NMR Structural Analysis of α-Bungarotoxin and Its Complex with the Principal α-Neurotoxin-binding Sequence on the α7 Subunit of a Neuronal Nicotinic Acetylcholine Receptor*

We report a new, higher resolution NMR structure of α-bungarotoxin that defines the structure-determining disulfide core and β-sheet regions. We further report the NMR structure of the stoichiometric complex formed between α-bungarotoxin and a recombinantly expressed 19-mer peptide (¹⁷⁸IPGKRTESFYECCKEPYPD¹⁹⁶) derived from the α7 subunit of the chick neuronal nicotinic acetylcholine receptor. A comparison of these two structures reveals binding-induced stabilization of the flexible tip of finger II in α-bungarotoxin. The conformational rearrangements in the toxin create an extensive binding surface involving both sides of the α7 19-mer hairpin-like structure. At the contact zone, Ala⁷, Ser⁹, and Ile¹¹ in finger I and Arg³⁶, Lys³⁸, Val³⁹, and Val⁴⁰ in finger II of α-bungarotoxin interface with Phe¹⁸⁶, Tyr¹⁸⁷, Glu¹⁸⁸, and Tyr¹⁹⁴ in the α7 19-mer underscoring the importance of receptor aromatic residues as critical neurotoxin-binding determinants. Superimposing the structure of the complex onto that of the acetylcholine-binding protein (1I9B), a soluble homologue of the extracellular domain of the α7 receptor, places α-bungarotoxin at the peripheral surface of the inter-subunit interface occluding the agonist-binding site. The disulfide-rich core of α-bungarotoxin is suggested to be tilted in the direction of the membrane surface with finger II extending into the proposed ligand-binding cavity.