The insecticidal spider toxin SFI1 is a knottin peptide that blocks the pore of insect voltage‐gated sodium channels via a large β‐hairpin loop

Abstract

Spider venoms contain a plethora of insecticidal peptides that act on neuronal ion channels and receptors. Because of their high specificity, potency and stability, these peptides have attracted much attention as potential environmentally friendly insecticides. Although many insecticidal spider venom peptides have been isolated, the molecular target, mode of action and structure of only a small minority have been explored. Sf1a, a 46-residue peptide isolated from the venom of the tube-web spider Segesteria florentina, is insecticidal to a wide range of insects, but nontoxic to vertebrates. In order to investigate its structure and mode of action, we developed an efficient bacterial expression system for the production of Sf1a. We determined a high-resolution solution structure of Sf1a using multidimensional 3D/4D NMR spectroscopy. This revealed that Sf1a is a knottin peptide with an unusually large b-hairpin loop that accounts for a third of the peptide length. This loop is delimited by a fourth disulfide bond that is not commonly found in knottin peptides. We showed, through mutagenesis, that this large loop is functionally critical for insecticidal activity. Sf1a was further shown to be a selective inhibitor of insect voltage-gated sodium channels, consistent with its 'depressant' paralytic phenotype in insects. However, in contrast to the majority of spider-derived sodium channel toxins that function as gating modifiers via interaction with one or more of the voltage-sensor domains, Sf1a appears to act as a pore blocker.

Insecticidal activity of a recombinant knottin peptide from Loxosceles intermedia venom and recognition of these peptides as a conserved family in the genus

“…Recombinant Li1b induced rapid paralysis upon injection into sheep blowflies that lasted until 24 h postinjection at the highest dose tested. Some flies at lower doses recovered from the paralytic effects within 24 h. The paralytic potency of rLi1b at 24 h postinjection is in the same range as the U 2 ‐segestritoxin‐Sf1a peptide, which was previously tested in the same assay (Bende et al ., ). The PD 50 of rLi1b (at 24 h) is two orders of magnitude lower than the highly potent insecticidal spider venom peptide β‐diguetoxin‐Dc1a (Bende et al ., ), so Li1b can be considered a moderately potent insecticidal toxin.…”

Toxin structures as evolutionary tools: Using conserved 3D folds to study the evolution of rapidly evolving peptides

“…Variations in the size of loop 4 in ICK defensins and spider‐venom ICK neurotoxins. Structures and sequence alignment of tachystatin A (PDB 1CIX; ), an arachnid ICK defensin, and the spider‐venom toxins κ‐theraphotoxin‐Ps1a (PDB 1V7F; ) and μ‐segestritoxin‐Sf1a (PDB 2MF3; ), highlighting the differences in the size of loop 4 (red).…”

Insect-Active Toxins with Promiscuous Pharmacology from the African Theraphosid Spider Monocentropus balfouri

“…Several spider toxins have been found to paralyze or kill the dipterans L. cuprina and M. domestica , with activities ranging from 198 pmoL/g (LD 50 ; U 1 -AGTX-Ta1a) to 2229 pmoL/g (PD 50 ; μ-SGTX-Sf1a) on L. cuprina , and 77 pmoL/g (LD 50 ; ω-HXTX-Hv1a) to 1380 pmoL/g (LD 50 ; μ-AGTX-Aa1b) on M. domestica [47,48,49,50]. With PD 50 values of 5600–5800 pmoL/g, Mb1a and Mb1b are only moderately potent against L. cuprina compared to other spider toxins that affect dipterans.…”