The adenovirus E4-6/7 protein transactivates the E2 promoter by inducing dimerization of a heteromeric E2F complex

Self-Labeling Enzyme Tags for Analyses of Translocation of Type III Secretion System Effector Proteins

Type III secretion systems (T3SS) are molecular machines in Gram-negative pathogens that translocate effector proteins with central roles in virulence. The analyses of the translocation, subcellular localization, and mode of action of T3SS effector proteins are of central importance for the understanding of host-pathogen interaction and pathogenesis of bacterial infections. The analysis of translocation requires dedicated techniques to address the temporal and spatial dynamics of translocation. Here we describe a novel approach to deploy self-labeling enzymes (SLE) as universal tags for localization and tracking of translocated effector proteins. Effector-SLE fusion proteins allow live-cell imaging of translocation by T3SS, superresolution microscopy, and single-molecule tracking of effector motility in living host cells. We describe the application of the approach to T3SS effector proteins for invasion and intracellular lifestyle of Salmonella enterica serovar Typhimurium and to a T3SS effector of Yersinia enterocolitica The novel approach enables analyses of the role of T3SS in host-pathogen interaction at the highest temporal and spatial resolution, toward understanding the molecular mechanisms of their effector proteins.IMPORTANCE Type III secretion systems mediate translocation of effector proteins into mammalian cells. These proteins interfere with host cell functions, being main virulence factors of Gram-negative pathogens. Analyses of the process of translocation, the subcellular distribution, and the dynamics of effector proteins in host cells have been hampered by the lack of suitable tags and detection systems. Here we describe the use of self-labeling enzyme tags for generation of fusions with effector proteins that are translocated and functional in host cell manipulation. Self-labeling reactions with cell-permeable ligand dyes are possible prior to or after translocation. We applied the new approach to superresolution microscopy for effector protein translocation. For the first time, we show the dynamic properties of effector proteins in living host cells after translocation by intracellular bacteria. The new approach of self-labeling enzyme tags fusions will enable analyses of type III secretion system effector proteins with new dimensions of temporal and spatial resolution.

Transcriptomic and proteomic analyses reveal new insights into the regulation of immune pathways during adenovirus type 2 infection

Background

Human adenovirus (Ad) infection leads to the changes of host cell gene expression and biosynthetic processes. Transcriptomics in adenovirus type 2 (Ad2)-infected lung fibroblasts (IMR-90) cells has previously been studied using RNA sequencing. However, this study included only two time points (12 and 24 hpi) using constrained 76 bp long sequencing reads. Therefore, a more detailed study of transcription at different phases of infection using an up-graded sequencing technique is recalled. Furthermore, the correlation between transcription and protein expression needs to be addressed.

Results

In total, 3556 unique cellular genes were identified as differentially expressed at the transcriptional level with more than 2-fold changes in Ad2-infected cells as compared to non-infected cells by using paired-end sequencing. Based on the kinetics of the gene expression changes at different times after infection, these RNAs fell into 20 clusters. Among them, cellular genes involved in immune response were highly up-regulated in the early phase before becoming down-regulated in the late phase. Comparison of differentially expressed genes at transcriptional and posttranscriptional levels revealed low correlation. Particularly genes involved in cellular immune pathways showed a negative correlation. Here, we highlight the genes which expose inconsistent expression profiles with an emphasis on key factors in cellular immune pathways including NFκB, JAK/STAT, caspases and MAVS. Different from their transcriptional profiles with up- and down-regulation in the early and late phase, respectively, these proteins were up-regulated in the early phase and were sustained in the late phase. A surprising finding was that the target genes of the sustained activators failed to show response.

Conclusion

There were features common to genes which play important roles in cellular immune pathways. Their expression was stimulated at both RNA and protein levels during the early phase. In the late phase however, their transcription was suppressed while protein levels remained stable. These results indicate that Ad2 and the host cell use different strategies to regulate cellular immune pathways. A control mechanism at the post-translational level must thus exist which is under the control of Ad2.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1375-5) contains supplementary material, which is available to authorized users.

Activation of E2F-dependent transcription by the mouse cytomegalovirus M117 protein affects the viral host range

Cytomegaloviruses (CMVs) have a highly restricted host range as they replicate only in cells of their own or closely related species. To date, the molecular mechanisms underlying the CMV host restriction remain poorly understood. However, it has been shown that mouse cytomegalovirus (MCMV) can be adapted to human cells and that adaptation goes along with adaptive mutations in several viral genes. In this study, we identify MCMV M117 as a novel host range determinant. Mutations in this gene enable the virus to cross the species barrier and replicate in human RPE-1 cells. We show that the M117 protein is expressed with early kinetics, localizes to viral replication compartments, and contributes to the inhibition of cellular DNA synthesis. Mechanistically, M117 interacts with members of the E2F transcription factor family and induces E2F target gene expression in murine and human cells. While the N-terminal part of M117 mediates E2F interaction, the C-terminal part mediates self-interaction. Both parts are required for the activation of E2F-dependent transcription. We further show that M117 is dispensable for viral replication in cultured mouse fibroblasts and endothelial cells, but is required for colonization of mouse salivary glands in vivo. Conversely, inactivation of M117 or pharmacological inhibition of E2F facilitates MCMV replication in human RPE-1 cells, whereas replacement of M117 by adenovirus E4orf6/7, a known E2F activator, prevents it. These results indicate that E2F activation is detrimental for MCMV replication in human cells. In summary, this study identifies MCMV M117 as a novel E2F activator that functions as a host range determinant by precluding MCMV replication in human cells.

Salmonella stimulates pro-inflammatory signalling through p21-activated kinases bypassing innate immune receptors

Microbial infections are most often countered by inflammatory responses that are initiated through the recognition of conserved microbial products by innate immune receptors and result in pathogen expulsion^1-6. However, inflammation can also lead to pathology. Tissues such as the intestinal epithelium, which are exposed to microbial products, are therefore subject to stringent negative regulatory mechanisms to prevent signalling through innate immune receptors^6-11. This presents a challenge to the enteric pathogen Salmonella Typhimurium, which requires intestinal inflammation to compete against the resident microbiota and to acquire the nutrients and electron acceptors that sustain its replication^12,13. We show here that S. Typhimurium stimulates pro-inflammatory signalling by a unique mechanism initiated by effector proteins that are delivered by its type III protein secretion system. These effectors activate Cdc42 and the p21-activated kinase 1 (PAK1) leading to the recruitment of TNF receptor-associated factor 6 (TRAF6) and mitogen-activated protein kinase kinase kinase 7 (TAK1), and the stimulation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) inflammatory signalling. The removal of Cdc42, PAK1, TRAF6 or TAK1 prevented S. Typhimurium from stimulating NF-κB signalling in cultured cells. In addition, oral administration of a highly specific PAK inhibitor blocked Salmonella-induced intestinal inflammation and bacterial replication in the mouse intestine, although it resulted in a significant increase in the bacterial loads in systemic tissues. Thus, S. Typhimurium stimulates inflammatory signalling in the intestinal tract by engaging critical downstream signalling components of innate immune receptors. These findings illustrate the unique balance that emerges from host-pathogen co-evolution, in that pathogen-initiated responses that help pathogen replication are also important to prevent pathogen spread to deeper tissues.

Effects of Adenovirus Type 5 E1A Isoforms on Viral Replication in Arrested Human Cells

Human adenovirus has evolved to infect and replicate in terminally differentiated human epithelial cells, predominantly those within the airway, the gut, or the eye. To overcome the block to viral DNA replication present in these cells, the virus expresses the Early 1A proteins (E1A). These immediate early proteins drive cells into S-phase and induce expression of all other viral early genes. During infection, several E1A isoforms are expressed with proteins of 289, 243, 217, 171, and 55 residues being present for human adenovirus type 5. Here we examine the contribution that the two largest E1A isoforms make to the viral life cycle in growth-arrested normal human fibroblasts. Viruses that express E1A289R were found to replicate better than those that do not express this isoform. Importantly, induction of several viral genes was delayed in a virus expressing E1A243R, with several viral structural proteins undetectable by western blot. We also highlight the changes in E1A isoforms detected during the course of viral infection. Furthermore, we show that viral DNA replication occurs more efficiently, leading to higher number of viral genomes in cells infected with viruses that express E1A289R. Finally, induction of S-phase specific genes differs between viruses expressing different E1A isoforms, with those having E1A289R leading to, generally, earlier activation of these genes. Overall, we provide an overview of adenovirus replication using modern molecular biology approaches and further insights into the contribution that E1A isoforms make to the life cycle of human adenovirus in arrested human fibroblasts.

