Subunit folding and alpha delta heterodimer formation in the assembly of the nicotinic acetylcholine receptor. Comparison of the mouse and human alpha subunits.

We have used the mouse alpha (alpha M) and human alpha (alpha H) subunits to investigate the molecular mechanisms of assembly of the mammalian acetylcholine receptor (AChR) transiently expressed in COS cells. COS cells expressing hybrid receptors incorporating alpha H along with other mouse subunits exhibited a 2-fold higher level of surface alpha-bungarotoxin (BuTx) binding than cells expressing the wild-type mouse AChR. When expressed either alone or with the delta subunit in COS cells, alpha H acquired the BuTx binding conformation (alpha Tx) more efficiently than did alpha M. By oligonucleotide-directed mutagenesis we showed that 2 residues in the amino-terminal domain were responsible for the differences between alpha M and alpha H. Alpha MST, the modified mouse alpha subunit, both folded more efficiently to form alpha Tx and was more effective in forming a stable alpha delta heterodimer than was alpha M. The kinetics of alpha Tx and alpha delta heterodimer formation revealed that the delta subunit increased the conversion of immature forms of the alpha subunit into the BuTx binding form and therefore provides evidence for interaction between the delta subunit and the immature form of the alpha subunit. These results provide evidence of the importance of the amino-terminal domains of the AChR subunits in the assembly process.