Purification and characterization of two hemolysins from Stichodactyla helianthus

Decoupling immunomodulatory properties from lipid binding in the α-pore-forming toxin Sticholysin II

Sticholysin II (StII), a pore-forming toxin from the marine anemone Stichodactyla helianthus, enhances an antigen-specific cytotoxic T lymphocyte (CTL) response when co-encapsulated in liposomes with a model antigen. This capacity does not depend exclusively on its pore-forming activity and is partially supported by its ability to activate Toll-like receptor 4 (TLR4) in dendritic cells, presumably by interacting with this receptor or by triggering signaling cascades upon binding to lipid membrane. In order to investigate whether the lipid binding capacity of StII is required for immunomodulation, we designed a mutant in which the aromatic amino acids from the interfacial binding site Trp110, Tyr111 and Trp114 were substituted by Ala. In the present work, we demonstrated that StII3A keeps the secondary structure composition and global folding of StII, while it loses its lipid binding and permeabilization abilities. Despite this, StII3A upregulates dendritic cells maturation markers, enhances an antigen-specific effector CD8+ T cells response and confers antitumor protection in a preventive scenario in C57BL/6 mice. Our results indicate that a mechanism independent of its lipid binding ability is involved in the immunomodulatory capacity of StII, pointing to StII3A as a promising candidate to improve the reliability of the Sts-based vaccine platform.

Unveiling Sticholysin II and plasmid DNA interaction: Implications for developing non-viral vectors

Non-viral gene delivery systems offer significant potential for gene therapy due to their versatility, safety, and cost advantages over viral vectors. However, their effectiveness can be hindered by the challenge of efficiently releasing the genetic cargo from endosomes to prevent degradation in lysosomes. To overcome this obstacle, functional components can be incorporated into these systems. Sticholysin II (StII) is one of the pore-forming proteins derived from the sea anemone Stichodactyla helianthus, known for its high ability to permeabilize cellular and model membranes. In this study, we aimed to investigate the interaction between StII, and a model plasmid (pDNA) as an initial step towards designing an improved vector with enhanced endosomal escape capability. The electrophoretic mobility shift assay (EMSA) confirmed the formation of complexes between StII and pDNA. Computational predictions identified specific residues involved in the StII-DNA interaction interface, highlighting the importance of electrostatic interactions and hydrogen bonds in mediating the binding. Atomic force microscopy (AFM) of StII-pDNA complexes revealed the presence of nodular fiber and toroid shapes. These complexes were found to have a predominantly micrometer size, as confirmed by dynamic light scattering (DLS) measurements. Despite increase in the overall charge, the complexes formed at the evaluated nitrogen-to-phosphorus (N/P) ratios still maintained a negative charge. Moreover, StII retained its pore-forming capacity regardless of its binding to the complexes. These findings suggest that the potential ability of StII to permeabilize endosomal membranes could be largely maintained when combined with nucleic acid delivery systems. Additionally, the still remaining negative charge of the complexes would enable the association of another positively charged component to compact pDNA. However, to minimize non-specific cytotoxic effects, it is advisable to explore methods to regulate the protein's activity in response to the microenvironment.

The Important Role of Membrane Fluidity on the Lytic Mechanism of the α-Pore-Forming Toxin Sticholysin I

Actinoporins have emerged as archetypal α-pore-forming toxins (PFTs) that promote the formation of pores in membranes upon oligomerization and insertion of an α-helix pore-forming domain in the bilayer. These proteins have been used as active components of immunotoxins, therefore, understanding their lytic mechanism is crucial for developing this and other applications. However, the mechanism of how the biophysical properties of the membrane modulate the properties of pores generated by actinoporins remains unclear. Here we studied the effect of membrane fluidity on the permeabilizing activity of sticholysin I (St I), a toxin that belongs to the actinoporins family of α-PFTs. To modulate membrane fluidity we used vesicles made of an equimolar mixture of phosphatidylcholine (PC) and egg sphingomyelin (eggSM), in which PC contained fatty acids of different acyl chain lengths and degrees of unsaturation. Our detailed single-vesicle analysis revealed that when membrane fluidity is high, most of the vesicles are partially permeabilized in a graded manner. In contrast, more rigid membranes can be either completely permeabilized or not, indicating an all-or-none mechanism. Altogether, our results reveal that St I pores can be heterogeneous in size and stability, and that these properties depend on the fluid state of the lipid bilayer. We propose that membrane fluidity at different regions of cellular membranes is a key factor to modulate the activity of the actinoporins, which has implications for the design of different therapeutic strategies based on their lytic action.

Cys mutants as tools to study the oligomerization of the pore-forming toxin sticholysin I

Sticholysin I (StI) is a water-soluble protein with the ability to bind membranes where it oligomerizes and forms pores leading to cell death. Understanding the assembly property of this protein may be valuable for designing potential biotechnological tools, such as stable or structurally defined nanopores. In order to get insights into the stabilization of StI oligomers by disulfide bonds, we designed and characterized single and double cysteine mutants at the oligomerization interface. The oligomer formation was induced in the presence of lipid membranes and visualized by SDS-PAGE. The contribution of the oligomeric structures to the membrane binding and pore-forming capacities of StI was assessed. Single and double cysteine introduction at the protein-protein oligomerization interface does not considerably affect the conformation and function of the monomeric protein. In the presence of membranes, a cysteine double mutation at positions 15 and 59 favored formation of different size oligomers stabilized by disulfide bonds. The results of this work highlight the relevance of these positions (15 and 59) to be considered for developing biosensors based on nanopores from StI.

Natural marine products as antiprotozoal agents against amitochondrial parasites

The goal of this work is to compile and discuss molecules of marine origin reported in the scientific literature with anti-parasitic activity against Trichomonas, Giardia, and Entamoeba, parasites responsible for diseases that are major global health problems, and Microsporidial parasites as an emerging problem. The presented data correspond to metabolites with anti-parasitic activity in human beings that have been isolated by chromatographic techniques from marine sources and structurally elucidated by spectroscopic and spectrometric procedures. We also highlight some semi-synthetic derivatives that have been successful in enhancing the activity of original compounds. The biological oceanic reservoir offers the possibility to discover new biologically active molecules as lead compounds to develop new drug candidates. The molecular variety is extensive and must be correctly explored and managed. Also, it will be necessary to take some actions to preserve the source species from extinction or overharvest (e.g., by cryopreservation of coral spermatozoa, oocytes, embryos, and larvae) and coordinate appropriate exploitation to increase the chemical knowledge of the natural products generated in the oceans. Additional initiatives such as the total synthesis of complex natural products and their derivatives can help to prevent overharvest of the marine ecosystems and at the same time contribute to the discovery of new molecules.

