Cloning and inducible synthesis of poliovirus nonstructural proteins

Fragmentation of the Golgi apparatus provides replication membranes for human rhinovirus 1A

All viruses with a positive-stranded RNA genome replicate their genomic RNA in association with membranes from the host cell. Here we demonstrate a novel organelle source of replication membranes for human rhinovirus 1A (HRV-1A). HRV-1A infection induces fragmentation of the Golgi apparatus, and Golgi membranes are rearranged into vesicles of approximately 250–500 nm diameter. The newly distributed Golgi membranes co-localize with viral RNA replication templates, strongly suggesting that the observed vesicles are the sites of viral RNA replication. Expression of the HRV-1A 3A protein induces alterations in the Golgi staining pattern similar to those seen during viral infection, and expressed 3A localizes to the Golgi-derived membranes. Taken together, these data show that in HRV-1A infection, the 3A protein plays a role in fragmenting the Golgi complex and generating vesicles that are used as the site of viral RNA replication.

Individual expression of poliovirus 2Apro and 3Cpro induces activation of caspase-3 and PARP cleavage in HeLa cells

The expression of individual viral genes enables the study of their effects on cellular functions. Our group previously generated stable HeLa cell lines that efficiently express poliovirus proteases 2A (clone 2A7d) and 3C (clone 3C7) under the control of tetracycline [Virology 266 (2000a) 352; J. Virol. 74 (2000b) 2383]. Upon induction of these proteases, the cells undergo drastic morphological alterations and eventually die. The present paper characterizes, in detail, the cellular and molecular events that lead to cell death in these lines. Several signs of apoptosis were observed in both 2A7d- and 3C7-induced cells, such as nuclear fragmentation, DNA breakdown (as determined by TUNEL), and phosphatidylserine translocation. Protease 2A induces the cleavage of poly-ADP-ribose-polymerase (PARP). This is blocked by the caspase-3 inhibitor DEVD in both 2A7d-On and 3C7-On cells suggesting that this enzyme might account for PARP cleavage in both cell lines. The results indicate that both poliovirus proteases induce apoptosis by mechanisms involving caspase activation, although the kinetics of apoptosis differs.

A stable HeLa cell line that inducibly expresses poliovirus 2A(pro): effects on cellular and viral gene expression

A HeLa cell clone (2A7d) that inducibly expresses the gene for poliovirus protease 2A (2A(pro)) under the control of tetracycline has been obtained. Synthesis of 2A(pro) induces severe morphological changes in 2A7d cells. One day after tetracycline removal, cells round up and a few hours later die. Poliovirus 2A(pro) cleaves both forms of initiation factor eIF4G, causing extensive inhibition of capped-mRNA translation a few hours after protease induction. Methoxysuccinyl-Ala-Ala-Pro-Val-chloromethylketone, a selective inhibitor of 2A(pro), prevents both eIF4G cleavage and inhibition of translation but not cellular death. Expression of 2A(pro) still allows both the replication of poliovirus and the translation of mRNAs containing a picornavirus leader sequence, while vaccinia virus replication is drastically inhibited. Translation of transfected capped mRNA is blocked in 2A7d-On cells, while luciferase synthesis from a mRNA bearing a picornavirus internal ribosome entry site (IRES) sequence is enhanced by the presence of 2A(pro). Moreover, synthesis of 2A(pro) in 2A7d cells complements the translational defect of a poliovirus 2A(pro)-defective variant. These results show that poliovirus 2A(pro) expression mimics some phenotypical characteristics of poliovirus-infected cells, such as cell rounding, inhibition of protein synthesis and enhancement of IRES-driven translation. This cell line constitutes a useful tool to further analyze 2A(pro) functions, to complement poliovirus 2A(pro) mutants, and to test antiviral compounds.

