilization and contact-dependent cytotoxicity. These 24 non-functional alleles fall into three classes: those with a known biochemical defect, those with a predicted enzymatic defect and those with an unknown defect. Six other alleles display partial activity in dot/icm phenotypic assays, thus constituting a fourth class. Two mutants in this class are unable to export a subset of T4SS substrates, providing the first evidence for a DotB function in substrate export and suggesting a possible role in substrate selection.

Coevolution between nonhomologous but functionally similar proteins and their conserved partners in the Legionella pathogenesis system

Legionella pneumophila, the causative agent of Legionnaires' disease, and other pathogenic Legionella species multiply inside protozoa and human macrophages by using the intracellular multiplication (Icm)/defect in organelle trafficking (Dot) type-IV secretion system. The IcmQ protein, which possesses pore-forming activity, and IcmR, which regulates the IcmQ activity, are two essential components of this system. Analysis of the region expected to contain these two genes from 29 Legionella species revealed the presence of a conserved icmQ gene and a large hypervariable gene family [functional homologues of icmR (fir) genes], located at the icmR genomic position. Although hypervariable in their sequence, the fir genes from all 29 Legionella species were found, together with their corresponding icmQ genes, to function similarly during infection. In addition, all FIR proteins we examined were found to interact with their corresponding IcmQ proteins. Detailed bioinformatic, biochemical, and genetic analysis of the interaction between the variable FIR proteins and conserved IcmQ proteins revealed that their interaction depends on a variable region located between two conserved domains of IcmQ. This variable region was also found to be critical for IcmQ self-interaction, and the region probably coevolved with the corresponding FIR protein. A FIR-IcmQ pair was also found in Coxiella burnetii, the only known non-Legionella bacterium that contains an Icm/Dot system, indicating the significance of this protein pair for the function of this type-IV secretion system. We hypothesize that this gene variation, which is probably mediated by positive selection, plays an important role in the evolutionary arms race between the protozoan host cell and the pathogen.

Specificity of Legionella pneumophila and Coxiella burnetii vacuoles and versatility of Legionella pneumophila revealed by coinfection

Legionella pneumophila and Coxiella burnetii are phylogenetically related intracellular bacteria that cause aerosol-transmitted lung infections. In host cells both pathogens proliferate in vacuoles whose biogenesis displays some common features. To test the functional similarity of their respective intracellular niches, African green monkey kidney epithelial (Vero) cells, A/J mouse bone marrow-derived macrophages, human macrophages, and human dendritic cells (DC) containing mature C. burnetii replication vacuoles were superinfected with L. pneumophila, and then the acidity, lysosome-associated membrane protein (LAMP) content, and cohabitation of mature replication vacuoles was assessed. In all cell types, wild-type L. pneumophila occupied distinct vacuoles in close association with acidic, LAMP-positive C. burnetii replication vacuoles. In murine macrophages, but not primate macrophages, DC, or epithelial cells, L. pneumophila replication vacuoles were acidic and LAMP positive. Unlike wild-type L. pneumophila, type IV secretion-deficient dotA mutants trafficked to lysosome-like C. burnetii vacuoles in Vero cells where they survived but failed to replicate. In primate macrophages, DC, or epithelial cells, growth of L. pneumophila was as robust in superinfected cell cultures as in those singly infected. Thus, despite their noted similarities, L. pneumophila and C. burnetii are exquisitely adapted for replication in unique replication vacuoles, and factors that maintain the C. burnetii replication vacuole do not alter biogenesis of an adjacent L. pneumophila replication vacuole. Moreover, L. pneumophila can replicate efficiently in either lysosomal vacuoles of A/J mouse cells or in nonlysosomal vacuoles of primate cells.

LidA, a translocated substrate of the Legionella pneumophila type IV secretion system, interferes with the early secretory pathway

Legionella pneumophila uses a type IV secretion system to deliver effector molecules into the host cell and establish its replication vacuole. In this study, we investigated the role of LidA, a translocated substrate associated with the surface of the L. pneumophila-containing vacuole. LidA is secreted into the host cell throughout the replication cycle of the bacteria and associates with compartments of the early secretory pathway. When overexpressed in mammalian cells or yeast, LidA interferes with the early secretory pathway, probably via a domain predicted to be rich in coiled-coil structure. Finally, during intracellular replication, the replication vacuoles are in close contact with the endoplasmic reticulum-Golgi intermediate compartment and the Golgi apparatus, suggesting a positive correlation between intracellular growth and association of the vacuole with compartments of the early secretory pathway. We propose that LidA is involved in the recruitment of early secretory vesicles to the L. pneumophila-containing vacuole and that the vacuole associates with the secretory pathway to facilitate this process.

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.

Bacterial interactions with the eukaryotic secretory pathway

Pathogenic bacteria exploit a wide variety of host cellular processes to adhere to, invade, replicate within and damage host cells. One such process is the eukaryotic secretory pathway, in which proteins and lipids are modified and transported from the endoplasmic reticulum through the Golgi network to the plasma membrane and other cellular destinations. Certain bacteria secrete toxins that utilise this transport pathway to reach their cellular targets. Some intracellular pathogens, including Legionella, Brucella and Chlamydia, engage other steps of the pathway to establish intracellular replicative organelles. Recent work has implicated specific virulence proteins of enterohaemorrhagic Escherichia coli and Salmonella enterica in secretory pathway interactions.

