Structural Determinants for α-Neurotoxin Sensitivity in Muscle nAChR and Their Implications for the Gating Mechanism

Abstract

Neurotoxins from snake venoms act as potent antagonists on the nicotinic acetylcholine receptors (nAChRs). α-Neurotoxins such as α-bungarotoxin (α-Btx) selectively bind to the skeletal muscle nAChRs among other subtypes, causing failure of the neuromuscular transmission. Through evolution, some species including snakes and mongoose have developed resistance to α-neurotoxins via specific amino acid substitutions in their muscle-type nAChR α1 subunit, which constitutes most of the toxin-binding site. Here we analyze these sequence variations in the context of our recent crystal structure of the extracellular domain of the mouse nAChR α1 bound to α-Btx. Our structure suggests that α-Btx has evolved as an extremely potent antagonist of muscle nAChR by binding the receptor tightly, blocking its ligand site, and locking its conformation in a closed state. Conversely, most toxin-resistant mutations occur at the α-Btx binding interface on nAChR α1 but away from the agonist binding site. These mutations can interfere with the binding of α-Btx without having deleterious effect on the gating function. These analyses not only help understand the structural determinants for neurotoxin sensitivity in muscle-type nAChR, but also shed light on its gating mechanism.