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 α1 - α5, β1 - β5, αβ1 - αβ4, depending on the subunit they are attached to). Only 2, i.e. the adjacent β3 and β5 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 β3 and β5 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 β3 and β5 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.