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Natural active components of marine organisms have specific biological properties.

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The marine compounds have multiple anti-parasitic activity.

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The semi-synthetic derivatives of natural active components of marine organism are candidates for new drugs.

The pore-forming activity of sticholysin I is enhanced by the presence of a phospholipid hydroperoxide in membrane

Sticholysin I (StI) is a pore-forming toxin (PFT) belonging to the actinoporin protein family characterized by high permeabilizing activity in membranes. StI readily associates with sphingomyelin (SM)-containing membranes originating pores that can lead to cell death. Binding and pore-formation are critically dependent on the physicochemical properties of membrane. 1-palmitoyl-2-oleoylphosphatidylcholine hydroperoxide (POPC-OOH) is an oxidized phospholipid (OxPL) containing an -OOH moiety in the unsaturated hydrocarbon chain which orientates towards the bilayer interface. This orientation causes an increase in the lipid molecular area, lateral expansion and decrease in bilayer thickness, elastic and bending modulus, as well as modification of lipid packing. Taking advantage of membrane structural changes promoted by POPC-OOH, we investigated its influence on the permeabilizing ability of StI. Here we report the action of StI on Giant Unilamellar Vesicles (GUVs) made of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and SM containing increasing amount of POPC-OOH to assess vesicle permeability changes when compared to OxPL-lacking membranes. Inclusion of POPC-OOH in membranes did not promote spontaneous vesicle leaking but resulted in increased membrane permeability due to StI action. StI activity did not modify the fluid-gel phase coexistence boundaries neither in POPC:SM or POPC-OOH:SM membranes. However, the StI insertion mechanism in membrane seems to differ between POPC:SM and POPC-OOH:SM mixtures as suggested by changes in the time course of monolayer surface tension measurements, even though a preferable binding of the toxin to OxPL-containing systems could not be here demonstrated. In summary, modifications in the membrane imposed by lipid hydroperoxidation favor StI permeabilizing activity.

Panorama of the Intracellular Molecular Concert Orchestrated by Actinoporins, Pore-Forming Toxins from Sea Anemones

Actinoporins (APs) are soluble pore-forming proteins secreted by sea anemones that experience conformational changes originating in pores in the membranes that can lead to cell death. The processes involved in the binding and pore-formation of members of this protein family have been deeply examined in recent years; however, the intracellular responses to APs are only beginning to be understood. Unlike pore formers of bacterial origin, whose intracellular impact has been studied in more detail, currently, we only have knowledge of a few poorly integrated elements of the APs’ intracellular action. In this review, we present and discuss an updated landscape of the studies aimed at understanding the intracellular pathways triggered in response to APs attack with particular reference to sticholysin II, the most active isoform produced by the Caribbean Sea anemone Stichodactyla helianthus. To achieve this, we first describe the major alterations these cytolysins elicit on simpler cells, such as non-nucleated mammalian erythrocytes, and then onto more complex eukaryotic cells, including tumor cells. This understanding has provided the basis for the development of novel applications of sticholysins such as the construction of immunotoxins directed against undesirable cells, such as tumor cells, and the design of a cancer vaccine platform. These are among the most interesting potential uses for the members of this toxin family that have been carried out in our laboratory.

Sticholysin II shows similar immunostimulatory properties to LLO stimulating dendritic cells and MHC-I restricted T cell responses of heterologous antigen

Induction of CD8⁺ T cell responses against tumor cells and intracellular pathogens is an important goal of modern vaccinology. One approach of translational interest is the use of liposomes encapsulating pore-forming proteins (PFPs), such as Listeriolysin O (LLO), which has shown efficacy at priming strong and sustained CD8⁺ T cell responses. Recently, we have demonstrated that Sticholysin II (StII), a PFP from the sea anemone Stichodactyla helianthus, co-encapsulated into liposomes with ovalbumin (OVA) was able to stimulate, antigen presenting cells, antigen-specific CD8⁺ T cells and anti-tumor activity in mice. In the present study, we aimed to compare StII and LLO in terms of their abilities to stimulate dendritic cells and to induce major histocompatibility complex (MHC) class I restricted T cell responses against OVA. Interestingly, StII exhibited similar abilities to LLO in vitro of inducing dendritic cells maturation, as measured by increased expression of CD40, CD80, CD86 and MHC-class II molecules, and of stimulating OVA cross-presentation to a CD8⁺ T cell line. Remarkably, using an ex vivo Enzyme-Linked ImmunoSpot Assay (ELISPOT) to monitor gamma interferon (INF-γ) producing effector memory CD8⁺ T cells, liposomal formulations containing either StII or LLO induced comparable frequencies of OVA-specific INF-γ producing CD8⁺ T cells in mice that were sustained in time. However, StII-containing liposomes stimulated antigen-specific memory CD8⁺ T cells with a higher potential to secrete IFN-γ than liposomes encapsulating LLO. This StII immunostimulatory property further supports its use for the rational design of T cell vaccines against cancers and intracellular pathogens. In summary, this study indicates that StII has immunostimulatory properties similar to LLO, despite being evolutionarily distant PFPs.

Sticholysins, pore-forming proteins from a marine anemone can induce maturation of dendritic cells through a TLR4 dependent-pathway

Sticholysins (Sts) I and II (StI and StII) are pore-forming proteins (PFPs), purified from the Caribbean Sea anemone Stichodactyla helianthus. StII encapsulated into liposomes induces a robust antigen-specific cytotoxic CD8⁺ T lymphocytes (CTL) response and in its free form the maturation of bone marrow-derived dendritic cells (BM-DCs). It is probable that the latter is partially supporting in part the immunomodulatory effect on the CTL response induced by StII-containing liposomes. In the present work, we demonstrate that the StII's ability of inducing maturation of BM-DCs is also shared by StI, an isoform of StII. Using heat-denatured Sts we observed a significant reduction in the up-regulation of maturation markers indicating that both PFP's ability to promote maturation of BM-DCs is dependent on their conformational characteristics. StII-mediated DC maturation was abrogated in BM-DCs from toll-like receptor (TLR) 4 and myeloid differentiation primary response gene 88 (MyD88)-knockout mice but not in cells from TLR2-knockout mice. Furthermore, the antigen-specific CTL response induced by StII-containing liposomes was reduced in TLR4-knockout mice. These results indicate that StII, and probably by extension StI, has the ability to induce maturation of DCs through a TLR4/MyD88-dependent pathway, and that this activation contributes to the CTL response generated by StII-containing liposomes.