Antigen-specific bacterial vaccine combined with anti-PD-L1 rescues dysfunctional endogenous T cells to reject long-established cancer

Immunogenic tumors grow progressively even when heavily infiltrated by CD8(+) T cells. We investigated how to rescue CD8(+) T cell function in long-established immunogenic melanomas that contained a high percentage of endogenous PD-1(+) tumor-specific CD8(+) T cells that were dysfunctional. Treatment with αPD-L1 and αCTLA-4 blocking antibodies did not prevent tumors from progressing rapidly. We then tested exogenous tumor-specific antigen delivery into tumors using Salmonella Typhimurium A1-R to increase antigen levels and generate a proinflammatory tumor microenvironment. Antigen-producing A1-R rescued the endogenous tumor-specific CD8(+) T cell response: proliferation was induced in the lymphoid organs and effector function was recovered in the tumor. Treatment with antigen-producing A1-R led to improved mouse survival and resulted in 32% rejection of long-established immunogenic melanomas. Following treatment with antigen-producing A1-R, the majority of tumor-specific CD8(+) T cells still expressed a high level of PD-1 in the tumor. Combining antigen-producing A1-R with αPD-L1 blocking antibody enhanced the expansion of tumor-specific CD8(+) T cells and resulted in 80% tumor rejection. Collectively, these data demonstrate a powerful new therapeutic approach to rescue dysfunctional endogenous tumor-specific CD8(+) T cells and eradicate advanced immunogenic tumors.

Alveolar macrophages and neutrophils are the primary reservoirs for Legionella pneumophila and mediate cytosolic surveillance of type IV secretion

Legionella pneumophila, an intracellular pathogen responsible for the severe pneumonia Legionnaires' disease, uses its dot/icm-encoded type IV secretion system (T4SS) to translocate effector proteins that promote its survival and replication into the host cell cytosol. However, by introducing bacterial products into the host cytosol, L. pneumophila also activates cytosolic immunosurveillance pathways, thereby triggering robust proinflammatory responses that mediate the control of infection. Thus, the pulmonary cell types that L. pneumophila infects not only may act as an intracellular niche that facilitates its pathogenesis but also may contribute to the immune response against L. pneumophila. The identity of these host cells remains poorly understood. Here, we developed a strain of L. pneumophila producing a fusion protein consisting of β-lactamase fused to the T4SS-translocated effector RalF, which allowed us to track cells injected by the T4SS. Our data reveal that alveolar macrophages and neutrophils both are the primary recipients of T4SS-translocated effectors and harbor viable L. pneumophila during pulmonary infection of mice. Moreover, both alveolar macrophages and neutrophils from infected mice produced tumor necrosis factor and interleukin-1α in response to T4SS-sufficient, but not T4SS-deficient, L. pneumophila. Collectively, our data suggest that alveolar macrophages and neutrophils are both an intracellular reservoir for L. pneumophila and a source of proinflammatory cytokines that contribute to the host immune response against L. pneumophila during pulmonary infection.

Full genome sequence analysis of a novel adenovirus of rhesus macaque origin indicates a new simian adenovirus type and species

Multiple novel simian adenoviruses have been isolated over the past years and their potential to cross the species barrier and infect the human population is an ever present threat. Here we describe the isolation and full genome sequencing of a novel simian adenovirus (SAdV) isolated from the urine of two independent, never co-housed, late stage simian immunodeficiency virus (SIV)-infected rhesus macaques. The viral genome sequences revealed a novel type with a unique genome length, GC content, E3 region and DNA polymerase amino acid sequence that is sufficiently distinct from all currently known human- or simian adenovirus species to warrant classifying these isolates as a novel species of simian adenovirus. This new species, termed Simian mastadenovirus D (SAdV-D), displays the standard genome organization for the genus Mastadenovirus containing only one copy of the fiber gene which sets it apart from the old world monkey adenovirus species HAdV-G, SAdV-B and SAdV-C.

Dormant intracellular Salmonella enterica serovar Typhimurium discriminates among Salmonella pathogenicity island 2 effectors to persist inside fibroblasts

Salmonella enterica uses effector proteins delivered by type III secretion systems (TTSS) to colonize eukaryotic cells. Recent in vivo studies have shown that intracellular bacteria activate the TTSS encoded by Salmonella pathogenicity island-2 (SPI-2) to restrain growth inside phagocytes. Growth attenuation is also observed in vivo in bacteria colonizing nonphagocytic stromal cells of the intestinal lamina propria and in cultured fibroblasts. SPI-2 is required for survival of nongrowing bacteria persisting inside fibroblasts, but its induction mode and the effectors involved remain unknown. Here, we show that nongrowing dormant intracellular bacteria use the two-component system OmpR-EnvZ to induce SPI-2 expression and the PhoP-PhoQ system to regulate the time at which induction takes place, 2 h postentry. Dormant bacteria were shown to discriminate the usage of SPI-2 effectors. Among the effectors tested, SseF, SseG, and SseJ were required for survival, while others, such as SifA and SifB, were not. SifA and SifB dispensability correlated with the inability of intracellular bacteria to secrete these effectors even when overexpressed. Conversely, SseJ overproduction resulted in augmented secretion and exacerbated bacterial growth. Dormant bacteria produced other effectors, such as PipB and PipB2, that, unlike what was reported for epithelial cells, did not to traffic outside the phagosomal compartment. Therefore, permissiveness for secreting only a subset of SPI-2 effectors may be instrumental for dormancy. We propose that the S. enterica serovar Typhimurium nonproliferative intracellular lifestyle is sustained by selection of SPI-2 effectors that are produced in tightly defined amounts and delivered to phagosome-confined locations.

Type III secretion system-dependent translocation of ectopically expressed Yop effectors into macrophages by intracellular Yersinia pseudotuberculosis

Yersinia pseudotuberculosis is a Gram-negative bacterial pathogen. Virulence in Y. pseudotuberculosis requires the plasmid-encoded Ysc type III secretion system (T3SS), which functions to translocate a set of effectors called Yops into infected host cells. The effectors function to antagonize phagocytosis (e.g., YopH) or to induce apoptosis (YopJ) in macrophages infected with Y. pseudotuberculosis. Additionally, when antiphagocytosis is incomplete and Y. pseudotuberculosis is internalized by macrophages, the bacterium can survive in phagosomes. Previous studies have shown that delivery of effectors into host cells occurs efficiently when Yersinia is extracellular. However, it is not clear whether the T3SS can be utilized by intracellular Y. pseudotuberculosis to translocate Yops. This possibility was investigated here using Y. pseudotuberculosis strains that express YopJ or YopH under the control of an inducible promoter. Bone marrow-derived murine macrophages were infected with these strains under conditions that prevented the survival of extracellular bacteria. Effector translocation was detected by measuring apoptosis or the activities of Yop-β-lactamase fusion proteins. Results showed that macrophages underwent apoptosis when YopJ expression was induced prior to phagocytosis, confirming that delivery of this effector prior to or during uptake is sufficient to cause cell death. However, macrophages also underwent apoptosis when YopJ was ectopically expressed after phagocytosis; furthermore, expression of the translocator YopB from intracellular bacteria also resulted in increased cell death. Analysis by microscopy showed that translocation of ectopically expressed YopH- or YopJ-β-lactamase fusions could be correlated with the presence of viable Y. pseudotuberculosis in macrophages. Collectively, our results suggest that the Ysc T3SS of Y. pseudotuberculosis can function within macrophage phagosomes to translocate Yops into the host cytosol.

