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Equinatoxin II (Eqt-II) is a member of the actinoporins, a unique
family of cytotoxins comprising 20 kDa pore-forming proteins
isolated from sea anemones. Actinoporins bind preferentially to
lipid membranes containing sphingomyelin, and create cationselective
pores by oligomerization of three to four monomers.
Previous studies have shown that regions of Eqt-II crucial for
its cytolytic mechanism are an exposed aromatic cluster and the
N-terminal region containing an amphipathic α-helix. In the present
study, we have investigated the transfer of the N-terminal
α-helix into the lipid membrane by the use of three mutants containing
an additional tryptophan residue in different positions
within the amphipathic α-helix (Ile18→Trp, Val22→Trp and
Ala25→Trp). The interaction of the mutants with different model
systems, such as lipid monolayers, erythrocytes and ghost membranes,
was extensively characterized. Intrinsic fluorescence measurements
and the use of vesicles containing brominated
phospholipids indicated a deep localization of the N-terminal amphipathic
helix in the lipid bilayer, except for the case of Val22→ Trp. This mutant is stabilized in a state immediately prior to
final pore formation. The introduction of additional tryptophan
residues in the sequence of Eqt-II has proved to be a suitable
approach to monitor the new environments that surround defined
regions of the molecule upon membrane interaction.
This work was supported by grant 042.310-13552/2001 from the University of the Basque
Country and BIO2003-BI09056 from the Basque Government. I. G.-A. was the recipient
of a predoctoral fellowship from the Basque Government. A. B. was the recipient of a
postdoctoral fellowship from the Basque Government. Slovenian authors acknowledge
financial support from the Slovenian Ministry of Education, Science and Sport.
Peer reviewed