Cryo-EM structure of the ASIC1a\u2013mambalgin-1 complex reveals that the peptide toxin mambalgin-1 inhibits acid-sensing ion channels through an unusual allosteric effect

Demeng Sun,Longhua Zhang,Xiaobin Xue,Qian Qu,You Yu,Maojun Yang,Siyu Li,Xueming Li,Man Pan,Ming Wen,Lei Liu,Changlin Tian,Xiaorun Li

doi:10.1038/s41421-018-0026-1

Abstract

Acid-sensing ion channels (ASICs) are neuronal voltage-independent Na+ channels that are activated by extracellular acidification. ASICs play essential roles in a wide range of physiological processes, including sodium homeostasis, synaptic plasticity, neurodegeneration, and sensory transduction. Mambalgins, a family of three-finger toxins isolated from black mamba venom, specifically inhibit ASICs to exert strong analgesic effects in vivo, thus are thought to have potential therapeutic values against pain. However, the interaction and inhibition mechanism of mambalgin on ASICs remains elusive. Here, we report a cryo-electron microscopy (cryo-EM) structure of chicken ASIC1a (cASIC1a) in complex with mambalgin-1 toxin at 5.4 Å resolution. Our structure provides the first experimental evidence that mambalgin-1 interacts directly with the extracellular thumb domain of cASIC1a, rather than inserting into the acid-sensing pocket, as previously reported. Binding of mambalgin-1 leads to relocation of the thumb domain that could disrupt the acidic pocket of cASIC1a, illustrating an unusual inhibition mechanism of toxins on ASIC channels through an allosteric effect. These findings establish a structural basis for the toxicity of the mambalgins, and provide crucial insights for the development of new optimized inhibitors of ASICs.

Highlights

Acid-sensing ion channels (ASICs) are neuronal voltage-independent Na+ channels that are activated by extracellular acidification
Electrophysiological analysis in Chinese hamster ovary (CHO) cells showed the synthetic mambalgin-1 caused a concentration-dependent inhibition with an IC50 of 123.6 ± 20.3 nM for chicken ASIC1a and 197.3 ± 18.7 nM for human ASIC1a (Fig. 1b, c)
A hydrophobic patch consisting of residues Met25/Pro26/ Phe27/Leu30/Leu[32] in Finger II could contribute to the chicken ASIC1a (cASIC1a)–mambalgin-1 interactions, especially with the hydrophobic region in α5 helix of cASIC1a thumb domain (Fig. 3d)

Summary

Introduction

Acid-sensing ion channels (ASICs) are neuronal voltage-independent Na+ channels that are activated by extracellular acidification. Several peptide toxins have been identified as selective and potent modulators for ASICs and function as channel agonizts, such as Texas coral snake toxin MitTx27; desensitization state promoters, such as psalmotoxin-1 (PcTx1) from the venom of the tarantula;[28,29] or inhibitors, such as the sea anemone toxin APETx230 and mambalgins isolated from mamba venom[31] These toxins bind to open, desensitized and closed states of the channels respectively, providing powerful tools to arrest ASICs in specific conformational states for pharmacological, biophysical, and structural studies[32,33]. It has been reported to be a potent, rapid and reversible inhibitor of ASIC1a or ASIC1b-containing channels in both central and peripheral neurons[31]

Methods

Results

Conclusion