Intracellular site of prolactin synthesis in rat pituitary cells in culture |
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Authors: | D K Biswas A H Tashjian |
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Institution: | 1. Laboratory of Pharmacology, Harvard School of Dental Medicine, Boston, Mass. 02115, USA;2. Department of Pharmacology, Harvard Medical School, Boston, Mass. 02115, USA |
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Abstract: | Free and membrane-bound polyribosomes were isolated from control and thyrotropin releasing hormone-treated GH3 cells. The two polysome fractions were used to direct {3H}leucine incorporation into prolactin in both heterologous and homologous cell-free protein-synthesizing systems. Prolactin was measured by immunoprecipitation and SDS-disc gel electrophoresis of the reaction products. Only membrane-bound polysomes directed incorporation of {3H}leucine into labeled prolactin. In additon, intact cells were pulselabeled with {3H}leucine, free and membrane-bound polysomes were isolated, and newly synthesized prolactin associated with each polysome fraction was measured. In control cells, {3H}prolactin represented about 0.4 and 4.2% of total acid-insoluble radioactivity in free and membrane-bound polysomes, respectively; whereas, in thyrotropin releasing hormone-treated cells, these values were about 1 and 20%, respectively. Added {3H}prolactin did not associate nonspecifically with membrane-bound polysomes. We conclude that prolactin is synthesized predominantly on membrane-bound polysomes in GH3 cells. |
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