Glycosphingolipid-facilitated membrane insertion and internalization of cobra cardiotoxin. The sulfatide.cardiotoxin complex structure in a membrane-like environment suggests a lipid-dependent cell-penetrating mechanism for membrane binding polypeptides

Assessing the Efficacy of Monovalent and Commercialized Antivenoms for Neutralizing Moroccan Cobra Naja haje Venom: A Comparative Study

In Morocco, eight species of venomous snakes belonging to the Viperidae and Elapidae families are responsible for severe envenomation cases. The species from the Elapidae family is only represented by the medically relevant cobra Naja haje, which is widely distributed in North Africa. However, there is little information on the systemic effects of Moroccan cobra venom on vital organs due to regional variations. It has been demonstrated that the venom of Naja haje from Egypt causes hemorrhage, while the venom of the Moroccan cobra is neurotoxic and devoid of systemic bleeding. This variability is known to significantly influence treatment efficacy against Naja haje cobra bites in the Middle East. In this study, we examined the pathophysiological mechanisms responsible for the lethality induced by Naja haje venom, as well as the evaluation of the neutralizing capacity of two antivenoms; the monospecific antivenom made for Naja haje only and the antivenom marketed in the Middle East and North Africa. We first determined the toxicity of Naja haje venom by LD₅₀ test, then compared the neutralizing capacity of the two antivenoms studied by determining the ED₅₀. We also performed histological analysis on Swiss mice envenomed and treated with these antivenoms to observe signs of cobra venom envenomation and the degree of reduction of induced systemic alterations. The results showed significant differences between both antivenoms in terms of neutralization. The monospecific antivenom was four times more effective than the marketed antivenom. These results were confirmed by a histological study, which showed that monospecific antivenoms neutralized severe signs of mortality, such as congestion of blood vessels in the heart and kidneys, pulmonary and renal edema, cytoplasmic vacuolization of hepatocytes in the liver, and infiltration of inflammatory cells in the brain and spleen. However, the polyvalent antivenom failed to protect all severe lesions induced by Naja haje venom in mice. These findings highlight the negative impact of geographic variation on the effectiveness of conventional antivenom therapy and confirm the need for a specific Naja haje antivenom for the effective treatment of cobra envenomation in Morocco.

Heparin prevents the cytotoxic activity of Bothrops jararacussu and Apis mellifera venoms in renal cells

Envenomation by Bothrops snakes and Apis mellifera bee may imply systemic disorders which affect well-perfused organs such as kidneys, a process that can lead to acute renal failure. Nevertheless, there is scarce information regarding a direct renal cell effect and the putative antagonism by antivenoms. Here the cytotoxic effect of B. jararacussu and A. mellifera venoms was evaluated in the renal proximal tubule cell line LLC-PK1, as well as the antagonism of this effect by heparin. B. jararacussu venom showed significant cytotoxicity as assessed by LDH release and MTT reduction, with a sharp decline of the cell number after 180 min (>90% at 50 μg/mL). A. mellifera venom produced a much faster and potent cytotoxic activity, conferring almost no viable cells after 15 min at 25 μg/mL. Phase contrast microscopy revealed that while B. jararacussu venom induced a progressive loss of cell adhesion and detachment, A. mellifera venom promoted a rapid plasma membrane disruption and nuclear condensation suggestive of necrotic cell death. Pre-incubation of both venoms with heparin for 30 min significantly reduced cytotoxicity. Our results demonstrate direct toxicity of B. jararacussu and A. mellifera venoms toward renal cells but with distinct kinetics and cell pattern, suggesting different mechanisms of action. In addition, the antagonistic, cytoprotective effect of heparin ascribes such compound as a promising drug for preventing renal failure from envenomation.

The secretory phenotypes of envenomed cells: Insights into venom cytotoxicity

Snake envenomation is listed as Category A Neglected Tropical Diseases (NTD) by World Health Organization, indicates a severe public health problem. The global figures for envenomation cases are estimated to be more than 1.8 million annually. Even if the affected victims survive the envenomation, they might suffer from permanent morbidity due to local envenomation. One of the most prominent local envenomation is dermonecrosis. Dermonecrosis is a pathophysiological outcome of envenomation that often causes disability in the victims due to surgical amputations, deformities, contracture, and chronic ulceration. The key venom toxins associated with this local symptom are mainly attributed to substantial levels of enzymatic and non-enzymatic toxins as well as their possible synergistic actions. Despite so, the severity of the local tissue damage is based on macroscopic observation of the bite areas. Furthermore, limited knowledge is known about the key biomarkers involved in the pathogenesis of dermonecrosis. The current immunotherapy with antivenom is also ineffective against dermonecrosis. These local effects eventually end up as sequelae. There is also a global shortage of toxins-targeted therapeutics attributed to inadequate knowledge of the actual molecular mechanisms of cytotoxicity. This chapter discusses the characterization of secretory phenotypes of dermonecrosis as an advanced tool to indicate its severity and pathogenesis in envenomation. Altogether, the secretory phenotypes of envenomed cells and tissues represent the precise characteristics of dermonecrosis caused by venom toxins.

