Evidence that free polysomes are not the precursors of membrane-bound polysomes in rat liver.
We tested, in rat liver, the postulate that free polysomes were precursors of membrane-bound polysomes. Three methods were used to isolate free and membrane-bound ribosomes from either post-nuclear or post-mitochondrial supernatants of rat liver. Isolation and quantitation of 28 S and 18 S rRNA allowed determination of the 40 S and 60 S subunit composition of free and membrane-bound ribosomal populations, while pulse labeling of 28 S and 18 S rRNA with [6-14C) orotic acid and inorganic (32P] phosphate allowed assessment of relative rates of subunit renewal. Throughout the extra-nuclear compartment, 40 S and 60 S subunits were present in essentially equal numbers, but, free ribosomes contained a stoichiometric excess of 40 S subunits, while membrane-bound ribosomes contained a complementary excess of 60 S subunits. Experiments with labeled precursors showed that throughout the extra-nuclear compartment, 40 S and 60 S subunits accumulated isotopes at essentially equal rates, however, free ribosomes accumulated isotopes faster than membrane-bound ribosomes. Among free ribosomes or polysomes, 40 S subunits accumulated isotopes faster than 60 S subunits, but, this relationship was not seen among membrane-bound ribosomes. Here, 40 S subunits accumulated isotope more slowly than 60 S subunits. This distribution of labeled precursors does not support the postulate that free polysomes are precursors of membrane-bound polysomes, but, these data suggest that membrane-bound polysomes could be precursors of free polysomes.
D000818:Animals; D002451:Cell Compartmentation; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005260:Female; D007425:Intracellular Membranes / Q000378:metabolism*; D008862:Microsomes, Liver / Q000378:metabolism; D011132:Polyribosomes / Q000378:metabolism*; D012335:RNA, Ribosomal / Q000378:metabolism; D051381:Rats; D012270:Ribosomes / Q000648:ultrastructure
D012335:RNA, Ribosomal