Dependence of agonist activation on an aromatic moiety in the DPLIY motif of the gonadotropin-releasing hormone receptor.

In the GnRH receptor, the NPX2-3Y motif that is present in the seventh transmembrane helix of most G protein-coupled receptors is unusual in containing Asp instead of Asn but retains the highly conserved Tyr residue. The importance of this aromatic residue in the DPLIY sequence of the GnRH receptor function was analyzed by replacing Tyr322 with Ala or Phe residues. The Y322A mutant receptor expressed in COS-7 cells had high agonist binding affinity, but its ability to interact with G protein(s) and to activate inositol phosphate production in response to GnRH was abolished. Although functionally inactive, the Y322A mutant receptor was internalized at about 50% of the rate of the wild type receptor in agonist-treated cells. When Tyr322 was replaced with Phe to preserve its aromatic nature, the Y322F mutant receptor displayed normal G protein activation and inositol phosphate responses to GnRH and was internalized in the same manner as the wild type receptor. These findings demonstrate that the aromatic moi...