Reductive alkylation of mammalian ribosomes

40- and 60-S ribosomal subunits and 80-S ribosomes from rat liver were highly labelled by reductive methylation using formaldehyde and sodium boro-[3H] hydride, under conditions which did not decrease their activity in poly-U-directed polyphenylalanine synthesis. Dissociation of the monosomes, subunits dimers, and polysomes into free subunits was observed after methylation. Free proteins labelled after extraction from the ribosomal subunits incorporated 7 times more radioactivity than when labelled in the subunits. Proteins extracted from methylated subunits and ribosomes were analyzed by two-dimensional gel electrophoresis, and the radioactivity of each protein was compared to that of the same free protein. A classification of the proteins was established according to their accessibility to the reagents in the subunits and the ribosomes.