Control of Torpedo acetylcholine receptor biosynthesis in Xenopus oocytes

The nicotinic acetylcholine receptor (AcChoR) of Torpedo electroplax is a multisubunit transmembrane glycoprotein complex with a subunit stoichiometry of alpha 2 beta gamma delta. The RNAs for the separate subunits were transcribed in vitro from cDNAs inserted in pSP64T vectors and microinjected in Xenopus oocytes. Microinjected in vitro-transcribed RNAs were stable, with a half-life of 72 hr. Xenopus oocytes assembled functional AcChoRs from the subunit-specific RNAs. These receptors were inserted in the cell membrane and could be detected as early as 6 hr after RNA microinjection. The oocyte-expressed AcChoR subunits could be immunoprecipitated with anti-Torpedo AcChoR subunit antibodies. Expression of the AcChoR in oocytes proceeded linearly for 72 hr after microinjection. While the amount of RNA injected did not alter the linearity of the expression time course, the rate of receptor expression in oocytes showed a saturable dependence on RNA concentration. Varying the relative amount of alpha-subunit RNA microinjected into oocytes had a striking effect on receptor expression. This effect was specific for the alpha-subunit. These results suggest that transcript availability may control receptor expression in Xenopus oocytes. In addition, the availability of the alpha-subunit may be a limiting factor for receptor expression.