A MODEL OF THE NICOTINIC RECEPTOR EXTRACELLULAR DOMAIN BASED ON SEQUENCE IDENTITY AND RESIDUE LOCATION

We have modeled the extracellular domains of individual subunits (amin o acids 31-200) in the nicotinic acetylcholine receptor using sequence homology with copper binding proteins of known crystal structure, pla stocyanin and pseudoazurin, and data from recent site-specific mutagen esis, antibody mapping, and site-directed labeling studies. These data formed an initial model that was refined using molecular dynamics and mechanics as well as electrostatic and solvation energy calculations. The sequences between residues 31 and 164 in the alpha(1)-subunit and corresponding residues in homologous receptor subunits show similarit y with the Gore sequence of the cation binding site in plastocyanin an d pseudoazurin, a region in the template proteins characterized by mul tiple hairpin loops. in addition to defining the subunit interfaces th at comprise the site for agonist and competitive antagonist binding in more detail, the findings show that negatively charged residues clust er in domains arranged to diminish electrostatic free energy of the co mplex. Electrostatic factors also appear to distinguish the ligand bin ding interfaces, alpha gamma and alpha delta, from the other three int erfaces on the pentameric receptor.