Importance of extracellular domains for ligand binding in the thyrotropin-releasing hormone receptor.

The role of putative extracellular sequences for ligand binding in the TRH receptor was examined using deletion or substitution mutations. Each mutant receptor was transiently expressed in TRH receptor-minus GH(1)2C(1)b rat pituitary cells, and binding of 4 Nu Mu [3H]pGlu-N(tau)-MeHis-Pro-NH2 ([3H] MeTRH) was measured. When binding was not detected, signal transduction at 10 microM MeTRH was measured to assess receptor expression. Deletion of most of the N-terminal sequences (Glu(2)-Leu(22)), including two potential glycosylation sites, had no effect on the affinity of the receptor for MeTRH. Segmental deletions or simultaneous substitution of multiple amino acid residues in the first, second, or third extracellular loop (EL1, EL2, or EL3) resulted, however, in total loss of [3H]MeTRH binding, suggesting important roles for the loop sequences in either receptor expression or ligand binding. Individual substitutions were made to test further the role of the specific extracellular loop sequences in TRH binding. In EL1, conversion of Tyr93 to Ala resulted in more than 20-fold decrease in affinity for MeTRH. In EL2 and the top portion of the fifth transmembrane helix, conversion of Tyr181 to Phe, Tyr188 to Ala, and Phe199 to Ala resulted in a large ( > 100-fold) decrease in affinity for MeTRH, and conversion of Tyr 188 to Phe and Phe196 to Ala caused an agonist-specific 4- to 5-fold decrease in affinity. In EL3, conversion of Asn289 to Ala and of Ser290 to Ala caused a large ( > 100-fold) decrease in affinity for MeTRH. These results suggest important roles for the extracellular loops in high affinity TRH binding and lead us to propose a model in which TRH binds to the extra-cellular domain of its receptor.