Abstract
The luteinizing hormone/chorionic gonadotropin receptor (LH/CG-R) is a member of the glycoprotein hormone receptor family which includes the follicle stimulating hormone receptor (FSH-R) and the thyroid stimulating hormone receptor (TSH-R). These receptors are characterized by a large extracellular hydrophilic N-terminal region and a membrane embedded C-terminal region containing three extracellular loops, seven putative membrane-embedded helices, three cytoplasmic loops and a domain containing the $\alpha$-carboxyl group within the cytoplasm. Sequence alignment of the gonadotropin receptors revealed high overall homology between the LH/CG, FSH and TSH receptors. Therefore, this suggests an important role for conserved residues in receptor binding and/or signal transduction.Site-directed mutagenesis was used to prepare and characterize single replacements of the rat LH/CG-R at positions Arg-341, Lys-368, Lys-401, Thr-424, Arg-459, Lys-455, His-460, Pro-479, Lys-488, Arg-526, Tyr-528, Met-560, Pro-562, Ser-564, Lys-583, Tyr-590, Pro-591, Pro-598 and Tyr-601. In addition, two reciprocal mutants were prepared: Asp-397 was replaced with Lys and Lys-583 was changed to Asp; Glu-441 was substituted with His and His-460 was replaced with Glu.The results demonstrated that Lys-583, Pro-562, Pro-591 and Tyr-601 are not required for hormone binding but participate in signal transduction. The Lys-401, Pro-479 and His-460 mutations resulted in LH/CG-Rs which are expressed; however, hormone binding was abolished. Replacements of Lys-455 and Pro-598 interfered with membrane localization of the receptor. Similarly, the Asp-397-Lys, Lys-583-Asp reciprocal mutation yielded a LH/CG-R which remains trapped intracellularly. Both Glu-441 and His-460 appear to be involved in achieving proper trafficking of the receptor; these residues are important in signal transduction as well.This study has identified several amino acid residues in the LH/CG-R that are important in signal transduction, hormone binding and cell-surface expression of the receptor. These structure-function relationships, coupled with other findings may lead to a better understanding of hormone binding, receptor activation and signal transduction for the glycoprotein hormone receptors.