Evidence that free polysomes are not the precursors of membrane-bound polysomes in rat liver

Association of maternal and newly synthesized ribosomes with membranous noncytoskeletal structures in Xenopus laevis embryonic cells

In Xenopus laevis embryos a high concentration of both KCl and 0.5% DOC (sodium deoxycholate) is needed for maximal extraction of ribosomes and polysomes. We studied the nature of the structures that keep ribosomes and polysomes immobilized within the cytoplasm of embryonic cells at cleavage through tailbud stages, using various combinations of a low-salt buffer (20 mM KCl), a high-salt buffer (500 mM KCl), 0.5% DOC, and 0.5% Triton X-100. With a low-salt buffer and 0.5% DOC, but not Triton X-100, 80S ribosomal monomers and polysomes were liberated from the cytoplasmic rapidly sedimenting structures (RSS) to the soluble fraction. With a high-salt buffer (500 mM KCl), ribosomes were solubilized as 60S and 40S subunits together with about one-half of the total polysomes. When cells were homogenized in a low-salt buffer with added inhibitors of the cytoskeleton (cytochalasin B or colchicine), the majority of polysomes but not ribosomes were solubilized. These results provide evidence for the following conclusions. 1) Polysomes are bound to cytoskeletal structures in Xenopus embryos, but ribosomes, both maternal and newly synthesized, are associated with membranous noncytoskeletal structures. 2) The membranous structures consist of two compartments, one high-salt sensitive and the other high-salt resistant. 3) Ribosomes of the high-salt resistant group increase in amount with developmental stage and appear to be the precursor to the ribosomes of the high-salt sensitive group.