The critical protein interactions and structures that elicit growth deregulation in cancer and viral replication

One of the greatest challenges in biomedicine is to define the critical targets and network interactions that are subverted to elicit growth deregulation in human cells. Understanding and developing rational treatments for cancer requires a definition of the key molecular targets and how they interact to elicit the complex growth deregulation phenotype. Viral proteins provide discerning and powerful probes to understand both how cells work and how they can be manipulated using a minimal number of components. The small DNA viruses have evolved to target inherent weaknesses in cellular protein interaction networks to hijack the cellular DNA and protein replication machinery. In the battle to escape the inevitability of senescence and programmed cell death, cancers have converged on similar mechanisms, through the acquisition and selection of somatic mutations that drive unchecked cellular replication in tumors. Understanding the dynamic mechanisms through which a minimal number of viral proteins promote host cells to undergo unscheduled and pathological replication is a powerful strategy to identify critical targets that are also disrupted in cancer. Viruses can therefore be used as tools to probe the system-wide protein-protein interactions and structures that drive growth deregulation in human cells. Ultimately this can provide a path for developing system context-dependent therapeutics. This review will describe ongoing experimental approaches using viruses to study pathways deregulated in cancer, with a particular focus on viral cellular protein-protein interactions and structures.

Alternative endogenous protein processing via an autophagy-dependent pathway compensates for Yersinia-mediated inhibition of endosomal major histocompatibility complex class II antigen presentation

Extracellular Yersinia pseudotuberculosis employs a type III secretion system (T3SS) for translocating virulence factors (Yersinia outer proteins [Yops]) directly into the cytosol of eukaryotic cells. Recently, we used YopE as a carrier molecule for T3SS-dependent secretion and translocation of listeriolysin O (LLO) from Listeria monocytogenes. We demonstrated that translocation of chimeric YopE/LLO into the cytosol of macrophages by Yersinia results in the induction of a codominant antigen-specific CD4 and CD8 T-cell response in orally immunized mice. In this study, we addressed the requirements for processing and major histocompatibility complex (MHC) class II presentation of chimeric YopE proteins translocated into the cytosol of macrophages by the Yersinia T3SS. Our data demonstrate the ability of Yersinia to counteract exogenous MHC class II antigen presentation of secreted hybrid YopE by the action of wild-type YopE and YopH. In the absence of exogenous MHC class II antigen presentation, an alternative pathway was identified for YopE fusion proteins originating in the cytosol. This endogenous antigen-processing pathway was sensitive to inhibitors of phagolysosomal acidification and macroautophagy, but it did not require the function either of the proteasome or of transporters associated with antigen processing. Thus, by an autophagy-dependent mechanism, macrophages are able to compensate for the YopE/YopH-mediated inhibition of the endosomal MHC class II antigen presentation pathway for exogenous antigens. This is the first report demonstrating that autophagy might enable the host to mount an MHC class II-restricted CD4 T-cell response against translocated bacterial virulence factors. We provide critical new insights into the interaction between the mammalian immune system and a human pathogen.

In macrophages, caspase-1 activation by SopE and the type III secretion system-1 of S. typhimurium can proceed in the absence of flagellin

The innate immune system is of vital importance for protection against infectious pathogens. Inflammasome mediated caspase-1 activation and subsequent release of pro-inflammatory cytokines like IL-1β and IL-18 is an important arm of the innate immune system. Salmonella enterica subspecies 1 serovar Typhimurium (S. Typhimurium, SL1344) is an enteropathogenic bacterium causing diarrheal diseases. Different reports have shown that in macrophages, S. Typhimurium may activate caspase-1 by at least three different types of stimuli: flagellin, the type III secretion system 1 (T1) and the T1 effector protein SopE. However, the relative importance and interdependence of the different factors in caspase-1 activation is still a matter of debate. Here, we have analyzed their relative contributions to caspase-1 activation in LPS-pretreated RAW264.7 macrophages. Using flagellar mutants (fliGHI, flgK) and centrifugation to mediate pathogen-host cell contact, we show that flagellins account for a small part of the caspase-1 activation in RAW264.7 cells. In addition, functional flagella are of key importance for motility and host cell attachment which is a prerequisite for mediating caspase-1 activation via these three stimuli. Using site directed mutants lacking several T1 effector proteins and flagellin expression, we found that SopE elicits caspase-1 activation even when flagellins are absent. In contrast, disruption of essential genes of the T1 protein injection system (invG, sipB) completely abolished caspase-1 activation. However, a robust level of caspase-1 activation is retained by the T1 system (or unidentified T1 effectors) in the absence of flagellin and SopE. T1-mediated inflammasome activation is in line with recent work by others and suggests that the T1 system itself may represent the basic caspase-1 activating stimulus in RAW264.7 macrophages which is further enhanced independently by SopE and/or flagellin.

Development of a method for effective amplification of human adenovirus 40

Human adenovirus 40 (Ad40) is an interesting candidate for vector construction because of its tropism for the gastrointestinal tract. Although effective preparation of the vector is necessary for its in vivo application, amplification of Ad40 has been very difficult. Ad40 E1 deletion mutants were detected by PCR in the viral DNA from Ad40 Dugan amplified by Ad5 E1-expressing human embryonic kidney (293) cells and in Ad40 Dugan plaques observed with Ad5 E1-expressing human retinoblastic cells. For the purpose of generating a single wild-type Ad40 clone, the entire Ad40 DNA was cloned into a plasmid by homologous recombination. A pure Ad40 was successfully generated by plasmid transfection and subsequently amplified with Ad5 E4orf6-inducible 293 (2V6.11) cells. 2V6.11 is an apposite cell line for effective Ad40 amplification and for future vector construction because Ad40 genetic integrity was maintained with this Ad5 E1 and E4orf6 trans-complementing cell line.

The C terminus of SipC binds and bundles F-actin to promote Salmonella invasion

Salmonella enterica serovar Typhimurium invade non-phagocytic cells by injecting bacterial effector proteins to exploit the host actin cytoskeleton network. SipC is such a Salmonella effector known to nucleate actin, bundle F-actin, and translocate type III effectors. The molecular mechanism of how SipC bundles F-actin and SipC domains responsible for these activities are not well characterized. We successfully separated these activities through a series of genetic deletion/insertions in SipC. We found that the C terminus (amino acids 200-409) of SipC bundled actin filaments using in vitro biochemical assays. We further demonstrated that amino acid residues 221-260 and 381-409 of full-length SipC were indispensable for its actin binding and bundling activities. Furthermore, Salmonella mutant strains lacking the actin bundling activity were less invasive into HeLa cells. These studies indicate that the C terminus of SipC bundles F-actin to promote Salmonella invasion.

Functional analysis of the Salmonella pathogenicity island 2-mediated inhibition of antigen presentation in dendritic cells

Salmonella enterica is a facultative intracellular pathogen that is able to modify host cell functions by means of effector proteins translocated by the type III secretion system (T3SS) encoded by Salmonella Pathogenicity Island 2 (SPI2). The SPI2-T3SS is also active in Salmonella after uptake by murine bone marrow-derived dendritic cells (BM-DC). We have previously shown that intracellular Salmonella interfere with the ability of BM-DC to stimulate antigen-dependent T-cell proliferation in an SPI2-T3SS-dependent manner. We observed that Salmonella-mediated inhibition of antigen presentation could be restored by external addition of peptides on major histocompatibility complex class II (MHC-II). The processing of antigens in Salmonella-infected cells was not altered; however, the intracellular loading of peptides on MHC-II was reduced as a function of the SPI2-T3SS. We set out to identify the effector proteins of the SPI2-T3SS involved in inhibition of antigen presentation and demonstrated that effector proteins SifA, SspH2, SlrP, PipB2, and SopD2 were equally important for the interference with antigen presentation, whereas SseF and SseG contributed to a lesser extent to this phenotype. These observations indicate the presence of a host cell-specific virulence function of a novel subset of SPI2-effector proteins.

Optimization of the delivery of heterologous proteins by the Salmonella enterica serovar Typhimurium type III secretion system for vaccine development

Type III protein secretion systems, which are organelles with the capacity to deliver bacterial proteins into host cells, have been adapted to deliver heterologous antigens for vaccine development. A limitation of these antigen delivery systems is that some proteins are not amenable to secretion through this pathway. We show here that proteins from the simian and human immunodeficiency viruses that are not permissive for secretion through a Salmonella enterica serovar Typhimurium type III secretion system can be modified to travel this secretion pathway by introduction of discrete mutations. Proteins optimized for secretion were presented more efficiently via the major histocompatibility complex class I pathway and were able to induce a better immune response.

