Flavivirus methyltransferase: A novel antiviral target

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 (m⁷GpppAmG). Flaviviruses encode a single MTase domain that catalyzes two sequential methylations of the viral RNA cap, GpppA-RNA → m⁷GpppA-RNA → m⁷GpppAm-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.