Poliovirus protease 3C(pro) kills cells by apoptosis

The tetracycline-based Tet-Off expression system has been used to analyze the effects of poliovirus protease 3C(pro) on human cells. Stable HeLa cell clones that express this poliovirus protease under the control of an inducible, tightly regulated promoter were obtained. Tetracycline removal induces synthesis of 3C protease, followed by drastic morphological alterations and cellular death. Degradation of cellular DNA in nucleosomes and generation of apoptotic bodies are observed from the second day after 3C(pro) induction. The cleavage of poly(ADP-ribose) polymerase, an enzyme involved in DNA repair, occurs after induction of 3C(pro), indicating caspase activation by this poliovirus protease. The 3C(pro)-induced apoptosis is blocked by the caspase inhibitor z-VAD-fmk. Our findings suggest that the protease 3C is responsible for triggering apoptosis in poliovirus-infected cells by a mechanism that involves caspase activation.

Intracellular membrane proliferation in E. coli induced by foot-and-mouth disease virus 3A gene products

During picornavirus infection replication of genomic RNA occurs in membrane-associated ribonucleoprotein complexes. These replication complexes contain different nonstructural viral proteins with mostly unknown function. To examine the function of nonstructural picornaviral proteins in more detail, cDNA of foot-and-mouth-disease virus (FMDV) strain O1 Lausanne was cloned into lambda ZAP II, and different parts of the P3-coding sequence were expressed in E. coli by the T7 polymerase system. Expression products constituted (a) fusion proteins composed of N-terminal leader peptide of bacteriophage T7 phi 10 protein fused to FMDV P3-sequences of different lengths, (b) translation products of authentic P3-region genes, and (c) carboxy-terminally truncated 3A proteins. Expression products were characterized by NaDodSO4-polyacrylamide gel electrophoresis, immunoblotting, as well as electron and immunoelectron microscopy. We show here that in the T7 polymerase system a high level of expression of 3A-containing peptides is achieved in E. coli. Remarkably, the expression of 3A-derived proteins induced a dramatic intracellular membrane proliferation in E. coli cells, similar to the vesicle induction observed in FMDV-infected cells. By immunoelectron microscopy, 3A-reactive material was found associated with these membranes. We hypothesize that the FMDV 3A protein is instrumental in eliciting intracellular membrane proliferation in infected cells as a prerequisite for viral RNA replication.

Membrane permeabilization by different regions of the human immunodeficiency virus type 1 transmembrane glycoprotein gp41

The transmembrane glycoprotein (gp41) of human immunodeficiency virus type 1 (HIV-1) has been implicated in the cytopathology observed during HIV infection. The first amino acids located at the amino terminus are involved in membrane fusion and syncytium formation, while sequences located at the carboxy terminus have been predicted to interact with membranes and modify membrane permeability. The HIV-1 gp41 gene has been cloned and expressed in Escherichia coli cells by using pET vectors to analyze changes in membrane permeability produced by this protein. This system is well suited for expressing toxic genes in an inducible manner and for analyzing the function of proteins that modify membrane permeability. gp41 enhances the permeability of the bacterial membrane to hygromycin B despite the low level of expression of this protein. To localize the regions of gp41 responsible for these effects, a number of fragments spanning different portions of gp41 were inducibly expressed in E. coli. Two regions of gp41 were shown to increase membrane permeability: one located at the carboxy terminus, where two highly amphipathic helices have been predicted, and another one corresponding to the membrane-spanning domain. Expression of the central region of gp41 comprising this domain was highly lytic for E. coli cells and increased membrane permeability to a number of compounds. These findings are discussed in the light of HIV-induced cytopathology and gp41 structure.

