The equinatoxin N-terminus is transferred across planar lipid membranes and helps to stabilize the transmembrane pore

Equinatoxin II is a cytolytic protein isolated from the sea anemone Actinia equina. It is a member of the actinoporins, a family of eukaryotic pore-forming toxins with a unique mechanism of pore formation. Equinatoxin II is a 20 kDa cysteineless protein, with sphingomyelin-dependent activity. Recent studies showed that the N-terminal region of the molecule requires conformational flexibility during pore formation. An understanding of the N-terminal position in the final pore and its role in membrane insertion and pore stability is essential to define the precise molecular mechanism of pore formation. The formation of pores and their electrophysiologic characteristics were studied with planar lipid membranes. We show that amino acids at positions 1 and 3 of equinatoxin II are exposed to the lumen of the pore. Moreover, sulfhydryl reagents and a hexa-histidine tag attached to the N-terminus revealed that the N-terminus of the toxin extends through the pore to the other (trans) side of the membrane and that negatively charged residues inside the pore are crucial to define the electrophysiologic characteristics of the channel. Finally, we detected a new, less stable, state with a lower conductance by using a deletion mutant in which the first five N-terminal amino acids were removed. We propose that the first five amino acids help to anchor the amphipathic helix on the trans side of the membrane and consequently stabilize the final transmembrane pore.