Processing and major histocompatibility complex class II presentation of Legionella pneumophila antigens by infected macrophages

To better understand interactions between the intracellular pathogen Legionella pneumophila and macrophages (Mphis), host and bacterial determinants important for presentation of antigens on major histocompatibility complex class II molecules (MHC-II) were investigated. It was determined that immune CD4 T-cell responses to murine bone marrow-derived Mphis (BMphis) infected with wild-type L. pneumophila were higher than the responses to avirulent dotA mutant bacteria. Although this enhanced response by immune T cells required modulation of vacuole transport mediated by the Dot/Icm system, it did not require intracellular replication of L. pneumophila. Intracellular cytokine staining identified a population of immune CD4 T cells that produced gamma interferon upon incubation with BMphis infected with wild-type L. pneumophila that did not respond to Mphi infection with dotA mutant bacteria. Endocytic processing was required for presentation of L. pneumophila antigens on MHC-II as determined by a defect in CD4 T-cell responses when the pH of BMphi endosomes was neutralized with chloroquine. Investigation of MHC-II presentation of antigens by BMphis infected with L. pneumophila icmR, icmW, and icmS mutants indicated that these mutants have an intermediate presentation phenotype relative to those of wild-type and dotA mutant bacteria. In addition, it was found that antigens from dot and icm mutants are presented earlier than antigens from wild-type L. pneumophila. Although immune CD4 T-cell responses to proteins secreted by the L. pneumophila Lsp system were not detected, it was found that the Lsp system is important for priming L. pneumophila-specific T cells in vivo. These data indicate that optimal antigen processing and MHC-II presentation to immune CD4 T cells involves synthesis of L. pneumophila proteins in an endoplasmic reticulum-derived compartment followed by transport to lysosomes.

A yeast genetic system for the identification and characterization of substrate proteins transferred into host cells by the Legionella pneumophila Dot/Icm system

The Dot/Icm system is a type IVb secretion system used by Legionella pneumophila to modulate vesicular transport in both protozoan and mammalian host cells. It has been shown that proteins and processes that are highly conserved in all eukaryotic cells are targets for some of the proteins injected by the Dot/Icm system. For example, the Legionella protein RalF was shown previously to be a Dot/Icm substrate that functions as a guanine nucleotide exchange factor (GEF) for the Arf family of eukaryotic small GTP-binding proteins. Here we show that ectopic production of the RalF protein in Saccharomyces cerevisiae interferes with yeast growth. Inhibition of yeast growth was found to be dependent on the ability of RalF to function as an Arf-GEF in vivo. The possibility that other Dot/Icm substrate proteins would have the capacity to interfere with yeast growth was used as a rationale to screen plasmid libraries containing random fragments of Legionella chromosomal DNA positioned downstream of a galactose-inducible promoter. This screen identified Legionella proteins that conferred a conditional growth defect when overproduced by yeast cultured in the presence of galactose. Most of the Legionella proteins identified were determined to be substrates of the Dot/Icm system. This screen led to the identification of a new Dot/Icm substrate protein that was called YlfA, for yeast lethal factor A. A paralogue of YlfA was identified on an unlinked region of the Legionella chromosome and this protein was also translocated by the Dot/Icm system. It was determined that a hydrophobic region near the N-terminus of the YlfA protein and an adjacent region predicted to form a coiled-coil domain were necessary for a biological activity that interfered with yeast growth. The YlfA protein did not decorate the Legionella-containing vacuole during the first 7 h of infection but could be observed on the endoplasmic reticulum (ER)-derived replicative vacuole and on punctate structures throughout the host cell at later stages. Ectopic production of YlfA in mammalian cells revealed that the N-terminal hydrophobic domain in YlfA was able to localize the protein to early secretory organelles, including endoplasmic reticulum. These studies show that yeast genetics can be exploited to identify and characterize proteins that are injected into host cells by bacterial pathogens that utilize type IV secretion systems for pathogenesis.

Recent advances on the development of bacterial poles

In rod-shaped bacteria, a surprisingly large number of proteins are localized to the cell poles. Polar positioning of proteins is crucial to many fundamental cellular processes. Formation of the pole occurs at the time of a prior cell division event and involves coordination of the cell division machinery with septal placement of newly-synthesized peptidoglycan. Development of polar peptidoglycan and outer membrane depends on the formation of the cytokinetic FtsZ ring at midcell. By contrast, positioning of at least two polar proteins depends on signals independent of both the assembly of the FtsZ ring and the synthesis of septal and polar peptidoglycan. We propose a model for distinct but interrelated developmental pathways for polar cell envelope synthesis and positional information recognized by polar proteins.

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.