Sea Anemone Heteractis crispa Actinoporin Demonstrates In Vitro Anticancer Activities and Prevents HT-29 Colorectal Cancer Cell Migration

Actinoporins are the most abundant group of sea anemone cytolytic toxins. Their membranolytic activity is of high interest for the development of novel anticancer drugs. However, to date the activity of actinoporins in malignant cells has been poorly studied. Here, we report on recombinant analog of Hct-S3 (rHct-S3), belonging to the combinatory library of Heteractis crispa actinoporins. rHct-S3 exhibited cytotoxic activity against breast MDA-MB-231 (IC₅₀ = 7.3 µM), colorectal HT-29 (IC₅₀ = 6.8 µM), and melanoma SK-MEL-28 (IC₅₀ = 8.3 µM) cancer cells. The actinoporin effectively prevented epidermal growth factor -induced neoplastic transformation of JB6 Cl41 cells by 34% ± 0.2 and decreased colony formation of HT-29 cells by 47% ± 0.9, MDA-MB-231 cells by 37% ± 1.2, and SK-MEL-28 cells by 34% ± 3.6. Moreover, rHct-S3 decreased proliferation and suppressed migration of colorectal carcinoma cells by 31% ± 5.0 and 99% ± 6.4, respectively. The potent anti-migratory activity was proposed to mediate by decreased matrix metalloproteinases-2 and -9 expression. In addition, rHct-S3 induced programmed cell death by cleavage of caspase-3 and poly (ADP-ribose) polymerase, as well as regulation of Bax and Bcl-2. Our results indicate rHct-S3 to be a promising anticancer drug with a high anti-migratory potential.

Functional and Structural Variation among Sticholysins, Pore-Forming Proteins from the Sea Anemone Stichodactyla helianthus

Venoms constitute complex mixtures of many different molecules arising from evolution in processes driven by continuous prey-predator interactions. One of the most common compounds in these venomous cocktails are pore-forming proteins, a family of toxins whose activity relies on the disruption of the plasmatic membranes by forming pores. The venom of sea anemones, belonging to the oldest lineage of venomous animals, contains a large amount of a characteristic group of pore-forming proteins known as actinoporins. They bind specifically to sphingomyelin-containing membranes and suffer a conformational metamorphosis that drives them to make pores. This event usually leads cells to death by osmotic shock. Sticholysins are the actinoporins produced by Stichodactyla helianthus. Three different isotoxins are known: Sticholysins I, II, and III. They share very similar amino acid sequence and three-dimensional structure but display different behavior in terms of lytic activity and ability to interact with cholesterol, an important lipid component of vertebrate membranes. In addition, sticholysins can act in synergy when exerting their toxin action. The subtle, but important, molecular nuances that explain their different behavior are described and discussed throughout the text. Improving our knowledge about sticholysins behavior is important for eventually developing them into biotechnological tools.

Structural and functional characterization of sticholysin III: A newly discovered actinoporin within the venom of the sea anemone Stichodactyla helianthus

Actinoporins are a family of pore-forming toxins produced by sea anemones as part of their venomous cocktail. These proteins remain soluble and stably folded in aqueous solution, but when interacting with sphingomyelin-containing lipid membranes, they become integral oligomeric membrane structures that form a pore permeable to cations, which leads to cell death by osmotic shock. Actinoporins appear as multigenic families within the genome of sea anemones: several genes encoding very similar actinoporins are detected within the same species. The Caribbean Sea anemone Stichodactyla helianthus produces three actinoporins (sticholysins I, II and III; StnI, StnII and StnIII) that differ in their toxic potency. For example, StnII is about four-fold more effective than StnI against sheep erythrocytes in causing hemolysis, and both show synergy. However, StnIII, recently discovered in the S. helianthus transcriptome, has not been characterized so far. Here we describe StnIII's spectroscopic and functional properties and show its potential to interact with the other Stns. StnIII seems to maintain the well-preserved fold of all actinoporins, characterized by a high content of β-sheet, but it is significantly less thermostable. Its functional characterization shows that the critical concentration needed to form active pores is higher than for either StnI or StnII, suggesting differences in behavior when oligomerizing on membrane surfaces. Our results show that StnIII is an interesting and unexpected piece in the puzzle of how this Caribbean Sea anemone species modulates its venomous activity.

Actinoporins: From the Structure and Function to the Generation of Biotechnological and Therapeutic Tools

Actinoporins (APs) are a family of pore-forming toxins (PFTs) from sea anemones. These biomolecules exhibit the ability to exist as soluble monomers within an aqueous medium or as constitutively open oligomers in biological membranes. Through their conformational plasticity, actinoporins are considered good candidate molecules to be included for the rational design of molecular tools, such as immunotoxins directed against tumor cells and stochastic biosensors based on nanopores to analyze unique DNA or protein molecules. Additionally, the ability of these proteins to bind to sphingomyelin (SM) facilitates their use for the design of molecular probes to identify SM in the cells. The immunomodulatory activity of actinoporins in liposomal formulations for vaccine development has also been evaluated. In this review, we describe the potential of actinoporins for use in the development of molecular tools that could be used for possible medical and biotechnological applications.

Membrane Remodeling by the Lytic Fragment of SticholysinII: Implications for the Toroidal Pore Model

Sticholysins are pore-forming toxins of biomedical interest and represent a prototype of proteins acting through the formation of protein-lipid or toroidal pores. Peptides spanning the N-terminus of sticholysins can mimic their permeabilizing activity and, together with the full-length toxins, have been used as a tool to understand the mechanism of pore formation in membranes. However, the lytic mechanism of these peptides and the lipid shape modulating their activity are not completely clear. In this article, we combine molecular dynamics simulations and experimental biophysical tools to dissect different aspects of the pore-forming mechanism of StII_1-30, a peptide derived from the N-terminus of sticholysin II (StII). With this combined approach, membrane curvature induction and flip-flop movement of the lipids were identified as two important membrane remodeling steps mediated by StII_1-30. Pore formation by this peptide was enhanced by the presence of the negatively curved lipid phosphatidylethanolamine in membranes. This lipid emerged not only as a facilitator of membrane interactions but also as a structural element of the StII_1-30 pore that is recruited to the ring upon its assembly. Collectively, these, to our knowledge, new findings support a toroidal model for the architecture of the pore formed by StII_1-30 and provide new molecular insight into the role of phosphatidylethanolamine as a membrane component that can easily integrate into the ring of toroidal pores, thus probably aiding in their stabilization. This study contributes to a better understanding of the molecular mechanism underlying the permeabilizing activity of StII_1-30 and peptides or proteins acting via a toroidal pore mechanism and offers an informative framework for the optimization of the biomedical application of this and similar molecules.

