The 5' end of eukaryotic
mRNA contains the type-1 (m7GpppNm) or type-2 (m7GpppNmNm) cap structure. Many viruses have evolved various mechanisms to develop their own capping
enzymes (e.g. flavivirus and coronavirus) or to 'steal' caps from host mRNAs (e.g. influenza virus). Other viruses have developed 'cap-mimicking' mechanisms by attaching a
peptide to the 5' end of
viral RNA (e.g. picornavirus and calicivirus) or by having a complex 5'
RNA structure (
internal ribosome entry site) for translation initiation (e.g. picornavirus, pestivirus and hepacivirus). Here we review the diverse
viral RNA capping mechanisms. Using flavivirus as a model, we summarize how a single
methyltransferase catalyses two distinct N-7 and 2'-O methylations of
viral RNA cap in a sequential manner. For
antiviral development, a structural feature unique to the flavivirus
methyltransferase was successfully used to design selective inhibitors that block viral
methyltransferase without affecting host
methyltransferases. Functionally, capping is essential for prevention of
triphosphate-triggered innate immune activation; N-7 methylation is critical for enhancement of viral translation; and 2'-O methylation is important for subversion of innate immune response during
viral infection. Flaviviruses defective in 2'-O
methyltransferase are replicative, but their viral RNAs lack 2'-O methylation and are recognized and eliminated by the host immune response. Such mutant viruses could be rationally designed as live
attenuated vaccines. This concept has recently been proved with Japanese encephalitis virus and dengue virus. The findings obtained with flavivirus should be applicable to other RNA viruses.