Conformational changes associated with activation of RIG-I. (A) Binding of dsRNA and ATP coverts RIG-I helicase from a flexible open form (orange) to an ordered closed form (gray) and involves significant relative rotation of Hel-1, Hel-2, and Hel-2i; based on duck RIG-I PDB entries 4A2P and 4A36, respectively. (B) The ATP-binding site is formed at the interface of Hel-1 and Hel-2 by conserved helicase motifs; based on duck RIG-I PDB entry 4A36 with ATP analog ADP:AlF3. (C) Model of the auto-repressed state of duck RIG-I (PDB entry 4A2W). The flexibly linked CTD (invisible in the crystal structure) has been included to illustrate that it is free to detect and bind 5′ppp-bpRNA as the first step in activation. The CARDs, bound via CARD2 to Hel-2i, are unavailable for signaling. (D) Model of the activated state of human RIG-I (PDB entry 3TMI) with added CARDs on a 55-residue long flexible linker. In C and D, the Hel-2i domains (yellow) are superposed to show that CARD2 binding to Hel-2i is incompatible with the bound position of the dsRNA and CTD.
