Protection of rat liver 80 S ribosomes against ricin A chain inactivation by proteins extracted from rat liver and wheat germ ribosomal subunits with ammonium chloridemagnesium chloride

Proteins extracted from wheat germ 60 S ribosomal subunits and rat liver 60 S and 40 S ribosomal subunits with $\text{3}\text{M NH}{}_4\text{Cl}\text{75}\text{mM MgCl}{}_2$ were able to prevent the ricin A chain-mediated inactivation of untreated 80 S rat liver ribosomes. The protection of polyphenylalanine synthetic capability of 80 S ribosomes was saturable and reached 100% protection in the presence of about 20 μg of extracted protein using a uniform set of assay conditions. No protection was observed using proteins extracted from wheat germ 40 S subunits or the core fraction of rat liver 60 S subunits or protein extracted from Escherichia coli ribosomes or ribosomal subunits. The conclusion that the protective effect of extracted 60 S subunit proteins was specific, was further strengthened by showing that unrelated proteins such as α-lactalbumin, bovine serum albumin and lysozyme, and polypeptides such as polylysine and poly(aspartic acid), also showed no protection. If 80 S ribosomes were first treated with ricin A chain and then incubated with proteins extracted from rat liver 60 S subunits, no protection was observed. Proteins extracted with $\text{NH}{}_4\text{Cl}\text{MgCl}{}_2$ from 60 S rat liver subunits were applied to a carboxymethylcellulose column equilibrated with 6 M urea. Stepwise elution with increasing concentrations of LiCl resulted in seven fractions. One fraction (D) contained most of the protective factor; one fraction (E) contained a lesser amount of the protective factor. Two-dimensional polyacrylamide gel electrophoresis of fraction D showed the presence of ten proteins. These data are consistent with the idea that the enzymatic target of ricin A chain is protein is nature and that fraction D contains one or more proteins that appear to act as a inhibitor against ricin A chain.