A sodium channel inhibitor ISTX-I with a novel structure provides a new hint at the evolutionary link between two toxin folds.

Mingqiang Rong,Meilin Zhang,Gang Zhao,Yaping Zhang,Jiangxin Liu,Guodong Wang,Gan Wang,Kaifeng Hu,Ren Lai

doi:10.1038/srep29691

Abstract

Members of arachnida, such as spiders and scorpions, commonly produce venom with specialized venom glands, paralyzing their prey with neurotoxins that specifically target ion channels. Two well-studied motifs, the disulfide-directed hairpin (DDH) and the inhibitor cystine knot motif (ICK), are both found in scorpion and spider toxins. As arachnids, ticks inject a neurotoxin-containing cocktail from their salivary glands into the host to acquire a blood meal, but peptide toxins acting on ion channels have not been observed in ticks. Here, a new neurotoxin (ISTX-I) that acts on sodium channels was identified from the hard tick Ixodes scapularis and characterized. ISTX-I exhibits a potent inhibitory function with an IC50 of 1.6 μM for sodium channel Nav1.7 but not other sodium channel subtypes. ISTX-I adopts a novel structural fold and is distinct from the canonical ICK motif. Analysis of the ISTX-I, DDH and ICK motifs reveals that the new ISTX-I motif might be an intermediate scaffold between DDH and ICK, and ISTX-I is a clue to the evolutionary link between the DDH and ICK motifs. These results provide a glimpse into the convergent evolution of neurotoxins from predatory and blood-sucking arthropods.

Highlights

IntroductionScorpions and spiders are well-studied terrestrial predators, and the major components of their secreted venom are small, disulfide-rich peptides that commonly interfere with voltage-gated ion channels[2]
It was previously suggested that the disulfide-directed β-hairpin (DDH) fold, which has only two mandatory disulfide bonds with a C1-C4 and C2-C3
To investigate disulfide bonds connectivity, ISTX-I was first partially reduced by Tris (2-carboxyethyl) phosphine (TCEP), and the intact peptide and partially reduced intermediates were analyzed and identified as four peaks utilizing MALDI-TOF mass spectrometry (Fig. 1A)

Summary

Introduction

Scorpions and spiders are well-studied terrestrial predators, and the major components of their secreted venom are small, disulfide-rich peptides that commonly interfere with voltage-gated ion channels[2]. Venoms containing spider toxins predominantly share the inhibitor cystine knot (ICK) fold[5,6,7]. In contrast to predators such as spiders and scorpions, ticks are obligate blood-feeding ectoparasites of a wide range of vertebrates and have drawn extensive attention as pathogen vectors. They successfully obtain blood meals using pharmacological components in their saliva, which contains a large number of peptides and proteins that have been classified as venoms[14]. The structure of ISTX-I is a novel scaffold that provides an evolutionary link between the ICK and DDH motifs

Methods

Results

Conclusion