Crystal structure of a Cbtx–AChBP complex reveals essential interactions between snake α-neurotoxins and nicotinic receptors

Abstract

The crystal structure of the snake long alpha-neurotoxin, alpha-cobratoxin, bound to the pentameric acetylcholine-binding protein (AChBP) from Lymnaea stagnalis, was solved from good quality density maps despite a 4.2 A overall resolution. The structure unambiguously reveals the positions and orientations of all five three-fingered toxin molecules inserted at the AChBP subunit interfaces and the conformational changes associated with toxin binding. AChBP loops C and F that border the ligand-binding pocket move markedly from their original positions to wrap around the tips of the toxin first and second fingers and part of its C-terminus, while rearrangements also occur in the toxin fingers. At the interface of the complex, major interactions involve aromatic and aliphatic side chains within the AChBP binding pocket and, at the buried tip of the toxin second finger, conserved Phe and Arg residues that partially mimic a bound agonist molecule. Hence this structure, in revealing a distinctive and unpredicted conformation of the toxin-bound AChBP molecule, provides a lead template resembling a resting state conformation of the nicotinic receptor and for understanding selectivity of curaremimetic alpha-neurotoxins for the various receptor species.

Pore conformations and gating mechanism of a Cys-loop receptor

“…The structural basis for coupling between motions in the neurotransmitter-binding domain and pore gating is still largely unknown (18)(19)(20). The distinct activitydependent orientations of the M2 segments and the superposition of the closed-over the open-pore model provide further insights into principal gating motions.…”

Structural basis for cooperative interactions of substituted 2-aminopyrimidines with the acetylcholine binding protein

“…Such interactions are evident in the crystal structures of acetylcholine, carbamylcholine, and other quaternary complexes (16,18), and the energetics have been established through mutagenesis studies modifying the polarizability, and electronegativity of the aromatic side chains in the binding pocket (28,29); (ii) secondary and tertiary amines (19)(20)(21) and imines (21), which, when in a protonated state, hydrogen bond to the backbone carbonyl of W143. The protonated dihydropyridine-bound state for the benzylidene anabaseines has been demonstrated by difference spectroscopy (30); and (iii) peptides, such as the α-conotoxins and α-neurotoxins, stabilized by multiple interactions on the C loop and interfacial residues on both the principal and complementary faces (31)(32)(33)(34). The binding interactions of the 2-aminopyrimidines do not appear to follow the above patterns and therefore these structures constitute a distinct fourth group of ligands.…”

Structure-guided drug design: Conferring selectivity among neuronal nicotinic receptor and acetylcholine-binding protein subtypes

“…Several structures of agonist (epibatidine, nicotine, lobeline, carbamylcholine) and antagonist (α-conotoxin, methyllycaconitine, α-bungarotoxin) bound to AChBP have been reported [6,[9][10][11][12], and other structures have been resolved at high resolution or are under study. Hence, one now has a fairly comprehensive perspective on structures of the AChBP complexes.…”

“…Hence, agonists have the requisite flexibility and steric features to allow C loop closure. Antagonists appear to keep the C loop in its open position or extend the C loop in a more radial direction [9,11].…”