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. Author manuscript; available in PMC: 2017 Apr 1.
Published in final edited form as: Curr Opin Struct Biol. 2016 Feb 7;37:134–144. doi: 10.1016/j.sbi.2016.01.011

Fig. 1. dsRNA-dependent oligomerization of RIG-I and MDA5.

Fig. 1

(A) Domain organization of RIG-I, MDA5 and MAVS. CTD refers to the C-terminal domain. TBS and TM refer to the TRAF-binding site and transmembrane domain, respectively.

(B) The electron micrographs of MDA5 filament and RIG-I filament formed on 112 bp dsRNA. The images were adapted from 26 and 32.

(C) A schematic of the filament assembly process of MDA5. The MDA5 filament nucleates on the dsRNA interior and unidirectionally propagates to the dsRNA end. Filament formation brings 2CARDs of nearby MDA5 molecules into close proximity and promotes 2CARD oligomerization, which is a pre-requisite for activation of MAVS.

(D) Schematics of the filament- and ubiquitin-dependent oligomerization of RIG-I 2CARD. (I) On > 40 bp dsRNA, RIG-I forms a signaling-competent filament that assembles from the dsRNA end and propagates to the dsRNA interior. Within this filament, 2CARD oligomerization can occur independent of K63-Ubn through the proximity-induced mechanism as with MDA5. (II) On short dsRNA (<20 bp), only a single RIG-I molecule can bind per RNA molecule, and thus 2CARD oligomerization exclusively depends on K63-Ubn. (III) In general, the two mechanisms synergize to obtain stable tetramerization of 2CARD and robust signal activation.