Mutations of the Conserved DRS Motif in the Second Intracellular Loop of the Gonadotropin-Releasing Hormone Receptor Affect Expression, Activation, and Internalization

The GnRH receptor is an unusual member of the G protein-coupled receptor (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 arginine residues of the triplet are highly conserved in almost all GPCRs. The functional importance of these residues was evaluated in wild type and mutant GnRH receptors expressed in COS-7 cells. Mutants in which Asp138 was replaced by Asn or Glu were poorly expressed, but showed significantly increased internalization and exhibited augmented inositol phosphate generation to maximal agonist stimulation compared with the wild type receptor. In contrast, receptors in which Arg139 was substituted with Gln, Ala, or Ser showed reduced internalization, and the GnRH-induced inositol phosphate response for the Arg139Gln mutant was significantly impaired in proportion to its low expression level. Replacing Ser140 with Ala affected neither internalization nor signal transduction. The role of the polar amino acids at the C terminus of the 2i loop was evaluated in two additional mutants (Ser151Ala, Ser153Ala, and Ser151Ala, Ser153Ala, Lys154Gln, Glu156Gln). Both of these mutants exhibited agonist-induced inositol phosphate responses similar to that of the wild type receptor, but showed increased receptor internalization. This mutational analysis indicates that the conserved Asp and Arg residues in the DRY/S triplet make important contributions to the structural integrity of the receptor and influence receptor expression, agonist-induced activation, and internalization.