Synthesis in vitro of corticosteroid-binding globulin from rat liver messenger ribonucleic acid

To show that corticosteroid-binding globulin (CBG), a plasma glycoprotein, is synthesized in the liver as well as to develop a tool with which to study the hormonal factors which control its synthesis, we undertook to translate rat CBG mRNA in vitro. Rat liver mRNA was translated in the presence of [35S]methionine in both a rabbit reticulocyte lysate (cell-free) System and a Xenopus laevis oocyte (whole cell) system. A monospecific antibody against highly purified rat CBG was used to precipitate a single 35S-labeled product from the mixture of newly synthesized hepatic proteins in the two systems. In the rabbit reticulocyte lysate system, the protein migrated with an apparent molecular weight of 41,000 on sodium dodecyl sulfate-gel electrophoresis; CBG isolated from rat serum migrated as a doublet with apparent molecular weights of 56,000 and 62,500. Since proteins synthesized by a cell-free system are not glycosylated, the difference in molecular weight can probably be accounted for by the carbohydrate content of plasma CBG. Translation of rat liver mRNA in a Xenopus laevis oocyte system again resulted in immunoprecipitation of a single 35S-labeled protein, but this time with an apparent molecular weight of 53,000 on sodium dodecyl sulfate-gel electrophoresis; this probably represents glycosylated CBG and corresponds to the apparent molecular weight of one of the species isolated from rat plasma.