Membrane tethering enables an extracellular domain of the acetylcholine receptor α subunit to form a heterodimeric ligand-binding site

Abstract

The first step of assembly of the nicotinic acetylcholine receptor (AChR) of adult skeletal muscle is the specific association of the α subunit with either δ or ε subunits to form a heterodimer with a ligand-binding site. Previous experiments have suggested that heterodimer formation in the ER arises from interaction between the luminal, NH2-terminal domains of the subunits. When expressed in COS cells with the δ subunit, however, the truncated NH2-terminal domain of the α subunit folded correctly but did not form a heterodimer. Association with the δ subunit occurred only when the NH2-terminal domain was retained in the ER and was tethered to the membrane by its own M1 transmembrane domain, by the transmembrane domain of another protein, or by a glycolipid link. In each case, the ligand-binding sites of the resulting heterodimers were indistinguishable from that formed when the full-length α subunit was used. Attachment to the membrane may promote interaction by concentrating or orienting the subunit; alternatively, a membrane-bound factor may facilitate subunit association.