The second intracellular loop of metabotropic glutamate receptors recognizes C termini of G-protein α-subunits

Abstract

Heptahelical receptor coupling selectivity to G-proteins is controlled by a large contact area that involves several portions of the receptor and each subunit of the G-protein. In the G-protein α subunit, the C-terminal 5 residues, the N terminus, and the αN-β1 and α4-α5 loops play important roles. On the receptor side, both the second and third (i2 and i3) intracellular loops as well as the C-terminal tail probably contact these different regions of the G-protein. It is now accepted that the C terminus of the α subunit binds in a cavity formed by the i2 and i3 loops. Among the various G-protein-coupled receptors (GPCRs), class III receptors that include metabotropic glutamate (mGlu) receptors greatly differ from the rhodopsin-like GPCRs, but the contact zone between these receptors and the G-protein is less understood. The C terminus of the α subunit has been shown to play a pivotal role in the selective recognition of class III GPCRs. Indeed, the mGlu2 and mGlu4 and -8 receptors can discriminate between α subunits that differ at the level of their C-terminal end only (such as Gqo and Gqz). Here, we examine the role of the i2 loop of mGluRs in the selective recognition of this region of the α subunit. To that aim, we analyzed the coupling properties of mGlu2 and mGlu4 or -8 receptors and chimeras containing the i2 loop of the converse receptor to G-protein α subunits that only differ by their C termini (Gqo, Gqz, and their point mutants). Our data demonstrate that the central portion of the i2 loop is responsible for the selective recognition of the C-terminal end of the α subunit, especially the residue on position -4. These data are consistent with the proposal that the C-terminal end of the G-protein α subunit interacts with residues in a cavity formed by the i2 and i3 loops in class III GPCRs, as reported for class I GPCRs.