Letter to the editor: Sequence-specific resonance assignments of the potent cytolysin equinatoxin II.

Sea anemone cytolysins constitute a novel class of toxins that functions by forming channels in cell membranes. In contrast to the bacterial pore-forming toxins, there is little detailed information on the mechanism of action of these toxins and as yet no 3-D structures have been determined. These highly basic toxins of mass ∼20 kDa generate pores in membranes containing sphingomyelin by forming assemblies of three or more monomers (Belmonte et al., 1993) in the membrane. The channels formed by this oligomerisation are permeable to small molecules and solutes and the resulting osmotic imbalance promotes cell lysis. The cytolysins share no significant sequence identity with the bacterial pore-forming proteins such as αhemolysin, where a heptameric complex forms the transmembrane pore (Gouaux, 1997), making it likely that the cytolysins have a unique structure. Equinatoxin II (EqtII) is a 179-residue, 19.8-kDa cytolysin isolated from the Mediterranean anemone Actinia equina L. (Maček and Lebez, 1988). It is essentially identical with tenebrosin-C, a cytolysin isolated from the Australian red waratah anemone, Actinia tenebrosa and the first of this class of toxins to be sequenced in full (Simpson et al., 1990); tenebrosin-C has a variant S177T and EqtII a variant P81D (Belmonte et al., 1994). EqtII and tenebrosinC show a high degree of sequence similarity (>60%) to cytolysins of Stichodactyla helianthus, Heteractis magnifica and other sea anemones (Wang et al., 2000). The transmembrane pore formed by EqtII consists