The amoebae plate test implicates a paralogue of lpxB in the interaction of Legionella pneumophila with Acanthamoeba castellanii

Legionella pneumophila is a bacterial parasite of freshwater amoebae which also grows in alveolar macrophages and thus causes the potentially fatal pneumonia Legionnaires' disease. Intracellular growth within amoebae and macrophages is mechanistically similar and requires the Icm/Dot type IV secretion system. This paper reports the development of an assay, the amoebae plate test (APT), to analyse growth of L. pneumophila wild-type and icm/dot mutant strains spotted on agar plates in the presence of Acanthamoeba castellanii. In the APT, wild-type L. pneumophila formed robust colonies even at high dilutions, icmT, -R, -P or dotB mutants failed to grow, and icmS or -G mutants were partially growth defective. The icmS or icmG mutant strains were used to screen an L. pneumophila chromosomal library for genes that suppress the growth defect in the presence of the amoebae. An icmS suppressor plasmid was isolated that harboured the icmS and flanking icm genes, indicating that this plasmid complements the intracellular growth defect of the mutant. In contrast, different icmG suppressor plasmids rendered the icmG mutant more cytotoxic for A. castellanii without enhancing intracellular multiplication in amoebae or RAW264.7 macrophages. Deletion of individual genes in the suppressor plasmids inserts identified lcs (Legionella cytotoxic suppressor) -A, -B, -C and -D as being required for enhanced cytotoxicity of an icmG mutant strain. The corresponding proteins show sequence similarity to hydrolases, NlpD-related metalloproteases, lipid A disaccharide synthases and ABC transporters, respectively. Overexpression of LcsC, a putative paralogue of the lipid A disaccharide synthase LpxB, increased cytotoxicity of an icmG mutant but not that of other icm/dot or rpoS mutant strains against A. castellanii. Based on sequence comparison and chromosomal location, lcsB and lcsC probably encode enzymes involved in cell wall maintenance and peptidoglycan metabolism. The APT established here may prove useful to identify other bacterial factors relevant for interactions with amoeba hosts.

Pathogen effector protein screening in yeast identifies Legionella factors that interfere with membrane trafficking

Legionella pneumophila invades and replicates intracellularly in human and protozoan hosts. The bacteria use the Icm/Dot type IVB secretion system to translocate effectors that inhibit phagosome maturation and modulate host vesicle trafficking pathways. To understand how L. pneumophila modulates organelle trafficking in host cells, we carried out pathogen effector protein screening in yeast, identifying L. pneumophila genes that produced membrane trafficking [vacuole protein sorting (VPS)] defects in yeast. We identified four L. pneumophila DNA fragments that perturb sorting of vacuolar proteins. Three encode ORFs of unknown function that are translocated via the Icm/Dot transporter from Legionella into macrophages. VPS inhibitor protein (Vip) A is a coiled-coil protein, VipD is a patatin domain-containing protein, and VipF contains an acetyltransferase domain. Processing studies in yeast indicate that VipA, VipD, and VipF inhibit lysosomal protein trafficking by different mechanisms; overexpressing VipA has an effect on carboxypeptidase Y trafficking, whereas VipD interferes with multivesicular body formation at the late endosome and endoplasmic reticulum-to-Golgi body transport. Such differences highlight the multiple strategies L. pneumophila effectors use to subvert host trafficking processes. Using yeast as an effector gene discovery tool allows for a powerful, genetic approach to both the identification of virulence factors and the study of their function.

IcmS-dependent translocation of SdeA into macrophages by the Legionella pneumophila type IV secretion system

Legionella pneumophila replicates inside alveolar macrophages and causes an acute, potentially fatal pneumonia called Legionnaires' disease. The ability of this bacterium to grow inside of macrophages is dependent on the presence of a functional dot/icm type IV secretion system (T4SS). Proteins secreted by the Dot/Icm T4SS are presumed to alter the host endocytic pathway, allowing L. pneumophila to establish a replicative niche within the host cell. Here we show that a member of the SidE family of proteins interacts with IcmS and is required for full virulence in the protozoan host Acanthamoeba castellanii. Using immunofluorescence microscopy and adenylate cyclase fusions, we show that SdeA is secreted into host cells by L. pneumophila in an IcmS-dependent manner. The SidE-like proteins are secreted very early during macrophage infection, suggesting that they are important in the initial formation of the replicative phagosome. Secreted SidE family members show a similar localization to other Dot/Icm substrates, specifically, to the poles of the replicative phagosome. This common localization of secreted substrates of the Dot/Icm system may indicate the formation of a multiprotein complex on the cytoplasmic face of the replicative phagosome.

A C-terminal translocation signal required for Dot/Icm-dependent delivery of the Legionella RalF protein to host cells

The Legionella pneumophila Dot/Icm system is a type IV secretion apparatus that transfers bacterial proteins into eukaryotic host cells. The RalF protein is a substrate engaged and translocated into host cells by the Dot/Icm system. In this study, the mechanism of Dot/Icm-mediated translocation of RalF has been investigated. It was determined that RalF translocation into host cells occurs before bacterial internalization. Sequences essential for RalF translocation were located at the C terminus of the RalF protein. A fusion protein consisting of a 20-aa C-terminal RalF peptide appended to the calmodulin-dependent adenylate cyclase domain of the Bordetella pertussis adenylate cyclase protein was translocated into host cells by the Dot/Icm system. A leucine (L372) residue at the -3 position in relation to the RalF C terminus was critical for translocation. Consistent with RalF L372 playing an important role in substrate recognition by the Dot/Icm system, most other Dot/Icm substrates were found to have amino acid residues with similar physical properties at their -3 or -4 C-terminal positions. These data demonstrate that the Dot/Icm system can transfer bacterial proteins that modulate host cellular functions before uptake and indicate that substrate recognition involves a C-terminal translocation signal. Thus, Legionella has the ability to engage synthesized substrate proteins and transfer them into host cells on contact, enabling Legionella to rapidly alter transport of the vacuole in which it resides.

