An investigation into the inhibition of interferon action by fowlpox virus proteins

This work is concerned with the ability of poxviruses to overcome the antiviral state induced by interferons. A major part of the antiviral state induced by interferons is mediated by PKR, a serine/threonine protein kinase activated by dsRNA which phosphorylates eIF-2a, thereby inhibiting translation initiation. Many viruses interfere with the PKR response. Vaccinia virus (VV), like all poxviruses, replicates in the cytoplasm where it is directly exposed to the action of PKR (and 2'-5'A synthetase). In order to replicate it must overcome the action of interferons and PKR (and 2,-5,A synthetase). VV encodes proteins which bind interferons and encodes two proteins known to interfere with PKR: E3L, encodes a dsRNA-binding protein and K3L, encodes a homologue of eIF-2cx. The investigation has shown that an avian poxvirus, fowlpox virus (FPV), is resistant to more than 8 U/ml chicken IFN. FPV is capable of rescuing an IFN-sensitive virus, Semliki Forest virus (SFV) from the effects of chicken IFN. FPV is also capable of promoting the replication of SFV in the presence or absence of IFN. These results suggest that FPV does make antagonists of IFN. Biochemical assays were used to test IFN-treated avian cell extracts for the ability to phosphorylate known substrates of PKR. Major changes occur in the phosphorylation profile of CEF treated with IFN. Poly IC-binding proteins from IFN-treated CEF are capable of phosphorylation of histone proteins and phosphorylation of mammalian eIF-2oc peptides. These data suggest the presence of a protein(s) with properties similar to PKR. Attempts to clone an avian homologue of PKR from a commercial chicken cDNA library, proved unsuccessful. A FPV homologue of E3L does not lie between the homologues of E2L and E4L. Southern blot analysis of FPV DNA suggested that homologues of E3L and K3L exist in a 5.3 kbp restriction fragment. The appropriate fragment has been cloned and sequenced. The sequence shows that FPV has potential ORFs to encode homologues of VV E10R, E11L, OIL, I1L, I2L, and I3L but no ORF for E3L, K3L or 02L could be found.