Huang_2013_Biochem.J_454_303

To identify high-affinity interactions between long-chain alpha-neurotoxins and nicotinic receptors, we determined the crystal structure of the complex between alpha-btx (alpha-bungarotoxin) and a pentameric ligand-binding domain constructed from the human alpha7 AChR (acetylcholine receptor) and AChBP (acetylcholine-binding protein). The complex buries ~2000 A2 (1 A=0.1 nm) of surface area, within which Arg36 and Phe32 from finger II of alpha-btx form a pi-cation stack that aligns edge-to-face with the conserved Tyr184 from loop-C of alpha7, while Asp30 of alpha-btx forms a hydrogen bond with the hydroxy group of Tyr184. These inter-residue interactions diverge from those in a 4.2 A structure of alpha-ctx (alpha-cobratoxin) bound to AChBP, but are similar to those in a 1.94 A structure of alpha-btx bound to the monomeric alpha1 extracellular domain, although compared with the monomer-bound complex, the alpha-btx backbone exhibits a large shift relative to the protein surface. Mutational analyses show that replacing Tyr184 with a threonine residue abolishes high-affinity alpha-btx binding, whereas replacing with a phenylalanine residue maintains high affinity. Comparison of the alpha-btx complex with that coupled to the agonist epibatidine reveals structural rearrangements within the binding pocket and throughout each subunit. The overall findings highlight structural principles by which alpha-neurotoxins interact with nicotinic receptors.