The Legionella IcmS-IcmW protein complex is important for Dot/Icm-mediated protein translocation

The intracellular pathogen Legionella pneumophila can infect and replicate within macrophages of a human host. To establish infection, Legionella require the Dot/Icm secretion system to inject protein substrates directly into the host cell cytoplasm. The mechanism by which substrate proteins are engaged and translocated by the Dot/Icm system is not well understood. Here we show that two cytosolic components of the Dot/Icm secretion machinery, the proteins IcmS and IcmW, play an important role in substrate translocation. Biochemical analysis indicates that IcmS and IcmW form a stable protein complex. In Legionella, the IcmW protein is rapidly degraded in the absence of the IcmS protein. Substrate proteins translocated into mammalian host cells by the Dot/Icm system were identified using the IcmW protein as bait in a yeast two-hybrid screen. It was determined that the IcmS-IcmW complex interacts with these substrates and plays an important role in translocation of these proteins into mammalian cells. These data are consistent with the IcmS-IcmW complex being involved in the recognition and Dot/Icm-dependent translocation of substrate proteins during Legionella infection of host cells.

Macrophages from mice with the restrictive Lgn1 allele exhibit multifactorial resistance to Legionella pneumophila

Although Legionella pneumophila can multiply in diverse cell types from a variety of species, macrophages from most inbred mouse strains are nonpermissive for intracellular replication and allow little or no growth of the bacteria. This phenomenon is likely genetically controlled by the mouse naip5 (birc1e) gene located within the Lgn1 locus. In this study, we have investigated the resistance of C57BL/6J macrophages to L. pneumophila infection by examining the fate of both the bacterium and the infected cells compared to that in macrophages from the permissive A/J strain. Our results indicate that although the trafficking of the L. pneumophila-containing vacuole is partially disrupted in C57BL/6J macrophages, this cannot account for the severity of the defect in intracellular growth observed in this strain. Infected macrophages are lost shortly after infection, and at later times a larger fraction of the C57BL/6J macrophages in which L. pneumophila undergoes replication are apoptotic compared to those derived from A/J mice. Finally, a loss of bacterial counts occurs after the first round of growth. Therefore, the resistance mechanism of C57BL/6J macrophages to L. pneumophila infection appears to be multifactorial, and we discuss how early and late responses result in clearing the infection.

Temporal analysis of Coxiella burnetii morphological differentiation

Coxiella burnetii undergoes a poorly defined developmental cycle that generates morphologically distinct small-cell variants (SCV) and large-cell variants (LCV). We developed a model to study C. burnetii morphogenesis that uses Vero cells synchronously infected with homogeneous SCV (Nine Mile strain in phase II) harvested from aged infected cell cultures. A time course transmission electron microscopic analysis over 8 days of intracellular growth was evaluated in conjunction with one-step growth curves to correlate morphological differentiations with growth cycle phase. Lag phase occurred during the first 2 days postinfection (p.i.) and was primarily composed of SCV-to-LCV morphogenesis. LCV forms predominated over the next 4 days, during which exponential growth was observed. Calculated generation times during exponential phase were 10.2 h (by quantitative PCR assay) and 11.7 h (by replating fluorescent focus-forming unit assay). Stationary phase began at approximately 6 days p.i. and coincided with the reappearance of SCV, which increased in number at 8 days p.i. Quantitative reverse transcriptase-PCR demonstrated maximal expression of scvA, which encodes an SCV-specific protein, at 8 days p.i., while immunogold transmission electron microscopy revealed degradation of ScvA throughout lag and exponential phases, with increased expression observed at the onset of stationary phase. Collectively, these results indicate that the overall growth cycle of C. burnetii is characteristic of a closed bacterial system and that the replicative form of the organism is the LCV. The experimental model described in this report will allow a global transcriptome and proteome analysis of C. burnetii developmental forms.

Differentiate to thrive: lessons from the Legionella pneumophila life cycle

When confronted by disparate environments, microbes routinely alter their physiology to tolerate or exploit local conditions. But some circumstances require more drastic remodelling of the bacterial cell, as sporulation by the Bacillus and Streptomyces species of soil bacteria vividly illustrates. Cellular differentiation is also crucial for pathogens, the challenge for which is to colonize one host, then be transmitted to the next. Using the Gram-negative Legionella pneumophila as a model intracellular pathogen, we describe how biogenesis of the replication vacuole is determined by the developmental state of the bacterium. Subsequently, when replicating bacteria have exhausted the nutrient supply, the pathogens couple their conversion to stationary phase physiology with expression of traits that promote transmission to a new host. The cellular differentiation of L. pneumophila is co-ordinated by a regulatory circuit that integrates several elements that are broadly conserved in the microbial world. The alarmone (p)ppGpp promotes transcription directed by the alternative sigma factors RpoS, FliA and, probably, RpoN, and also post-transcriptional control mediated by a two-component regulatory system, LetA/S (GacA/S), and an mRNA-binding protein, CsrA (RsmA). By applying knowledge of microbial differentiation in combination with tools to screen the complete genomes of pathogens, experiments can be designed to identify two distinct classes of virulence traits: factors that promote replication and those dedicated to transmission.

