Residues within transmembrane helices 2 and 5 of the human gonadotropin-releasing hormone receptor contribute to agonist and antagonist binding

To understand the ligand binding properties of the human GnRH receptor (hGnRH-R), 24 site-specific mutants within transmembrane helices (TMH) 1, 2, and 5 and the extracellular loop 2 (E2) were generated. These mutants were analyzed by using a functional reporter gene assay, monitoring receptor signaling via adenylate cyclase to a cAMP-responsive element fused to Photinus pyralis luciferase. The functional behavior of 14 receptor mutants, capable of G-protein coupling and signaling, was studied in detail with different well described agonistic and antagonistic peptide ligands. Furthermore, the binding constants were determined in displacement binding experiments with the antagonist[ 125I]Cetrorelix. The substitution of residues K36, Q204, W205, H207, Q208, F210, F213, F216, and S217 for alanine had no or only a marginal effect on ligand binding and signaling. In contrast, substitution of N87, E90, D98, R179, W206, Y211, F214, and T215 for alanine resulted in receptor proteins neither capable of ligand bind...