ASSEMBLY OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR - THE FIRST TRANSMEMBRANE DOMAINS OF TRUNCATED ALPHA-SUBUNIT AND DELTA-SUBUNIT ARE REQUIRED FOR HETERODIMER FORMATION IN-VIVO

To investigate the mechanism of assembly of the mouse muscle acetylcho line receptor, we have expressed truncated N-terminal fragments of the alpha and delta subunits in COS cells and have examined their ability to fold, to associate into heterodimers, and to form a ligand-binding site, Truncated fragments of the alpha subunit that include all, part , or none of the first transmembrane domain (M1) folded to acquire cu- bungarotoxin binding activity, Neither the full-length alpha subunit n or any of the fragments were expressed on the cell surface, although t he shortest folded fragment lacking a transmembrane domain was secrete d into the medium, When coexpressed with the delta subunit, the alpha subunit fragment possessing M1 formed a heterodimer containing a ligan d-binding site, but shorter fragments, which lack transmembrane segmen ts, did not associate with the delta subunit, N-terminal delta subunit fragments gave similar results. An N-terminal delta subunit fragment that contains M1 associated with the alpha subunit to form a heterodim er, while a fragment lacking M1 did not. These results show that a com plete M1 domain is necessary for association of truncated N-terminal a lpha and delta subunits into a heterodimer with high affinity ligand b inding activity.