Interaction of the COOH-terminal domain of the neurotensin receptor with a G protein does not control the phospholipase C activation but is involved in the agonist-induced internalization.

The agonist-induced internalization of the neurotensin receptor was studied in transfected Chinese hamster ovary cells expressing either the wild-type or a truncated rat neurotensin receptor, lacking the complete intracellular COOH-terminal end. Incubation of cells expressing the wild-type neurotensin receptor in the presence of the peptide resulted in a dramatic decrease in the [3H]neurotensin binding at the cell surface. This disappearance of cell surface binding sites resulted from the internalization of the receptor after the binding of the peptide. The receptor/peptide complexes were internalized in an intracellular compartment resistant to acid washes. The truncated receptor displayed high affinity binding properties for neurotensin in cell homogenates and activated phospholipase C as did the wild-type receptor. However, in cells expressing the truncated receptor, incubation with neurotensin only induced a partial decrease in cell surface binding, and internalization of the bound peptide was also impaired. On cell homogenates, the GTP analogue Gpp(NH)p was found to decrease the affinity of [3H]neurotensin for the wild-type receptor, whereas no similar effect was observed with the truncated receptor. These results show that the intracellular COOH-terminal region of the rat neurotensin receptor is not required for its functional coupling with intracellular G protein but is involved in the shift of the affinity of the receptor for the agonist, which occurs as a consequence of receptor activation and coupling. Because the truncated receptor was shown to internalize poorly, it may be proposed that internalization is not directly related to the activation of G protein but rather is a consequence of modification of receptor affinity, after activation by the agonist.