HCG-RECEPTOR BINDING AND TRANSMEMBRANE SIGNALING

The technique of site-directed mutagenesis has proven to be quite powe rful in elucidating contact sites involved in the interaction of the h eterodimeric glycoprotein hormones and their respective seven transmem brane (TM) G protein-coupled receptors. Our laboratory has focused on identification of the minimum core sequences of the alpha and beta sub units required for bioactivity, the minimum length of a conjoined (yok ed) single-chain hCG, the amino acid residues on hCG and the LH/CG-rec eptor (LH/CG-R) responsible for high-affinity binding, and the regions of the receptor that are involved in TM signaling. A number of amino acid residues have been mapped on the alpha and beta subunits of hCG t hat appear important in receptor binding. When projected onto the crys tal structure of HF-treated hCG, these residues, by and large, cluster on one side of the molecule and cover a sizeable surface area, indica ting that the hormone-receptor binding interface is rather extensive. Based on mutagenesis studies of several conserved ionizable amino acid residues in the extracellular domain (ECD) of LH/CG-R and a model tha t we, in collaboration with Drs Lapthorn and Isaacs, have developed fo r this region based on the crystal structure of porcine ribonuclease i nhibitor, a charged region that appears to play an important role in h ormone-receptor recognition has been identified. We have also delineat ed several regions of LH/CG-R that do not appear to participate in hCG binding but are involved in hCG-mediated signaling. These regions are located in the ECD and extracellular loop III just prior to entry int o the membrane via TM helices I and VII: respectively, and in TM helic es VI and VII. Similarly, a homologous region in the ECD of the FSH re ceptor, located with ten residues of TM helix I, is important in signa ling but not hormone binding. These results suggest that ligand bindin g and ligand-mediated receptor activation are quasi-distinct, albeit s equential phenomena. Collectively, our mutagenesis and modeling studie s, coupled with results from other laboratories, argue for a ligand-in duced conformational change of the receptor that may involve a relativ e reorientation of the TM helices. Copyright (C) 1996 Elsevier Science Ireland Ltd.