Differential ricin sensitivity of rat liver and wheat germ ribosomes in polyuridylic acid translation

Rat liver and wheat germ ribosomes were treated with ricin or ricin A chain and assayed for polyuridylic acid-directed protein synthesis. Ricin A chain was not able to inhibit polyphenylalanine synthesis by wheat germ ribosomes at 5000 times the concentration that was effective in preventing synthesis by rat liver ribosomes. Wheat germ S-100 did not prevent inactivation of rat liver ribosomes by ricin and the presence of rat liver S-100 did not cause wheat germ ribosomes to become sensitive to ricin. Ricintreated and control 40 and 60 S ribosomal subunits from wheat germ and rat liver were mixed in all possible combinations and assayed for protein synthesis activity. The only inhibited ribosome was that containing a ricin-treated 60 S subunit from rat liver. The binding of ricin A chain to wheat germ ribosomes was compared to that observed with rat liver ribosomes. Gel filtration of the ricin A chain-ribosome complex on agarose confirmed results obtained by sedimentation and showed that rat liver and wheat germ ribosomes bind ricin A chain in a similar manner. Escherichia coli ribosomes bound very small amounts of ricin A chain compared to rat liver and wheat germ ribosomes. These results indicate that binding alone is not sufficient to result in ribosome inactivation.