Copyright © by Biophysical Society. Final full-text version of the paper available at: http://www.biophysj.org/cgi/content/abstract/80/3/1343
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 m21) 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 m21), 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.
This work was supported by grants PI-1998–110 and UE-1998–43 from
the Basque Government. J.M.M.C., I.E., and I.G.A. were recipients of
predoctoral fellowships from the Basque Government.
Peer reviewed