Adenovirus ubiquitin-protein ligase stimulates viral late mRNA nuclear export

Theadenovirus type 5 (Ad5) E1B-55K and E4orf6 proteins are required together to stimulate viral late nuclear mRNA export to the cytoplasm and to restrict host cell nuclear mRNA export during the late phase of infection. Previous studies have shown that these two viral proteins interact with the cellular proteins elongins B and C, cullin 5, RBX1, and additional cellular proteins to form an E3 ubiquitin-protein ligase that polyubiquitinates p53 and probably one or more subunits of the MRE11-RAD50-NBS1 (MRN) complex, directing their proteasomal degradation. The MRN complex is required for cellular DNA double-strand break repair and induction of the DNA damage response by adenovirus infection. To determine if the ability of E1B-55K and E4orf6 to stimulate viral late mRNA nuclear export requires the ubiquitin-protein ligase activity of this viral ubiquitin-protein ligase complex, we designed and expressed a dominant-negative mutant form of cullin 5 in HeLa cells before infection with wild-type Ad5 or the E1B-55K null mutant dl1520. The dominant-negative cullin 5 protein stabilized p53 and the MRN complex, indicating that it inhibited the viral ubiquitin-protein ligase but had no effect on viral early mRNA synthesis, early protein synthesis, or viral DNA replication. However, expression of the dominant-negative cullin 5 protein caused a decrease in viral late protein synthesis and viral nuclear mRNA export similar to the phenotype produced by mutations in E1B-55K. We conclude that the stimulation of adenovirus late mRNA nuclear export by E1B-55K and E4orf6 results from the ubiquitin-protein ligase activity of the adenovirus ubiquitin-protein ligase complex.

In vivo antigen delivery by a Salmonella typhimurium type III secretion system for therapeutic cancer vaccines

Bacterial vectors may offer many advantages over other antigen delivery systems for cancer vaccines. We engineered a Salmonella typhimurium vaccine strain to deliver the NY-ESO-1 tumor antigen (S. typhimurium-NY-ESO-1) through a type III protein secretion system. The S. typhimurium-NY-ESO-1 construct elicited NY-ESO-1-specific CD8+ and CD4+ T cells from peripheral blood lymphocytes of cancer patients in vitro. Oral administration of S. typhimurium-NY-ESO-1 to mice resulted in the regression of established NY-ESO-1-expressing tumors. Intratumoral inoculation of S. typhimurium-NY-ESO-1 to NY-ESO-1-negative tumors resulted in delivery of antigen in vivo and led to tumor regression in the presence of preexisting NY-ESO-1-specific CD8+ T cells. Specific T cell responses against at least 2 unrelated tumor antigens not contained in the vaccine were observed, demonstrating epitope spreading. We propose that antigen delivery through the S. typhimurium type III secretion system is a promising novel strategy for cancer vaccine development.

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.

Adenovirus E1B 55-kilodalton protein is required for both regulation of mRNA export and efficient entry into the late phase of infection in normal human fibroblasts

The human adenovirus type 5 (Ad5) E1B 55-kDa protein is required for selective nuclear export of viral late mRNAs from the nucleus and concomitant inhibition of export of cellular mRNAs in HeLa cells and some other human cell lines, but its contributions(s) to replication in normal human cells is not well understood. We have therefore examined the phenotypes exhibited by viruses carrying mutations in the E1B 55-kDa protein coding sequence in normal human fibroblast (HFFs). Ad5 replicated significantly more slowly in HFFs than it does in tumor cells, a difference that is the result of delayed entry into the late phase of infection. The A143 mutation, which specifically impaired export of viral late mRNAs from the nucleus in infected HeLa cells (R. A. Gonzalez and S. J. Flint, J. Virol. 76:4507-4519, 2002), induced a more severe defect in viral mRNA export in HFFs. This observation indicates that the E1B 55-kDa protein regulates mRNA export during the late phase of infection of normal human cells. Other mutants exhibited phenotypes not observed in HeLa cells. In HFFs infected by the null mutant Hr6, synthesis of viral late mRNAs and proteins was severely impaired. Such defects in late gene expression were the result of inefficient progression into the late phase of infection, for viral DNA synthesis was 10-fold less efficient in Hr6-infected HFFs than in cells infected by Ad5. Similar, but less severe, defects in viral DNA synthesis were induced by the insertion mutation H224, which has been reported to inhibit binding of the E1B 55-kDa protein to p53 (C. C. Kao, P. R. Yew, and A. J. Berk, Virology 179:806-814, 1990).

Development of transcriptionally regulated oncolytic adenoviruses

Changes initiated at the cellular and systemic levels as a result of viral infection or neoplastic transformation share significant overlap. Therefore, the use of replicating viruses to treat tumors has long been postulated as a promising avenue for oncolytic therapy. Over the last 10 years, transcriptionally regulated adenoviruses have become a popular platform for the development of such oncolytic viruses. Placement of heterologous promoters in front of key adenoviral transcription units to achieve tumor- or tissue-specific viral replication is well documented. Various derivatives of this general strategy have led to considerable insight into its limitations, pitfalls, and potential. Although a general process can be described by which to develop transcriptionally regulated adenoviruses, it is apparent that few set rules can yet be defined as to what constitutes a safe, stable, and therapeutically effective vector. Clinical experiences to date suggest the short-term potential for this class of therapeutics lies in combination therapy regimens. Such lessons from the clinic suggest the next generation of transcriptionally regulated oncolytic adenoviruses take advantage of the ability of the platform to carry transgenes in order to deliver a multimodal therapy from a single agent. Beyond this 'arming' of the vectors lies the detargeting, retargeting, and coating of adenoviruses to improve the delivery of the agent to the treatment site(s). As a therapeutic platform, transcriptionally regulated adenoviruses are at an early stage of development with considerable opportunities for advancement.

Adenovirus exploits the cellular aggresome response to accelerate inactivation of the MRN complex

Results reported here indicate that adenovirus 5 exploits the cellular aggresome response to accelerate inactivation of MRE11-RAD50-NBS1 (MRN) complexes that otherwise inhibit viral DNA replication and packaging. Aggresomes are cytoplasmic inclusion bodies, observed in many degenerative diseases, that are formed from aggregated proteins by dynein-dependent retrograde transport on microtubules to the microtubule organizing center. Viral E1B-55K protein forms aggresomes that sequester p53 and MRN in transformed cells and in cells transfected with an E1B-55K expression vector. During adenovirus infection, the viral protein E4orf3 associates with MRN in promyelocytic leukemia protein nuclear bodies before MRN is bound by E1B-55K. Either E4orf3 or E4orf6 is required in addition to E1B-55K for E1B-55K aggresome formation and MRE11 export to aggresomes in adenovirus-infected cells. Aggresome formation contributes to the protection of viral DNA from MRN activity by sequestering MRN in the cytoplasm and greatly accelerating its degradation by proteosomes following its ubiquitination by the E1B-55K/E4orf6/elongin BC/Cullin5/Rbx1 ubiquitin ligase. Our results show that aggresomes significantly accelerate protein degradation by the ubiquitin-proteosome system. The observation that a normal cellular protein is inactivated when sequestered into an aggresome through association with an aggresome-inducing protein has implications for the potential cytotoxicity of aggresome-like inclusion bodies in degenerative diseases.

The adenovirus E4-6/7 protein directs nuclear localization of E2F-4 via an arginine-rich motif

E2F transcription factors are key participants in the regulation of proliferation, apoptosis, and differentiation in mammalian cells. E2Fs are negatively regulated by members of the retinoblastoma protein (pRb) family. During adenovirus (Ad) infection, viral proteins that displace pRb family members from E2Fs and recruit E2F complexes to viral and cellular promoter regions are expressed. This recruitment of E2F involves the induction of stable E2F binding to inverted E2F binding sites in the Ad E2a and cellular E2F-1 promoters and induces both viral and cellular gene expression. E2F-4 has abundant E2F activity within cells, and the majority of E2F-4 in asynchronous cells is found in the cytoplasm. Upon expression of the adenovirus E4-6/7 protein, a significant portion of E2F-4 is translocated to the nucleus, and its activity constitutes the majority of Ad-induced nuclear E2F DNA binding activity. This redirection of E2F-4 from cytoplasm to the nucleus requires an N-terminal arginine-rich nuclear localization sequence within E4-6/7. The directed targeting of E4-6/7 to the nucleus is important for the function of this protein in the context of viral infection. This function of E4-6/7 has a redundant component as well as nonredundant components in cooperation with the adenovirus E1A oncoproteins to deregulate and usurp host cell E2F function.

