Sticholysins, two pore-forming toxins produced by the Caribbean Sea anemone Stichodactyla helianthus: Their interaction with membranes

Abstract

Sticholysins (Sts) I and II (StI/II) are pore-forming toxins (PFTs) produced by the Caribbean Sea anemone Stichodactyla helianthus belonging to the actinoporin family, a unique class of eukaryotic PFTs exclusively found in sea anemones. As for the rest of the members of this family, Sts are cysteine-less proteins, with molecular weights around 20 kDa, high isoelectric points (>9.5), and a preference for sphingomyelin-containing membranes. A three-dimensional structure of StII, solved by X-ray crystallography, showed that it is composed of a hydrophobic beta-sandwich core flanked on the opposite sides by two alpha helices comprising residues 14-23 and 128-135. A variety of experimental results indicate that the first thirty N-terminal residues, which include one of the helices, are directly involved in pore formation. This region contains an amphipathic stretch, well conserved in all actinoporins, which is the only portion of the molecule that can change conformation without perturbing the general protein fold; in fact, binding to model membranes only produces a slight increase in the regular secondary structure content of Sts. Sts are produced in soluble form but they readily bind to different cell and model membrane systems such as lipidic monolayers, micelles, and lipid vesicles. Remarkably, both the binding and pore-formation steps are critically dependent on the physico-chemical nature of the membrane. In fact, a large population of toxin irreversibly binds with high affinity in membranes containing sphingomyelin whereas binding in membranes lacking this sphingolipid is relatively low and reversible. The joint presence of SM and cholesterol largely promotes binding and pore formation. Minor amounts of lipids favoring a non-lamellar organization also augment the efficiency of pore formation. The functional pore formed in cellular and model membranes has a diameter of approximately 2.0 nm and is presumably formed by the N-terminal alpha helices of four monomers tilted 31 degrees in relation to the bilayer normal. Experimental evidence supports the hypothesis that sticholysins, as well as equinatoxin II, another actinoporin, form a toroidal pore in membranes in which the polypeptide chains as well as the polar head groups of phospholipids are involved.

The effect of sea anemone (H. magnifica) venom on two human breast cancer lines: death by apoptosis

“…Upon chemical or physical stimulation, the thread tubule folded in the nematocyst is discharged and penetrates the epithelium of the victim (Hutton and Valerie 1996;Marino et al 2004;Albert 2008;Andreev et al 2008;Alvarez et al 2009). Some of the most potent marine toxins known are from sea anemones, and include a rich source of peptide toxins, sodium channel toxins and potassium channel toxins.…”

Biophysical and biochemical strategies to understand membrane binding and pore formation by sticholysins, pore-forming proteins from a sea anemone

“…Sticholysins I and II (Sts, StI/II) are PFT produced by the Caribbean Sea anemone Stichodactyla helianthus (Lanio et al 2001;Alvarez et al 2009); they belong to the actinoporin protein family, a unique class of PFT found in sea anemones. Actinoporins are monomeric, soluble, α-helical barrel PFP characterized by a molecular mass of about 20 kDa, high pI (>9.0), lack of Cys residues and a high affinity for sphingomyelin (SM)-containing membranes (Anderluh and Macek 2002).…”

Differential Effect of Solution Conditions on the Conformation of the Actinoporins Sticholysin II and Equinatoxin II

“…Actinoporins are a family of potent pore-forming proteins produced by sea anemones (Cnidiaria: Anthozoa: Actiniaria) (Álvarez et al 2009, Bakrac and Anderluh 2010. These toxins are synthesized as stable water-soluble monomers that assemble into oligomeric porelike structures to promote target cell lysis (Álvarez et al 2009(Álvarez et al , Mancheño et al 2001(Álvarez et al , 2003.…”

“…These toxins are synthesized as stable water-soluble monomers that assemble into oligomeric porelike structures to promote target cell lysis (Álvarez et al 2009(Álvarez et al , Mancheño et al 2001(Álvarez et al , 2003. Model actinoporins include Sticholysin II (StnII) and Equinatoxin II (EqtxII), obtained from Stichodactyla helianthus and Actina equine, respectively (Kristan et al 2009, Parker and Feil 2005).…”

“…The main conformational differences between NMR and crystal structures of these toxins lie on the C-terminus α-helix (Gln128-Tyr136 in StnII and Gln130-Tyr138 in EqtxII), at the phosphocholine binding site (Álvarez et al 2009, Miles et al 2008, Hinds et al 2002. Since actinoporins are considered a potential target for biotechnological applications ), the current work intends to characterize the dynamics of StnII and EqtxII under distinct physical-chemical conditions as a strategy to gain insights into the structural basis for the actinoporins hemolytic activity.…”