Nicotinic receptors in wonderland.

The structure of a soluble homopentameric homologue of the N-terminal extracellular domain of the nicotinic acetylcholine (ACh) receptor has recently been determined at the atomic level. These data reveal the three-dimensional structure of the binding site for ACh and nicotinic ligands. The ACh-binding sites are located at subunit boundaries in an equatorial position and are framed by residues previously identified in nicotinic receptors, by photoaffinity labelling and mutagenesis experiments, as being crucial for ligand binding. On this basis, a hypothetical mechanism for the allosteric transitions of the nicotinic receptors is suggested. Despite the proposal by Langley (1905) of the concept of a 'receptive substance' mediating the effect of nicotine and curare on muscle, receptors remained enigmatic entities. Their identification was made possible largely by a few wonders of the animal world. First, the electric organ of fish offers both an extremely rich and homogeneous source of synaptic material and single cells (or electroplaques) for pharmacological studies, notably the response to acetylcholine (ACh) in vivo 1. However, none of the many pharmacological agents, including thiol group reagents, tested in vivo by following the electrophysiological response of single electroplaques was selective enough to characterize the detergent-solubilized protein isolated from crude extracts of electric organ that bound ACh in vitro. Second, because of its exquisite selectivity and high affinity, an α-toxin from the venom of the snake Bungarus 2 enabled the isolation and purification of the nicotinic ACh receptor (nAChR) from the electric organ 3,4. Finally, a soluble protein that binds nicotinic ligands and α-bungarotoxin has recently been discovered in a snail (Lymnaea stagnalis) from European fresh waters 5. This ACh-binding protein (AChBP) is produced and stored in glial cells and is released in an ACh-dependent manner in the synaptic cleft where it regulates synaptic transmission 5. X-ray crystallographic analysis of AChBP at 2.7 Å resolution reveals a striking resemblance between the AChBP structure and the organization for the ligand-binding domain of nAChR proposed from biochemical and structure-prediction experiments 6. The nAChR from fish electric organ is a heteropentamer of ~300 kDa with an (α1) 2 .β1.γ/ε.δ stoichiometry. The receptor has two distinct ACh-binding sites (located at interfaces between the α and γ, and the α and δ, subunits), a unique ion channel along the transmembrane axis of (pseudo)symmetry, and all the structural elements that mediate allosteric coupling of the two binding sites during activation and desensitization 4. In response to ACh, single …