Euroconference on the Biology of Type IV Secretion Processes: bacterial gates into the outer world

Type IV secretion systems (T4SSs) mediate both protein and ssDNA secretion from a wide range of bacteria into virtually any cell type or into the milieu. It is this versatility that confers on them the ability to participate in many processes of bacterial life that imply communication with their environment. Type IV secretion systems are involved in horizontal DNA transfer to other bacteria and to plant cells, in DNA uptake from the milieu, in toxin secretion into the milieu, and in the injection of virulence factors into the eukaryotic host cell in a number of mammalian and plant pathogens. Recently, a EuroConference addressed the different aspects of the biology of these transmembrane multiprotein complexes, from the crystal structure of the individual components to the modification that the secreted substrates induce in the recipient cell. Significant progress has been made in the understanding of the molecular architecture and mechanism of secretion. The analysis of protein-protein interactions confirms the role of coupling proteins as substrate recruiters for the transporter. The VirB10 component of the complex has come up as a strong candidate for signal transducer. The wide range of effects on the recipient suggests that many effector proteins are secreted. New effector proteins are being identified for both plant and animal pathogens, as are their targets within the host cells. New T4SS members are being identified that perform novel roles, beyond DNA transfer and virulence, such as establishment of symbiotic processes. Our current knowledge of the Biology of Type IV Secretion Processes increases our ability to exploit them as biotechnological tools or to use them as new targets for inhibitors that could constitute a new generation of antimicrobials in the near future.

Thirty-eight C-terminal amino acids of the coupling protein TraD of the F-like conjugative resistance plasmid R1 are required and sufficient to confer binding to the substrate selector protein TraM

Coupling proteins (CPs) are present in type IV secretion systems of plant, animal, and human pathogens and are essential for DNA transfer in bacterial conjugation systems. CPs connect the DNA-processing machinery to the mating pair-forming transfer apparatus. In this report we present in vitro and in vivo data that demonstrate specific binding of CP TraD of the IncFII R1 plasmid transfer system to relaxosomal protein TraM. With overlay assays and enzyme-linked immunosorbent assays we showed that a truncated version of TraD, termed TraD11 (DeltaN155), interacted strongly with TraM. The apparent TraD11-TraM association constant was determined to be 2.6 x 10(7) liters/mol. Electrophoretic mobility shift assays showed that this variant of TraD also strongly bound to TraM when it was in complex with its target DNA. When 38 amino acids were additionally removed from the C terminus of TraD, no binding to TraM was observed. TraD15, comprising the 38 amino-acid-long C terminus of TraD, bound to TraM, indicating that the main TraM interaction domain resides in these 38 amino acids of TraD. TraD15 exerted a dominant negative effect on DNA transfer but not on phage infection by pilus-specific phage R17, indicating that TraM-TraD interaction is important for conjugative DNA transfer but not for phage infection. We also observed that TraD encoded by the closely related F factor bound to TraM encoded by the R1 plasmid. Our results thus provide evidence that substrate selection within the IncF plasmid group is based on TraM's capability to select the correct DNA molecule for transport and not on substrate selection by the CP.

Branching sites and morphological abnormalities behave as ectopic poles in shape-defective Escherichia coli

Certain mutants in Escherichia coli lacking multiple penicillin-binding proteins (PBPs) produce misshapen cells containing kinks, bends and branches. These deformed regions exhibit two structural characteristics of normal cell poles: the peptidoglycan is inert to dilution by new synthesis or turnover, and a similarly stable patch of outer membrane caps the sites. To test the premise that these aberrant sites represent biochemically functional but misplaced cell poles, we assessed the intracellular distribution of proteins that localize specifically to bacterial poles. Green fluorescent protein (GFP) hybrids containing polar localization sequences from the Shigella flexneri IcsA protein or from the Vibrio cholerae EpsM protein formed foci at the poles of wild-type E. coli and at the poles and morphological abnormalities in PBP mutants. In addition, secreted wild-type IcsA localized to the outer membrane overlying these aberrant domains. We conclude that the morphologically deformed sites in these mutants represent fully functional poles or pole fragments. The results suggest that prokaryotic morphology is driven, at least in part, by the controlled placement of polar material, and that one or more of the low-molecular-weight PBPs participate in this process. Such mutants may help to unravel how particular proteins are targeted to bacterial poles, thereby creating important biochemical and functional asymmetries.

Evidence in the Legionella pneumophila genome for exploitation of host cell functions and high genome plasticity

Legionella pneumophila, the causative agent of Legionnaires' disease, replicates as an intracellular parasite of amoebae and persists in the environment as a free-living microbe. Here we have analyzed the complete genome sequences of L. pneumophila Paris (3,503,610 bp, 3,077 genes), an endemic strain that is predominant in France, and Lens (3,345,687 bp, 2,932 genes), an epidemic strain responsible for a major outbreak of disease in France. The L. pneumophila genomes show marked plasticity, with three different plasmids and with about 13% of the sequence differing between the two strains. Only strain Paris contains a type V secretion system, and its Lvh type IV secretion system is encoded by a 36-kb region that is either carried on a multicopy plasmid or integrated into the chromosome. Genetic mobility may enhance the versatility of L. pneumophila. Numerous genes encode eukaryotic-like proteins or motifs that are predicted to modulate host cell functions to the pathogen's advantage. The genome thus reflects the history and lifestyle of L. pneumophila, a human pathogen of macrophages that coevolved with fresh-water amoebae.

