The gonadotropin-releasing hormone (GnRH) receptor in catfish differs from its mammalian counterparts in showing a very low affinity for the hypothalamic GnRH form [i.e. catfish GnRH (cfGnRH)] and a very high affinity for the highly conserved mesencephalic GnRH, chicken GnRH-II (cGnRH-II). In the present study we investigated the molecular interactions between ligand and receptor involved in determining the ligand selectivity of the catfish GnRH receptor. Studies on the binding characteristics of the catfish GnRH receptor for cfGnRH and cGnRH-II as well as for mammalian GnRH (mGnRH) and synthetic chimaeric GnRHs, differing at positions 5, 7 and 8, revealed that the low affinity of the catfish receptor for cfGnRH can be improved by replacing Leu7 by a tryptophan residue and/or Asn8 by either a tyrosine or an arginine residue. Testing cfGnRH and cGnRH-II as well as mGnRH and the chimaeric GnRHs on Asp304 → Ala, Asp304 → Glu and Asp304 → Asn mutant catfish GnRH receptors revealed that Asp304 of the catfish receptor mediates the recognition of Arg8 in mGnRH, as well as in the chimaeric peptides [Arg8]cfGnRH and [Arg8]cGnRH-II, but seems to be less important for the recognition of Tyr8 in cGnRH-II. On the basis of these results, a three-dimensional model for the binding of [Arg8]cGnRH-II to the catfish GnRH receptor is proposed.

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