Primary and secondary structure of a pore-forming toxin from the sea anemone, Actinia equina L., and its association with lipid vesicles.

The complete amino acid sequence of equinatoxin II, a potent pore-forming toxin with hemolytic, cytotoxic and cardiotoxic activity from the venom of the sea anemone, Actinia equina L., is reported. In addition, circular dicroism was used to estimate the secondary structure of this toxin either in the water-soluble or in the membrane-anchored form. Equinatoxin II when in water was found to contain about 29-33% of alpha-helical structure, 53-58% of beta-strand+beta-turn and 10-16% of random structure. Upon association with phospholipids, in particular with sphingomyelin, a rearrangement of the secondary structure occurs resulting in an increase of the alpha-helix content. An amphiphilic alpha-helical segment is predicted at the N-terminus, which shares structural homology with membrane active peptides like melittin and viral fusion peptides. In analogy to the behaviour of these peptides we propose that at least part of the alpha-helix content increase of equinatoxin II is due to the insertion of its N-terminus into the lipid bilayer. As in the case of melittin, association of 3-4 equinatoxin molecules is necessary to induce membrane permeabilisation.