Reconstitution of solubilized atrial cholinergic muscarinic receptors in liposomes

The reconstitution of solubilized bovine atrial cholinergic muscarinic receptor into liposomes made of exogenous lipids has been achieved by polyethyleneglycol precipitation. Of the different lipid mixtures used, soybean lecithins were shown to be the best on the basis of receptor recovery. The receptor reconstituted into soybean lecithins liposomes exhibited ligand binding properties very similar to those of the native receptor. The dissociation constant of [3H]-N-methyl-scopolamine ([3H]NMS) was 0.46 and 0.30 nM as determined by equilibrium and kinetics experiments respectively. The potency of a range of muscarinic ligands in displacing [3H]NMS binding was atropine greater than methyl-atropine greater than scopolamine greater than pirenzepine oxotremorine greater than gallamine greater than carbamylcholine greater than pilocarpine bethanechol. The Hill slopes of the displacement curves were near 1 for the antagonists and smaller than 1 for the agonists and for gallamine. The agonist binding may be modulated by guanine nucleotides. These results indicate that soybean lecithins fulfill the lipid requirements for the reconstitution of the atrial muscarinic receptor.