Differential interaction of equinatoxin II with model membranes in response to lipid composition

Equinatoxin II is a 179-amino-acid pore-forming protein isolated from the v enom of the sea anemone Acfinia equina. Large unilamellar vesicles and lipi d monolayers of different lipid compositions have been used to study its in teraction with membranes. The critical pressure for insertion is the same i n monolayers made of phosphatidylcholine or sphingomyelin (similar to 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 ph osphatidylcholine-sphingomyelin (1:1) monolayers, the critical pressure is higher (similar to 33 mN m(-1)), thus permitting the insertion of equinatox in II in large unilamellar vesicles, a process that is accompanied by major conformational changes. In the presence of vesicles made of phosphatidylch oline, a fraction of the protein molecules remains associated with the memb ranes. This interaction is fully reversible, does not involve major conform ational changes, and is governed by the high affinity for membrane interfac es of the protein region comprising amino acids 101-120. We conclude that a lthough the presence of sphingomyelin within the membrane creates condition s for irreversible insertion and pore formation, this lipid is not essentia l for the initial partitioning event, and its role as a specific receptor f or the toxin is not so clear-cut.