CHIMERIC MUSCARINIC CHOLINERGIC - BETA-ADRENERGIC RECEPTORS THAT ARE FUNCTIONALLY PROMISCUOUS AMONG G-PROTEINS

We evaluated the G protein selectivity of chimeric M1 and M2 muscarini c cholinergic receptors in which either the third intracellular (I3) l oop or the N-terminal portion of this loop (the I3N peptide) was repla ced by the corresponding sequence from the beta(1)-adrenergic receptor . The chimeras retained agonist-dependent G protein regulatory activit y, but were completely promiscuous among potential G protein targets. When expressed in transfected cells, the chimeric receptors activated adenylyl cyclase, the major target of the beta-adrenergic receptor, an d activated phospholipase C via a pertussis toxin-insensitive G protei n, presumably a G(q). G(s) is not a target of either muscarinic recept or, and G(q) is not a cellular target of either the M2 muscarinic or b eta-adrenergic receptor. When co-reconstituted into phospholipid vesic les with purified G proteins, the chimeric receptors were completely n onselective among all G proteins tested. They activated G(i), G(o), G( z), G(q), and G(s) with similar efficiencies. This promiscuity was lar gely suppressed, both in transfected cells and in reconstituted vesicl es, by the additional replacement of the second intracellular (I2) loo p of the beta-adrenergic receptor. Such double substitutions created r eceptors specific for G(s), the target of the beta-adrenergic receptor . These findings suggest that G protein specificity depends on the pro per combination of multiple regions on a receptor's cytoplasmic surfac e. In addition, the promiscuous receptors described here may be useful for regulating novel G proteins whose natural regulators are not yet known.