a, Domain architecture of RIG-I and top and side views of the 2CARD tetramer (chains A–D, surface representation) bound by Ub chains (cartoon representation). The second CARDs are demarcated by black lines, b, Two-dimensional (2D) representation and ‘lock-washer model of the RIG-I 2CARD tetramer, using the same colour code as for a. c, Definition of the surface types in the first and second CARDs. Six types of intermolecular interactions were observed; three in the A–B, B–C and C–D interactions (solid lines on the right), and another three in the D–A interaction (dotted lines). A single type of intramolecular interaction was observed between IIb of the first CARD and IIa of the second CARD (curved line). See Extended Data Fig. 4. d, Tetramer formation of wild-type and mutant 2CARD fused to an Alexa647-labelled SNAP tag (2CARD-S) with and without K63-Ubn (n > 8) as measured by EMSA. Mutations are on the indicated surface as defined in c.