Coexpression of two distinct muscle acetylcholine receptor alpha-subunits during development.

The nicotinic acetylcholine receptor is a ligand-gated channel that mediates signalling at the vertebrate neuromuscular junction. It is a pentameric complex of four different subunits, assembled with a stoichiometry of alpha 2 beta gamma delta. Muscle-like alpha-subunits have been cloned from Torpedo, mouse, calf, rat, chicken, human and Xenopus, and only a single alpha-subunit complementary DNA from each species has been detected. We report here the cloning and characterization of a second muscle alpha-subunit cDNA from Xenopus, and show that this and a previously reported Xenopus alpha-subunit cDNA are encoded by distinct genes. The novel alpha-subunit reported here is expressed uniquely in oocytes; but both types of alpha-subunit are coexpressed throughout muscle development. This latter observation indicates that the expression of these two alpha-subunits is different from a previously reported developmental 'subunit-switch' mechanism used to generate channel diversity.