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.