THE SPECIES SELECTIVITY OF CHEMICALLY DISTINCT TACHYKININ NONPEPTIDE ANTAGONISTS IS DEPENDENT ON COMMON DIVERGENT RESIDUES OF THE RAT AND HUMAN NEUROKININ-1 RECEPTORS

During evolution mutations have occurred in peptide receptors that are neutral with respect to binding of the natural peptide ligand but fre quently affect the binding of nonpeptide antagonists. By systematicall y introducing the nonconserved residues from the human neurokinin (NK) -1 receptor into the corresponding rat receptor we have attempted to l ocalize the structural elements that are responsible for 15-76-fold hi gher affinity of three tachykinin nonpeptide antagonists for the human receptor, compared with the corresponding rat receptor. Surprisingly, exchange of the four divergent residues located around the previously located apparent binding site for CP 96,345 and FK 888 at the top of transmembrane segment (TM) V and VI, either alone or as a group, did n ot affect the binding of these nonpeptide compounds. However, substitu tion of Ser(290) in TM VII of the rat receptor with isoleucine present in the human receptor increased the affinity for FK 888 20-fold and t hat for CP 96345 6-fold, corresponding to an affinity that was only ab out 4-fold less than the affinity for the human NK-1 receptor. Full hu man-like affinity for FK 888 and CP 96,345 could be conveyed to the ra t receptor by the combined substitution of Ser(290) in TM VII to isole ucine and Leu(116) in TM III to valine. The NK-2 receptor-selective co mpound SR 48,968 was found to bind with low affinity to the human NK-1 receptor but with 15-fold even lower affinity to the rat receptor. Su bstitution of residue 290, which is situated within the previously loc ated binding site for this compound, could completely account for this difference. These data demonstrate that the species selectivities of the nonpeptide antagonists CP 96345, FK 888, and SR 48,968, independen tly of clear differences in their chemical structures and modes of dis covery, have a similar structural basis, being dependent on two diverg ent residues that apparently are not involved in peptide agonist bindi ng.