The transmembrane topology of the amino terminus of the alpha subunit of the nicotinic acetylcholine receptor

We have investigated the transmembrane topology of the amino-terminal domain of the alpha subunit of the mouse muscle nicotinic acetylcholine receptor synthesized in vitro and in vivo. Using oligonucleotide-directed mutagenesis we introduced new glycosylation consensus sequences at alpha 154 and at alpha 200. For each novel site, additional constructs were made in which the original site at alpha N141 was eliminated. Glycosylation at the new sites, as exhibited in a rabbit reticulocyte cell-free translation system supplemented with canine pancreatic microsomes and in a transient transfection system with COS cells, was taken as evidence of the transmembrane translocation of the new site. Each of the new sites was glycosylated in both systems. In separate experiments we found that an alpha subunit fragment terminating at alpha M207 could be extracted from microsomal membranes with sodium carbonate after in vitro translation, indicating that this fragment is not an integral membrane protein. Our results, taken together with previous experiments, indicate that the amino terminus of the alpha subunit up to at least residue alpha 207 is translocated across the membrane of the endoplasmic reticulum. This topology probably represents the orientation of the amino terminus of the alpha subunit in the assembled receptor.