Salmonella pathogenicity island 2-mediated overexpression of chimeric SspH2 proteins for simultaneous induction of antigen-specific CD4 and CD8 T cells

Salmonella enterica serovar Typhimurium employs two different type III secretion systems (TTSS) encoded within Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2) for targeting of effector proteins into the cytosol of eukaryotic cells during different stages of the infection cycle. The SPI1 TTSS translocates virulence factors across the plasma membrane when the bacterium initially contacts the host cell. In contrast, the SPI2 TTSS functions to translocate proteins across the membrane of the Salmonella-containing vacuole and promotes intracellular survival and replication. The aim of the present study was to directly compare the potentials of SPI1 and SPI2 type III effector proteins to act as carrier molecules for a heterologous antigen. The p60 protein of Listeria monocytogenes was used as a model antigen to construct chimeric SopE2 (SPI1), SifA (SPI2), and SspH2 (SPI2) proteins. SPI1- and SPI2-dependent up- and down-regulation of hybrid gene expression led to sequential translocation of p60 fusion proteins into the cytosol of Salmonella-infected macrophages. Mice orally immunized with recombinant Salmonella strains expressing these hybrid proteins revealed comparable numbers of p60-specific CD8 T cells. However, only overexpression of translocated SspH2/p60 from a medium-copy-number vector induced simultaneous antigen-specific CD4 and CD8 T-cell responses, suggesting that SspH2 is an attractive carrier molecule for foreign-protein delivery.

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.

Genetic analysis of the Salmonella enterica type III secretion-associated ATPase InvC defines discrete functional domains

An essential component of all type III secretion systems is a highly conserved ATPase that shares significant amino acid sequence similarity to the beta subunit of the F(0)F(1) ATPases and is thought to provide the energy for the secretion process. We have performed a genetic and functional analysis of InvC, the ATPase associated with the Salmonella enterica type III secretion system encoded within its pathogenicity island 1. Through a mutagenesis analysis, we have identified amino acid residues that are essential for specific activities of InvC, such as nucleotide hydrolysis and membrane binding. This has allowed us to define discrete domains of InvC that are specifically associated with different essential activities of this protein.

InvB is required for type III-dependent secretion of SopA in Salmonella enterica serovar Typhimurium

The Salmonella effector protein SopA is translocated into host cells via the SPI-1 type III secretion system (TTSS) and contributes to enteric disease. We found that the chaperone InvB binds to SopA and slightly stabilizes it in the bacterial cytosol and that it is required for its transport via the SPI-1 TTSS.

InvB is a type III secretion-associated chaperone for the Salmonella enterica effector protein SopE

SopE is a bacteriophage-encoded effector protein of Salmonella enterica serovar Typhimurium that is translocated into the cytosol of eukaryotic cells by a type III secretion system (TTSS) (W.-D. Hardt, H. Urlaub, and J. E. Galán, Proc. Natl. Acad. Sci. USA 95:2574-2579, 1998; M. W. Wood, R. Rosqvist, P. B. Mullan, M. H. Edwards, and E. E. Galyov, Mol. Microbiol. 22:327-338, 1996). In this study, we provide evidence that an unlinked gene carried within the Salmonella pathogenicity island 1 (SPI-1), invB (K. Eichelberg, C. Ginocchio, and J. E. Galán, J. Bacteriol. 176:4501-4510, 1994), is required for the secretion of SopE through the SPI-1 TTSS. Furthermore, far-Western blotting analysis shows that SopE directly interacts with InvB through a domain located at its amino terminus. We conclude that InvB is the TTSS-associated chaperone for SopE.

Role of the Salmonella pathogenicity island 1 (SPI-1) protein InvB in type III secretion of SopE and SopE2, two Salmonella effector proteins encoded outside of SPI-1

Salmonella enterica subspecies 1 serovar Typhimurium encodes a type III secretion system (TTSS) within Salmonella pathogenicity island 1 (SPI-1). This TTSS injects effector proteins into host cells to trigger invasion and inflammatory responses. Effector proteins are recognized by the TTSS via signals encoded in their N termini. Specific chaperones can be involved in this process. The chaperones InvB, SicA, and SicP are encoded in SPI-1 and are required for transport of SPI-1-encoded effectors. Several key effector proteins, like SopE and SopE2, are located outside of SPI-1 but are secreted in an SPI-1-dependent manner. It has not been clear how these effector proteins are recognized by the SPI-1 TTSS. Using pull-down and coimmunoprecipitation assays, we found that SopE is copurified with InvB, the known chaperone for the SPI-1-encoded effector protein Sip/SspA. We also found that InvB is required for secretion and translocation of SopE and SopE2 and for stabilization of SopE2 in the bacterial cytosol. Our data demonstrate that effector proteins encoded within and outside of SPI-1 use the same chaperone for secretion via the SPI-1 TTSS.

Strategic attack on host cell gene expression during adenovirus infection

To understand the interaction between the virus and its host, we used three sources of cDNA microarrays to examine the expression of 12,309 unique genes at 6 h postinfection of HeLa cells with high multiplicities of adenovirus type 2. Seventy-six genes with significantly changed expression ratios were identified, suggesting that adenovirus only modulates expression of a limited set of cellular genes. Quantitative real-time PCR analyses on selected genes were performed to confirm the microarray results. Significantly, a pronounced transcriptional activation by the promiscuous E1A-289R transcriptional activator was not apparent. Instead, promoter sequences in 45% of the upregulated genes harbored a potential E2F binding site, suggesting that the ability of the amino-terminal domain of E1A to regulate E2F-dependent transcription may be a major pathway for regulation of cellular gene expression. CDC25A was the only upregulated gene directly involved in cell cycle control. In contrast, several genes implicated in cell growth arrest were repressed. The transforming growth factor beta superfamily was specifically affected in the expression of both the upstream ligand and an intracellular regulator. In agreement with previous reports, adenovirus also targeted the innate immune response by downregulating several cytokines, including CLL2, CXCL1, and interleukin-6. Finally, stress response genes encoding GADD45B, ATF3, and TP53AP1 were upregulated. Importantly, we also found a novel countermeasure-activation of the apoptosis inhibitor survivin.

Attenuated Yersinia pseudotuberculosis carrier vaccine for simultaneous antigen-specific CD4 and CD8 T-cell induction

Yersinia pseudotuberculosis employs a type III secretion system for targeting of several virulence factors directly to the cytosol of eukaryotic cells. This protein translocation mechanism mediates the ability of Yersinia to resist phagocytosis and is required for sustained extracellular bacterial replication. In the present study, the Yersinia outer protein E (YopE) was used as a carrier molecule for type III-dependent secretion and translocation of listeriolysin O (LLO) from Listeria monocytogenes. In comparison to wild-type Yersinia, an attenuated Y. pseudotuberculosis yopK-null mutant strain hypertranslocates chimeric YopE/LLO into the cytosol of macrophages, resulting in enhanced major histocompatibility complex (MHC) class I-restricted antigen presentation of an LLO-derived CD8 T-cell epitope. Remarkably, T-cell activation assays also revealed a superior ability of translocated over secreted LLO to induce MHC class II-restricted antigen presentation. These in vitro observations were confirmed after immunization of mice with a single dose of the yopK-null mutant strain. Animals orally inoculated with recombinant Yersinia expressing translocated chimeric YopE/LLO revealed high numbers of gamma interferon-producing LLO-specific CD4 and CD8 T cells. For the first time, it is shown that cytosolic antigen display mediated by an extracellular bacterial carrier vaccine results in simultaneous CD4 and CD8 T-cell priming, conferring protection against an intracellular pathogen.

