The structure of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor of the electric ray is currently one of the most extensively studied integral membrane proteins. A very speculative model for the structure of acetylcholine receptor in the membrane is shown in Fig. 1. This model does not take into account the extremely high density of receptor in the subsynaptic region of the membrane. The receptor is an oligomeric glycoprotein composed of five subunits, all of which are exposed externally, span the lipid bilayer, and are exposed cytoplasmically. Although the subunits have considerable sequence homology, they arise from the translation of different messenger RNAs. The naturally occurring form of the Torpedo receptor is a dimer, but monomer is also functionally active. Each receptor monomer has two binding sites for agonists and competitive antagonists but probably only one site for specific binding of local anesthetics. This latter site appears to be in the open ion channel near the center of the membrane.