Elongation‐Factor‐2‐Induced Sensitization of Ribosomes to Modeccin

Isolated rabbit reticulocyte ribosomes were treated with the toxic lectin modeccin for increasing periods of time. The toxin treatment was stopped by adding specific antitoxin and the residual activity of the ribosomes was tested in a polyphenylalanine‐synthesizing system. With increasing time of toxin treatment the ability of the ribosomes to support polymerization of phenylalanine decreased. Addition of anti‐toxin immediately stopped further inactivation of the ribosomes. The presence of elongation factor 2 (EF‐2) strongly increased the rate of ribosome inactivation by modeccin. There was no similar effect with EF‐1. Maximal rate of inactivation was observed when the molar ratio of EF‐2 to ribosomes was about 1. The rate of ribosome inactivation was about 4 ribosomes min −1 (molecule of modeccin) −1 under optimal conditions. The ability of EF‐2 to sensitize ribosomes to modeccin was strongly descreased in the presence of GTP, GDP and guanosine 5′‐(β,γ‐methylene]triphosphate. EF‐2, which had been ADP‐ribosylated in the presence of diphtheria toxin, was fully active in sensitizing the ribosomes whereas N ‐ethylmaleimide‐treated EF‐2 had lost this ability. Isolated 60‐S subunits were sensitized to modeccin by EF‐2 in a similar way as whole ribosomes. The data indicate that modeccin inactivates ribosomes catalytically and that in the absence of nucleotides it binds to the 60‐S subunit in a specific way. The possibility of two binding sites for EF‐2 on the ribosomes is discussed.