The N-terminal extracellular region (amino acids 1-209) of the ␣-subunit of the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue was expressed as inclusion bodies in Escherichia coli using the pET 3a vector. Employing a novel protocol of unfolding and refolding, in the absence of detergent, a water-soluble globular protein of 25 kDa was obtained displaying approximately 15% ␣-helical and 45% -structure. The fragment bound ␣- So far, most of our knowledge on the three-dimensional structure of neuroreceptors is based on a combination of electronmicroscopical, biochemical and immunological data obtained for the nicotinic acetylcholine receptor (nAChR) 1 from the electric ray Torpedo (1, 2). These studies have elucidated the overall dimensions of the receptor protein (3), its position with respect to the surrounding lipid bilayer, the locations of functional domains and amino acid residues belonging to the integral ion channel (4, 5), its gating structures (6, 7), and the binding sites for several classes of ligands (1, 2, 7-12). However, a high resolution three-dimensional structure of the nAChR or any other neuroreceptor is still missing. If available, it could provide a molecular correlate for the recognition function of the receptor and thereby also for rational drug design.Prompted by the fact that all attempts to crystallize the detergent-solubilized whole nAChR protein have been unsuccessful for the past more than 20 years, we have begun to overexpress selected domains of the receptor in bacterial expression systems. If successfully renatured, the domains that are not transmembranous should be water-soluble and thus should provide a better material for protein crystallization than the detergent-solubilized whole receptor protein. Moreover, if small enough in size, such fragments should be suited for multidimensional NMR analysis (13). As presented here for the N-terminal extracellular region of the nAChR ␣-subunit, we attempted and achieved expression as a water-soluble globular protein that displays ligand binding properties comparable to or better than those of the SDS gel-isolated ␣-subunit. To achieve appropriate renaturation, we developed a protocol for the complete unfolding (by means of chaotropic agents and disulfide-reducing agents) and refolding (by means of an oxido shuffling system and L-arginine as structure-stabilizing agent) of the expressed polypeptide (14). The experimental conditions were such that self-organization of the unfolded protein was favored at the expense of (unwanted) aggregation and denaturation. In this way, we are able to prepare large quantities of the functional ligand binding domain from inclusion bodies of transformed Escherichia coli bacteria. Our results confirm that the N-terminal extracellular domain indeed harbors major elements of the ligand recognition function of the nAChR, as has long been suggested on the basis of affinity labeling and immunological studies (for recent reviews, see Refs. 1,2,8,9,15,and 16).
EXPERIMENTAL PROCEDURE...