Architecture of the pore forming toxin sticholysin I in membranes

Sticholysin I (StI) is a toxin produced by the sea anemone Stichodactyla helianthus and belonging to the actinoporins family. Upon binding to sphingomyelin-containing membranes StI forms oligomeric pores, thereby leading to cell death. According to recent controversial experimental evidences, the pore architecture of actinoporins is a debated topic. Here, we investigated the StI topology in membranes by site-directed spin labeling and electron paramagnetic resonance spectroscopy. The results reveal that StI in membrane exhibits an oligomeric architecture with heterogeneous stoichiometry of predominantly eight or nine protomers, according to the available structural models. The StI topology resembles the conic pore structure reported for the actinoporin fragaceatoxin C. Our data show that StI coexists in two membrane-associated conformations, with the N-terminal segment either attached to the protein core or inserted in the membrane forming the pore. This finding suggests a 'pre-pore' to 'pore' transition determined by a conformational change that detaches the N-terminal segment.

Multiple Pleomorphic Tetramers of Thermostable Direct Hemolysin from Grimontia hollisae in Exerting Hemolysis and Membrane Binding

Oligomerization of protein into specific quaternary structures plays important biological functions, including regulation of gene expression, enzymes activity, and cell-cell interactions. Here, we report the determination of two crystal structures of the Grimontia hollisae (formally described as Vibrio hollisae) thermostable direct hemolysin (Gh-TDH), a pore-forming toxin. The toxin crystalized in the same space group of P21212, but with two different crystal packing patterns, each revealing three consistent tetrameric oligomerization forms called Oligomer-I, -II, and -III. A central pore with comparable depth of ~50 Å but differing in shape and size was observed in all determined toxin tetrameric oligomers. A common motif of a toxin dimer was found in all determined structures, suggesting a plausible minimum functional unit within the tetrameric structure in cell membrane binding and possible hemolytic activity. Our results show that bacterial toxins may form a single or highly symmetric oligomerization state when exerting their biological functions. The dynamic nature of multiple symmetric oligomers formed upon release of the toxin may open a niche for bacteria survival in harsh living environments.

Susceptibility of erythrocytes from different sources to xenobiotics-induced lysis

As erythrocytes are continuously exposed to plenty of metabolites and toxicants, the aim of the present work is to show whether erythrocytes from different sources may exhibit different susceptibility to hemolysis induced by two classes of xenobiotics, Cnidaria venoms, acting via pore forming mechanism on cell membrane, and Cd²⁺, inducing cell damage mainly via lipid peroxidation. To this end, the hemolytic power of crude venom from stinging cells of the anthozoan Aiptasia mutabilis and from the scyphozoan Pelagia noctiluca has been tested, along with Cd²⁺ (5 to 20 mM), on erythrocytes (0.05% v/v) obtained from either rabbit, or dog, or chicken, or human source. Our results show a more significant susceptibility of rabbit erythrocytes to hemolysis induced by both crude venom and Cd²⁺ than erythrocytes from other sources. This difference seems to rely neither on the different specimens used to extract crude venom, nor on the different mechanism of toxicants. In this light, the present study may contribute: i) to confirm hemolytic test as a suitable biological assay to verify erythrocytes resistance to toxicants; ii) to show variability in hemolytic response to xenobiotics; iii) to propose rabbit erythrocytes as more sensitive to the lytic action of xenobiotics, adding more knowledge to the field of toxicology.

Lipase and Phospholipase Activity Methods for Marine Organisms

Lipases are very important enzymes having a role in fat digestion and lipid metabolism in marine animals, plants, and microorganisms. The methods for measuring lipase and phospholipase activity have been applied in several studies; however, considering that lipases are water-soluble molecules and their substrates are generally water-insoluble molecules, several steps are required for measuring their digestion products. After a general review of the main type of methods used in marine lipase studies, and experimental procedures, a proposal of new or improved methods is described in order to facilitate the lipase activity measurements in marine organisms.

The Vacuolar Pathway in Macrophages Plays a Major Role in Antigen Cross-Presentation Induced by the Pore-Forming Protein Sticholysin II Encapsulated Into Liposomes

Cross-presentation is an important mechanism for the differentiation of effector cytotoxic T lymphocytes (CTL) from naïve CD8⁺ T-cells, a key response for the clearance of intracellular pathogens and tumors. The liposomal co-encapsulation of the pore-forming protein sticholysin II (StII) with ovalbumin (OVA) (Lp/OVA/StII) induces a powerful OVA-specific CTL activation and an anti-tumor response in vivo. However, the pathway through which the StII contained in this preparation is able to induce antigen cross-presentation and the type of professional antigen presenting cells (APCs) involved have not been elucidated. Here, the ability of mouse bone marrow-derived dendritic cells (BM-DCs) and macrophages (BM-MΦs) stimulated with Lp/OVA/StII to activate SIINFEKL-specific B3Z CD8⁺ T cells was evaluated in the presence of selected inhibitors. BM-MΦs, but not BM-DCs were able to induce SIINFEKL-specific B3Z CD8⁺ T cell activation upon stimulation with Lp/OVA/StII. The cross-presentation of OVA was markedly decreased by the lysosome protease inhibitors, leupeptin and cathepsin general inhibitor, while it was unaffected by the proteasome inhibitor epoxomicin. This process was also significantly reduced by phagocytosis and Golgi apparatus function inhibitors, cytochalasin D and brefeldin A, respectively. These results are consistent with the concept that BM-MΦs internalize these liposomes through a phagocytic mechanism resulting in the cross-presentation of the encapsulated OVA by the vacuolar pathway. The contribution of macrophages to the CTL response induced by Lp/OVA/StII in vivo was determined by depleting macrophages with clodronate-containing liposomes. CTL induction was almost completely abrogated in mice depleted of macrophages, demonstrating the relevance of these APCs in the antigen cross-presentation induced by this formulation.

