Molecular determinants of selectivity in 5-hydroxytryptamine1B receptor-G protein interactions

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-HT1B) receptor to discriminate between G protein heterotrimers containing G alpha(i1) or G alpha(t). G(i1) can interact with the 5-HT1B receptor and stabilize a high affinity agonist binding state of this receptor, but G(t) cannot. A series of G alpha(t)/G alpha(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 G alpha(t)/G alpha(i1) chimeras to 5-HT1B receptors using high affinity agonist binding and receptor-stimulated guanosine 5'-3-0-(thio)triphosphate (GTP gamma S) binding, In the presence of beta gamma subunits, amino acid residues 299-318 of G alpha(i1) increase agonist binding to the 5-HT1B receptor and receptor stimulation of GTP gamma S binding, Moreover, G alpha(i1) containing only G alpha(t) amino acid sequences from this region does not show any coupling ability to 5-HT1B receptors, Our studies suggest that the alpha 4 helix and alpha 4-beta 6 loop region of G alpha s are an important region for specific recognition between receptors and G(i) family members.