We evaluated the G protein selectivity of chimeric M1 and M2 muscarinic cholinergic receptors in which either the third intracellular (I3) loop or the N-terminal portion of this loop (the I3N peptide) was replaced by the corresponding sequence from the β1-adrenergic receptor. The chimeras retained agonist-dependent G protein regulatory activity, 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 β-adrenergic receptor, and activated phospholipase C via a pertussis toxin-insensitive G protein, presumably a G(q), G(s) is not a target of either muscarinic receptor, and G(q) is not a cellular target of either the M2 muscarinic or β-adrenergic receptor. When co-reconstituted into phospholipid vesicles with purified G proteins, the chimeric receptors were completely nonselective among all G proteins tested. They activated G(i), G(o), G(z), G(q), and G(s) with similar efficiencies. This promiscuity was largely suppressed, both in transfected cells and in reconstituted vesicles, by the additional replacement of the second intracellular (I2) loop of the β-adrenergic receptor. Such double substitutions created receptors specific for G(s), the target of the β-adrenergic receptor. These findings suggest that G protein specificity depends on the proper combination of multiple regions on a receptor's cytoplasmic surface. In addition, the promiscuous receptors described here may be useful for regulating novel G proteins whose natural regulators are not yet known.
|Original language||English (US)|
|Number of pages||9|
|Journal||Journal of Biological Chemistry|
|State||Published - Jul 22 1994|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology