Intracellular Distribution of Rat-Liver Polyribosomes Synthesizing Coeruloplasmin

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Abstract
Three independent approaches (binding of 125I-antibodies to polysomes, cell-free translation of polyadenylated mRNA in a wheat germ system and hybridization of RNA with a specific complementary DNA probe) were used to study the intracellular distribution of polysomes involved in the synthesis of ceruloplasmin as well as of ceruloplasmin mRNA sequences in rat liver. It was shown that only membrane-bound polysomes contain both nascent chains of ceruloplasmin and ceruloplasmin mRNA sequences. The size of ceruloplasmin polysomes is 16-18 monomers/mRNA molecule and their proportion is about 0.4-0.6% of total liver polysomes. The proportion of ceruloplasmin mRNA is 0.0025% of the total polysomal RNA and 0.29% of poly(A)-containing mRNA. These values correspond to ceruloplasmin mRNA concentration about 400-535 molecules/parenchymatous liver cell, 90% of those mRNA molecules were recovered from polysomes while the remaining 10% were from postpolysomal supernatant.