Evidence that avian reovirus σA protein is an inhibitor of the double-stranded RNA-dependent protein kinase

The results of a previous study demonstrated that avian reovirus is highly resistant to the antiviral effects of interferon and suggested that the double-stranded RNA (dsRNA)-binding sigma A protein might play an important role in that resistance. To gather more evidence on the interferon-inhibitory activity of sigma A protein, its gene was cloned into the prokaryotic maltose-binding protein (MBP) gene fusion vector pMalC and into the recombinant vaccinia virus WRS2. The two recombinant sigma A proteins displayed a dsRNA-binding affinity similar to that of sigma A protein synthesized in avian reovirus-infected cells. Interestingly, MBP- sigma A, but not MBP, was able to relieve the translation-inhibitory activity of dsRNA in reticulocyte lysates by blocking the activation of endogenous dsRNA-dependent enzymes. In addition, transient expression of sigma A protein in HeLa cells rescued gene expression of a vaccinia virus mutant lacking the E3L gene, and insertion of the sigma A-encoding gene into vaccinia virus conferred protection for the virus against interferon in chicken cells. Further studies demonstrated that expression of recombinant sigma A in mammalian cells interfered with dsRNA-dependent protein kinase (PKR) function. From these results we conclude that sigma A is capable of reversing the interferon-induced antiviral state by down-regulating PKR activity in a manner similar to other virus-encoded dsRNA-binding proteins.