Homology between ricin and Ricinus communie agglutinin: Amino terminal sequence analysis and protein synthesis inhibition studies

The existence of three forms of ricin and two forms of the Ricinus communis agglutinin (RCA) was established using cation exchange chromatography, isoelectric focusing, and polyacrylamide gel electrophoresis. The preparation of the RCA we obtained was 60-75 times more potent than ricin in the agglutination of erythrocytes, but was about 4% as effective as an inhibitor of cell-free protein synthesis. When reduced with 2-mercaptoethanol, the RCA was activated 3000-fold as an inhibitor of cell-free protein synthesis, whereas ricin was activated about 600-fold by the same treatment. A mixture of the RCA A chains was about one-fifth as effective as the ricin A chain in the inhibition of cell-free protein synthesis. The purified polypeptide subunits of the castor bean lectins were subjected to automated Edman degradation. The sequence for 17 of the first 19 residues of the agglutinin A chain was determined. The first seven residues of the ricin A chain were determined and they are identical with those of the RCA A chain. Nineteen turns of Edman degradation on the RCA B chain resulted in the identification of 18 amino acids. The sequence determined for the first 17 residues of the ricin B chain was identical with that of the RCA B chain. It is likely that the identity of the ricin/RCA A and B chain sequences extends further along the polypeptide chains than the sequences we have determined. The similar structural and catalytic potentials of the RCA and ricin suggest that they bear a precursor-product relationship.