mRNA-directed biosynthesis of alpha subunit of thyrotropin: translation in cell-free and whole-cell systems

mRNA from mouse thyrotropic pituitary tumors was translated in frog oocytes (a whole-cell system) and in wheat germ extract and reticulocyte lysate (cell-free systems) in the presence of [(35)S]methionine. Synthesized peptides related to thyrotropin were identified in the three systems by immunoprecipitation with subunit-specific antisera developed against the alpha subunit of ovine lutropin (luteinizing hormone) and the beta subunit of bovine thyrotropin. In wheat germ extract and reticulocyte lysate, a single immunoprecipitable form of the alpha subunit of thyrotropin was synthesized with an apparent molecular weight of 14,000 by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. In the frog oocyte, three forms of immunoprecipitable alpha subunit of thyrotropin were synthesized with apparent molecular weights of 20,000, 14,000, and 10,000. The 20,000 form is similar to unlabeled rat pituitary standard alpha subunit and (35)S-labeled mouse tumor alpha subunit in cell cultures (20,000-21,000); thus, it may represent a precursor-cleaved and glycosylated form. The 14,000 form synthesized in all three systems probably represents the pre-alpha subunit of thyrotropin; the 10,000 form, synthesized only in the frog oocyte, could be a proteolytically cleaved but unglycosylated form. Because only the alpha subunit of thyrotropin was identified and no larger molecular weight immunoprecipitable form of either subunit was detected in any of the translation systems, alpha and beta subunits of thyrotropin appear to be translated from separate mRNAs.