Sea anemones are soft-bodied, sedentary animals equipped with cnidae, which are microscopic stinging organelles located in their ectoderm, especially in tentacles. Cnidae (nematocysts and spirocysts) correspond to one of the earliest mechanisms of intradermal delivery of bioactive peptides and proteins such as cytolysins, protease inhibitors and a variety of ion channel blockers. Many of these components have medical potential. For example, the potassium channel blocker ShK-186 (an analogue of ShK toxin from the sea anemone Stichodactyla helianthus) is being tested in clinical trials as a therapy for autoimmune diseases [1].
Some species of the family Actiniidae possess specialized vesicular organs called acrorhagi, used in combat against competitors. The cnidae composition in the acrorhagi is distinct from the other tissues as they contain very large holotrichs. Honma et al. [2] reported a new acrorhagi peptide toxin, acrorhagin I, which is involved in conspecific aggression in the sea anemone Actinia equina. Acrorhagin I has been shown to have paralytic and neurotoxic activity, with an LD50 value of 520 µg/kg in crabs. Acrorhagin I was also shown to induce necrosis in mouse fibroblasts and insect gonadal cells by intracellular formation of reactive oxygen species (ROS) [3].
Acrorhagin I does not show sequence homology to any documented sea anemone toxins. The unique distribution of its four disulfide bonds and lack of toxin homologues suggests that acrorhagin I may adopt a novel fold. The aim of this work is to determine the three-dimensional structure of recombinant acrorhagin I by NMR and evaluate its biological activity against a panel of ion channels. The development of a recombinant expression system will also assist in identification of the molecular target of acrorhagin I.