Pokeweed antiviral protein (PAP) is a ribosome-inactivating protein characterized by its ability to depurinate the sarcin/ricin (S/R) loop of the large rRNA of prokaryotic and eukaryotic ribosomes. Here, a series of PAP mutants were used to examine the relationship between depurination of the S/R loop and inhibition of +1 programmed ribosomal frameshifting (PRF) and to define PAP sequences critical for inhibition of +1 PRF and Ty1 retrotransposition in the yeast Saccharomyces cerevisiae. Using three different classes of mutants we present evidence that strong binding of a C-terminal PAP mutant (PAPc) to ribosomes is sufficient to inhibit +1 PRF and Ty1 retrotransposition in the absence of S/R loop depurination. PAPc did not affect the totivirus ScV-L-A and HIV-1-directed -1 PRF efficiencies or the ability of cells to maintain the M1-dependent killer phenotype, demonstrating the specificity of the effect of PAPc on +1 PRF.
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