Identification of mRNAs encoding the α and β subunits of human choriogonadotropin

RNA derived from first trimester placenta was translated in wheat germ cell-free extracts containing [³H]-proline. Antisera generated against highly purified, denatured α and β subunits of human choriogonadotropin were employed to demonstrate the in vitro synthesis of both proteins. The immunoprecipitated products of the cell-free system were resolved on sodium dodecyl sulfate-polyacrylamide slab gels. It was observed that the molecular weight of the immunoprecipitable α subunit protein was approximately 14,000 while the size of the immunoprecipitable β protein was estimated at 18,000. Since the molecular weights of the authentic α and β apoproteins corresponded to 10,500 and 15,500, respectively, the cell-free products presumably represented their pre-protein forms. The quantities of α and β proteins synthesized were determined using different labeled amino acids in the translation reactions. It was observed that the ratio of α and β subunits synthesized was dependent on the magnesium concentration in the translation mixtures. At a magnesium concentration of 1.8 mM, the ratio of α to β subunit synthesized was about 6:1. However, at 1.5 mM, this ratio was reversed, the β subunit synthesized exceeded the α subunit by a factor of about 2. RNA isolated from first trimester placenta directed the synthesis of 8-fold more α protein compared to that synthesized in the presence of term RNA. No detectable β protein was synthesized by term RNA at 1.5 mM magnesium or at a concentration of 1.8 mM. These differences between first trimester and term mRNA were observed using either [³H]proline or [³⁵S]Met-tRNA(f)(Met) as a source of label. First trimester RNA was resolved on sucrose gradients and it was observed that the mRNAs encoding the α and β subunit proteins sedimented differently. These results showed that the subunits were translated from separate mRNAs, the translatable subunit mRNA levels paralleled the blood levels of the intact hormone, and the synthesis of the β subunit may be a rate-limiting step in the secretion of human choriogonadotropin in vivo.