MUTATIONS OF THE CONSERVED DRS MOTIF IN THE 2ND INTRACELLULAR LOOP OFTHE GONADOTROPIN-RELEASING-HORMONE RECEPTOR AFFECT EXPRESSION, ACTIVATION, AND INTERNALIZATION

The GnRH receptor is an unusual member of the G protein-coupled recept or (GPCR) superfamily with several unique features. One of these is a variant of the conserved DRY motif that is located at the junction of the third transmembrane domain and the second intracellular (2i) loop of most GPCRs. In the GnRH receptor, the Tyr residue of the conserved triplet is replaced by Ser, giving a DRS sequence. The aspartate and a rginine residues of the triplet are highly conserved in almost all GPC Rs. The functional importance of these residues was evaluated in wild type and mutant GnRH receptors expressed in COS-7 cells. Mutants in wh ich Asp(138) was replaced by Asn or Glu were poorly expressed, but sho wed significantly increased internalization and exhibited augmented in ositol phosphate generation to maximal agonist stimulation compared wi th the wild type receptor. In contrast, receptors in which Arg(139) wa s substituted With Gin, Ala, or Ser showed reduced internalization, an d the GnRH-induced inositol phosphate response for the Arg(139)Gln mut ant was significantly impaired in proportion to its low expression lev er. Replacing Ser(140) with Ala affected neither internalization nor s ignal transduction. The role of the polar amino acids at the C terminu s of the 2i loop was evaluated in two additional mutants (Ser(151)Ala, Ser(153)Ala, and Ser(151)Ala, Ser(153)Ala, Lys(154)Gln, Glu(156)Gln). Both of these mutants exhibited agonist-induced inositol phosphate re sponses similar to that of the wild type receptor, but showed increase d receptor internalization. This mutational analysis indicates that th e conserved Asp and Arg residues in the DRY/S triplet make important c ontributions to the structural integrity of the receptor and influence receptor expression, agonist-induced activation, and internalization.