Activities of nucleoprotein particles derived from rat liver ribosome

80-S ribosomes and 60-S subunits from rat liver were treated at increasing KC1 concentrations giving protein-deficient ribosomal particles whose components were analyzed and their activity tested. Most of the activities assayed stand treatment up to KC1 concentrations of around 0.6 M; peptidyl transferase, measured by the fragment reaction, however was 50% inhibited by 0.5 M KC1 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 KC1 the proteins of the large sub-unit can be divided into three groups: (1) 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; (2) 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; (3) 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 KC1 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.