The Second Intracellular Loop of Metabotropic Glutamate Receptor 1 Cooperates with the Other Intracellular Domains to Control Coupling to G-proteins

Metabotropic glutamate receptors (mGluR) share no sequence homology with any other G-protein-eoupled receptors (GPCRs). The characterization of their G-protein coupling domains will therefore help define the general rules for receptor-G-protein interaction. To this end, the intracellular domains of mGluR3 and mGluR1, receptors coupled negatively to adenylyl cyclase and positively to phospholipase C, respectively, were systematically exchanged. The ability of these chimeric receptors to induce Ca²⁺ signals were examined in Xenopus oocytes and HEK 293 cells. The chimeric receptors that still possessed the second intracellular loop (i2) of mGluR3 induced little or no Ca²⁺ signals, even though these proteins were targeted correctly to the plasma membrane. Consistent Ca²⁺ signals could be recorded only with chimeric mGluR3 receptors that contains i2 and at least one other intracellular domain of mGluR1. However, most intracellular domains of mGluR3 have to be replaced by their mGluR1 equivalent to produce optimal coupling to G protein. These observations indicate that i2 of mGluR1 is a critical element in determining the transduction mechanism of this receptor. These results suggest that i2 of mGluRs may play a role similar to i3 of most other GPCRs in the specificity of coupling to the G-proteins. Moreover, as in many other GPCRs, our data revealed cooperation between the different mGluR intracellular domains to control efficient coupling to G-proteins.