Expression of mouse-Torpedo acetylcholine receptor subunit chimeras and hybrids in Xenopus oocytes.

In this study, in vitro synthesized mRNA encoding mouse and Torpedo nicotinic acetylcholine receptor subunits was injected into Xenopus oocytes, followed by assays for assembly onto the oocyte surface (using [125I]alpha-bungarotoxin binding) and for acetylcholine-induced conductances (using voltage clamp). We constructed hybrid acetylcholine receptors in Xenopus oocytes by injecting all 8 possible combinations of 4 subunit-specific mRNAs in which a single subunit is derived from the other species. For each hybrid combination, there is detectable assembly and conductance. We also constructed cDNA clones that encode chimeric acetylcholine receptor subunits in which part of the gamma subunit from Torpedo was replaced by the homologous region of the delta subunit from mouse. None of the chimeric subunits was able to replace the Torpedo gamma, mouse delta, or Torpedo delta subunit with regard to assembly or function. We therefore conclude that widely spaced (and unknown) parts of the protein chain are required for the intersubunit interactions that eventually lead to functional assembly of the receptor.