Contrasting internalization kinetics of human and chicken gonadotropin- releasing hormone receptors mediated by C-terminal tail

The chicken gonadotropin-releasing hormone receptor (GnRH-R) is notable for having a cytoplasmic C-terminal tail, which is not present in the mammalian GnRH-Rs. We report here that the cytoplasmic tail mediates rapid agonist-promoted receptor internalization. The chicken GnRH-R mediated internalization of gonadotropin-releasing hormone (GnRH) agonist (125I- [His5-D-Tyr6]GnRH) at a rate of 11.3 %.min-1, compared with only 0.71%.min-1 for the human GnRH-R. To determine whether the presence of the cytoplasmic tail was responsible for the more rapid internalization kinetics of the chicken GnRH-R, we truncated the tail after the Ile336 residue (S337stop). Receptor-mediated internalization of GnRH agonist by the S337stop-chicken GnR-R was much slower than in the wild-type chicken receptor, and was similar to the wild-type human GnRH-R (0.55 %.min-1). These data indicate that rapid agonist-promoted internalization of the chicken GnRH-R is mediated through elements in the cytoplasmic C-terminal tail, distal to or including Ser337 and suggests that elimination of the C-terminal tail during evolution of mammalian GnRH-Rs may be related to its effects on internalization.