Covalitoxin-II

Covalitoxin-II is a peptide toxin that is produced by the spider 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]

References
  1. Balaji RA, Sasaki T, Gopalakrishnakone P, Sato K, Kini RM, Bay BH (2000). "Purification, structure determination and synthesis of covalitoxin-II, a short insect-specific neurotoxic peptide from the venom of the Coremiocnemis validus (Singapore tarantula)". FEBS Lett. 474 (2–3): 208–12. doi:10.1016/s0014-5793(00)01593-3. PMID 10838086. S2CID 26196883.
  2. UniProt, UniProt Knowledge Base; P82601 (TXVL2_CORVA); published December 1, 2000; Last modified October 3, 2012; Date of retrieval October 16, 2012;
  3. Grishin E; Polypeptide neurotoxins from spider venoms; Eur. J. Biochem. 1999 Sep;264(2):276-80. Grishin, E. (1999). "Polypeptide neurotoxins from spider venoms". European Journal of Biochemistry. 264 (2): 276–280. doi:10.1046/j.1432-1327.1999.00622.x. PMID 10491071.
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