title: Epitope mapping of the receptor-bound agonistic form of human chorionic gonadotropin (hCG) in comparison to the antagonistic form (deglycosylated hCG).
abstract: On the surface of free human chorionic gonadotropin (hCG), we can distinguish with our panel of monoclonal antibodies (MCA) 14 topographically distinct epitopes (designated alpha 1 - alpha 5, beta 1 - beta 5, alpha beta 1 - alpha beta 4, depending on the subunit they are attached to). Only 2, i.e. the adjacent beta 3 and beta 5 epitopes, of these 14 are accessible to 125I-labeled MCA binding, when hCG is first allowed to bind to the rat testis hCG receptor. This result indicates that the agonist hCG assumes a defined orientation in its receptor-bound state and that, except for that small area comprising the beta 3 and beta 5 epitopes, most of its surface is masked by the hCG receptor. We therefore asked whether the competitive antagonist deglycosylated hCG (degly-hCG), which, when free, is antigenically (as to number and topography of epitopes) indistinguishable from native hCG, would interact with the receptor differently, that is, in a way that can be discerned by this epitope accessibility paradigm. Here we describe that on receptor-bound degly-hCG the beta 3 and beta 5 epitopes were concealed as were all other epitopes. This observation, together with finding the receptor affinity of degly-hCG to be 4 times higher than that of native hCG, suggests that degly-hCG assumes a signal transduction-incompetent ligand orientation and at the same time interacts with the receptor more intensively, i.e. establishes additional ("antagonist accessory") protein-protein contacts besides those involved in agonist binding. It thus appears that the carbohydrate moieties function to prevent formation of such accessory contacts.
authors.affiliation: Department of General & Experimental Pathology, University of Innsbruck, Faculty of Medicine, Austria.
authors.forename: S
authors.initials: S
authors.lastname: Schwartz
authors.forename: H
authors.initials: H
authors.lastname: Krude
authors.forename: W E
authors.initials: WE
authors.lastname: Merz
authors.forename: C
authors.initials: C
authors.lastname: Lottersberger
authors.forename: G
authors.initials: G
authors.lastname: Wick
authors.forename: P
authors.initials: P
authors.lastname: Berger
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D002462:Cell Membrane / Q000378:metabolism; D006063:Chorionic Gonadotropin / Q000276:immunology / Q000378:metabolism*; D000939:Epitopes / Q000032:analysis*; D007700:Kinetics; D008297:Male; D008961:Models, Structural; D011487:Protein Conformation; D011869:Radioligand Assay; D051381:Rats; D011967:Receptors, Gonadotropin / Q000378:metabolism*; D013737:Testis / Q000378:metabolism
chemical_list: D000911:Antibodies, Monoclonal; D006063:Chorionic Gonadotropin; D000939:Epitopes; D011967:Receptors, Gonadotropin