Effect of Abrin on Peptide Chain Initiation

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Abstract
Cell‐free systems from wheat germ were preincubated before and after addition of immunoglobulin mRNA to generate systems dependent and independent on peptide chain initiation, respectively. When abrin A chain was added, the system dependent on peptide chain initiation was much more strongly inhibited than the system measuring primarily chain elongation.Sucrose density gradient analysis showed that abrin A chain did not affect the binding of labelled methionine or labelled mRNA to the 40‐S subunit. However, it caused a strong reduction in the amount of labelled mRNA present in the 80‐S and polysome region. The results indicate that abrin A chain interferes with initiation of protein synthesis by inhibiting the binding of the 40‐S initiation complex to the 60‐S subunit to form the 80‐S initiation complex.The inhibiting effect of abrin A chain on poly(U)‐directed synthesis of polyphenylalanine was partly overcome by increasing the Mg2+ concentration. A similar effect of Mg2+ was found also in a cell‐free system from Krebs II ascites cells under conditions where primarily chain elongation was measured. Experiments with ribosomes treated with abrin A chain at different concentrations of MgCl2 and then tested for their ability to support polymerization of phenylalanine showed that high magnesium concentrations do not protect the ribosomes against the attack of abrin A chain. The data suggest that high Mg2+ concentrations tend to reverse toxin‐induced conformation changes in the ribosomes.