Closely Related G-protein-coupled Receptors Use Multiple and Distinct Domains on G-protein α-Subunits for Selective Coupling*

The molecular basis of selectivity in G-protein receptor coupling has been explored by comparing the abilities of G-protein heterotrimers containing chimeric Gα subunits, comprised of various regions of Gi1α, Gtα, and Gqα, to stabilize the high affinity agonist binding state of serotonin, adenosine, and muscarinic receptors. The data indicate that multiple and distinct determinants of selectivity exist for individual receptors. While the A1 adenosine receptor does not distinguish between Gi1α and Gtα sequences, the 5-HT1A and 5-HT1B serotonin and M2 muscarinic receptors can couple with Gi1 but not Gt. It is possible to distinguish domains that eliminate coupling and are defined as “critical,” from those that impair coupling and are defined as “important.” Domains within the N terminus, α4-helix, and α4-helix-α4/β6-loop of Gi1α are involved in 5-HT and M2 receptor interactions. Chimeric Gi1α/Gqα subunits verify the critical role of the Gα C terminus in receptor coupling, however, the individual receptors differ in the C-terminal amino acids required for coupling. Furthermore, the EC50 for interactions with Gi1 differ among the individual receptors. These results suggest that coupling selectivity ultimately involves subtle and cooperative interactions among various domains on both the G-protein and the associated receptor as well as the G-protein concentration.