Comparative Structure and Function Analysis of the RIG-I-Like Receptors: RIG-I and MDA5

Brisse, Morgan; Ly, Hinh

The series of steps required for RIG-I and MDA5 activation have been described in depth elsewhere (80) (81) (82) (83) (84) . Briefly summarized, these proteins endogenously exist in the cytoplasm of the cell in a phosphorylated and inactivated conformation when they are not activated by PAMP (dsRNA) (85) (86) (87) (Figures 4A,F) . Phosphorylation is mediated at the N terminal CARD domains (S8 and T170) of RIG-I by PKC-α/β (88, 89) and at the C terminal RNA interaction domain (S854, S855, and T770) by CKβ (90) . On the other hand, MDA5 is phosphorylated at S828 by RIOK3 (91) as well as by other yet unknown kinases (92, 93) . RIG-I is also acetylated at K909 in its C terminal domain that requires deacetylation by HDAC6 to be able to recognize RNA in its activated form (94) . Upon recognition of PAMP (dsRNA), RIG-I unfolds into an open and activated state that is mediated by the flexible hinge regions between the CARD domains and the helicase domain, and between the helicase and the C terminal domain (64, 87, (95) (96) (97) (98) (Figure 4B) . On the contrary, there is evidence to suggest that MDA5 has a more dynamic structure (99) . Unlike a model of RIG-I activation described above, MDA5 exists in a conformational equilibrium between close and open forms, with close forms favored in the dsRNA unliganded state. While not yet formally demonstrated, it is possible that MDA5 may be inhibited in the absence of the dsRNA ligand by its structural dynamics, which may prevent strong protein-protein interactions ( Figure 4F) . However, upon binding to dsRNA ligand, MDA5 adopts an open and activated form, which is perhaps more conducive for protein-protein interactions (Figures 4G,H) .