The allosteric activation of α7 nAChR by α-conotoxin MrIC is modified by mutations at the vestibular site

Abstract

-conotoxins are 13-19 amino acid toxin peptides that bind various nicotinic acetylcholine receptor (nAChR) subtypes. -conotoxin Mr1.7c (MrIC) is a 17 amino acid peptide that targets 7 nAChR. Although MrIC has no activating effect on 7 nAChR when applied by itself, it evokes a large response when co-applied with the type II positive allosteric modulator PNU-120596, which potentiates 7 nAChR response by recovering it from a desensitized state. Lack of standalone activity despite activation upon co-application with a positive allosteric modulator was previously observed for molecules that bind to an extracellular domain allosteric activation (AA) site at the vestibule of the receptor. We hypothesized that MrIC may activate 7 nAChR allosterically through this site. We ran voltage-clamp electrophysiology experiments and in silico peptide docking calculations to gather evidence in support of 7 nAChR activation by MrIC through the AA site. The experiments with the wild-type 7 nAChR supported an allosteric mode of action, which was confirmed by the increased MrIC + PNU-120596 responses of three 7 nAChR AA site mutants that were designed in silico to improve MrIC binding. Overall, our results shed light on allosteric activation of 7 nAChR by MrIC and suggest involvement of the AA site. Significance Statement-conotoxin MrIC (MrIC) is an allosteric agonist of the 7 nicotinic acetylcholine receptor (nAChR). This mode of action is unique among -conotoxins since these peptides typically act as orthosteric antagonists of nAChR. However, the mechanism of 7 nAChR activation by MrIC has been elusive so far. This work demonstrates that activation by MrIC is independent of the 7 nAChR orthosteric site and is related to a vestibular allosteric activation site at the extracellular domain of the receptor. Our experimental and computational studies identified the residues that play a role in allosteric activation and confirmed the utility of ensemble docking methods in understanding peptide - nAChR interactions, thus providing a basis for the design of peptides for allosteric modulation of nAChR.