A highly conserved glycine within linker I and the extreme C terminus of G protein α subunits interact cooperatively in switching G protein-coupled receptor-to-effector specificity

Numerous studies have attested to the importance of the extreme C terminus of G protein α subunits in determining their selectivity of receptor recognition. We have previously reported that a highly conserved glycine residue within linker I is important for constraining the fidelity of receptor recognition by Gα_q proteins. Herein, we explored whether both modules (linker I and extreme C terminus) interact cooperatively in switching G protein-coupled receptor (GPCR)-to-effector specificity and created as models mutant Gα_q proteins in which glycine was replaced with various amino acids and the C-terminal five Gα_q residues with the corresponding Gα_i or Gα_s sequence. Coupling properties of the mutated Gα_q proteins were determined after coexpression with a panel of 13 G_i-and G_s-selective receptors and compared with those of Gα proteins modified in only one module. Gα proteins modified in both modules are significantly more efficacious in channeling non-G_q-selective receptors to G _q-mediated signaling events compared with those containing each module alone. Additive effects of both modules were observed even if individual modules lacked an effect on GPCR-to-effector specificity. Dually modified Gα proteins were also superior in conferring high-affinity agonist sites onto a coexpressed GPCR in the absence, but not in the presence, of guanine nucleotides. Together, our data suggest that receptor-G protein coupling selectivity involves cooperative interactions between the extreme C terminus and linker I of Gα proteins and that distinct determinants of selectivity exist for individual receptors.