title: Effect of modeccin on the steps of peptide-chain elongation.
abstract: Modeccin inhibits polypeptide-chain elongation catalysed by Artemia salina (brine shrimp) ribosomes by inactivating the 60 S ribosomal subunit. Among the individual steps of elongation, peptide-bond formation, catalysed by 60 S peptidyltransferase, is unaffected by the toxin, whereas the binding of EF 2 (elongation factor 2) to ribosomes is strongly inhibited. Modeccin does not affect the poly(U)-dependent non-enzymic binding of either deacylated tRNAPhe or phenylalanyl-tRNA to ribosomes. The inhibitory effect of modeccin on the EF 1 (elongation factor 1)-dependent binding of phenylalanyl-tRNA is discussed, since it is decreased by tRNAPhe, which stimulates the binding reaction. The analysis of the distribution of ribosome-bound radioactivity during protein synthesis shows that modeccin consistently inhibits the radioactivity bound as long-chain peptides, but depending on the experimental conditions, can leave unchanged or even greatly stimulates the radioactivity bound as phenylalanyl-tRNA and/or short-chain peptides. It is concluded that, during the complete elongation cycle, modeccin does not affect the binding of the first aminoacyl-tRNA to ribosomes, but inhibits some step in the subsequent repetitive activity of either EF 1 or EF 2. The results obtained indicate that the mechanism of action of modeccin is very similar to that of ricin and related plant toxins such as abrin and crotin.
mesh_terms: D000818:Animals; D001156:Artemia; D066298:In Vitro Techniques; D037102:Lectins / Q000494:pharmacology*; D010441:Peptide Chain Elongation, Translational / Q000187:drug effects*; D010445:Peptide Elongation Factors / Q000378:metabolism; D010649:Phenylalanine / Q000378:metabolism; D011072:Poly U / Q000494:pharmacology; D014176:Protein Biosynthesis; D011691:Puromycin / Q000378:metabolism; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism; D054790:Ribosome Inactivating Proteins, Type 2; D012270:Ribosomes / Q000187:drug effects / Q000378:metabolism; D014118:Toxins, Biological / Q000494:pharmacology*
chemical_list: D037102:Lectins; D010445:Peptide Elongation Factors; D012346:RNA, Transfer, Amino Acyl; D054790:Ribosome Inactivating Proteins, Type 2; D014118:Toxins, Biological; D011072:Poly U; D010649:Phenylalanine; D011691:Puromycin; C016004:modeccin
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