ACTIONS OF POKEWEED ANTIVIRAL PROTEIN ON VIRUS-INFECTED PROTOPLASTS

Pokeweed antiviral protein (PAP) belongs to a group of ribosome-inacti vating proteins (RIPs) that inactivate ribosomes by depurinating rRNA at a specific site, To study the mechanism for the antiviral activity of PAP, the actions of PAP on TMV-infected and uninfected tobacco prot oplasts were investigated, The addition of 0.33 mu M PAP to TMV-inocul ated protoplasts caused a complete inhibition of TMV production, The s ame concentration of PAP was found to inhibit protein synthesis in the virus-infected protoplasts and to kill the cells, but it had no effec t on the uninfected protoplasts. The concentration dependence of prote in synthesis-inhibition by PAP was related to that of inhibition of vi ral multiplication, Furthermore, two other RIPs (ricin A-chain and luf fin-a), which showed 240 and 430-fold less activity on tobacco ribosom es than PAP in a cell-free system, did not inhibit viral multiplicatio n even at a concentration of 3.3 mu M. The analysis of RNAs from the v irus-infected and PAP-treated protoplasts demonstrated that 25S rRNA w as depurinated by PAP in the infected cells, These results suggest tha t PAP, which is normally unable to penetrate the plasma membrane of un infected protoplasts, gains entrance to the cytosol of infected cells and prevents viral multiplication by inactivating ribosomes.