Abstract

The recognition between G protein and cognate receptor plays a key role in specific cellular responses to environmental stimuli. Here we explore specificity in receptor-G protein coupling by taking advantage of the ability of the 5-hydroxytryptamine_1B (5-HT_1B) receptor to discriminate between G protein heterotrimers containing Gα_i1 or Gα_t. G_i1 can interact with the 5-HT_1B receptor and stabilize a high affinity agonist binding state of this receptor, but G_t cannot. A series of Gα_t/Gα_i1 chimeric proteins have been generated in Escherichia coli, and their functional integrity has been reported previously (Skiba, N. P., Bae, H., and Hamm, H. E. (1996) J. Biol. Chem. 271, 413–424). We have tested the functional coupling abilities of the Gα_t/Gα_i1 chimeras to 5-HT_1Breceptors using high affinity agonist binding and receptor-stimulated guanosine 5′-3-O-(thio)triphosphate (GTPγS) binding. In the presence of βγ subunits, amino acid residues 299–318 of Gα_i1 increase agonist binding to the 5-HT_1Breceptor and receptor stimulation of GTPγS binding. Moreover, Gα_i1 containing only Gα_t amino acid sequences from this region does not show any coupling ability to 5-HT_1B receptors. Our studies suggest that the α4 helix and α4-β6 loop region of Gαs are an important region for specific recognition between receptors and G_i family members.