Studies with chimeras of the gonadotropin receptors reveal the importance of third intracellular loop threonines on the formation of the receptor/nonvisual arrestin complex

Using chimeras and more discrete exchange mutations of the rat (r) and human (h) gonadotropin receptors, we had previously identified multiple noncontiguous residues of the lutropin (LHR) and follitropin (FSHR) receptors that dictate their rates of internalization. Since the internalization of the LHR and the FSHR is driven by their abilities to associate with the nonvisual arrestins, we hypothesized that one or more of the residues previously identified by the internalization assays are involved in the formation of the receptor/nonvisual arrestin complex. In the studies reported herein, we tested this hypothesis by measuring the association of arrestin-3 with a large number of rLHR/hLHR and rFSHR/hFSHR exchange mutants that affect internalization. The results presented show that the same residues that dictate the rate of internalization of these two receptor pairs affect their ability to associate with arrestin-3. Although these residues are located in distinct topological domains, our analyses show that threonine residues in the third intracellular loop of both receptor pairs are particularly important for the formation of the receptor/arrestin-3 complexes and internalization. We conclude that the different rates of internalization of the gonadotropin receptors are dictated by their different abilities to associate with the nonvisual arrestins and that this association is, in turn, largely dictated by the presence of threonine residues in their third intracellular loops.