Covalitoxin-II

Covalitoxin-II is a peptide toxin that is produced by the spider species Coremiocnemis validus. It can induce excitatory, non-lethal behavioral symptoms like quivering and jerking in crickets.

Sources

Covalitoxin-II (Cvtx-II) is a toxin in the venom of Coremiocnemis validus (Singapore or Blue femur tarantula).^[1][2] This spider lives in South East Asian tropical forests.^[1]

Chemistry

Structure

The toxin consists of 31 amino acids (ACSRAGENCYKSGRCCDGLYCKAYVVTCYKP). This sequence forms a peptide with a molecular weight of 3.4 kDa. It has six cysteine residues which form three disulphide bonds, between the specific amino acid locations 2&16, 19&21 and 15&28.^[2] There is an equal distribution of hydrophilic, hydrophobic and neutral amino acids. Near the C-terminus there is a positively charged surface formed by conserved basic residues, which is thought to interact with an ion channel.^[1][3] The inhibitor cystine knot motif that is formed by the cysteine residues shows analogy to other spider toxins and can also be found in ω-conotoxins, which are present in the venom of cone snails. It has low homology with two peptides:

• 47% homology with spider peptide PLTX-II (calcium channel blocker)
• 47% homology with conotoxin MrVIB (sodium channel blocker)

Target and mechanism of action

The specific target and mechanism of action of Cvtx-II are not yet clear. Based on its effect on behavior, it has been speculated that Cvtx-II targets sodium channel inactivation, analogous to some excitotoxins.^[1]

Toxicity

Cvtx-II induces the following non-lethal symptoms in crickets:

• Quivering, jerking, and hyperextension of the legs
• Rapid movement of antenna, mandibles and maxillae
• Abdominal contraction
• Frequent body arching
• Loss of righting reflex
• Greatly reduced locomotion
• Indications of paralysis ^[1]

Experiments have shown that these symptoms are not in present in cockroaches or mice after Cvtx-II injections. Therefore, Cvtx-II is thought to be an insect species-specific neurotoxic peptide. A dose of 0.2 μmol/g was necessary to immobilize or inactivate 50% of the insects (ID50). After 40–60 minutes, the immobilizing effects disappear.^[1]