IcmF and DotU are required for optimal effector translocation and trafficking of the Legionella pneumophila vacuole

The gram-negative bacterium Legionella pneumophila causes a severe form of pneumonia called Legionnaires' disease, characterized by bacterial replication within alveolar macrophages. Prior to intracellular replication, the vacuole harboring the bacterium must first escape trafficking to the host lysosome, a process that is dependent on the Dot/Icm type IV secretion system. To identify genes required for intracellular growth, bacterial mutants were isolated that were delayed in escape from the macrophage but which retain a minimally functional Dot/Icm machinery. The mutations were found in eight distinct genes, including three genes known to be required for optimal intracellular growth. Two of these genes, icmF and dotU, are located at one end of a cluster of genes that encode the type IV secretion system, yet both icmF and dotU lack orthologs in other type IV translocons. DotU protein is degraded in the early postexponential phase in wild-type L. pneumophila and at all growth phases in an icmF mutant. IcmF contains an extracytoplasmic domain(s) based on accessibility to a membrane-impermeant amine-reactive reagent. In the absence of either gene, L. pneumophila targets inappropriately to LAMP-1-positive compartments during macrophage infection, is defective in the formation of replicative vacuoles, and is impaired in the translocation of the effector protein SidC. Therefore, although IcmF and DotU do not appear to be part of the core type IV secretion system, these proteins are necessary for an efficiently functioning secretion apparatus.

Integration host factor is involved in transcriptional regulation of the Brucella abortus virB operon

Type IV secretion systems (T4SSs) are multicomponent machineries that play an essential role in pathogenicity of many facultative intracellular bacteria. The virB operon of Brucella abortus codes for a T4SS essential for virulence and intracellular multiplication. Here, virB expression analyses carried out using lacZ transcriptional fusions showed that virB promoter (PvirB) is temporally activated within J774 cells. Primer extension experiments revealed that virB transcription starts at 27 bp upstream of the first gene of the virB operon. Structural analyses showed that PvirB and regulatory sequences involved in intracellular regulation span 430 bp upstream of the transcription start site. A protein able to bind PvirB was isolated and identified. This protein, homologue to integration host factor (IHF), specifically interacts with PvirB and induces a DNA bending with an angle of 50.36 degrees . DNAse I footprinting experiments showed that IHF protects a 51 bp region that contains two overlapped IHF binding consensus motifs. VirB expression experiments carried out with PvirB-lacZ fusions showed that in B. abortus IHF participates in the regulation of PvirB activity during the intracellular and vegetative growth in different media. A mutant strain with a 20 bp IHF binding site replacement failed to turn on the virB operon during the initial stages of macrophage infection and displayed severe intracellular multiplication defects. These data indicate that IHF plays a key role during intracellular virB operon expression being required for the biogenesis of the endoplasmic reticulum-derived replicative vacuole.

Shaping bacterial genomes with integrative and conjugative elements

Integrative and conjugative elements (ICEs) are self-transmissible mobile genetic elements that are increasingly recognized to contribute to lateral gene flow in prokaryotes. ICEs, like most temperate bacteriophages integrate into the genome and like conjugative plasmids disseminate by conjugative transfer to new hosts. Thought of schematically, the structure of ICEs is similar to that of other types of the mobile elements; ICEs have a backbone composed of three modules ensuring maintenance, dissemination and regulation. This backbone can acquire additional functions probably through the action of insertion sequences, transposons and specific recombinases. Previously, ICEs were thought of as only vectors for transfer of antibiotic resistance genes, but it is now evident that ICEs can mediate the transfer of a very diverse set of functions. ICEs allow bacteria to rapidly adapt to new environmental conditions and to colonize new niches. Like phages and conjugative plasmids they also likely mediate the transfer of virulence determinants. ICEs shape the bacterial genome, promoting variability between strains of the same species and distributing genes between unrelated bacterial genera. Finally, we propose that by utilizing conserved integration sites, ICEs may promote the mobilization of genomic islands.

A specific genomic location within the icm/dot pathogenesis region of different Legionella species encodes functionally similar but nonhomologous virulence proteins

Legionella pneumophila, the major causative agent of Legionnaires' disease, is a facultative intracellular pathogen that grows within human macrophages and amoebae. Intracellular growth involves the formation of a replicative phagosome that requires the Icm/Dot type IV secretion system. Part of the icm/dot region in L. pneumophila contains the icmTSRQPO genes. The proteins encoded by the icmR and icmQ genes were shown to exhibit a chaperone-substrate relationship. Analysis of this region from other pathogenic Legionella species, i.e., L. micdadei and L. longbeachae, indicated that the overall organization of this region is highly conserved, but it was found to contain a favorable site for gene variation. In the place where the icmR gene was expected to be located, other open reading frames that are nonhomologous to each other or to any entry in the GenBank database were found (migAB in L. micdadei and ligB in L. longbeachae). Examination of these unique genes revealed an outstanding phenomenon; by use of interspecies complementation, the icmR, migB, and ligB gene products were found to be functionally similar. In addition, the function of these proteins was usually dependent on the presence of the corresponding IcmQ proteins. Moreover, each of these proteins (IcmR, LigB, and MigB) was found to interact with the corresponding IcmQ proteins, and the genes encoding these proteins were found to be regulated by CpxR. This study reveals new evidence of gene variation occurring in the same genomic location within the icm/dot locus in various Legionella species. The genes found at this site were shown to be similarly regulated and to encode species-specific, nonhomologous, but functionally similar proteins.

