The hepatitis C virus RNA-dependent RNA polymerase membrane insertion sequence is a transmembrane segment

The hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) belongs to a class of membrane proteins termed tail-anchored proteins. Here, we show that the HCV RdRp C-terminal membrane insertion sequence traverses the phospholipid bilayer as a transmembrane segment. Moreover, the HCV RdRp was found to be retained in the endoplasmic reticulum (ER) or an ER-derived modified compartment both following transient transfection and in the context of a subgenomic replicon. An absolutely conserved GVG motif was not essential for membrane insertion but possibly provides a docking site for transmembrane protein-protein interactions. These findings have important implications for the functional architecture of the HCV replication complex.     

Insertion of green fluorescent protein into nonstructural protein 5A allows direct visualization of functional hepatitis C virus replication complexes

Hepatitis C virus (HCV) replicates its genome in a membrane-associated replication complex, composed of viral proteins, replicating RNA and altered cellular membranes. We describe here HCV replicons that allow the direct visualization of functional HCV replication complexes. Viable replicons selected from a library of Tn7-mediated random insertions in the coding sequence of nonstructural protein 5A (NS5A) allowed the identification of two sites near the NS5A C terminus that tolerated insertion of heterologous sequences. Replicons encoding green fluorescent protein (GFP) at these locations were only moderately impaired for HCV RNA replication. Expression of the NS5A-GFP fusion protein could be demonstrated by immunoblot, indicating that the GFP was retained during RNA replication and did not interfere with HCV polyprotein processing. More importantly, expression levels were robust enough to allow direct visualization of the fusion protein by fluorescence microscopy. NS5A-GFP appeared as brightly fluorescing dot-like structures in the cytoplasm. By confocal laser scanning microscopy, NS5A-GFP colocalized with other HCV nonstructural proteins and nascent viral RNA, indicating that the dot-like structures, identified as membranous webs by electron microscopy, represent functional HCV replication complexes. These findings reveal an unexpected flexibility of the C-terminal domain of NS5A and provide tools for studying the formation and turnover of HCV replication complexes in living cells.     

Expression of hepatitis C virus proteins interferes with the antiviral action of interferon independently of PKR-mediated control of protein synthesis

Hepatitis C virus (HCV) of genotype 1 is the most resistant to interferon (IFN) therapy. Here, we have analyzed the response to IFN of the human cell line UHCV-11 engineered to inducibly express the entire HCV genotype 1a polyprotein. IFN-treated, induced UHCV cells were found to better support the growth of encephalomyocarditis virus (EMCV) than IFN-treated, uninduced cells. This showed that expression of the HCV proteins allowed the development of a partial resistance to the antiviral action of IFN. The nonstructural 5A (NS5A) protein of HCV has been reported to inhibit PKR, an IFN-induced kinase involved in the antiviral action of IFN, at the level of control of protein synthesis through the phosphorylation of the initiation factor eIF2alpha (M. Gale, Jr., C. M. Blakely, B. Kwieciszewski, S. L. Tan, M. Dossett, N. M. Tang, M. J. Korth, S. J. Polyak, D. R. Gretch, and M. G. Katze, Mol. Cell. Biol. 18:5208-5218, 1998). Accordingly, cell lines inducibly expressing NS5A were found to rescue EMCV growth (S. J. Polyak, D. M. Paschal, S. McArdle, M. J. Gale, Jr., D. Moradpour, and D. R. Gretch, Hepatology 29:1262-1271, 1999). In the present study we analyzed whether the resistance of UHCV-11 cells to IFN could also be attributed to inhibition of PKR. Confocal laser scanning microscopy showed no colocalization of PKR, which is diffuse throughout the cytoplasm, and the induced HCV proteins, which localize around the nucleus within the endoplasmic reticulum. The effect of expression of HCV proteins on PKR activity was assayed in a reporter assay and by direct analysis of the in vivo phosphorylation of eIF2alpha after treatment of cells with poly(I)-poly(C). We found that neither PKR activity nor eIF2alpha phosphorylation was affected by coexpression of the HCV proteins. In conclusion, expression of HCV proteins in their biological context interferes with the development of the antiviral action of IFN. Although the possibility that some inhibition of PKR (by either NS5A or another viral protein) occurs at a very localized level cannot be excluded, the resistance to IFN, resulting from the expression of the HCV proteins, cannot be explained solely by inhibition of the negative control of translation by PKR.     

Genetically engineered phage harbouring the lethal catabolite gene activator protein gene with an inducer-independent promoter for biocontrol of Escherichia coli

Complications of chemotherapy, such as appearance of multidrug resistance, have persuaded researchers to consider phage therapy as a new method to combat bacterial infections. In vitro experiments were performed to assess the therapeutic value of genetically modified phages for controlling gastrointestinal Escherichia coli O157:H7 cells in Luria–Bertani (LB) media and contaminated cow milk. We constructed a modified nonreplicating M13-derived phage expressing a lethal catabolite gene activator protein (CAP) that is a Glu181Gln mutant of CAP. The modified phagemid was propagated in the lethal CAP-resistant strain XA3DII. Time–kill assay experiments showed a considerable reduction in the number of surviving bacteria in both LB media and contaminated cow milk. Our further study using other test strains demonstrated that the host range of lethal phage is limited to E. coli strains that produce pili. This study provides a possible strategy for the exploitation of genetically engineered nonlytic phages as bactericidal agents by minimizing the risk of release of progeny phages and endotoxins into the environment. The phage was engineered to remain lethal to its bacterial target, but incapable of replicating therein. Furthermore, the addition of an inducer to express the lethal protein is not required.