Self-association and folding in membrane determine the mode of action of peptides from the lytic segment of sticholysins

Sticholysin I and II (Sts: St I and St II) are proteins of biomedical interest that form pores upon the insertion of their N-terminus in the plasma membrane. Peptides spanning the N-terminal residues of StI (StI_1-31) or StII (StII_1-30) can mimic the permeabilizing ability of these toxins, emerging as candidates to rationalize their potential biomedical applications. These peptides have different activities that correlate with their hydrophobicity. However, it is not clear how this property contributes to peptide folding in solution or upon binding to membranes. Here we compared the conformational properties of these peptides and shorter versions lacking the hydrophobic segment 1-11 of StI (StI_12-31) or 1-10 of StII (StII_11-30). Folding of peptides was assessed in solution and in membrane mimetic systems and related with their ability to bind to membranes and to permeabilize lipid vesicles. Our results suggest that the differences in activity among peptides could be ascribed to their different folding propensity and different membrane binding properties. In solution, StII_1-30 tends to acquire α-helical conformation coexisting with self-associated structures, while StI_1-31 remains structureless. Both peptides fold as α-helix in membrane; but StII_1-30 also self-associates in the lipid environment, a process that is favored by its higher affinity for membrane. We stress the contribution of the non-polar/polar balance of the 1-10 amino acid sequence of the peptides as a determining factor for different self-association capabilities. Such difference in hydrophobicity seems to determine the molecular path of peptides folding upon binding to membranes, with an impact in their permeabilizing activity. This study contributes to a better understanding of the molecular mechanisms underlying the permeabilizing activity of Sts N-terminal derived peptides, with connotation for the exploitation of these small molecules as alternative of the full-length toxins in clinical settings.

Response of Cellular Innate Immunity to Cnidarian Pore-Forming Toxins

A group of stable, water-soluble and membrane-bound proteins constitute the pore forming toxins (PFTs) in cnidarians. They interact with membranes to physically alter the membrane structure and permeability, resulting in the formation of pores. These lesions on the plasma membrane causes an imbalance of cellular ionic gradients, resulting in swelling of the cell and eventually its rupture. Of all cnidarian PFTs, actinoporins are by far the best studied subgroup with established knowledge of their molecular structure and their mode of pore-forming action. However, the current view of necrotic action by actinoporins may not be the only mechanism that induces cell death since there is increasing evidence showing that pore-forming toxins can induce either necrosis or apoptosis in a cell-type, receptor and dose-dependent manner. In this review, we focus on the response of the cellular immune system to the cnidarian pore-forming toxins and the signaling pathways that might be involved in these cellular responses. Since PFTs represent potential candidates for targeted toxin therapy for the treatment of numerous cancers, we also address the challenge to overcoming the immunogenicity of these toxins when used as therapeutics.

Dissecting the mechanism of action of actinoporins. Role of the N-terminal amphipathic α-helix in membrane binding and pore activity of sticholysins I and II

Actinoporins sticholysin I and sticholysin II (St I, St II) are proposed to lyse model and biomembranes via toroidal pore formation by their N-terminal domain. Based on the hypothesis that peptide fragments can reproduce the structure and function of this domain, the behavior of peptides containing St I residues 12–31 (StI_12-31), St II residues 11–30 (StII_11-30), and its TOAC-labeled analogue (N-TOAC-StII_11-30) was examined. Molecular modeling showed a good match with experimental structures, indicating amphipathic α-helices in the same regions as in the toxins. CD spectra revealed that the peptides were essentially unstructured in aqueous solution, acquiring α-helical conformation upon interaction with micelles and large unilamellar vesicles (LUV) of variable lipid composition. Fluorescence quenching studies with NBD-containing lipids indicated that N-TOAC-StII_11-30’s nitroxide moiety is located in the membranes polar head group region. Pyrene-labeled phospholipid inter-leaflet redistribution suggested that the peptides form toroidal pores, according to the mechanism of action proposed for the toxins. Binding occurred only to negatively charged LUV, indicating the importance of electrostatic interactions; in contrast the peptides bound to both negatively charged and zwitterionic micelles, pointing to a lesser influence of these interactions. In addition, differences between bilayers and micelles in head group packing and in curvature led to differences in peptide-membrane interaction. We propose that the peptides topography in micelles resembles that of the toxins in the toroidal pore. The peptides mimicked the toxins permeabilizing activity, St II peptides being more effective than StI_12-31. To our knowledge, this is the first demonstration that differences in the toxins N-terminal amphipathic α-helix play a role in the difference between St I and St II activities.

Pharmacological characterization of cnidarian extracts from the Caribbean Sea: evaluation of anti-snake venom and antitumor properties

BACKGROUND

Cnidarians produce toxins, which are composed of different polypeptides that induce pharmacological effects of biotechnological interest, such as antitumor, antiophidic and anti-clotting activities. This study aimed to evaluate toxicological activities and potential as antitumor and antiophidic agents contained in total extracts from five cnidarians: Millepora alcicornis, Stichodactyla helianthus, Plexaura homomalla, Bartholomea annulata and Condylactis gigantea (total and body wall).

METHODS

The cnidarian extracts were evaluated by electrophoresis and for their phospholipase, proteolytic, hemorrhagic, coagulant, fibrinogenolytic, neuromuscular blocking, muscle-damaging, edema-inducing and cytotoxic activities.

RESULTS

All cnidarian extracts showed indirect hemolytic activity, but only S. helianthus induced direct hemolysis and neurotoxic effect. However, the hydrolysis of NBD-PC, a PLA2 substrate, was presented only by the C. gigantea (body wall) and S. helianthus. The extracts from P. homomalla and S. helianthus induced edema, while only C. gigantea and S. helianthus showed intensified myotoxic activity. The proteolytic activity upon casein and fibrinogen was presented mainly by B. annulata extract and all were unable to induce hemorrhage or fibrinogen coagulation. Cnidarian extracts were able to neutralize clotting induced by Bothrops jararacussu snake venom, except M. alcicornis. All cnidarian extracts were able to inhibit hemorrhagic activity induced by Bothrops moojeni venom. Only the C. gigantea (body wall) inhibited thrombin-induced coagulation. All cnidarian extracts showed antitumor effect against Jurkat cells, of which C. gigantea (body wall) and S. helianthus were the most active; however, only C. gigantea (body wall) and M. alcicornis were active against B16F10 cells.

CONCLUSION

The cnidarian extracts analyzed showed relevant in vitro inhibitory potential over the activities induced by Bothrops venoms; these results may contribute to elucidate the possible mechanisms of interaction between cnidarian extracts and snake venoms.

