Localization of Agonist and Antagonist Binding Domains of the Human Neurokinin-1 Receptor

To identify the molecular determinants of ligand-receptor interactions, the extracellular domain of the human neurokinin-1 receptor was systematically substituted with the corresponding sequences from the other two neurokinin receptor subtypes. Three residues within the first extracellular segment and 2 residues of the second segment are required for the optimal binding of all three natural peptide agonists. The divergent nature of 4 of the 5 residues supports the hypothesis that the peptide binding site on the neurokinin-1 receptor is not highly conserved in the other two receptor subtypes. In contrast, substitution of part of the third extracellular segment and the fourth extracellular segment with the corresponding amino acids of the human neurokinin-3 receptor results in an increase in neurokinin B affinity without affecting substance P binding, suggesting that the two peptides do not interact with the same set of functional groups on the receptor. Among the four extracellular regions, only parts of the third and fourth segments affect the binding of the quinuclidine antagonist L-703,606, and these two regions may partially account for the neurokinin-1 receptor subtype specificity of this non-peptide antagonist. These studies demonstrate that both the extracellular and transmembrane domains of the neurokinin-1 receptor are involved in the binding of substance P and related peptides.