Interaction of hepatitis C virus polymerase with host cell proteins

Abstract

Hepatitis C virus (HCV) interacts with host cell proteins to modify cellular pathways creating a favourable environment that facilitates its replication and persistence. The purpose of the work presented in this thesis was to identify cellular proteins that can interact with NS5B, the virus's RNA-dependent RNA polymerase, that may contribute to the virus's biology. A number of cellular proteins were found to interact with NS5B using the yeast two-hybrid system. These proteins were involved in cellular pathways such as interferon signalling, lipid transport and metabolism, protein trafficking, cell proliferation and apoptosis. Of these, phospholipid scramblase 1 (PLSCR1) and zinc finger protein 143 (ZNF143) were selected for further investigation. The interactions were confirmed in vitro, and, for PLSCR1, the region that interacted with NS5B was determined to be within the amino-terminal region of the protein (61-137 a.a.). NS5B interacted with PLSCR1 and ZNF143 via a single interacting region localized in its N-terminus (1-153 a.a.).Expression of PLSCR1 or ZNF143 enhanced the ability of interferon to stimulate transcription from an interferon-stimulated response element (ISRE) reporter construct. Co-expression with NS5B was found to down-regulate this activity. Expression of a number of interferon-stimulated genes was investigated in the presence of NS5B, PLSCR1 or ZNF143 but no significant effect was observed. Overexpression of PLSCR1 had no effect on HCV sub-genomic replicon replication, while reduction of its expression by short hairpin RNA (shRNA) enhanced replication. Overexpression of ZNF143 was found to have a suppressive effect on replication but downregulating its expression did not enhance replication. In addition to using the yeast two-hybrid system to identify NS5B- interacting proteins, an in vitro pulldown assay coupled with mass spectrometry identified α- and β -tubulin associated with NS5B in vitro and in vivo. Subsequently this association was demonstrated to be an indirect interaction but the intermediatory partner was not identified. The domain that mediated the association with α- and β-tubulin was determined to be within the N-terminus of NS5B (1-153 a.a.). Nocodazole, an inhibitor of tubulin polymerization, had a marked effect on the association of α -tubulin with NS5B displacing it from the complex but had no effect on β -tubulin's association. Utilizing an HCV sub- genomic replicon, nocodazole was shown to have a significant inhibitory effect on replication. Taken together the data presented in this thesis showed that NS5B had a multitude of potential interactions with a variety of cellular proteins. The biological significance of some of these interactions on the cellular response to IFN and replicon replication was investigated. This work has generated a number of novel observations on the interaction between the virus and the cell that warrant future investigation