Ricin from Ricinus communis was isolated and the binding of 3H-reductively alkylated or 125I-iodinated ricin was studied by incubating the toxic protein with ribosomes and isolating the ricin-ribosome complex by centrifugation. Neither of the labeled ricin derivatives nor 3H-labeled A chain bound Escherichia coli ribosomes, but both bound rat liver ribosomes in a reproducible manner. 3H-labeled ricin bound in a ratio of 1 mol/mol of ribosomes with a dissociation constant of 3 μm as calculated from a Scatchard plot. Similarly, 3H-labeled B chain isolated from ricin also bound in a one-to-one complex with a dissociation constant of 1 μm. The binding of ricin and ricin B chain was sensitive to lactose, while the binding of reduced ricin or ricin A chain was not prevented by lactose. Reduced 125I-labeled ricin in the presence of lactose and 3H-labeled A chain bound with a ratio of 2 mol/mol of ribosomes. It was further demonstrated that 3H-labeled ricin A chain bound only to the 60S ribosomal subunit and not to the 40S ribosomal subunit. The dissociation constant for the binding was 2 μm both in the presence and absence of lactose and 2 mol of A chain were bound per mole of 60S ribosomal subunit.
ASJC Scopus subject areas
- Molecular Biology