Pokeweed antiviral protein accesses ribosomes by binding to L3

Pokeweed antiviral protein (PAP), a 29-kDa ribosome-inactivating protein, catalytically removes an adenine residue from the conserved alpha-sarcin loop of the large rRNA, thereby preventing the binding of eEF-2.GTP complex during protein elongation. Because the alpha-sarcin loop has been placed near the peptidyltransferase center in Escherichia coli ribosomes, we investigated the effects of alterations at the peptidyltransferase center on the activity of PAP. We demonstrate here that a chromosomal mutant of yeast, harboring the mak8-1 allele of peptidyltransferase-linked ribosomal protein L3 (RPL3), is resistant to the cytostatic effects of PAP. Unlike wild-type yeast, ribosomes from mak8-1 cells are not depurinated when PAP expression is induced in vivo, indicating that wild-type L3 is required for ribosome depurination. Co-immunoprecipitation studies show that PAP binds directly to L3 or Mak8-1p in vitro but does not physically interact with ribosome-associated Mak8-1p. L3 is required for PAP to bind to ribosomes and depurinate the 25 S rRNA, suggesting that it is located in close proximity to the alpha-sarcin loop. These results demonstrate for the first time that a ribosomal protein provides a receptor site for an ribosome-inactivating protein and allows depurination of the target adenine.