Solution structure of two insect‐specific spider toxins and their pharmacological interaction with the insect voltage‐gated Na⁺ channel

Abstract

Delta-paluIT1 and delta-paluIT2 are toxins purified from the venom of the spider Paracoelotes luctuosus. Similar in sequence to mu-agatoxins from Agelenopsis aperta, their pharmacological target is the voltage-gated insect sodium channel, of which they alter the inactivation properties in a way similar to alpha-scorpion toxins, but they bind on site 4 in a way similar to beta-scorpion toxins. We determined the solution structure of the two toxins by use of two-dimensional nuclear magnetic resonance (NMR) techniques followed by distance geometry and molecular dynamics. The structures of delta-paluIT1 and delta-paluIT2 belong to the inhibitory cystine knot structural family, i.e. a compact disulfide-bonded core from which four loops emerge. Delta-paluIT1 and delta-paluIT2 contain respectively two- and three-stranded anti-parallel beta-sheets as unique secondary structure. We compare the structure and the electrostatic anisotropy of those peptides to other sodium and calcium channel toxins, analyze the topological juxtaposition of key functional residues, and conclude that the recognition of insect voltage-gated sodium channels by these toxins involves the beta-sheet, in addition to loops I and IV. Besides the position of culprit residues on the molecular surface, difference in dipolar moment orientation is another determinant of receptor binding and biological activity differences. We also demonstrate by electrophysiological experiments on the cloned insect voltage-gated sodium channel, para, heterologuously co-expressed with the tipE subunit in Xenopus laevis oocytes, that delta-paluIT1 and delta-paluIT2 procure an increase of Na+ current. delta-PaluIT1-OH seems to have less effect when the same concentrations are used.

Four disulfide-bridged scorpion beta neurotoxin CssII: Heterologous expression and proper folding in vitro

“…It has been observed in sodium channel neurotoxins from spiders and scorpions that C-terminus amidated peptides have stronger affinity for their sodium channel receptors [9,24]; that is, the exchange of an anionic charge for a neutral one at the C-terminus increases the overall positive charge of amidated peptides which is correlated with their receptor affinities [31].…”

SPider Neurotoxins Targeting Voltage-Gated Sodium Channels

“…Using the isolated cockroach axon preparation and cloned para/tipE insect Nav channels expressed in Xenopus oocytes they have been shown to slow insect Nav channel inactivation, with no shift in the voltage dependence of activation. They also have no effect on Nav1.2/β1 channels at concentrations up to 10 µM (Ferrat et al, 2005). This action is similar to site 3 neurotoxins such as…”

Unique Bell-shaped Voltage-dependent Modulation of Na+ Channel Gating by Novel Insect-selective Toxins from the Spider Agelena orientalis

“…antagonize channel inactivation. However, profound differences between ␦-Palu-IT1 and -2 and scorpion ␣-toxins were noted, the former competing with scorpion ␤-toxins for binding to receptor site 4 and not influencing the kinetics of channel inactivation (23,53). It was therefore suggested that ␦-Palu-IT1 and -2 only macroscopically resemble typical scorpion ␣-toxins but, unlike them, influence the inactivated-to-closed state transition process (53).…”

“…Apart for overall sequence similarity, all of these toxins (except for ␦-Palu-IT3) are C-terminally amidated and contain eight strictly conserved halfcystine residues that most probably account for a presumably common molecular fold. To date, spatial structures of -Aga-1 and ␦-palutoxins-IT1 and -2 are known, all showing a high degree of similarity (53,54) and conforming to the inhibitor cystine knot (ICK) motif most common for spider venom neurotoxins (4,55). Even in the absence of experimental data proving the exact disulfide connectivity, it is therefore not unreasonable to surmise the typical ICK motif for the ␤/␦-agatoxins.…”