Concomitant cytosolic delivery of two immunodominant listerial antigens by Salmonella enterica serovar typhimurium confers superior protection against murine listeriosis

During its interaction with host cells, Salmonella enterica serovar Typhimurium employs a type III secretion system for cytosolic targeting of virulence factors. This protein translocation mechanism is a useful tool for heterologous antigen delivery by attenuated Salmonella vaccine carrier strains. In the present study, we used the Yersinia outer protein E (YopE) as a carrier molecule for Salmonella type III-dependent cytosolic delivery of the immunodominant CD8 T-cell antigens listeriolysin O (LLO) and p60 of Listeria monocytogenes. It is shown that concomitant translocation of hybrid YopE/LLO and YopE/p60 proteins by Salmonella led to antigen presentation and CD8 T-cell priming efficacies comparable to those of translocation of single listerial antigens. However, simultaneous translocation of LLO and p60 significantly surpassed single cytosolic antigen delivery in the ability to protect against Listeria. For the first time, this study demonstrates that concomitant expression of two independent antigens via the same recombinant plasmid leads to superior protection against a challenge with an intracellular bacterial pathogen. In conclusion, these findings emphasize the versatility of Salmonella type III-mediated heterologous antigen delivery for the induction of cytotoxic T-lymphocyte-mediated immunity.

SpiC is required for translocation of Salmonella pathogenicity island 2 effectors and secretion of translocon proteins SseB and SseC

The Salmonella pathogenicity island 2 (SPI2) type III secretion system (TTSS) promotes Salmonella enterica serovar Typhimurium virulence for mice and increased survival and replication within eukaryotic cells. After phagocytosis, Salmonella serovar Typhimurium assembles the SPI2 TTSS to translocate over a dozen effector proteins across the phagosome membrane. SpiC has been previously shown to be a translocated effector with a large contribution to virulence (K. Uchiya, M. A. Barbieri, K. Funato, A. H. Shah, P. D. Stahl, and E. A. Groisman, EMBO J. 18:3924-3933, 1999). This report demonstrates by competitive index that the virulence phenotype of a spiC mutant is equivalent to that of a secretion component mutant. In addition, translocation of SPI2 effector proteins was shown to require SpiC. Thus, the severe virulence phenotype resulting from deletion of spiC is likely due to the inability to translocate all SPI2 effectors. SpiC was also required to secrete translocon proteins SseB and SseC but not translocated effector SseJ, indicating that lack of assembly of the translocon explains the spiC mutant phenotype.

Salmonella type III secretion-associated protein InvE controls translocation of effector proteins into host cells

Salmonella enterica encodes a type III secretion system (TTSS) within a pathogenicity island located at centisome 63 (SPI-1), which is essential for its pathogenicity. This system mediates the transfer of a battery of bacterial proteins into the host cell with the capacity to modulate cellular functions. The transfer process is dependent on the function of protein translocases SipB, SipC, and SipD. We report here that Salmonella protein InvE, which is also encoded within SPI-1, is essential for the translocation of bacterial proteins into host cells. An S. enterica serovar Typhimurium mutant carrying a loss-of-function mutation in invE shows reduced secretion of SipB, SipC, and SipD while exhibiting increased secretion of other TTSS effector proteins. We also demonstrate that InvE interacts with a protein complex formed by SipB, SipC, and their cognate chaperone, SicA. We propose that InvE controls protein translocation by regulating the function of the Sip protein translocases.

Analysis of the adenovirus E1B-55K-anchored proteome reveals its link to ubiquitination machinery

During the early phase of infection, the E1B-55K protein of adenovirus type 5 (Ad5) counters the E1A-induced stabilization of p53, whereas in the late phase, E1B-55K modulates the preferential nucleocytoplasmic transport and translation of the late viral mRNAs. The mechanism(s) by which E1B-55K performs these functions has not yet been clearly elucidated. In this study, we have taken a proteomics-based approach to identify and characterize novel E1B-55K-associated proteins. A multiprotein E1B-55K-containing complex was immunopurified from Ad5-infected HeLa cells and found to contain E4-orf6, as well as several cellular factors previously implicated in the ubiquitin-proteasome-mediated destruction of proteins, including Cullin-5, Rbx1/ROC1/Hrt1, and Elongins B and C. We further demonstrate that a complex containing these as well as other proteins is capable of directing the polyubiquitination of p53 in vitro. These ubiquitin ligase components were found in a high-molecular-mass complex of 800 to 900 kDa. We propose that these newly identified binding partners (Cullin-5, Elongins B and C, and Rbx1) complex with E1B-55K and E4-orf6 during Ad infection to form part of an E3 ubiquitin ligase that targets specific protein substrates for degradation. We further suggest that E1B-55K functions as the principal substrate recognition component of this SCF-type ubiquitin ligase, whereas E4-orf6 may serve to nucleate the assembly of the complex. Lastly, we describe the identification and characterization of two novel E1B-55K interacting factors, importin-alpha 1 and pp32, that may also participate in the functions previously ascribed to E1B-55K and E4-orf6.

Effects of mutations in the adenoviral E1B 55-kilodalton protein coding sequence on viral late mRNA metabolism

The human subgroup C adenoviral E1B 55-kDa protein cooperates with the viral E4 Orf6 protein to induce selective export of viral, late mRNAs from the nucleus to the cytoplasm. Previous studies have suggested that such preferential transport of viral mRNA and the concomitant inhibition of export of cellular mRNAs are the result of viral colonization of specialized microenvironments within the nucleus. However, neither the molecular basis of this phenomenon nor the mechanism by which the E1B 55-kDa protein acts has been elucidated. We therefore examined viral late mRNA metabolism in HeLa cells infected with a series of mutant viruses that carry insertions at various positions in the E1B protein coding sequence (P. R. Yew, C. C. Kao, and A. J. Berk, Virology 179:795-805, 1990). All the mutations examined impaired cytoplasmic accumulation of viral L2 mRNAs and reduced L2 mRNA export efficiency. However, in most cases these defects could be ascribed to reduced E1B 55-kDa protein concentration or the unexpected failure of the altered E1B proteins to enter the nucleus efficiently. The latter property, the pleiotropic defects associated with all the mutations that impaired nuclear entry of the E1B protein, and consideration of its primary sequence suggest that these insertions result in misfolding of the protein. Insertion of four amino acids at residue 143 also inhibited viral mRNA export but resulted in increased rather than decreased accumulation of the E1B 55-kDa protein in the nucleus. This mutation specifically impaired the previously described association of the E1B protein with intranuclear structures that correspond to sites of adenoviral DNA replication and transcription (D. Ornelles and T. Shenk, J. Virol. 65:424-439, 1991) and the colocalization of the E1B and E4 Orf6 proteins. As this insertion has been shown to inhibit the interaction of the E1B with the E4 Orf6 protein in infected cell extracts (S. Rubenwolf, H. Schütt, M. Nevels, H. Wolf, and T. Dobner, J. Virol. 71:1115-1123, 1997), these phenotypes provide direct support for the hypothesis that selective viral mRNA export is determined by the functional organization of the infected cell nucleus.

Type III secretion of Salmonella enterica serovar Typhimurium translocated effectors and SseFG

The type III secretion system (TTSS) encoded by Salmonella enterica serovar Typhimurium pathogenicity island 2 (SPI2) is employed by Salmonella enterica for interaction with host cells during the intracellular phase of pathogenesis. This TTSS secretes a set of SPI2-encoded proteins in vitro and translocates Salmonella serovar Typhimurium translocated effectors (STE) that are encoded by genes outside of SPI2 into host cells. Using an epitope-tagging approach, we analyzed secretion of proteins by the TTSS of SPI2 and identified SseF and SseG as further secreted substrate proteins. Three members of the STE family, SifA, SifB, and SseJ, were secreted under conditions that also induce secretion of SPI2-encoded substrate proteins.

Adenovirus early E4 genes in viral oncogenesis

Previous investigations into potential transforming activities of adenovirus (Ad) early genes were largely overshadowed by the more obvious roles of E1A and E1B products. One exception was an Ad9 E4 protein (ORF1) shown to enhance transformation of cultured cells and promote mammary tumors in female rats. Recently, significant advances in understanding Ad E4 gene products at the molecular level have revealed that these proteins possess an unexpectedly diverse collection of functions, which not only orchestrate many viral processes, but overlap with oncogenic transformation of primary mammalian cells. Operating through a complex network of protein interactions with key viral and cellular regulatory components, Ad E4 products are apparently involved in transcription, apoptosis, cell cycle control, DNA repair, cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as PML oncogenic domains (PODs). Some of these functions directly relate to known transforming and oncogenic processes, or implicate mechanisms such as modulating the function and subcellular localization of cellular PDZ domain-containing proteins, POD reorganization, targeted proteolytic degradation, inhibition of DNA double-strand break repair and 'hit-and-run' mutagenesis. Here, we summarize the recent data and discuss how E4 gene product interactions may contribute to viral oncogenesis.

