How do Cone Snails hunt Fish?

Cone snails (Conidae) are marine animals which contain venoms which, in the worst case, can prove fatal to human beings. Many species of cone snails hunt fish by using teeth which for cone snails have evolved into small harpoons. The harpoons which are hollow and barbed are used to inject the venom into the victim. Some species of cone snails have developed extremely effective toxins which immobilize the prey almost instantaneously. The venom employed by the Conus purpurascens snail contains com­ponents which induce complete paralysis in their victims within one to two seconds. To elucidate the question which components of the venom of this cone snail is responsible for this fast immo­bilization of prey, snail venom was collected and was separated by HPLC chromatography into its individual components. The biological activity of these components was examined.

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It emerged that no single component of the snail venom alone is able to induce this effective paralysis in the victim, but that at least two components are required for this. These two components were purified, their amino acid sequences determined, and both peptides were chemically synthesized. One component (k-conotoxin PVIIA) is a peptide comprising 27 amino acids. Looking for the target molecule of this peptide it became clear that it blocks voltage-dependent potassium channels. With measure­ments using the Xenopus expression system it was demonstrated that the peptide inhibits Shaker mediated K+ currents but not currents mediated the rat homologue potassium channels Kv1.1-Kv1.4 or voltage gated sodium channels. By constructing chimeras between Shaker and the k-PVIIA insensitive Kv1.1 K+ channel it could be shown that the putative pore forming region of the channel protein is at least part of the receptor for k-PVIIA.

Furthermore, other residues in that region of the Shaker channel have been identified where amino acid substi­tution influenced the toxin binding substantially. The second component of the venom involved in the extremely rapid immo­bilization of the prey (d-conustoxin PVIA) is a peptide comprising 29 amino acids. d-PVIA inhibits the inactivation of sodium chan­nels resulting in an increased sodium-influx. Both mechanism together - increase of Na+-influx and inhibition of K+-efflux - appear to induce a powerful overexcitation of the nerve cells which leads to a "tetanus rigidity" in the victim. The fish is immobilized so quickly that it is not longer able to escape, and will then be devoured by the snail.

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This work is a collaboration with the department of Prof. Stühmer
and the department of Prof. B.M. Olivera, Salt Lake City, USA

Literature:

Terlau, H., Shon, K.-J., Grilley, M., Stocker, M., Stühmer, W. and Olivera, B.M.: Strategy for rapid immobilization of prey by a fish-hunting marine snail.
Nature, 381, 148 - 151 (1996).

Shon, K.-J., Stocker, M., Terlau, H., Stühmer, W., Jacobson, R., Walker, C., Grilley, M., Marsh, M., Hillyard, D., Gray, W.R. and Olivera, B.M.: k-conotoxin PVIIa: A peptide inhibiting the Shaker K+ channel. J. Biological Chemistry, 273, 33 - 38 (1998).