Crystal structure of the soluble form of equinatoxin II, a pore-forming toxin from the sea anemone Actinia equina.
BACKGROUND
Membrane pore-forming toxins have a remarkable property: they adopt a stable soluble form structure, which, when in contact with a membrane, undergoes a series of transformations, leading to an active, membrane-bound form. In contrast to bacterial toxins, no structure of a pore-forming toxin from an eukaryotic organism has been determined so far, an indication that structural studies of equinatoxin II (EqtII) may unravel a novel mechanism.


RESULTS
The crystal structure of the soluble form of EqtII from the sea anemone Actinia equina has been determined at 1.9 A resolution. EqtII is shown to be a single-domain protein based on a 12 strand beta sandwich fold with a hydrophobic core and a pair of alpha helices, each of which is associated with the face of a beta sheet.


CONCLUSIONS
The structure of the 30 N-terminal residues is the largest segment that can adopt a different structure without disrupting the fold of the beta sandwich core. This segment includes a three-turn alpha helix that lies on the surface of a beta sheet and ends in a stretch of three positively charged residues, Lys-30, Arg-31, and Lys-32. On the basis of gathered data, it is suggested that this segment forms the membrane pore, whereas the beta sandwich structure remains unaltered and attaches to a membrane as do other structurally related extrinsic membrane proteins or their domains. The use of a structural data site-directed mutagenesis study should reveal the residues involved in membrane pore formation.
Athanasiadis
A
A
Department of Biochemistry and Molecular Biology, Jozef Stefan Institute, Ljubljana, Slovenia.
Anderluh
G
G
Macek
P
P
Turk
D
D
D000595:Amino Acid Sequence; D000818:Animals; D002463:Cell Membrane Permeability; D003064:Cnidarian Venoms / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D003603:Cytotoxins / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012615:Sea Anemones / Q000737:chemistry*; D012995:Solubility; D055672:Static Electricity
D003064:Cnidarian Venoms; D003603:Cytotoxins; C012307:equinatoxin