Surface retention of an inactivating lutropin receptor mutant in exoloop 3

The heptahelical lutropin receptor (LHR) signals primarily via the G(s)-adenylyl cyclase pathway and undergoes ligand-mediated receptor desensitization and internalization. A loss-of-function rat LHR mutant was recently described in which a single amino acid residue replacement in exoloop 3, K583E, had no effect on human choriogonadotropin (hCG) binding but essentially abolished signaling. This LHR mutant is a prime candidate for which to study hCG-mediated receptor internalization since it is highly unlikely that an amino acid residue in exoloop 3, i.e. an extracellular portion of LHR connecting transmembrane helices 6 and 7, could have any direct interaction with Gas, which is located on the cytoplasmic face of the plasma membrane. A method to study endocytosis was adapted that involves concanavalin A binding to the glycoproteins on the cell surface, thus facilitating separation of the plasma membrane fraction from other cellular membrane fractions by sucrose gradient centrifugation. Conditions were used such that a single round of endocytosis could be determined with [I-125]hCG. Endocytic rate constants of 0.03 and 0 min(-1) were obtained for LHR and the mutant, respectively, in transfected human embryonic kidney 293 cells; moreover, internalization of the mutant could not be restored by the addition of 8-Br-cAMP. Thus, the presence of the second messenger cAMP is not sufficient for internalization of ligand-occupied LHR. Rather, it appears that ligand-mediated activation and subsequent internalization of LHR results from an altered conformational state or a conformation-dependent post-ligand binding modification such as phosphorylation.