Cloning, purification and characterization of nigrelysin, a novel actinoporin from the sea anemone Anthopleura nigrescens

Actinoporins constitute a unique class of pore-forming toxins found in sea anemones that being secreted as soluble monomers are able to bind and permeabilize membranes leading to cell death. The interest in these proteins has risen due to their high cytotoxicity that can be properly used to design immunotoxins against tumor cells and antigen-releasing systems to cell cytosol. In this work we describe a novel actinoporin produced by Anthopleura nigrescens, an anemone found in the Central American Pacific Ocean. Here we report the amino acid sequence of an actinoporin as deduced from cDNA obtained from total body RNA. The synthetic DNA sequence encoding for one cytolysin variant was expressed in BL21 Star (DE3) Escherichia coli and the protein purified by chromatography on CM Sephadex C-25 with more than 97% homogeneity as verified by MS-MS and HPLC analyses. This actinoporin comprises 179 amino acid residues, consistent with its observed isotope-averaged molecular mass of 19 661 Da. The toxin lacks Cys and readily permeabilizes erythrocytes, as well as L1210 cells. CD spectroscopy revealed that its secondary structure is dominated by beta structure (58.5%) with 5.5% of α-helix, and 35% of random structure. Moreover, binding experiments to lipidic monolayers and to liposomes, as well as permeabilization studies in vesicles, revealed that the affinity of this toxin for sphingomyelin-containing membranes is quite similar to sticholysin II (StII). Comparison by spectroscopic techniques and modeling the three-dimensional structure of nigrelysin (Ng) showed a high homology with StII but several differences were also detectable. Taken together, these results reinforce the notion that Ng is a novel member of the actinoporin pore-forming toxin (PFT) family with a HA as high as that of StII, the most potent actinoporin so far described, but with peculiar structural characteristics contributing to expand the understanding of the structure-function relationship in this protein family.

Identification of the two new, functional actinoporins, CJTOX I and CJTOX II, from the deep-sea anemone Cribrinopsis japonica

Actinoporins are pore-forming proteins found in sea anemones. Although we now have a large collection of data on actinoporins, our knowledge is based heavily on those identified in shallow-water anemones. Because the deep sea differs considerably from shallow waters in hydrostatic pressures, temperatures, and the prey composition, the deep-sea actinoporin may have evolved in unique ways. This study, therefore, aimed to obtain new actinoporins in the deep-sea anemone Cribrinopis japonica (Actiniaria, Actiniidae). An actinoporin-like sequence was identified from the previously established C. japonica RNA-Seq database, and the complete length (663 bp) of the deep-sea actinoporin gene, Cjtox I, was obtained. In addition, a similar gene, Cjtox II (666 bp), was also identified from RNA of actinopharynx. CJTOX I and CJTOX II were similar in their primary structures, but CJTOX I lacked one residue in the middle of the protein. There was also a difference in the gene expression in live animals, where only Cjtox I was expressed in tentacles of C. japonica. In the heterologous expression where BL21 (DE3) strain was retransformed with the plasmid containing either Cjtox I or Cjtox II gene, the supernatants of both cell lysates showed hemolytic activity on the equine erythrocytes. Preincubation of the supernatants with sphingomyelin caused reduced activity, implying that the CJTOX I and II would target sphingomyelin as with other actinoporins. Because of the structures similarity to the known actinoporins and the sphingomyelin-inhibitable hemolytic activity, both CJTOX I and II were concluded to be new actinoporins, which were identified for the first time from a deep-sea anemone.

Multigene Family of Pore-Forming Toxins from Sea Anemone Heteractis crispa

Sea anemones produce pore-forming toxins, actinoporins, which are interesting as tools for cytoplasmic membranes study, as well as being potential therapeutic agents for cancer therapy. This investigation is devoted to structural and functional study of the Heteractis crispa actinoporins diversity. Here, we described a multigene family consisting of 47 representatives expressed in the sea anemone tentacles as prepropeptide-coding transcripts. The phylogenetic analysis revealed that actinoporin clustering is consistent with the division of sea anemones into superfamilies and families. The transcriptomes of both H. crispa and Heteractis magnifica appear to contain a large repertoire of similar genes representing a rapid expansion of the actinoporin family due to gene duplication and sequence divergence. The presence of the most abundant specific group of actinoporins in H. crispa is the major difference between these species. The functional analysis of six recombinant actinoporins revealed that H. crispa actinoporin grouping was consistent with the different hemolytic activity of their representatives. According to molecular modeling data, we assume that the direction of the N-terminal dipole moment tightly reflects the actinoporins' ability to possess hemolytic activity.

Stichodactyla helianthus' de novo transcriptome assembly: Discovery of a new actinoporin isoform

Transcriptomic profiling of venom producing tissues from different animals is an effective approach for discovering new toxins useful in biotechnological and pharmaceutical applications, as well in evolutionary comparative studies of venomous animals. Stichodactyla helianthus is a Caribbean sea anemone which produces actinoporins as part of its toxic venom. This family of pore forming toxins is multigenic and at least two different isoforms, encoded by separate genes, are produced by S. helianthus. These isoforms, sticholysins I and II, share 93% amino acid identity but differ in their pore forming activity and act synergistically. This observation suggests that other actinoporin isoforms, if present in the venomous mixture, could offer an advantageous strategy to modulate whole venom activity. Using high-throughput sequencing we generated a de novo transcriptome of S. helianthus and determined the relative expression of assembled transcripts using RNA-Seq to better characterize components of this species' venom, focusing on actinoporin diversity. Applying this approach, we have discovered at least one new actinoporin variant from S. helianthus in addition to several other putative venom components.

Self-homodimerization of an actinoporin by disulfide bridging reveals implications for their structure and pore formation

The Trp111 to Cys mutant of sticholysin I, an actinoporin from Stichodactyla helianthus sea anemone, forms a homodimer via a disulfide bridge. The purified dimer is 193 times less hemolytic than the monomer. Ultracentrifugation, dynamic light scattering and size-exclusion chromatography demonstrate that monomers and dimers are the only independent oligomeric states encountered. Indeed, circular dichroism and fluorescence spectroscopies showed that Trp/Tyr residues participate in homodimerization and that the dimer is less thermostable than the monomer. A homodimer three-dimensional model was constructed and indicates that Trp147/Tyr137 are at the homodimer interface. Spectroscopy results validated the 3D-model and assigned 85° to the disulfide bridge dihedral angle responsible for dimerization. The homodimer model suggests that alterations in the membrane/carbohydrate-binding sites in one of the monomers, as result of dimerization, could explain the decrease in the homodimer ability to form pores.

