Role of two acetylcholine receptor subunit domains in homomer formation and intersubunit recognition, as revealed by alpha 3 and alpha 7 subunit chimeras.

Differential expression of subunit genes from the nicotinic acetylcholine receptor (AChR) superfamily yields distinct receptor subtypes. As each AChR subtype has a specific subunit composition and many subunit combinations appear not to be expressed, each subunit must contain some information leading to proper assembly. The neuronal AChR subunits alpha 3 and alpha 7 are expressed in bovine chromaffin cells, probably as constituents of two different AChR subtypes. These subunits have different assembly behavior when expressed in heterologous expression systems: alpha 7 subunits are able to produce homomeric AChRs, whereas alpha 3 subunits require other "structural" subunits for functional expression of AChRs. This feature allows the dissection of the requirements for subunit interactions during AChR formation. Analysis of alpha 7/alpha 3 chimeric constructs identified two regions essential to homomeric assembly and intersubunit recognition: an N-terminal extracellular region, controlling the initial association between subunits, and a second domain within a region comprising the first putative transmembrane segment, M1, and the cytoplasmic loop coupling it to the pore-forming segment, M2, involved in the subsequent interaction and stabilization of the oligomeric complex.