An RNA cap (nucleoside‐2′‐O‐)‐methyltransferase in the flavivirus RNA polymerase NS5: crystal structure and functional characterization

Viruses represent an attractive system with which to study the molecular basis of mRNA capping and its relation to the RNA transcription machinery. The RNA‐dependent RNA polymerase NS5 of flaviviruses presents a characteristic motif of S ‐adenosyl‐ L ‐methionine‐dependent methyltransferases at its N‐terminus, and polymerase motifs at its C‐terminus. The crystal structure of an N‐terminal fragment of Dengue virus type 2 NS5 is reported at 2.4 Å resolution. We show that this NS5 domain includes a typical methyltransferase core and exhibits a (nucleoside‐2′‐ O ‐)‐methyltransferase activity on capped RNA. The structure of a ternary complex comprising S ‐adenosyl‐ L ‐homocysteine and a guanosine triphosphate (GTP) analogue shows that 54 amino acids N‐terminal to the core provide a novel GTP‐binding site that selects guanine using a previously unreported mechanism. Binding studies using GTP‐ and RNA cap‐analogues, as well as the spatial arrangement of the methyltransferase active site relative to the GTP‐binding site, suggest that the latter is a specific cap‐binding site. As RNA capping is an essential viral function, these results provide a structural basis for the rational design of drugs against the emerging flaviviruses.