Many flaviviruses are significant human pathogens. No effective
antiviral therapy is currently available for treatment of
flavivirus infections. Development of
antiviral treatment against these viruses is urgently needed. The flavivirus
methyltransferase (MTase) responsible for N-7 and 2'-O methylation of the
viral RNA cap has recently been mapped to the N-terminal region of nonstructural
protein 5. Structural and functional studies suggest that the MTase represents a novel
antiviral target. Here we review current understanding of flavivirus
RNA cap methylation and its implications for development of
antivirals. The 5' end of the flavivirus plus-strand
RNA genome contains a type 1 cap structure (
m(7)GpppAmG). Flaviviruses encode a single MTase domain that catalyzes two sequential methylations of the
viral RNA cap, GpppA-
RNA-->
m(7)GpppA-
RNA-->
m(7)GpppAm-
RNA, using
S-adenosyl-L-methionine (SAM) as the methyl donor. The two reactions require different
viral RNA elements and distinct biochemical assay conditions. Despite exhibiting two distinct methylation activities, flavivirus MTase contains a single binding site for SAM in its crystal structure. Therefore, substrate GpppA-
RNA must be re-positioned to accept the N-7 and 2'-O methyl groups from SAM during the two methylation reactions. Structure-guided mutagenesis studies indeed revealed two distinct sets of
amino acids on the
enzyme surface that are specifically required for N-7 and 2'-O methylation. In the context of virus, West Nile viruses (WNVs) defective in N-7 methylation are non-replicative; however, WNVs defective in 2'-O methylation are attenuated and can protect mice from subsequent wild-type WNV challenge. Collectively, the results demonstrate that the N-7 MTase represents a novel target for flavivirus
therapy.