Shope fibroma virus (SFV) N1R is a member of a family of poxvirus proteins that is associated with virulence and largely defined by the presence of a C-terminal RING finger motif and localization to virus factories within the cytoplasm of infected cells. Altered proteins, with deletions and site-specific mutations, were transiently expressed in vaccinia virus infected cells to discern regions of the protein that are required for localization. Deletion mutagenesis implicated a requirement of a small central region of the RING for localization, but the RING motif alone was not sufficient. A chimeric protein, however, in which the RING motif of the herpes simplex virus-1 ICP0 protein replaced the SFV N1R RING motif did localize to virus factories, indicating that the specificity for factory localization resided outside the RING motif of N1R. Critical evaluation of an alignment of poxviral N1R homologs identified a short, highly conserved N-terminal sequence 24-YINIT-28. When this sequence was deleted from N1R localization was abolished.
Recombinant N1R protein isolated from vaccinia virus (VV) infected cells bound to calf-thymus DNA cellulose. Elution from this matrix required 0.5–0.75M NaCl, suggesting N1R localizes to the factory through an inherent DNA binding activity. Structural prediction analysis inferred that the conserved N-terminal region required for N1Rs factory localization forms a short β strand and subsequent alignment analysis with β sheet DNA binding proteins uncovered significant homology with the ribbon-helix-helix motif family which utilize a short β sheet for specific DNA interaction. Characterization of the factory localization of five N1R mutants, each having a single potential β strand residue replaced with Ala revealed that Asn 26 was the most important residue for factory localization.
In contrast to N1R, which strongly binds DNA and rapidly sediments with the virus factories, SFV-N1RAsn26ΔAla mutant protein was found in the soluble fraction of infected cell lysates and failed to bind DNA cellulose. These results indicate that the N1R RING finger motif may not be central to DNA interactions and that N1R β strand residues particularly Asn 26 are involved in DNA binding and targeting N1R to the virus factories.
Overexpression of N1R in vaccinia virus (VV) infected cells was found to inhibit virus induced apoptosis. To clarify the role of N1R protein with respect to apoptosis and to examine whether the related ectromelia virus virulence factor p28 (EVp28) might also play a role in apoptosis protection, a p28-mutant EV virus and the VV-N1R virus were tested for their ability to interfere with apoptosis induced by different signals.
VV and EV infection were found to protect cells from Ultra Violet (UV) light, Tumor necrosis factor alpha (TNFα) and anti-Fas induced apoptosis. Expression of SFV N1R and EVp28 however, only protected infected HeLa cells from apoptosis induced by UV light, and did not protect from apoptosis induced by TNFα or anti-Fas antibody. Immunoblot analysis indicated EVp28 blocks processing of procaspase-3 suggesting EVp28 acts upstream of this protease in response to UV induced apoptotic signals. The requirement of EVp28 to promote replication and virulence in vivo may be related to apoptosis suppression because the number of progeny virus harvested from p28-mutant EV virus infected cells compared to wild type EV was similar following mock UV induced apoptosis, but significantly reduced following apoptosis induction by UV. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9400 |
| Date | 28 May 2018 |
| Creators | Brick, David Joseph |
| Contributors | Upton, Christopher |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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