Inhibitor of protein synthesis co-isolating with polyribosomal RNA

Although low concentrations of total polyribosomal RNA from porcine parotid glands or rat pituitary cells in culture (GH3) isolated by standard dodecylsulphate/phenol, chloroform extraction techniques can direct the incorporation of radiolabeled amino acids into proteins using a cell-free protein-synthesizing system derived from wheat germ embryos, higher concentrations inhibit the translation of endogenous wheat germ mRNA, or added rabbit globin mRNA or polyribouridylic acid. This inhibitory activity is separated from poly(A)-rich RNA by oligo(dT)-cellulose chromatography. The inhibitory activity appears to reside in a heat-stable protein since it is inactivated by incubation with various proteases but not by DNAase I, pancreatic ribonuclease, alkaline hydrolysis, or treatment with formamide. Specificity of the inhibition is suggested since the inhibitory fraction prepared from GH3 cells also inhibits protein synthesis in a cell-free protein-synthesizing system derived from porcine parotid gland, while the inhibitory fraction prepared from porcine parotid gland has no inhibitory activity in this homologous system. Radioiodination and dodecylsulphate/polyacrylamide gel electrophoresis reveal several protein bands, the most prominent with an apparent molecular weight of 78 000.