Icm/dot-independent entry of Legionella pneumophila into amoeba and macrophage hosts

Legionella pneumophila, the causative agent of Legionnaires' disease, expresses a type IVB secretion apparatus that translocates bacterial proteins into amoeba and macrophage hosts. When stationary-phase cultures are used to infect hosts, the type IVB apparatus encoded by the icm/dot genes is required for entry, delay of phagosome-lysosome fusion, and intracellular multiplication within host cells. Null mutants with mutations in icm/dot genes are defective in these phenotypes. Here a new model is described in which hosts are infected with stationary-phase cultures that have been incubated overnight in pH 6.5 buffer. This model is called Ers treatment because it enhances the resistance to acid, hydrogen peroxide, and antibiotic stress beyond that of stationary-phase cultures. Following Ers treatment entry into amoeba and macrophage hosts does not require dotA, which is essential for Legionella virulence phenotypes when hosts are infected with stationary-phase cultures, dotB, icmF, icmV, or icmX. Defective host entry is also suppressed for null mutants with mutations in the KatA and KatB catalase-peroxidase enzymes, which are required for proper intracellular growth in amoeba and macrophage hosts. Ers treatment-induced suppression of defective entry is not associated with increased bacterial adhesion to host cells or with morphological changes in the bacterial envelope but is dependent on protein expression during Ers treatment. By using proteomic analysis, Ers treatment was shown to induce a protein predicted to contain eight tetratricopeptide repeats, a motif previously implicated in enhanced entry of L. pneumophila. Characterization of Ers treatment-dependent changes in expression is proposed as an avenue for identifying icm/dot-independent factors that function in the entry of Legionella into amoeba and macrophage hosts.

Characterization of the icmH and icmF genes required for Legionella pneumophila intracellular growth, genes that are present in many bacteria associated with eukaryotic cells

Legionella pneumophila, the causative agent of Legionnaires' disease, replicates intracellularly within a specialized phagosome of mammalian and protozoan host cells, and the Icm/Dot type IV secretion system has been shown to be essential for this process. Unlike all the other known Icm/Dot proteins, the IcmF protein, which was described before, and the IcmH protein, which is characterized here, have homologous proteins in many bacteria (such as Yersinia pestis, Salmonella enterica, Rhizobium leguminosarum, and Vibrio cholerae), all of which associate with eukaryotic cells. Here, we have characterized the L. pneumophila icmH and icmF genes and found that both genes are present in 16 different Legionella species examined. The icmH and icmF genes were found to be absolutely required for intracellular multiplication in Acanthamoeba castellanii and partially required for intracellular growth in HL-60-derived human macrophages, for immediate cytotoxicity, and for salt sensitivity. Mutagenesis of the predicted ATP/GTP binding site of IcmF revealed that the site is partially required for intracellular growth in A. castellanii. Analysis of the regulatory region of the icmH and icmF genes, which were found to be cotranscribed, revealed that it contains at least two regulatory elements. In addition, an icmH::lacZ fusion was shown to be activated during stationary phase in a LetA- and RelA-dependent manner. Our results indicate that although the icmH and icmF genes probably have a different evolutionary origin than the rest of the icm/dot genes, they are part of the icm/dot system and are required for L. pneumophila pathogenesis.

Legionella pneumophila replication vacuole formation involves rapid recruitment of proteins of the early secretory system

Legionella pneumophila vacuole biogenesis was analyzed by using a cell-free system. We show that calnexin, Sec22b, and Rab1 are recruited to the vacuole very shortly after bacterial uptake, and we have identified Rab1 as a potential host factor involved in the endoplasmic reticulum recruitment process.

The Legionella pneumophila PilT homologue DotB exhibits ATPase activity that is critical for intracellular growth

The ability of Legionella pneumophila to grow and cause disease in the host is completely dependent on a type IV secretion system known as the Dot/Icm complex. This membrane-spanning apparatus translocates effector molecules into host cells in a process that is poorly understood but that is known to require the putative ATPase DotB. One possible role for DotB is suggested by its similarity to the PilT family of proteins, which mediate pilus retraction. To better understand the molecular behavior of DotB, we have purified the protein and shown that it forms stable homohexameric rings and hydrolyzes ATP with a specific activity of 6.4 nmol of ATP/min/mg of protein. ATPase activity is critical to the function of DotB, as alteration of the conserved Walker box lysine residue resulted in a mutant protein, DotB K162Q, which failed to bind or hydrolyze ATP and which could not complement a DeltadotB strain for intracellular growth in macrophages. Consistent with the ability of DotB to interact with itself, the dotBK162Q allele exhibited transdominance over wild-type dotB, providing the first example of such a mutation in L. pneumophila. Finally, the DotB K162Q mutant protein had a significantly enhanced membrane localization in L. pneumophila compared to wild-type DotB, suggesting a relationship between nucleotide binding and membrane association. These results are consistent with a model in which DotB cycles between the cytoplasm and the Dot/Icm complex at the membrane, where it hydrolyzes nucleotides to provide energy to the complex.

The versatile bacterial type IV secretion systems

Bacteria use type IV secretion systems for two fundamental objectives related to pathogenesis--genetic exchange and the delivery of effector molecules to eukaryotic target cells. Whereas gene acquisition is an important adaptive mechanism that enables pathogens to cope with a changing environment during invasion of the host, interactions between effector and host molecules can suppress defence mechanisms, facilitate intracellular growth and even induce the synthesis of nutrients that are beneficial to bacterial colonization. Rapid progress has been made towards defining the structures and functions of type IV secretion machines, identifying the effector molecules, and elucidating the mechanisms by which the translocated effectors subvert eukaryotic cellular processes during infection.

