Equilibrium properties of mouse-Torpedo acetylcholine receptor hybrids expressed in Xenopus oocytes

and 6) of the nicotinic acetylcholine (ACh) receptor from mouse BC3H-1 cells and from Torpedo electric organ. The mRNA was synthesized in vitro by transcription with SP6 polymerase from cDNA clones. All 16 possible combi-nations that include one mRNA for each of a, fl, % and ~ were injected into oocytes. After allowing 2-8 d for translation and assembly, we assayed each oocyte for (a) receptor assembly, measured by the binding of [12sI]a-bungaro-toxin to the oocyte surface, and (b) ACh-induced conductance, measured under voltage clamp at various membrane potentials. All combinations yielded detect-able assembly (30-fold range among different combinations) and ACh-induced conductances (>l,000-fold range at 1 #M). On double-logarithmic coordinates, the dose-response r lations all had a slope near 2 for low concentrations of ACh. Data were corrected for variations in efficiency of translation among identically injected oocytes by expressing ACh-induced conductance per fem-tomole of a-bungarotoxin-binding sites. Five combinations were tested for d-tubocurarine inhibition by the dose-ratio method; the apparent dissociatio