Double-stranded RNA (dsRNA)-activated protein kinase (PKR), a major component of the cellular antiviral system, is activatedby the binding of either dsRNA or the cellular PKR activator, the PACT protein. The suppression of PKR activation is one of themain strategies that viruses employ to circumvent interferon signaling. Orf virus (ORFV), a parapoxvirus from the Poxviridaefamily, causes contagious pustular dermatitis in small ruminants. Previous studies have demonstrated that various OV20.0 isoforms,encoded by the OV20.0L gene, are able to inhibit PKR activation both by sequestering dsRNA and by physically interactingwith PKR in vitro. Thus, this gene acts as a virulence factor of ORFV when tested using a mouse infection model. In the presentstudy, the regions within OV20.0 that interact with dsRNA and with PKR have been mapped. Furthermore, this studydemonstrates for the first time that OV20.0 is also able to interact with the dsRNA binding domain of PACT and that the presenceof dsRNA strengthened the interaction of these two molecules. The presence of OV20.0 diminishes PKR phosphorylationwhen this is stimulated by PACT. Nevertheless, the association of OV20.0 with PKR, rather than with PACT, was found to beessential for reducing PACT-mediated PKR phosphorylation. These observations elucidate a new strategy whereby innate immunity
can be evaded by ORFV