CdtA, CdtB, and CdtC form a tripartite complex that is required for cytolethal distending toxin activity

Campylobacter jejuni encodes a cytolethal distending toxin (CDT) that causes cells to arrest in the G(2)/M transition phase of the cell cycle. Highly related toxins are also produced by other important bacterial pathogens. CDT activity requires the function of three genes: cdtA, cdtB, and cdtC. Recent studies have established that CdtB is the active subunit of CDT, exerting its effect as a nuclease that damages the DNA and triggers cell cycle arrest. Microinjection of CdtB into target cells led to G(2)/M arrest and cytoplasmic distention, in a manner indistinguishable from that caused by CDT treatment. Despite this progress, nothing is known about the composition of the CDT holotoxin or the function of CdtA and CdtC. We show here that, when applied individually, purified CdtA, CdtB, or CdtC does not exhibit toxic activity. In contrast, CdtA, CdtB, and CdtC when combined, interact with one another to form an active tripartite holotoxin that exhibits full cellular toxicity. CdtA has a domain that shares similarity with the B chain of ricin-related toxins. We therefore proposed that CDT is a tripartite toxin composed of CdtB as the enzymatically active subunit and of CdtA and CdtC as the heterodimeric B subunit required for the delivery of CdtB.

The p107 tumor suppressor induces stable E2F DNA binding to repress target promoters

E2F transcription factors are key players in the regulation of proliferation, apoptosis, and differentiation in mammalian cells. E2Fs are negatively regulated by members of the retinoblastoma protein family, Rb, p107 and p130. During adenovirus infection, viral proteins are expressed that displace Rb family members from E2Fs and recruit E2F complexes to viral and cellular promoter regions. This recruitment of E2F involves the induction of stable E2F binding to inverted E2F binding sites in the Ad E2a and cellular E2F-1 promoters and induces both viral and cellular gene expression. The cellular p107 tumor suppressor also displays such regulation of E2F DNA binding activity. p107 induces stable E2F-4/DP binding to inverted E2F binding sites in the Ad E2a and cellular E2F-1 promoters. The induction of E2F DNA binding by p107 minimally requires the sequences in p107 that mediate E2F interaction. The related tumor suppressor, p130, also effects this function. p107 levels increase substantially as cells progress through S phase. p107 induction of E2F DNA binding was observed primarily in S phase cells coincident with the increase in p107 protein levels. The results of promoter activity assays directly correlate the induction of E2F DNA binding by p107 with effective transcriptional repression. These results support a model in which p107 and p130 induce the stable binding of E2F complexes to promoters that drive expression of critical regulatory proteins such as E2F-1. Since p107 and p130 bind histone deacetylase complexes (HDACs) which repress promoter activity, p107-E2F and p130-E2F would stably recruit repressor complexes to effect efficient promoter repression.

A functional complex of adenovirus proteins E1B-55kDa and E4orf6 is necessary to modulate the expression level of p53 but not its transcriptional activity

In adenovirus-infected cells, binding of E1B-55kDa and E4orf6 to the tumor suppressor protein p53 inhibits its transcriptional activity and causes rapid turnover of the protein. To investigate the requirements of the E1B-E4orf6 complex to modulate p53 function, we generated an E4orf6 mutant that failed to associate functionally and physically with E1B-55kDa but still interacted with p53. We confirm that E4orf6 and E1B-55kDa reduce p53 transactivation individually and show that their combined inhibition is additive rather than synergistic. Furthermore, we found that downregulation of p53's expression level, but not transcriptional inhibition of p53, depends on a functional E1B-E4 complex. A functional interaction of E1B-55kDa with p53, on the other hand, is a prerequisite for both transcriptional repression and downregulation of p53. The separation of these two functions will enable further dissection of the requirements for oncogenicity by the E4orf6 protein.

Identification and subcellular localization of the Legionella pneumophila IcmX protein: a factor essential for establishment of a replicative organelle in eukaryotic host cells

The gram-negative respiratory pathogen Legionella pneumophila infects and grows within mammalian macrophages and protozoan host cells. Upon uptake into macrophages, L. pneumophila establishes a replicative organelle that avoids fusion with endocytic vesicles. There are 24 dot/icm genes on the L. pneumophila chromosome required for biogenesis of this vacuole. Many of the Dot/Icm proteins are predicted to be components of a membrane-bound secretion apparatus similar to type IV conjugal transfer systems. We have been investigating the function of L. pneumophila dot/icm gene products that do not have obvious orthologs in other type IV transfer systems, since these determinants could govern processes unique to phagosome biogenesis. The icmX gene product falls into this category. To understand the role of the IcmX protein in pathogenesis, we have detailed interactions between an L. pneumophila icmX deletion mutant and murine bone marrow-derived macrophages. These data demonstrate that icmX is required for biogenesis of the L. pneumophila replicative organelle. Immunoblot analysis indicates that the icmX gene product is a polypeptide with an estimated molecular mass of 50 kDa. The IcmX protein was localized to the bacterial periplasm, and periplasmic translocation was mediated by an N-terminal sec-dependent leader peptide. A truncated IcmX product was secreted into culture supernatants by wild-type L. pneumophila growing extracellularly in liquid media; however, transport of the IcmX protein into eukaryotic host cells was not detected. Proteins similar in molecular weight to IcmX were identified in other Legionella species by immunoblot analysis using a monoclonal antibody specific for L. pneumophila IcmX protein. From these data, we conclude that the IcmX protein is an essential component of the dot/icm secretion apparatus, and that a conserved mechanism of host cell parasitism exists for members of the Legionellaceae family.

Adenovirus late gene expression does not require a Rev-like nuclear RNA export pathway

Adenovirus late mRNA export is facilitated by viral early proteins of 55 and 34 kDa. The 34-kDa protein contains a leucine-rich nuclear export signal (NES) similar to that of the human immunodeficiency virus Rev protein. It was proposed that the 34-kDa protein might facilitate the export of adenovirus late mRNA through a Rev-like NES-mediated export pathway. We have tested the role of NES-mediated RNA export during adenovirus infection, and we find that it is not essential for the expression of adenovirus late genes.

The E4-6/7 protein functionally compensates for the loss of E1A expression in adenovirus infection

The E1A gene products are required and sufficient for activation of adenovirus gene expression in cultured cells. The E4-6/7 gene product induces the binding of the cellular transcription factor E2F to the viral E2a promoter region. The induction of E2F binding to the E2a promoter in vitro is directly correlated with transcriptional activation of the E2a promoter in vivo. The E2 region encodes the viral replication proteins, yet adenoviruses lacking E4-6/7 function demonstrate no defective phenotype in infected cells. Here we show that the E4-6/7 protein can functionally compensate for E1A expression in virus infection. In the absence of the E1A gene products, expression of the E4-6/7 protein is sufficient to displace retinoblastoma protein family members from E2Fs, activate expression of early region 2 via induction of E2F DNA binding to the E2a promoter region, and significantly enhance replication of an E1A-defective adenovirus. These results have implications in the regulation of viral gene expression and for the development of recombinant adenovirus vectors.

Two distinct activities contribute to the oncogenic potential of the adenovirus type 5 E4orf6 protein

Previous studies have shown that the adenovirus type 5 (Ad5) E4orf6 gene product displays features of a viral oncoprotein. It initiates focal transformation of primary rat cells in cooperation with Ad5 E1 genes and confers multiple additional transformed properties on E1-expressing cells, including profound morphological alterations and dramatically accelerated tumor growth in nude mice. It has been reported that E4orf6 binds to p53 and, in the presence of the Ad5 E1B-55kDa protein, antagonizes p53 stability by targeting the tumor suppressor protein for active degradation. In the present study, we performed a comprehensive mutant analysis to assign transforming functions of E4orf6 to distinct regions within the viral polypeptide and to analyze a possible correlation between E4orf6-dependent p53 degradation and oncogenesis. Our results show that p53 destabilization maps to multiple regions within both amino- and carboxy-terminal parts of the viral protein and widely cosegregates with E4orf6-dependent acceleration of tumor growth, indicating that both effects are related. In contrast, promotion of focus formation and morphological transformation require only a carboxy-terminal segment of the E4 protein. Thus, these effects are completely independent of p53 stability, but may involve other interactions with the tumor suppressor. Our results demonstrate that at least two distinct activities contribute to the oncogenic potential of Ad5 E4orf6. Although genetically separable, both activities are largely mediated through a novel highly conserved, cysteine-rich motif and a recently described arginine-faced amphipathic alpha helix, which resides within a carboxy-terminal "oncodomain" of the viral protein.

