Effects of ricin on the ribosomal sites involved in the interaction of the elongation factors

The effects of ricin on the different steps of the elongation cycle of protein synthesis in a rabbit reticulocyte cell-free system are studied in this paper. The toxin most probably acts by catalytically inactivating the ribosomes, since a single molecule of the toxin can inactivate 300 ribosomes for poly(U)-directed phenylalanine incorporation. The effect of the toxin on the ribosome is irreversible. Ricin specifically inhibits elongation-factor-1-dependent aminoacyl-tRNA binding to ribosomes but has no effect on the non-enzymic binding of aminoacyl-tRNA. Ricin also inhibits formation of the complex elongation-factor-2 - ribosome - nucleotide with GTP, GDP or GMP-P(CH2)P. However, the toxin has no effect on translocation. These apparently conflicting results are discussed in this study.

The involvement of sulphydryl groups in the peptidyl transferase centre of eukaryotic ribosomes

Treatment of mammalian ribosomes with N-ethylmaleimide enhances up to 100% the ribosome efficiency in the "fragment reaction assay" for peptide bond formation by increasing the affinity of the substrate C-A-C-C-A-Leu-Ac for the donor site. This stimulation in peptidyl transferase activity was not observed when yeast ribosomes were treated in a similar manner. Stimulation of the peptidyl transferase activity of mammalian ribosomes was also observed by treatment with either p-chloromercuribenzoic acid or 5,5'-dithiobis-(2-nitrobenzoic acid) or 5,5'-dithiobis-(2-nitropyridine) or the maleimide-derived antibiotic showdomycin. N-Ethylmaleimide treatment also enhances C-A-C-C-A-Leu binding to the acceptor site of the peptidyl transferase centre. However, neither binding of N-Ac-Phe-tRNA in the presence of ethanol, nor binding of Phe-tRNA to the ribosomes is stimulated by N-ethylmaleimide. The antibiotic tenuazionic acid (a selective inhibitor of peptide bond formation by mammalian ribosomes) appears to require for its inhibitory effect the ribosome sulphydryl residues, since its inhibitory action on the fragment reaction is greatly decreased in ribosomes treated with N-ethylmaleimide.