SUBSTITUTION OF 3 AMINO-ACIDS SWITCHES RECEPTOR SPECIFICITY OF G(Q)ALPHA TO THAT OF G(I)ALPHA

AGONIST-BOUND receptors activate heterotrimeric (alphabetagamma) G pro teins by catalysing replacement of GDP bound to the alpha-subunit by G TP1-5. Mutations in the C terminus of the alpha-subunit6,7, its covale nt modification by pertussis toxin-catalysed ribosylation of ADP8, pep tide-specific antibodies directed against it9-11, and peptides mimicki ng C-terminal sequences12, all inhibit receptor-mediated activation of G proteins. The logical prediction-that specific amino-acid residues at the C-termini of alpha-subunits can determine the abilities of indi vidual G proteins to discriminate among specific subsets of receptors- has so far not been tested experimentally. Different hormone receptors specifically activate G(q) or G(i), whose alpha-subunits (alpha(q) or alpha(i)) stimulate phosphatidylinositol-specific phospholipase C or inhibit adenylyl cyclase, respectively1-5. Here we replace C-terminal amino acids of alpha(q) with the corresponding residues of alpha(i2) t o create alpha(q)/alpha(i2) chimaeras that can mediate stimulation of phospholipase C by receptors otherwise coupled exclusively to G(i). A minimum of three alpha(i2) amino acids, including a glycine three resi dues from the C terminus, suffices to switch the receptor specificity of the alpha(q)/alpha(i2) chimaeras. We propose that a C-terminal turn , centred on this glycine, plays an important part in specifying recep tor interactions of G proteins in the G(i)/G(o)/G(z) family.