CARBOXYL-TERMINAL MUTATIONS OF G(Q-ALPHA) AND G(S-ALPHA) THAT AFTER THE FIDELITY OF RECEPTOR ACTIVATION

The carboxyl terminus of the G protein alpha subunit is a key determin ant of the fidelity of receptor activation. We have previously shown t hat the G(q alpha), subunit (alpha(q)) can be made to respond to alpha (i)-coupled receptors by replacing its carboxyl terminus with the corr esponding alpha(i2), alpha(o), or alpha(z), residues. We now extend th ese findings in three ways: 1) carboxyl-terminal mutations of alpha(q) /alpha(i) chimeras show that the critical amino acids are in the -3 an d -4 positions, 2) exchange of carboxyl termini between alpha(q) and a lpha(s) allows activation by receptors appropriate to the carboxylterm inal residues, and 3) we identify receptors that either do or do not a ctivate the expected carboxyl-terminal chimeras (alpha(o)/alpha(i), al pha(q)/alpha(s), alpha(s)/alpha(q)). Replacement of the five carboxyl- terminal amino acids of alpha(q) with the alpha(s) sequence permitted an alpha(s)-coupled receptor (the V2 vasopressin receptor but not the beta(2)-adrenergic receptor) to stimulate phospholipase C. Replacement of the five carboxyl-terminal amino acids of alpha(s) with residues o f permitted certain alpha(q)-coupled receptors (bombesin and V vasopre ssin receptors but not the oxytocin receptor) to stimulate adenylyl cy clase. Thus, the relative importance of the G(alpha) carboxyl terminus in permitting coupling to a new receptor depends on the receptor with which it is paired. These studies refine our understanding and provid e new tools with which to study the fidelity of receptor/G(()alpha act ivation.