Identification of amino acid residues in transmembrane helices VI and VII of the lutropin choriogonadotropin receptor involved in signaling

The lutropin/choriogonadotropin receptor (LH/CG-R) is a member of the G protein-coupled receptor superfamily containing a relatively large extracellular domain responsible for high affinity ligand binding. There is a paucity of information on the mechanism of ligand-mediated transmembrane signaling via the seven transmembrane helices (TMH) of these receptors. In the present study we have used site directed mutagenesis to replace each of nine conserved amino acid residues in the TMHs of rat LH/CG-R. COS-7 cells were transiently transfected with the eukaryotic expression vector pSVL-LH/CG-R, wild type and mutants, followed by measurements of human CG binding and human CG-mediated cAMP production. Three point mutants of LH/CG-R were prepared that diminished signaling but not binding: Pro-562-Leu (TMH VI), Pro-591-Leu (TMH VII), and Tyr-601-Ala (TMH VII). Two point mutants of LH/CG-R, Pro-479-Leu (TMH IV) and Pro-598-Leu (TMH VII), resulted in impaired localization, and four receptor mutants, Thr-424-Ala (TMH III), Ser-562-Ala (TMH VI), Met-560-Leu (TMH VI), and Tyr-590-Ala (TMH VII), were similar to wild-type LH/CC-R. In summary, these findings indicate a critical role of Tyr-601 in transmembrane signaling of LH/CG-R since an Ala replacement results in almost total abolition of cAMP production in response to human CG; prolines 562 and 591 also appear to be important for full signaling.