Legionella effectors that promote nonlytic release from protozoa

Legionella pneumophila, the bacterial agent of legionnaires' disease, replicates intracellularly within a specialized vacuole of mammalian and protozoan host cells. Little is known about the specialized vacuole except that the Icm/Dot type IV secretion system is essential for its formation and maintenance. The Legionella genome database contains two open reading frames encoding polypeptides (LepA and LepB) with predicted coiled-coil regions and weak homology to SNAREs; these are delivered to host cells by an Icm/Dot-dependent mechanism. Analysis of mutant strains suggests that the Lep proteins may enable the Legionella to commandeer a protozoan exocytic pathway for dissemination of the pathogen.

IcmR-regulated Membrane Insertion and Efflux by the Legionella pneumophila IcmQ Protein

Legionella pneumophila proliferates within alveolar macrophages as a central property of Legionnaires' disease. Intracellular growth involves formation of a replicative phagosome, which requires the bacterial Dot/Icm system, a multiprotein secretion apparatus that translocates proteins from the bacterium across the macrophage plasma membrane. Two components of this system, IcmR and IcmQ, are proposed to exhibit a chaperone/substrate relationship similar to that observed in other protein translocation systems. We report here that IcmQ inserts into lipid membranes and forms pores that allow the efflux of the dye calcein but not Dextran 3000. Both membrane insertion and pore formation were inhibited by IcmR. Trypsin digestion mapping demonstrated that IcmQ is subdivided into two functional domains. The N-terminal region of IcmQ was necessary and sufficient for insertion into lipid membranes and calcein efflux. The C-terminal domain was necessary for efficient association of the protein with lipid bilayers. IcmR was found to bind to the N-terminal portion of the protein thus providing a mechanism for its ability to inhibit IcmQ pore-forming activity. Localization of IcmQ on the surface of the L. pneumophila shortly after infection as well as its pore-forming capacities suggest a role for IcmQ in forming a channel that leads translocated effectors out of the bacterium.

Multiple substrates of the Legionella pneumophila Dot/Icm system identified by interbacterial protein transfer

Legionella pneumophila is an intracellular pathogen that multiplies in a specialized vacuole within host cells. Biogenesis of this vacuole requires the Dot/Icm type IV protein translocation system. By using a Cre/loxP-based protein translocation assay, we found that proteins translocated by the Dot/Icm complex across the host phagosomal membrane can also be transferred from one bacterial cell to another. The flexibility of this system allowed the identification of several families of proteins translocated by the Dot/Icm complex. When analyzed by immunofluorescence microscopy, a protein identified by this procedure, SidC, was shown to translocate across the phagosomal membranes to the cytoplasmic face of the L. pneumophila phagosome. The identification of large numbers of these substrates, and the fact that the absence of any one substrate rarely results in strong defects in intracellular growth, indicate that there is significant functional redundancy among the Dot/Icm translocation targets.

Molecular and cell biology of Legionella pneumophila

Legionella pneumophila is a facultative intracellular pathogen that can replicate within phagocytic host cells such as protozoa and macrophages. Evasion of phagocytic killing is mediated by the type IV Dot/Icm secretion system, which exports bacterial effectors that modulate biogenesis of the phagosome to evade endocytic fusion and also to intercept vesicles derived from the endoplasmic reticulum. Bacterial replication is associated with activation of caspase-3 in infected macrophages and is culminated in apoptosis and pore formation-mediated cytolysis of the host.

Activation of caspase-3 by the Dot/Icm virulence system is essential for arrested biogenesis of the Legionella-containing phagosome

The Dot/Icm type IV secretion system of Legionella pneumophila is essential for evasion of endocytic fusion and for activation of caspase-3 during early stages of infection of macrophages, but the mechanisms of manipulating these host cell processes are not known. Here, we show that caspase-3 activation by L. pneumophila is independent of all the known apoptotic pathways that converge on the activation of caspase-3. The cytoplasmic proteins IcmS, IcmR and IcmQ, which are involved in secretion of Dot/Icm effectors, are required for caspase-3 activation. Pretreatment of U937 macrophages and human peripheral blood monocytes (hPBM) with the capase-3 inhibitor (DEVD-fmk) or the paninhibitor of caspases (Z-VAD-fmk) before infection blocks intracellular replication of L. pneumophila in a dose-dependent manner. Inhibition of caspase-3 results in co-localization of the L. pneumophila-containing phagosome (LCP) with the late endosomal/lysosomal marker Lamp-2, and the LCP contains lysosomal enzymes, similar to the dotA mutant, which is defective in caspase-3 activation. However, activation of caspase-3 before infection does not rescue the replication defect of the dotA mutant. Interestingly, inhibition of caspase-3 after a 15 or 30 min infection period by the parental strain has no detectable effect on the formation of a replicative niche. The Dot/Icm-mediated activation of caspase-3 by L. pneumophila specifically cleaves, in a dose- and time-dependent manner, the Rab5 effector Rabaptin-5, which maintains Rab5-GTP on the endosomal membrane. In addition, PI3 kinase, which is a crucial effector of Rab5 downstream of Rababptin-5, is not required for intracellular replication. Using single-cell analysis, we show that apoptosis is not evident in the infected cell until bacterial replication results in > 20 bacteria per cell. We conclude that activation of caspase-3 by the Dot/Icm virulence system of L. pneumophila is essential for halting biogenesis of the LCP through the endosomal/lysosomal pathway, and that this is associated with the cleavage of Rabpatin-5.