De Robertis_1971_Science_171_963

A special proteolipid (a hydrophobic protein) has been extracted and purified from nerve-ending membranes and total particulate matter of gray areas of the central nervous system. Such a proteolipid shows a high affinity for binding d-tubocurarine, serotonin, and atropine and has been called receptor proteolipid. The interaction of this proteolipid with atropine sulfate was studied with light scattering and polarization of fluorescence. The changes observed, which follow a cooperative type of curve, were attributed to the aggregation of the proteolipid macromolecules. Such a phenomenon was then observed under the electron microscope. A receptor proteolipid having a high affinity for binding acetylcholine, hexamethonium, and other cholinergic drugs was isolated and purified from electric tissue of fishes and from electroplax membranes. Such a proteolipid was also extracted from membranes from which acetylcholinesterase had been removed, and it was concluded that this enzyme and the receptor proteolipid are two different macromolecules. A high affinity binding site with a dissociation constant of K1 equal to 10(-7) and about ten sites with K2 equal to 10(-5) were recognized in the receptor proteolipid. Under the electron microscope the receptor proteolipid of brain appears as a rod-shaped macromolecule which may assume paracrystalline arrays with 10(-8) molar atropine sulfate. Similarly the receptor proteolipid from electric tissue and from skeletal muscle may form paracrystalline arrays under the action of acetylcholine and hexamethonium. A model of the cholinergic receptor based on the properties of the proteolipid is presented. Preliminary work indicates the possibility of obtaining a biophysical response to acetylcholine when the receptor proteolipid is embedded in artificial bilayered lipid membrance.