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Effects of Viral Genome‐Linked Protein on Depurination of Viral RNA by Pokeweed Antiviral Protein

Pokeweed antiviral protein (PAP) is a ribosome inactivating protein and is an N‐glycosidase that removes specific purine residues from the sarcin/ricin loop of large rRNA and arrests protein synthesis at the translocation step. PAP, a potent antiviral agent against many plant, animal, and human viruses, inhibits translation in cell extracts by binding to the cap structure of eukaryotic mRNA and viral RNA depurinating them at sites downstream of the cap structure. To understand the molecular basis of PAP's antiviral activity, using fluorimetric methods, we examine interactions occurring between PAP and Potyvirus genome linked protein (VPg) that is shown to perform as a cap analog in viral cap‐independent translation of RNA bearing internal ribosome entry site (IRES). VPg interacts with cap‐binding proteins, stimulates translation of uncapped IRES‐containing RNA, and inhibits capped RNA translation in wheat germ extract. We have characterized interaction of PAP with VPg, and structured RNA derived from tobacco etch virus (TEV). When VPg complexes with PAP, the affinity for uncapped TEV RNA tripled compared with PAP alone, whereas affinity of PAP for capped RNA was not altered significantly. This selective preference of PAP for uncapped RNA in the presence of VPg stimulates the idea of creating a chimeric PAP‐VPg mutant that possesses selectivity to depurinate uncapped viral RNA by directing it to viral IRES.