Functional isoforms of IkappaB kinase alpha (IKKalpha) lacking leucine zipper and helix-loop-helix domains reveal that IKKalpha and IKKbeta have different activation requirements

The activity of the NF-kappaB family of transcription factors is regulated principally by phosphorylation and subsequent degradation of their inhibitory IkappaB subunits. Site-specific serine phosphorylation of IkappaBs by two IkappaB kinases (IKKalpha [also known as CHUK] and IKKbeta) targets them for proteolysis. IKKalpha and -beta have a unique structure, with an amino-terminal serine-threonine kinase catalytic domain and carboxy-proximal helix-loop-helix (HLH) and leucine zipper-like (LZip) amphipathic alpha-helical domains. Here, we describe the properties of two novel cellular isoforms of IKKalpha: IKKalpha-DeltaH and IKKalpha-DeltaLH. IKKalpha-DeltaH and IKKalpha-DeltaLH are differentially spliced isoforms of the IKKalpha mRNA lacking its HLH domain and both its LZip and HLH domains, respectively. IKKalpha is the major RNA species in most murine cells and tissues, except for activated T lymphocytes and the brain, where the alternatively spliced isoforms predominate. Remarkably, IKKalpha-DeltaH and IKKalpha-DeltaLH, like IKKalpha, respond to tumor necrosis factor alpha stimulation to potentiate NF-kappaB activation in HEK293 cells. A mutant, catalytically inactive form of IKKalpha blocked IKKalpha-, IKKalpha-DeltaH-, and IKKalpha-DeltaLH-mediated NF-kappaB activation. Akin to IKKalpha, its carboxy-terminally truncated isoforms associated with the upstream activator NIK (NF-kappaB-inducing kinase). In contrast to IKKalpha, IKKalpha-DeltaLH failed to associate with either itself, IKKalpha, IKKbeta, or NEMO-IKKgamma-IKKAP1, while IKKalpha-DeltaH complexed with IKKbeta and IKKalpha but not with NEMO. Interestingly, each IKKalpha isoform rescued HEK293 cells from the inhibitory effects of a dominant-negative NEMO mutant, while IKKalpha could not. IKKalpha-DeltaCm, a recombinant mutant of IKKalpha structurally akin to IKKalpha-DeltaLH, was equally functional in these assays, but in sharp contrast, IKKbeta-DeltaCm, a structurally analogous mutant of IKKbeta, was inactive. Our results demonstrate that the functional roles of seemingly analogous domains in IKKalpha and IKKbeta need not be equivalent and can also exhibit different contextual dependencies. The existence of cytokine-inducible IKKalpha-DeltaH and IKKalpha-DeltaLH isoforms illustrates potential modes of NF-kappaB activation, which are not subject to the same in vivo regulatory constraints as either IKKalpha or IKKbeta.

Complex function for SicA, a Salmonella enterica serovar typhimurium type III secretion-associated chaperone

Salmonella enterica encodes a type III secretion system within a pathogenicity island located at centisome 63 that is essential for virulence. All type III secretion systems require the function of a family of low-molecular-weight proteins that aid the secretion process by acting as partitioning factors and/or secretion pilots. One such protein is SicA, which is encoded immediately upstream of the type III secreted proteins SipB and SipC. We found that the absence of SicA results in the degradation of both SipB and SipC. Interestingly, in the absence of SipC, SipB was not only stable but also secreted at wild-type levels in a sicA mutant background, indicating that SicA is not required for SipB secretion. We also found that SicA is capable of binding both SipB and SipC. These results are consistent with a SicA role as a partitioning factor for SipB and SipC, thereby preventing their premature association and degradation. We also found that introduction of a sicA null mutation results in the lack of expression of SopE, another type III-secreted protein. Such an effect was shown to be transcriptional. Introduction of a loss-of-function sipC mutation into the sicA mutant background rescued sopE expression. These results indicate that the effect of sicA on sopE expression is indirect and most likely exerted through a regulatory factor(s) partitioned by SicA from SipC. These studies therefore describe a surprisingly complex function for the Salmonella enterica type III secretion-associated chaperone SicA.

Induction of the cellular E2F-1 promoter by the adenovirus E4-6/7 protein

The adenovirus type 5 (Ad5) E4-6/7 protein interacts directly with different members of the E2F family and mediates the cooperative and stable binding of E2F to a unique pair of binding sites in the Ad5 E2a promoter region. This induction of E2F DNA binding activity strongly correlates with increased E2a transcription when analyzed using virus infection and transient expression assays. Here we show that while different adenovirus isolates express an E4-6/7 protein that is capable of induction of E2F dimerization and stable DNA binding to the Ad5 E2a promoter region, not all of these viruses carry the inverted E2F binding site targets in their E2a promoter regions. The Ad12 and Ad40 E2a promoter regions bind E2F via a single binding site. However, these promoters bind adenovirus-induced (dimerized) E2F very weakly. The Ad3 E2a promoter region binds E2F very poorly, even via a single binding site. A possible explanation of these results is that the Ad E4-6/7 protein evolved to induce cellular gene expression. Consistent with this notion, we show that infection with different adenovirus isolates induces the binding of E2F to an inverted configuration of binding sites present in the cellular E2F-1 promoter. Transient expression of the E4-6/7 protein alone in uninfected cells is sufficient to induce transactivation of the E2F-1 promoter linked to chloramphenicol acetyltransferase or green fluorescent protein reporter genes. Further, expression of the E4-6/7 protein in the context of adenovirus infection induces E2F-1 protein accumulation. Thus, the induction of E2F binding to the E2F-1 promoter by the E4-6/7 protein observed in vitro correlates with transactivation of E2F-1 promoter activity in vivo. These results suggest that adenovirus has evolved two distinct mechanisms to induce the expression of the E2F-1 gene. The E1A proteins displace repressors of E2F activity (the Rb family members) and thus relieve E2F-1 promoter repression; the E4-6/7 protein complements this function by stably recruiting active E2F to the E2F-1 promoter to transactivate expression.

Overexpression of cyclin A inhibits augmentation of recombinant adeno-associated virus transduction by the adenovirus E4orf6 protein

The 34-kDa product of adenovirus E4 region open reading frame 6 (E4orf6) dramatically enhances transduction by recombinant adeno-associated virus vectors (rAAV). This is achieved by promoting the conversion of incoming single-stranded viral genomes into transcriptionally competent duplex molecules. The molecular mechanism for enhancing second-strand synthesis is not fully understood. In this study, we analyzed the cellular consequences of E4orf6 expression and the requirements for efficient rAAV transduction mediated by E4orf6. Expression of E4orf6 in 293 cells led to an inhibition of cell cycle progression and an accumulation of cells in S phase. This was preceded by specific degradation of cyclin A and p53, while the levels of other proteins involved in cell cycle control remained unchanged. In addition, the kinase activity of cdc2 was inhibited. We further showed that p53 expression is not necessary or inhibitory for augmentation of rAAV transduction by E4orf6. However, overexpression of cyclin A inhibited E4orf6-mediated enhancement of rAAV transduction. A cyclin A mutant incapable of recruiting protein substrates for cdk2 was unable to inhibit E4orf6-mediated augmentation. In addition, we created an E4orf6 mutant that is selectively defective in rAAV augmentation of transduction. Based on these findings, we suggest that cyclin A degradation represents a viral mechanism to disrupt cell cycle progression, resulting in enhanced viral transduction. Understanding the cellular pathways used during transduction will increase the utility of rAAV vectors in a wide range of gene therapy applications.