Studies on proteins of animal ribosomes. XXIV. Localisation of proteins in ribosomal subunits of rat liver studied by chemical substitution with p-nitrophenyl acetate and methyl acetimidate.

The reaction of [3H]p-nitrophenyl acetate (NPA) or [14C]methyl acetimidate (MAI) with amino groups of ribosomal pade between the reactivity of the proteins in situ in the ribosomal subunit with that of isolated protein mixtures. In the small subunit reactivity compared with the protein mixture was only 10-65% in the case of NPA but 45 to more than 100% in the case of MAI. In the large subunit reactivity to MAI was 10-60% that of the isas a denser structure than the small one. In agreement with earlier experiments with iodoacetamide the proteins S2, 5, 7, 8, 10 and 13 of the small subunit and L15, 17, 20, 24, 25, 27, 29, 33, 34, 35 and 38 in the large subunit are quite accessible while proteins S9, 14, 19, 20, 24, 25, 27, 29 and 30 of the small subunit and L1, 7, 8, 10, 11, 19, 28, 31, and 32 of the large one are relatively inaccessible.