Conserved HisVI-17 of the NK-1 receptor is involved in binding of non-peptide antagonists but not substance P.

Residue number 17 in transmembrane segment VI has been shown to be crucial for the binding of agonists in G-protein-coupled receptors for the monoamines. In many peptide receptors a histidyl residue has been conserved at this position. We find that replacement of HisVI-17 in the NK-1 receptor with either glutamine, phenylalanine, or alanine has no apparent effect on the binding of the natural peptide ligand substance P or on the agonist induced increase in inositolphosphate turnover. However, the binding of certain non-peptide antagonists was impaired; for example, replacement of HisVI-17 with alanine decreased the affinity for FK888 and RP67,580 5- to 12-fold, respectively. A glutamine side chain was a good substitute for the imidazole in the binding of all non-peptide antagonists. It is concluded that the conserved HisVI-17 in the NK-1 receptor is involved in the binding of certain non-peptide antagonists, but is not important for the action of the natural peptide agonist, substance P.