Identification of receptor contact site involved in receptor-G protein coupling

The mammalian G proteins transduce information from extracellular signals, including neurotransmitters, hormones and sensory stimuli, into regulation of effector enzymes or ion channels within cells. Triggered by appropriate extracellular signals, receptor proteins specifically activate members of the G protein family by catalysing replacement of GDP by GTP at the guanine nucleotide binding site. Like the receptor proteins¹, the heterotrimeric G proteins exhibit impressive structural similarities^2,3, suggesting that all receptor-G protein interactions use homologous structural elements and a single molecular mechanism. Topologically equivalent portions of each G protein may therefore interact with the appropriate receptor. We recently predicted the secondary structure of a composite G protein α-chain and proposed that a predicted amphipathic α-helix at the extreme carboxy-terminus of the polypeptide directly contacts receptors⁴. This proposal has now been confirmed by sequencing complementary DNAs of the gene that encodes the α-chain (α_s) of the stimulatory regulator (G_s) of adenylyl cyclase in wild-type cells and in a mutant mouse S49 lymphoma cell line, unc, in which G_s cannot be activated by hormone receptors⁵. The sequences reveal a point mutation in the unc gene that substitutes a proline residue for an arginine near the carboxy-terminus of the α_s-polypeptide. Expression of recombinant α_s-unc in genetically α_s-deficient S49 cells reproduces the unc phenotype.