A sequence in the main cytoplasmic loop of the alpha subunit is required for assembly of mouse muscle nicotinic acetylcholine receptor.

We have investigated the role of intracellular cytoplasmic sequences in the assembly of the mouse muscle nicotinic acetylcholine receptor (AChR) transiently expressed in COS cells. A chimeric protein in which the region from M1 to M4 of the alpha subunit was replaced by the corresponding region in the beta subunit was unable to support AChR assembly when substituted for the alpha subunit; a chimeric alpha subunit containing only the long cytoplasmic loop from the beta subunit was likewise inactive. Systematic mutation of short segments of the loop identified a sequence of 17 amino acids near the C-terminal end of the loop for which the beta sequence could not be substituted. Each of the inactive chimeric and mutated alpha subunits bound alpha-bungarotoxin when expressed alone and formed a heterodimer when expressed with the delta subunit. An alpha subunit truncated after M1 formed both an alpha delta heterodimer and an alpha delta beta heterotrimer, demonstrating that the cytoplasmic loop is dispensable for the early steps of assembly. A sequence in the long cytoplasmic loop of the alpha subunit thus appears to play a role in a late step of AChR assembly.