Protective Effect of Elongation Factor 2 on the Inactivation of Ribosomes by the Toxic Lectins Abrin and Ricin

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Abstract
The inactivation of rabbit reticulocyte ribosomes by abrin and ricin A-chains was studied by incubating ribosomes with the A-chains and testing, after various periods of time, aliquots of the ribosomes for their ability to polymerize phenylalanine. The presence of elongation factor 2 (EF-2) reduced the rate of inactivation of ribosomes by the A-chains. The protective effect of EF-2 was strongly enhanced by GTP and, to a lesser extent, also by GDP or dGTP. Other nucleotides had no demonstrable effect. Much less protection was found after binding of Phe-tRNA to ribosomes in the presence of EF-1 (enzymic binding) or in the presence of high Mg2+ concentration (non-enzymic binding). The data indicate that when EF-2 binds to the ribosomes it completely or partially covers the target site for abrin and ricin A-chains. The possibility that EF-1 also binds to this site is discussed