Channel properties of an insect neuronal acetylcholine receptor protein reconstituted in planar lipid bilayers

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Abstract
A pentameric membrane protein composed of four types of polypeptide has been identified as the minimal structural unit responsible for the electrogenic action of acetylcholine on electrocytes and muscle cells1–3. Because many populations of central and peripheral neurones also have nicotinic acetylcholine receptors (AChRs), considerable effort has recently gone into identifying the neuronal receptor4,5. The central nervous tissue of insects contains very high concentrations of nicotinic AChRs6,7, and we have recently purified an α-toxin binding protein, a putative AChR, from neuronal membranes of locusts8,9. It is a component of high relative molecular mass, clearly composed of identical subunits, a structure predicted for an ancestral AChR protein10,11. To verify that the purified polypeptides not only represent ligand binding sites but that they are indeed functional receptors, we have now reconstituted the isolated protein in a planar lipid bilayer. We show that in this system cholinergic agonists activate functional ion channels, that have properties comparable to those exhibited by the peripheral AChRs in vertebrates; thus, for the first time a functional acetylcholine receptor channel has been indentified in nerve cells.