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

Brisse, Morgan; Ly, Hinh

For RIG-I, a highly conserved residue in the C-terminal RNA binding pocket (H830) has been found to sterically exclude canonical self-RNA by the means of the N 1 -2 ′ O-methyl self-RNA motif, also known as Cap1 RNA (61, 230) . This results in a low binding affinity of RIG-I to cellular Cap1 RNA and decreased ATPase activity as compared to PAMP (dsRNA) (61, 231) . Flaviviruses take advantage of this precise discrimination by encoding a viral 2 ′ -O-methyltransferase capable of N 1 -2 ′ O-methylating its positive-strand RNA genome in order to evade RIG-I recognition and IFN1 activation (230) . Conversely, the mutations E373A and C268F found in the RIG-I protein in patients with auto-immune disorder Singleton-Merten syndrome confer the ability of the protein to recognize Cap1 RNA and become activated by ATP dependent and independent mechanisms, respectively (232) . Furthermore, the E373Q mutation of RIG-I, which was designed to constitutively bind ATP, was found to increase the affinity of RIG-I with ribosomal RNA (233) . It is noteworthy that host RNA contains additional internal RNA modifications and non-Watson-Crick base pairing which can inhibit activation of the other known dsRNA-sensing protein, the interferon-induced double-stranded RNA-activated protein kinase (PKR) (234) , and it is known that synthetic 5 ′ triphosphorylated RNA containing pseudouridine, 2-thiouridine or 2 ′ -O-methylated uridine has significantly decreased ability to activate RIG-I (67), which has been demonstrated to occur by preventing RIG-I filament formation in-situ (142) . N-6-methyladenosine (m6A) nucleotides, which are well-known nucleotide modifications among viruses (235) , have also been found to ablate dsRNA binding to RIG-I (236) .