Mapping the Ligand Binding Pocket of the Human Muscarinic Cholinergic Receptor Hml: Contribution of Tyrosine-82

The ligand binding pocket of many G protein-coupled receptors is thought to be located within the core formed by their seven transmembrane domains (TMDs). Previous results suggested that muscarinic antagonists bind to a pocket located toward the extracellular region of the TMDs, primarily at TMDs 2, 3, 6, and 7. Tyrosine-82 (Y82) is located in TMD2 only one helical turn from the presumed membrane surface of Hm1, whereas a phenylalanine (F124) is found in the equivalent position of the closely related Hm3. In order to determine the contribution of Y82 to Hm1 ligand binding and selectivity versus Hm3, we constructed the point mutation Y82 F of Hm1 and measured binding affinities of various ligands, with 3H-N-methylscopolamine (3H-NMS) as the tracer. The Hm1 wild-type receptor and the Y82F mutant were transfected into human embryonic kidney U293 cells. Whereas the affinities of NMS, carbachol, and atropine were either unchanged (carbachol) or enhanced by less than twofold (atropine and NMS), the affinity of the Hm1-selective piren-zepine was reduced threefold by the Y82 F mutation. These changes parallel affinity differences of Hm1 and Hm3, indicating that the Y82 F mutation affects the binding pocket and that Y82 contributes to the binding selectivity among closely related muscarinic receptors.