Proteomic Analyses of the Unexplored Sea Anemone Bunodactis verrucosa

Cnidarian toxic products, particularly peptide toxins, constitute a promising target for biomedicine research. Indeed, cnidarians are considered as the largest phylum of generally toxic animals. However, research on peptides and toxins of sea anemones is still limited. Moreover, most of the toxins from sea anemones have been discovered by classical purification approaches. Recently, high-throughput methodologies have been used for this purpose but in other Phyla. Hence, the present work was focused on the proteomic analyses of whole-body extract from the unexplored sea anemone Bunodactis verrucosa. The proteomic analyses applied were based on two methods: two-dimensional gel electrophoresis combined with MALDI-TOF/TOF and shotgun proteomic approach. In total, 413 proteins were identified, but only eight proteins were identified from gel-based analyses. Such proteins are mainly involved in basal metabolism and biosynthesis of antibiotics as the most relevant pathways. In addition, some putative toxins including metalloproteinases and neurotoxins were also identified. These findings reinforce the significance of the production of antimicrobial compounds and toxins by sea anemones, which play a significant role in defense and feeding. In general, the present study provides the first proteome map of the sea anemone B. verrucosa stablishing a reference for future studies in the discovery of new compounds.

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

Actinoporins constitute a unique class of pore-forming toxins found in sea anemones that are able to bind and oligomerize in membranes, leading to cell swelling, impairment of ionic gradients and, eventually, to cell death. In this review we summarize the knowledge generated from the combination of biochemical and biophysical approaches to the study of sticholysins I and II (Sts, StI/II), two actinoporins largely characterized by the Center of Protein Studies at the University of Havana during the last 20 years. These approaches include strategies for understanding the toxin structure-function relationship, the protein-membrane association process leading to pore formation and the interaction of toxin with cells. The rational combination of experimental and theoretical tools have allowed unraveling, at least partially, of the complex mechanisms involved in toxin-membrane interaction and of the molecular pathways triggered upon this interaction. The study of actinoporins is important not only to gain an understanding of their biological roles in anemone venom but also to investigate basic molecular mechanisms of protein insertion into membranes, protein-lipid interactions and the modulation of protein conformation by lipid binding. A deeper knowledge of the basic molecular mechanisms involved in Sts-cell interaction, as described in this review, will support the current investigations conducted by our group which focus on the design of immunotoxins against tumor cells and antigen-releasing systems to cell cytosol as Sts-based vaccine platforms.

Novel Adjuvant Based on the Pore-Forming Protein Sticholysin II Encapsulated into Liposomes Effectively Enhances the Antigen-Specific CTL-Mediated Immune Response

Vaccine strategies to enhance CD8⁺ CTL responses remain a current challenge because they should overcome the plasmatic and endosomal membranes for favoring exogenous Ag access to the cytosol of APCs. As a way to avoid this hurdle, sticholysin (St) II, a pore-forming protein from the Caribbean Sea anemone Stichodactyla helianthus, was encapsulated with OVA into liposomes (Lp/OVA/StII) to assess their efficacy to induce a CTL response. OVA-specific CD8⁺ T cells transferred to mice immunized with Lp/OVA/StII experienced a greater expansion than when the recipients were injected with the vesicles without St, mostly exhibiting a memory phenotype. Consequently, Lp/OVA/StII induced a more potent effector function, as shown by CTLs, in vivo assays. Furthermore, treatment of E.G7-OVA tumor-bearing mice with Lp/OVA/StII significantly reduced tumor growth being more noticeable in the preventive assay. The contribution of CD4⁺ and CD8⁺ T cells to CTL and antitumor activity, respectively, was elucidated. Interestingly, the irreversibly inactive variant of the StI mutant StI W111C, encapsulated with OVA into Lp, elicited a similar OVA-specific CTL response to that observed with Lp/OVA/StII or vesicles encapsulating recombinant StI or the reversibly inactive StI W111C dimer. These findings suggest the relative independence between StII pore-forming activity and its immunomodulatory properties. In addition, StII-induced in vitro maturation of dendritic cells might be supporting these properties. These results are the first evidence, to our knowledge, that StII, a pore-forming protein from a marine eukaryotic organism, encapsulated into Lp functions as an adjuvant to induce a robust specific CTL response.

Disrupting a key hydrophobic pair in the oligomerization interface of the actinoporins impairs their pore-forming activity

Crystallographic data of the dimeric and octameric forms of fragaceatoxin C (FraC) suggested the key role of a small hydrophobic protein-protein interaction surface for actinoporins oligomerization and pore formation in membranes. However, site-directed mutagenesis studies supporting this hypothesis for others actinoporins are still lacking. Here, we demonstrate that disrupting the key hydrophobic interaction between V60 and F163 (FraC numbering scheme) in the oligomerization interface of FraC, equinatoxin II (EqtII), and sticholysin II (StII) impairs the pore formation activity of these proteins. Our results allow for the extension of the importance of FraC protein-protein interactions in the stabilization of the oligomeric intermediates of StII and EqtII pointing out that all of these proteins follow a similar pathway of membrane disruption. These findings support the hybrid pore proposal as the universal model of actinoporins pore formation. Moreover, we reinforce the relevance of dimer formation, which appears to be a functional intermediate in the assembly pathway of some different pore-forming proteins.

Damage of eukaryotic cells by the pore-forming toxin sticholysin II: Consequences of the potassium efflux

Pore-forming toxins (PFTs) form holes in membranes causing one of the most catastrophic damages to a target cell. Target organisms have evolved a regulated response against PFTs damage including cell membrane repair. This ability of cells strongly depends on the toxin concentration and the properties of the pores. It has been hypothesized that there is an inverse correlation between the size of the pores and the time required to repair the membrane, which has been for long a non-intuitive concept and far to be completely understood. Moreover, there is a lack of information about how cells react to the injury triggered by eukaryotic PFTs. Here, we investigated some molecular events related with eukaryotic cells response against the membrane damage caused by sticholysin II (StII), a eukaryotic PFT produced by a sea anemone. We evaluated the change in the cytoplasmic potassium, identified the main MAPK pathways activated after pore-formation by StII, and compared its effect with those from two well-studied bacterial PFTs: aerolysin and listeriolysin O (LLO). Strikingly, we found that membrane recovery upon StII damage takes place in a time scale similar to LLO in spite of the fact that they form pores by far different in size. Furthermore, our data support a common role of the potassium ion, as well as MAPKs in the mechanism that cells use to cope with these toxins injury.

