Crystal structures of free and antagonist-bound states of human α9 nicotinic receptor extracellular domain

We determined the X-ray crystal structures of the extracellular domain (ECD) of the monomeric state of human neuronal alpha 9 nicotinic acetylcholine receptor (nAChR) and of its complexes with the antagonists methyllycaconitine and alpha-bungarotoxin at resolutions of 1.8 angstrom, 1.7 angstrom and 2.7 angstrom, respectively. The structure of the monomeric alpha 9 ECD superimposed well with the structures of homologous proteins in pentameric assemblies, denoting native folding, despite the absence of a complementary subunit and transmembrane domain. The interaction motifs of both antagonists were similar to those in the complexes with homologous pentameric proteins, thus highlighting the major contribution of the principal side of alpha 9 ECD to their binding. The structures revealed a functionally important beta 7-beta 10 strand interaction in alpha 9-containing nAChRs, involving their unique Thr147, a hydration pocket similar to that of mouse alpha 1 ECD and a membrane-facing network coordinated by the invariant Arg210.