ISOLATION AND AMINO-ACID-SEQUENCE OF A PROTEIN-SYNTHESIS INHIBITOR FROM THE SEEDS OF RYE (SECALE-CEREALE)

A protein-synthesis inhibitor, designated RPSI, was isolated from the seeds of rye (Secale cereale) using gel filtration and S-Sepharose col umn chromatography. RPSI is a basic protein with an isoelectric point of over 10, and the concentration of protein required for 50% inhibiti on of protein synthesis (IC50) of purified RPSI was about ten-fold the concentration of ricin A-chain. The complete amino acid sequence of R PSI was discovered by analyzing the peptides and fragments obtained fr om the proteolytic digests and by the cyanogen bromide- and hydroxylam ine-cleavages of RPSI. RPSI consists of 280 amino acid residues and ha s a molecular weight of 30,171. RPSI has only 21% sequence identity wi th that of ricin A-chain, but all five amino acid residues involved in the active site of ricin A-chain are conserved in RPSI.