Molecular basis for the host range function of the poxvirus PKR inhibitor E3

Area of Science:

• Virology
• Immunology
• Molecular Biology

Background:

• The antiviral protein kinase R (PKR) is crucial for innate immunity, activated by viral double-stranded RNA (dsRNA) to inhibit protein synthesis.
• Poxviruses, like vaccinia virus (VACV), encode inhibitors such as E3 and K3 to evade PKR-mediated antiviral responses, influencing viral host range.
• The prevailing model posits that E3 inhibits PKR by sequestering dsRNA, preventing PKR activation.

Approach:

• Investigated species-specific interactions between hamster protein kinase R (PKR) variants and vaccinia virus (VACV) E3 and K3 inhibitors.
• Utilized mutational analyses of hamster PKR to identify key residues determining sensitivity to E3 and K3.
• Employed congenic cells expressing specific hamster PKR variants to assess their impact on VACV infection and recapitulate species-specific phenotypes.

Key Points:

• Syrian hamster PKR is resistant to E3 inhibition but sensitive to K3, while Armenian hamster PKR shows the opposite sensitivity.
• Sensitivity to E3 is primarily determined by the linker region between PKR's dsRNA-binding and kinase domains.
• Two amino acid residues in the kinase domain's helix αG dictate PKR sensitivity to K3 inhibition.

Conclusions:

• The species-specific interactions between PKR and VACV E3/K3 proteins are key determinants of viral host range.
• The resistance of Syrian hamster PKR to E3 challenges the model that E3 functions mainly through non-specific dsRNA sequestration.
• These findings highlight the complexity of viral evasion mechanisms and suggest alternative modes of action for dsRNA-binding PKR inhibitors.