Deletion mutation in the putative third intracellular loop of the rat neurotensin receptor abolishes polyphosphoinositide hydrolysis but not cyclic AMP formation in CHO-K1 cells

The tridecapeptide neurotensin is a putative neurotransmitter in the central nervous system. Previously, we showed that the rat neurotensin receptor expressed in CHO-K1 cells mediates polyphosphoinositide hydrolysis and cAMP formation. To further investigate these neurotensin receptor- mediated signal transduction systems, we constructed three deletion mutations in the putative third intracellular loop of this receptor and transfected these mutated genes into CHO-K1 cells. The equilibrium dissociation constants for specific [³H]neurotensin binding to these mutants were not different from that for the wild-type receptor. However, one mutant, which lacked 27 amino acids (amino acids 270-296), did not stimulate polyphosphoinositide hydrolysis, whereas it retained its ability to stimulate cAMP formation. In addition, as found for the wild-type receptor, sequestration occurred with this mutant. We demonstrated here that the putative third intracellular loop of this receptor plays a role in coupling to certain G proteins that induce polyphosphoinositide hydrolysis but not in coupling to cAMP formation or in ligand binding. The two different signal transduction systems may be induced by different G proteins that couple at different sites of the neurotensin receptor protein in CHO-K1 cells. Furthermore, our data show that neurotensin receptor sequestration is independent of agonist-induced polyphosphoinositide hydrolysis.