Summary The influenza non-structural protein 1 (NS1) plays a critical role in antagonizing the innate immune response to infection. One interaction that facilitates this function is between NS1 and RIG-I, one of the main sensors of influenza virus infection. While NS1 and RIG-I are known to interact, it is currently unclear whether this interaction is direct or if it is mediated by other biomolecules. Here we demonstrate a direct, strain-dependent interaction between the NS1 RNA binding domain (NS1RBD) of the influenza A/Brevig Mission/1918 H1N1 (1918H1N1) virus and the second caspase activation and recruitment domain of RIG-I. Solving the solution structure of the 1918H1N1 NS1RBD revealed features in a functionally novel region that may facilitate the observed interaction. The biophysical and structural data herein suggest a possible mechanism by which strain-specific differences in NS1 modulate influenza virulence.
