ACTIVITIES OF NUCLEOPROTEIN PARTICLES DERIVED FROM RAT-LIVER RIBOSOME

The 80-S ribosomes and 60-S subunits from rat liver were treated at increasing KCl concentrations giving protein-deficient ribosomal particles whose components were analyzed and their activity tested. Most of the activities assayed stand treatment up to KCl concentrations of around 0.6 M; peptidyl transferase, measured by the fragment reaction, however was 50% inhibited by 0.5 M KCl in 60-S subunits but not in 80-S ribosomes. Three proteins, L21, L26 and L31, might be implicated in this loss of activity. 60-S subunits forming part of the 80-S ribosome are more resistant to the salt treatment and the pattern of proteins released by the treatment differs from the one obtained from free 60-S subunits, implying perhaps a change of conformation of this subunit upon association to form 80-S couples. According to their resistance to release by KCl the proteins of the large subunit can be divided into 3 groups: easily removed, including proteins, L1, L11, L17 and L25 in 80-S subunits and in addition L5, L8, L9, L13, L20, L22, L26, L29, L31 and L32/33 in 60-S subunits; proteins resistant to release by high salt concentrations in 80-S ribosomes as well as in 60-S subunits, namely proteins L3, L14, L27, L36, L40, L41, X1 and X2; the rest of the proteins which are released in a more or less continuous way throughout the treatment. 5 S RNA is not released by KCl treatment at the concentrations used. The binding sites for the antibiotics trichodermin and anisomycin are affected in a different way by the salt treatment, indicating that they are structurally different.