Pokeweed antiviral protein (PAP) from Phytolacca americana is a ribosome-inactivating protein (RIP) and an RNA N-glycosidase that removes specific purine residues from the sarcin/ricin loop of large rRNA, arresting protein synthesis at the translocation step. PAP is also a cap-binding protein and is a potent antiviral agent against many plant, animal, and human viruses. To elucidate the mechanism of RNA depurination, and to understand how PAP recognizes and targets various RNAs, the interactions between PAP and turnip mosaic virus genomelinked protein (VPg) were investigated. VPg can function as a cap analog in cap-independent translation and potentially target PAP to uncapped IRES-containing RNA. In this work, fluorescence spectroscopy andHPLCtechniques were used to quantitatively describe PAP depurination activity and PAP-VPg interactions. PAP binds to VPg with high affinity (29.5 nM); the reaction is enthalpically driven and entropically favored. Further, VPg is a potent inhibitor of PAP depurination of RNA in wheat germ lysate and competes with structured RNA derived from tobacco etch virus for PAP binding. VPg may confer an evolutionary advantage by suppressing one of the plant defense mechanisms and also suggests the possible use of this protein against the cytotoxic activity of ribosome-inactivating proteins.
Background: PAP is a ribosome-inactivating protein that depurinates RNA and inhibits protein synthesis.
Results: Turnip mosaic VPg inhibits enzymatic activity of PAP in wheat germ extract.
Conclusion: VPg may play a role in overcoming viral resistance by suppressing the plant defense mechanism.
Significance: Depurination inhibition by VPg suggests a novel viral strategy to evade host cell defense and possible anticytotoxic activity against RIPs.