Heterooligomers of the muscarinic receptor and G proteins purified from porcine atria

Muscarinic receptor extracted from porcine atria in digitonin-cholate copurified with Gαo, Gαi1-3, and caveolins. The presence of complexes was confirmed by coimmunoprecipitation of the receptor, α-subunits, and caveolins in various combinations. Homooligomers of αi2 were detected on Western blots, and heterooligomers of αi2 and αo were identified by coimmunoprecipitation; thus, a complex may contain at least two α-subunits. Other combinations of α-subunit were not detected. The ratio of total α-subunit to receptor was near 1, as measured by [35S]GTPγS and the antagonist [3H]quinuclidinylbenzilate, and the binding of [35S]GTPγS was manifestly biphasic. The ratio of αo to αi1,2 also was near 1, as determined from the intensity of Western blots. Cardiac muscarinic receptors therefore can be purified as a mixture of complexes that contain caveolins and oligomers of α-subunit, some of which are heteromeric. Each complex would appear to contain equal numbers of α-subunit and the receptor.