1.C.38 The Pore-forming Equinatoxin (Equinatoxin) Family

Sea anemones such as Actinia equina, Heteractis magnifica, and Stichodactyla helianthus produce a variety of sequence related toxins (called actinoporins) including Equinatoxins 1A,-1D, II, III, IV, V, etc. They have been given alternative designations such as Tenebrosin C (for Equinatoxin II), cytolysin, and hemolytic toxin. These cardiac stimulatory hemolysins penetrate membranes forming ion permeable, cation-selective pores, also permeable to small neutral solutes. They cause a variety of phenotypes in mammals including platelet aggregation, cytotoxicity of a variety of animal cells, lysis of some cells, and vasospasm of coronary vessels. They are of 175-179 aas in length and form tetrameric pore-forming structures in membranes. The 3-dimensional structure of the soluble form of equinatoxin II has been solved (Anastasiadis et al., 2001). The radius of the Sticholysin pore has been shown to be about 1.2 nm (diameter, ~2 nm). Pore size is independent of toxin concentration and is the same in biological and artificial membranes (Tejuca et al., 2001). Pore formation requires a flexible N-terminal region and a stable β-sandwich (Kristan et al., 2004). Actinoporins comprise a multigene family consisting of 47 representatives expressed in the sea anemone tentacles as prepropeptide-coding transcripts. Phylogenetic analysis revealed that actinoporin clustering is consistent with the division of sea anemones into superfamilies and families (Leychenko et al. 2018).  The structural mechanisms of pore formation and host-pathogen interaction of PFTs at an atomic level have been reviewed (Li et al. 2021).

Equinatoxin II inserts into the membrane via a two-step membrane-binding process involving an exposed cluster of aromatic residues (step 1) and a flexible N-terminal amphipathic α-helix (step 2) (Hong et al., 2002). The first step is similar to that of the evolutionarily distant cholesterol-dependent cytolysins. Interaction is dependent on sphingomyelin, and lipid phase coexistence favors membrane insertion (Barlic et al., 2004; Biserka et al., 2008; Schoen et al., 2008).

Equinatoxin II (EqtII) from Actinia equina and Sticholysin II (StnII) from Stichodactyla helianthus are the actinoporins that have been studied in greatest detail. Both proteins display a beta-sandwich fold composed of 10 β-strands flanked on each side by two short alpha-helices. Two-dimensional crystallization on lipid monolayers has allowed the determination of low-resolution models of tetrameric structures distinct from the pore. Wild-type EqtII and StnII, as well as a nice collection of natural and artificially made variants of both proteins, have been produced in Escherichia coli and purified. Four regions of the actinoporin structure seem to play an important role. The phosphatidyl choline or sphingomyelin-binding site and a cluster of exposed aromatic residues, together with a basic region, may be involved in the initial interaction with the membrane, whereas the amphipathic N-terminal region is essential for oligomerization and pore formation (Alegre-Cebollaba et al., 2007). Pore formation proceeds in at least four steps: Monomer binding to the membrane interface, assembly of four monomers, and at least two distinct conformational changes driving to the final formation of the functional pore. Sticholysin I is almost identical to sticholysin II. Conformational flexibility at the N-terminus of the protein does not provide higher affinity for the membrane, although it is necessary for correct pore formation (Alegre-Cebollada et al., 2008). An AF domain superfamily (abbreviated from actinoporin-like proteins and fungal fruit-body lectins) has been defined. It contains members from at least three animal and two plant phyla. On the basis of functional properties of some members, Crnigoj Kristan et al., 2009 hypothesised that AF domains mediate peripheral membrane interactions.

Fragaceatoxin C (FraC) is an α-barrel pore-forming toxin (PFT). The crystal structures of FraC at four different stages of the lytic mechanism have been determined at 3.1Å resolution, namely the water-soluble state, the monomeric lipid-bound form, an assembly intermediate and the fully assembled transmembrane pore (Tanaka et al. 2015). The structure of the transmembrane pore exhibits a unique architecture composed of both protein and lipids, with some of the lipids lining the pore wall, acting as assembly cofactors. The pore exhibits lateral fenestrations that expose the hydrophobic core of the membrane to the aqueous environment. The incorporation of lipids from the target membrane within the structure of the pore provides a membrane-specific trigger for the activation of this haemolytic toxin.

The assembly of the functional transmembrane pore requires several intermediate steps ranging from a water-soluble monomeric species to the multimeric ensemble inserted in the cell membrane. The non-lytic oligomeric intermediate is known as a prepore. Morante et al. 2016 employed single-particle cryo-electron microscopy (cryo-EM) and atomic force microscopy (AFM) to identify a prepore species of the actinoporin fragaceatoxin C (FraC) (TC# 1.C.38.1.12) bound to lipid vesicles. The size of the prepore coincides with that of the functional pore except for the transmembrane region, which is absent in the prepore. In the prepore species, the N-terminus is not inserted in the bilayer but is exposed to the aqueous solution. 

Sea anemones (Cnidaria, Anthozoa, and Actiniaria) use toxic peptides to incapacitate and immobilize prey and to deter potential predators. Their toxin arsenal is complex, targeting a variety of functionally important protein complexes and macromolecules involved in cellular homeostasis. Among these, actinoporins are one of the better characterized toxins; these venom proteins form a pore in cellular membranes containing sphingomyelin. Macrander and Daly 2016 used a combined bioinformatic and phylogenetic approach to investigate how actinoporins have evolved across three superfamilies of sea anemones (Actinioidea, Metridioidea, and Actinostoloidea). Their analysis identified 90 candidate actinoporins across 20 species. They also found clusters of six actinoporin-like genes in five species of sea anemone (Nematostella vectensis, Stomphia coccinea, Epiactis japonica, Heteractis crispa, and Diadumene leucolena); these actinoporin-like sequences resembled actinoporins but have a higher sequence similarity with toxins from fungi, cone snails, and Hydra. Comparative analysis of the candidate actinoporins highlighted variable and conserved regions within actinoporins that may pertain to functional variation.Multiple residues are involved in initiating sphingomyelin recognition and membrane binding. Residues thought to be involved with oligomerization were variable, while those forming the phosphocholine (POC) binding site and the N-terminal region involved with cell membrane penetration were highly conserved (Macrander and Daly 2016).

Equinatoxin II (EqtII), fragaceatoxin C (FraC), and sticholysins I and II (StnI and StnII, respectively), produced by three different sea anemone species, are the only actinoporins whose molecular structures had been studied in depth as of Jan, 2017. These four proteins show high sequence identities and practically coincident three-dimensional structures, but, their pore-forming activities are quite different depending on the model lipid system employed (García-Linares et al. 2016). These varied responses to lipid composition may be a consequence of their distinct specificities and/or membrane binding affinities. Trp residues play a major role in membrane recognition and binding, but these residues have only a minor influence on the diffusion and oligomerization steps needed to assemble a functional pore (García-Linares et al. 2016).

The generalized transport reaction catalyzed by members of the equinatoxin family is:

Small molecule (in) small molecule (out)