An efficient screening system for influenza virus cap-dependent endonuclease inhibitors.

The synthesis of influenza virus mRNA is primed by capped (m(7)GpppNm-) short RNAs that are cleaved from RNA polymerase II transcripts by a virally encoded endonuclease. This cap-dependent endonuclease activity called "cap-snatching" may provide a unique target for novel anti-viral agents. To screen candidate inhibitors, it is essential to establish a method for producing efficiently a capped RNA substrate and a convenient assay for the cap-snatching activity. A 3'-biotinylated short RNA was prepared by in vitro transcription, purified by C18 reverse-phase column chromatography, and subjected to a capping reaction involving three recombinant capping enzymes. This capped RNA was shown to be an efficient substrate for the cap-snatching assay. Cap-snatching activity was then measured with the novel pull-down assay developed in this study, which is based on the streptavidin-biotin interaction. A known inhibitor for the cap-snatching reaction was evaluated by the pull-down assay, demonstrating the efficacy of the established screening system.