COEXPRESSION OF DEFECTIVE LUTEINIZING-HORMONE RECEPTOR FRAGMENTS PARTIALLY RECONSTITUTES LIGAND-INDUCED SIGNAL GENERATION

Gonadotropin receptors are unique members of the seven-transmembrane ( TM), G protein-coupled receptor family with a large extracellular (EC) sequence forming the high-affinity ligand binding domain. In a patien t with Leydig cell hypoplasia, we identified a mutant LH receptor that is truncated at TM5. This protein retains limited ligand binding abil ity but cannot mediate cAMP responses, To study interactions between r eceptor fragments defective in either ligand binding or signal transdu ction, we co-expressed this truncated receptor together with a chimeri c receptor containing the EC region of the FSH receptor and the TM reg ion of the LH receptor. Although the chimeric receptor could not respo nd to human chorionic gonadotropin in producing cAMP, co-expression wi th the truncated LH receptor allowed partial restoration of ligand sig naling through intermolecular interactions, In addition, co-expression of the same truncated LH receptor with an N-terminally truncated LH r eceptor that lacked the EC ligand binding domain also partially restor ed ligand signaling, Further shortening of the TM region in the mutant receptor found in the patient indicated that the EC domain and TM1 we re sufficient for interactions with the N terminally truncated recepto r. In contrast, co-expression of the N terminally truncated receptor t ogether with cell-associated or soluble EC region of the LH receptor d id not allow ligand signaling, Unlike thrombin receptors, co-expressio n of the anchored EC region of the LH receptor together with the N-ter minally truncated receptor did not allow ligand signaling despite mode rate levels of human chorionic gonadotropin binding in transfected cel ls, These studies demonstrate that the co-expression of binding (+)/si gnaling (-) and binding (-)/signaling (+) receptor fragments partially restores ligand-induced signal generation and indicate the importance of TM1 of the LH receptor in the proper orientation of the EC ligand binding domain.