Membrane-active peptides from marine organisms--antimicrobials, cell-penetrating peptides and peptide toxins: applications and prospects

Marine organisms are known to be a rich and unique source of bioactive compounds as they are exposed to extreme conditions in the oceans. The present study is an attempt to briefly describe some of the important membrane-active peptides (MAPs) such as antimicrobial peptides (AMPs), cell-penetrating peptides (CPPs) and peptide toxins from marine organisms. Since both AMPs and CPPs play a role in membrane perturbation and exhibit interchangeable role, they can speculatively fall under the broad umbrella of MAPs. The study focuses on the structural and functional characteristics of different classes of marine MAPs. Further, AMPs are considered as a potential remedy to antibiotic resistance acquired by several pathogens. Peptides from marine organisms show novel post-translational modifications such as cysteine knots, halogenation and histidino-alanine bridge that enable these peptides to withstand harsh marine environmental conditions. These unusual modifications of AMPs from marine organisms are expected to increase their half-life in living systems, contributing to their increased bioavailability and stability when administered as drug in in vivo systems. Apart from AMPs, marine toxins with membrane-perturbing properties could be essentially investigated for their cytotoxic effect on various pathogens and their cell-penetrating activity across various mammalian cells. The current review will help in identifying the MAPs from marine organisms with crucial post-translational modifications that can be used as template for designing novel therapeutic agents and drug-delivery vehicles for treatment of human diseases.

The Kunitz-Type Protein ShPI-1 Inhibits Serine Proteases and Voltage-Gated Potassium Channels

The bovine pancreatic trypsin inhibitor (BPTI)-Kunitz-type protein ShPI-1 (UniProt: P31713) is the major protease inhibitor from the sea anemone Stichodactyla helianthus. This molecule is used in biotechnology and has biomedical potential related to its anti-parasitic effect. A pseudo wild-type variant, rShPI-1A, with additional residues at the N- and C-terminal, has a similar three-dimensional structure and comparable trypsin inhibition strength. Further insights into the structure-function relationship of rShPI-1A are required in order to obtain a better understanding of the mechanism of action of this sea anemone peptide. Using enzyme kinetics, we now investigated its activity against other serine proteases. Considering previous reports of bifunctional Kunitz-type proteins from anemones, we also studied the effect of rShPI-1A on voltage-gated potassium (Kv) channels. rShPI-1A binds Kv1.1, Kv1.2, and Kv1.6 channels with IC₅₀ values in the nM range. Hence, ShPI-1 is the first member of the sea anemone type 2 potassium channel toxins family with tight-binding potency against several proteases and different Kv1 channels. In depth sequence analysis and structural comparison of ShPI-1 with similar protease inhibitors and Kv channel toxins showed apparent non-sequence conservation for known key residues. However, we detected two subtle patterns of coordinated amino acid substitutions flanking the conserved cysteine residues at the N- and C-terminal ends.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

The Presence of Sterols Favors Sticholysin I-Membrane Association and Pore Formation Regardless of Their Ability to Form Laterally Segregated Domains

Sticholysin I (St I) is a pore-forming toxin (PFT) produced by the Caribbean Sea anemone Stichodactyla helianthus belonging to the actinoporin protein family, a unique class of eukaryotic PFT. As for actinoporins, it has been proposed that the presence of cholesterol (Chol) and the coexistence of lipid phases increase binding to the target membrane and pore-forming ability. However, little is known about the role of membrane structure and dynamics (phase state, fluidity, and the presence of lipid domains) on the activity of actinoporins or which regions of the membrane are the most favorable for protein insertion, oligomerization, and eventually pore formation. To gain insight into the role of membrane properties on the functional activity of St I, we studied its binding to monolayers and vesicles of phosphatidylcholine (PC), sphingomyelin (SM), and sterols inducing (ergosterol -Erg and cholesterol -Chol) or not (cholestenone - Cln) membrane phase segregation in liquid ordered (Lo) and liquid disordered (Ld) domains. This study revealed that St I binds and permeabilizes with higher efficiency sterol-containing membranes independently of their ability to form domains. We discuss the results in terms of the relevance of different membrane properties for the actinoporins mechanism of action, namely, molecular heterogeneity, specially potentiated in membranes with sterols inducers of phase separation (Chol or Erg) or Cln, a sterol noninducer of phase separation but with a high propensity to induce nonlamellar phase. The role of the Ld phase is pointed out as the most suitable platform for pore formation. In this regard, such regions in Chol-containing membranes seem to be the most favored due to its increased fluidity; this property promotes toxin insertion, diffusion, and oligomerization leading to pore formation.

Differential effect of solution conditions on the conformation of the actinoporins Sticholysin II and Equinatoxin II

Actinoporins are a family of pore-forming proteins with hemolytic activity. The structural basis for such activity appears to depend on their correct folding. Such folding encompasses a phosphocholine binding site, a tryptophan-rich region and the activity-related N-terminus segment. Additionally, different solution conditions are known to be able to influence the pore formation by actinoporins, as for Sticholysin II (StnII) and Equinatoxin II (EqtxII). In this context, the current work intends to characterize the influence of distinct solution conditions in the conformational behavior of these proteins through molecular dynamics (MD) simulations. The obtained data offer structural insights into actinoporins dynamics in solution, characterizing its conformational behavior at the atomic level, in accordance with previous experimental data on StnII and EqtxII hemolytic activities.

Inactivation of the pore-forming toxin Sticholysin I by peroxynitrite: protection by cys groups incorporated in the toxin

Sea anemones synthesize a variety of toxic peptides and proteins of biological interest. The Caribbean Sea anemone Stichodactyla helianthus, produces two pore-forming toxins, Sticholysin I (St I) and Stichloysin II (St II), with the ability to form oligomeric pores in cell and lipid bilayers characteristically lacking cysteine in their amino acid sequences. Recently, two mutants of a recombinant variant of Sticholysin I (rSt I) have been obtained with a Cys residue in functionally relevant regions for the pore-forming activity of the toxin: r St I F15C (in the amino terminal sequence) and r St I R52C (in the binding site). Aiming at characterizing the effects of oxidants in toxins devoid (r St I) or containing -SH moieties (r St I F15C and r St I R52C), we measured their hemolytic activity and pore forming capacity prior and after their incubation with peroxynitrite (ONOO(-)). At low ONOO(-)/Toxin ratios, nearly 0.8 Trp groups are modified by each added peroxynitrite molecule, and the toxin activity is reduced in ca. 20 %. On the other hand, in -SH bearing mutants only 0.5 Trp groups are modified by each peroxynitrite molecule and the toxin activity is only reduced in 10 %. The results indicated that Cys is the initial target of the oxidative damage and that Trp residues in Cys-containing toxins were less damaged than those in r St I. This relative protection of Trp groups correlates with a smaller loss of hemolytic activity and permeabilization ability in liposomes and emphasizes the relevance of Trp groups in the pore forming capacity of the toxins.