Equinatoxin II is a 179-amino-acid pore-forming protein isolated from the venom of the sea anemone Actinia equina. Large unilamellar vesicles and lipid monolayers of different lipid compositions have been used to study its interaction with membranes. The critical pressure for insertion is the same in monolayers made of phosphatidylcholine or sphingomyelin (∼26 mN m-1) and explains why the permeabilization of large unilamellar vesicles by equinatoxin II with these lipid compositions is null or moderate. In phosphatidylcholine-sphingomyelin (1:1) monolayers, the critical pressure is higher (∼33 mN m-1), thus permitting the insertion of equinatoxin II in large unilamellar vesicles, a process that is accompanied by major conformational changes. In the presence of vesicles made of phosphatidylcholine, a fraction of the protein molecules remains associated with the membranes. This interaction is fully reversible, does not involve major conformational changes, and is governed by the high affinity for membrane interfaces of the protein region comprising amino acids 101-120. We conclude that although the presence of sphingomyelin within the membrane creates conditions for irreversible insertion and pore formation, this lipid is not essential for the initial partitioning event, and its role as a specific receptor for the toxin is not so clear-cut.
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