Structure and Mechanism of IFN-γ Antagonism by an Orthopoxvirus IFN-γ-Binding Protein

Ectromelia virus (ECTV) encodes an IFN-γ-binding protein $({\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}})$ that disrupts IFN-γ signaling and its ability to induce an antiviral state within cells. ${\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}}$ is an important virulence factor that is highly conserved (>90%) in all orthopoxviruses, including variola virus, the causative agent of smallpox. The 2.2-Å crystal structure of the ${\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}}/{\rm IFN}\text{-}\gamma $ complex reveals ${\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}}$ consists of an IFN-γR1 ligand-binding domain and a 57-aa helix-turn-helix (HTH) motif that is structurally related to the transcription factor TFIIA. The HTH motif forms a tetramerization domain that results in an ${\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}}/{\rm IFN}\text{-}\gamma $ complex containing four ${\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}}$ chains and two IFN-γ dimers. The structure, combined with biochemical and cell-based assays, demonstrates that ${\rm IFN}\text{-}\gamma {\rm BP}^{{\rm ECTV}}$ tetramers are required for efficient IFN-γ antagonism.