Mutations in the hydrophobic domain of poliovirus protein 3AB abrogate its permeabilizing activity

Poliovirus protein 3AB contains a predicted amphipathic helix that could lead to pore formation in membranes. We have introduced various mutations in the hydrophobic domain of the protein and the membrane-modifying properties of the resulting mutants have been analyzed. Expression of wild type 3AB protein in E. coli increases the influx and efflux of different molecules such as nucleosides, lactose analogues and antibiotics. Thus, 3AB expression makes E. coli cells two orders of magnitude more sensitive to hygromycin B, a non-permeant inhibitor of translation, and causes a 15-20-fold enhancement in the efflux of uridine. Changes in membrane permeability take place under conditions where no cellular lysis is detected and when other molecules such as beta-galactosidase or polyribonucleotides are kept inside the cell. These membrane modifications can be blocked to different extents by amino acid substitutions in the membrane-spanning region of the protein. These results suggest that poliovirus protein 3AB could possess an intrinsic ability to form pores in natural membranes, thus allowing the flux of small hydrophylic molecules through them.

Cloning and inducible synthesis of poliovirus non-structural proteins in Saccharomyces cerevisiae

Several coding regions of the poliovirus (PV) genome were cloned in the yeast Saccharomyces cerevisiae, and placed under the control of the inducible hybrid promoter pGAL/CYC, such that expression was triggered by incubating the cells in galactose (Gal)-containing medium. A number of PV genes encoding non-structural proteins, including 2Apro, 2B, 2C, 3A, 3AB and 3Cpro, were cloned and expressed in this eukaryotic system. The presence of these proteins after induction was detected by immunoblot analysis using specific antisera against each protein. The levels and the kinetics of protein synthesis after induction varied according to the PV protein analyzed. Thus, 2C was detected soon after Gal addition (3-5 h) and was one of the major polypeptides synthesized by yeast cells after 16 h of induction. In contrast, only low levels of synthesis were observed for 3A or 3AB, and then only after several hours of growth in Gal. The induction of the PV protease, 2Apro, was highly toxic for the cells such that growth was arrested after 5 h of induction and cell survival sharply declined.

Modification of membrane permeability by animal viruses

Animal viruses permeabilize cells at two well-defined moments during infection: (1) early, when the virus gains access to the cytoplasm, and (2) during the expression of the virus genome. The molecular mechanisms underlying both events are clearly different; early membrane permeability is induced by isolated virus particles, whereas late membrane leakiness is produced by newly synthesized virus protein(s) that possess activities resembling ionophores or membrane-active toxins. Detailed knowledge of the mechanisms, by which animal viruses permeabilize cells, adds to our understanding of the steps involved in virus replication. Studies on early membrane permeabilization give clues about the processes underlying entry of animal viruses into cells; understanding gained on the modification by viral proteins of membrane permeability during virus replication indicates that membrane leakiness is required for efficient virus release from infected cells or virus budding, in the case of enveloped viruses. In addition, the activity of these membrane-active virus proteins may be related to virus interference with host cell metabolism and with the cytopathic effect that develops after virus infection.

Hybrid proteins between Pseudomonas exotoxin A and poliovirus protease 2Apro

Two hybrid proteins between Pseudomonas aeruginosa exotoxin A (PE) and poliovirus protease 2Apro have been generated. One hybrid protein contains the poliovirus 2Apro sequence replacing the region of PE corresponding to amino acids 413-607. The other hybrid contains in addition the transforming growth factor sequence. The two hybrid proteins were efficiently synthesized in E. coli cells using the inducible pET vectors. Both hybrid toxins cleaved p220 (eIF-4 gamma) when the recombinant plasmids were transfected in COS cells infected with recombinant vaccinia virus bearing the T7 RNA polymerase gene.

Influenza virus M2 protein modifies membrane permeability in E. coli cells

The M2 protein of influenza virus is an integral membrane protein with ion channel activity. This protein has been expressed in E. coli cells in an inducible manner. Expression of the M2 protein causes rapid lysis of BL21(DE3) pLysS E. coli cells upon induction with IPTB. M2 protein increases membrane permeability to a number of hydrophylic molecules, such as ONPG, uridine or impermeant translation inhibitors. The behaviour of M2 in bacteria resembles that of other viral proteins, such as poliovirus 3A and Semliki Forest virus 6K.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.