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RIG-I recognizes molecular patterns in viral RNA to regulate the induction of type I interferons. The C-terminal domain (CTD) of RIG-I exhibits high affinity for 5′ triphosphate (ppp) dsRNA as well as blunt-ended dsRNA. Structures of RIG-I CTD bound to 5′-ppp dsRNA showed that RIG-I recognizes the termini of dsRNA and interacts with the ppp through electrostatic interactions. However, the structural basis for the recognition of non-phosphorylated dsRNA by RIG-I is not fully understood. Here, we show that RIG-I CTD binds blunt-ended dsRNA in a different orientation compared to 5′ ppp dsRNA and interacts with both strands of the dsRNA. Overlapping sets of residues are involved in the recognition of blunt-ended dsRNA and 5′ ppp dsRNA. Mutations at the RNA-binding surface affect RNA binding and signaling by RIG-I. These results provide the mechanistic basis for how RIG-I recognizes different RNA ligands.

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