Mutations that induce constitutive activation and mutations that impair signal transduction modulate the basal and/or agonist-stimulated internalization of the lutropin/choriogonadotropin receptor

Previous results from this laboratory suggested that the same active confor mation of the lutropin/choriogonadotropin receptor (LHR) is involved in the stimulation of G proteins and in triggering the internalization of the bou nd agonist, We have now analyzed two naturally occurring, constitutively active mutants of the human LHR, These mutations were introduced into the rat LHR (rLHR) and are designated L435R and D556Y, Cells expressing rLHR-D556Y bind human choriogonadotropin (hCG) with normal affinity, exhibit a 25-fold increase i n basal cAMP and respond to hCG with a normal increase in cAMP accumulation . This mutation does not affect the internalization of the free receptor, b ut it enhances the internalization of the agonist-occupied receptors simila r to 3-fold. Cells expressing rLHR-L435R also bind hCG with normal affinity , exhibit a 47-fold increase in basal cAMP, and do not respond to hCG with a further increase in cAMP accumulation. This mutation enhances the interna lization of the free and agonist-occupied receptors similar to 2- and simil ar to 17-fold, respectively, We conclude that the state of activation of th e rLHR can modulate its basal and/or agonist-stimulated internalization. Since the internalization of hCG is involved in the termination of hCG acti ons, we suggest that the lack of responsiveness detected in cells expressin g rLHR-L435R is due to the fast rate of internalization of the bound hCG. T he finding that membranes expressing rLHR-L435R (a system where internaliza tion does not occur) respond to hCG with an increase in adenylyl cyclase ac tivity supports this suggestion.