Cell-free synthesis of alpha and beta subunits of human chorionic gonadotrophin with polyribosomes from early placenta.

Abstract

Abstract. Polyribosomes were prepared from first trimester human placenta and assayed for protein synthetic activity in the cell-free translatory system derived from wheat germ or from first trimester placental tissue. High incorporation of [3H]proline was observed, whereas incorporation of [3H]glucosamine did not occur. The radiolabelled proteins synthesized by the wheat germ cell-free system were immunoprecipitated with specific antisera for hCG, α- and β-subunit and analyzed by electrophoresis on sodium dodecyl sulphate-polyacrylamide gels. The apparent molecular weight of the immunoprecipitable CG-α-protein was estimated to be 15 000 daltons, while that of the immunoprecipitable β-protein was about 24 000 daltons. Since the molecular weights of the polypeptide part of authentic α and β subunits are 10 200 and 16 000 daltons, respectively, the cell-free products may represent their precursor protein forms; pre-CG-α and pre-CG-β. The rates of production of hCG, α and β proteins by in vitro incubation of placental polyribosomes with the wheat germ translating system were determined by the respective homologous radioimmunoassays. It is noteworthy that CG-β-protein production was highest during the initial 15 min of incubation and became limited subsequently, whereas an increase in hCG and α-protein formation was observed at 90 min of incubation. The present results suggest that the placental polyribosomes contain mRNA for α- and β-subunits. Earlier accelerated synthesis of CG-β protein might be required for the subsequent association with the newly synthesized α-subunit protein to form hCG.