Evidence for an important functional role of intracellular loop II of the lutropin receptor

The lutropin receptor (LHR) is a G protein-coupled receptor in which high affinity ligand binding occurs to the relatively large extracellular N-terminal domain. Various portions of the receptor have been mapped for their relative importance in localization and in hormone-mediated signaling. There is, however, a paucity of information available on the intracellular loops (ICL), where, along with the C-terminal cytoplasmic tail, G protein coupling is expected to occur. Site-directed mutagenesis was used to investigate the role of several conserved ionizable groups and one tyrosyl residue in ICLs I-III of the rat LHR. The pSVL expression vector, containing the LHR cDNA (wild-type and mutants), was transiently transfected into COS-7 cells, and human choriogonadotropin (hCG) binding and hCG-mediated cAMP production were determined. Several point mutants of amino acid residues in ICL II were prepared and characterized with the following results: replacements of Lys-455 and of His-460 with Glu gave mutant LHRs that failed to localize or fold properly at the cell surface as evidenced by the lack of significant binding to intact cells, although hCG binding could be detected in broken cell preparations, and a neighboring Arg-459 --> Glu replacement had no apparent effect on receptor trafficking, hCG binding or hCG-mediated cAMP-production. A reversal mutant in ICL II in which Glu-441, at the boundary of transmembrane helix III and ICL II, and His-460, at the interface between ICL II and transmembrane helix IV, were interchanged, exhibited hCG binding to intact cells, but the maximal cAMP level at high concentrations of ligand was less than that obtained with COS-7 cells transfected with wild-type LHR. The total number of cell surface receptors determined with the reversal mutant was less than that found with wild-type LHR. This difference, however, is not believed to be responsible for the reduced signaling, since maximal cAMP responses to hCG were obtained with comparable receptor densities of wild-type and various mutant LHRs. Other single replacements in ICL I, Lys-368 --> Glu and to Gin, and in ICL III, Arg-526 --> Glu and Tyr-528 --> Ser, resulted in mutant LHRs with characteristics of wild-type LHR in trafficking, hCG binding and hCG-mediated cAMP production. These findings suggest an important functional role of several amino acid residues in ICL II of LHR. (C) 1997 Elsevier Science Ireland Ltd.