Lipid phase coexistence favors membrane insertion of equinatoxin-II, a pore-forming toxin from Actinia equina

Equinatoxin-II is a eukaryotic pore-forming toxin belonging to the family of actinoporins. Its interaction with model membranes is largely modulated by the presence of sphingomyelin. We have used large unilamellar vesicles and lipid monolayers to gain further information about this interaction. The coexistence of gel and liquid-crystal lipid phases in sphingomyelin/phosphatidylcholine mixtures and the coexistence of liquid-ordered and liquid-disordered lipid phases in phosphatidylcholine/cholesterol or sphingomyelin/phosphatidylcholine/ cholesterol mixtures favor membrane insertion of equinatoxin-II. Phosphatidylcholine vesicles are not permeabilized by equinatoxin-II. However, the localized accumulation of phospholipase C-generated diacylglycerol creates conditions for toxin activity. By using epifluorescence microscopy of transferred monolayers, it seems that lipid packing defects arising at the interfaces between coexisting lipid phases may function as preferential binding sites for the toxin. The possible implications of such a mechanism in the assembly of a toroidal pore are discussed.