IDENTIFICATION OF A NEW COMPONENT OF THE AGONIST BINDING-SITE OF THE NICOTINIC ALPHA-7 HOMOOLIGOMERIC RECEPTOR

Tryptophan 54 of the alpha 7 neuronal nicotinic homooligomeric receptor is homologous to gamma-Trp-55 and delta-Trp-57 of non-alpha subunits of Torpedo receptor labeled by d-tubocurarine. This residue was mutated on the alpha 7-V201-5-hydrorrytryptamine (5HT)(3) homooligomeric chimera, which displays alpha 7 nicotinic pharmacology, and for which both equilibrium binding studies and electro physiological recordings could be carried out in parallel. Replacement of Trp-54 by a Phe, Ala, or His causes a progressive decrease both in binding affinity and in responses (EC(50) or IC50) for acetylcholine, nicotine, and dihydro-beta-erythroidine, without significant modification in alpha-Bgtx binding. Except for Gln-56, comparatively small effects are observed when the other residues of the 52-58 region are mutated into alanine. These data support the participation of Trp-54 to ligand binding, and provide evidence for a new ''complementary component'' of the alpha 7 nicotinic binding site, distinct from its three-loop ''principal component,'' and homologous to the ''non-alpha component'' present on gamma and delta subunits.