FUNCTIONAL NONEQUIVALENCE OF STRUCTURALLY HOMOLOGOUS DOMAINS OF NEUROKININ-1 AND NEUROKININ-2 TYPE TACHYKININ RECEPTORS

The neurokinin-1 (NK-1) and neurokinin-2 (NK-2) receptors are both mem bers of the tachykinin receptor family. Although both receptors bind p eptide ligands synthesized from common precursors and activate inosito l 1,4,5-triphosphate and cAMP responses, differences between these rec eptors have been observed in the extent and kinetics of agonist-induce d responses. Here, to test if structurally homologous domains of the N K-1 and NK-2 receptors are functionally distinct, stably transfected C hinese hamster ovary (CHO) cell lines expressing receptors that had ei ther their putative third cytoplasmic loop (C3) or carboxyl tail (CT) domains replaced with the equivalent domain of the other receptor were compared with stably transfected CHO cell lines expressing wild-type receptors. Radioligand binding demonstrated that each of these chimeri c receptors had agonist binding affinities indistinguishable from wild -type receptors. However, not all chimeric receptors were equivalent i n their ability to stimulate inositol phospholipid turnover and cAMP p roduction. The data suggest that the NK-1 C3 and the NK-2 CT domains p lay important roles in G-protein activation that cannot be replaced by the analogous domain of the other receptor. The characterization of C HO cell lines expressing truncated forms of both receptors supported t he hypothesis that the CT domain of the NK-2, but not the NK-1, recept or plays a critical role in G-protein activation. The data suggest a p otential mechanism for the differences observed in response characteri stics in tissues expressing NK-1 and NK-2 receptors and demonstrate th at the mechanisms whereby highly homologous receptors activate G-prote ins can be different.