Molecular Determinants of Species-Specific Interactions Between Protein Kinase R and Poxvirus K3 Orthologs

Protein kinase R (PKR) is an antiviral protein that is involved in molecular "arms races" with viral antagonists. As a result, some PKR inhibitors, including the vaccinia virus (VACV) protein K3 and its orthologs from other poxviruses only inhibit PKRs of selected species. We previously reported contrasting inhibition patterns of human, sheep, and cow PKRs by VACV K3 and the sheeppox virus (SPPV) K3 ortholog, SPPV 011. Here we show that the differential sensitivities of cow and sheep PKRs to VACV K3 were mediated by only two residues in PKR helix αG. In contrast, SPPV 011 sensitivities were governed by additional residues and regions. Analysis of the PKR sensitivities from 20 mammalian species to VACV K3 and SPPV 011 revealed four different sensitivity patterns: some PKRs were inhibited by only one K3 ortholog, as previously reported, whereas other PKRs were either resistant or sensitive to both inhibitors. Furthermore, we characterized a residue (K45) in VACV K3 that is involved in the species-specific inhibition of PKR. Mutating this residue increased the inhibition of sheep but not human PKR, whereas it decreased the inhibition of mouse PKR, highlighting that a single mutation in a viral protein can result in distinct species-dependent inhibition changes.