Flaviviruses encode a single methyltransferase domain that sequentially catalyzes two methylations of the viral RNA cap, GpppA-RNA→m 7GpppA-RNA→m7GpppAm-RNA, by using S-adenosyl-L-methionine (SAM) as a methyl donor. Crystal structures of flavivirus methyltransferases exhibit distinct binding sites for SAM, GTP, and RNA molecules. Biochemical analysis of West Nile virus methyltransferase shows that the single SAM-binding site donates methyl groups to both N7 and 2′-O positions of the viral RNA cap, the GTP-binding pocket functions only during the 2′-O methylation, and two distinct sets of amino acids in the RNA-binding site are required for the N7 and 2′-O methylations. These results demonstrate that flavivirus methyltransferase catalyzes two cap methylations through a substrate-repositioning mechanism. In this mechanism, guanine N7 of substrate GpppA-RNA is first positioned to SAM to generate m 7GpppA-RNA, after which the m7G moiety is repositioned to the GTP-binding pocket to register the 2′-OH of the adenosine with SAM, generating m7GpppAm-RNA. Because N7 cap methylation is essential for viral replication, inhibitors designed to block the pocket identified for the N7 cap methylation could be developed for flavivirus therapy.
ASJC Scopus subject areas
- Insect Science