The repertoire of protein-sulfatide interactions reveal distinct modes of sulfatide recognition

Sulfatide is an abundant glycosphingolipid in the mammalian nervous system, kidney, trachea, gastrointestinal tract, spleen, and pancreas and is found in low levels in other tissues. Sulfatide is characterized by the presence of a sulfate group in the hydrophilic galactose moiety, with isoforms differing in their sphingosine base and the length, unsaturation, and hydroxylation of their acyl chain. Sulfatide has been associated with a variety of cellular processes including immune responses, cell survival, myelin organization, platelet aggregation, and host-pathogen interactions. Structural studies of protein-sulfatide interactions markedly advanced our understanding of their molecular contacts, key-interacting residues, orientation of the sulfatide in its binding site, and in some cases, sulfatide-mediated protein oligomerization. To date, all protein-sulfatide interactions are reported to display dissociation constants in the low micromolar range. At least three distinct modes of protein-sulfatide binding were identified: 1) protein binding to short consensus stretches of amino acids that adopt α-helical-loop-α-helical conformations; 2) sulfatide-bound proteins that present the sulfatide head group to another protein; and 3) proteins that cage sulfatides. The scope of this review is to present an up-to-date overview of these molecular mechanisms of sulfatide recognition to better understand the role of this glycosphingolipid in physiological and pathological states.

The myth of cobra venom cytotoxin: More than just direct cytolytic actions

Cobra venom cytotoxin (CTX) is a non-enzymatic three-finger toxin that constitutes 40-60% of cobra venom. Thus, it plays an important role in the pathophysiology of cobra envenomation, especially in local dermonecrosis. The three-finger hydrophobic loops of CTX determine the cytotoxicity. Nevertheless, the actual mechanisms of cytotoxicity are not fully elucidated as they involve not only cytolytic actions but also intracellular signalling-mediated cell death pathways. Furthermore, the possible transition cell death pattern remains to be explored. The actual molecular mechanisms require further studies to unveil the relationship between different CTXs from different cobra species and cell types which may result in differential cell death patterns. Here, we discuss the biophysical interaction of CTX with the cell membrane involving four binding modes: electrostatic interaction, hydrophobic partitioning, isotropic phase, and oligomerisation. Oligomerisation of CTX causes pore formation in the membrane lipid bilayer. Additionally, the CTX-induced apoptotic pathway can be executed via death receptor-mediated extrinsic pathways and mitochondrial-mediated intrinsic pathways. We also discuss lysosomal-mediated necrosis and the occurrence of necroptosis following CTX action. Collectively, we provided an insight into concentration-dependent transition of cell death pattern which involves different mechanistic actions. This contributes a new direction for further investigation of cytotoxic pathways activated by the CTXs for future development of biotherapeutics targeting pathological effects caused by CTX.

The Heparan Sulfate Binding Peptide in Tumor Progression of Triple-Negative Breast Cancer

Angiogenesis is the formation of new vessels from pre-existing vasculature. The heparan sulfate chains from endothelial cell proteoglycans interact with the major angiogenic factors, regulating blood vessels´ formation. Since the FDA´s first approval, anti-angiogenic therapy has shown tumor progression inhibition and increased patient survival. Previous work in our group has selected an HS-binding peptide using a phage display system. Therefore, we investigated the effect of the selected peptide in angiogenesis and tumor progression. The HS-binding peptide showed a higher affinity for heparin N-sulfated. The HS-binding peptide was able to inhibit the proliferation of human endothelial umbilical cord cells (HUVEC) by modulation of FGF-2. It was verified a significant decrease in the tube formation of human endothelial cells and capillary formation of mice aorta treated with HS-binding peptide. HS-binding peptide also inhibited the formation of sub-intestinal blood vessels in zebrafish embryos. Additionally, in zebrafish embryos, the tumor size decreased after treatment with HS-binding peptide.

Effects of 3FTx Protein Fraction from Naja ashei Venom on the Model and Native Membranes: Recognition and Implications for the Mechanisms of Toxicity

Three-finger toxins are naturally occurring proteins in Elapidae snake venoms. Nowadays, they are gaining popularity because of their therapeutic potential. On the other hand, these proteins may cause undesirable reactions inside the body's cells. A full assessment of the safety of Naja ashei venom components for human cell application is still unknown. The aim of the study was to determine the effect of the exogenous application of three-finger toxins on the cells of monocytes (U-937) and promyelocytes (HL-60), with particular emphasis on the modification of their membranes under the influence of various doses of 3FTx protein fraction (0-120 ng/mL). The fraction exhibiting the highest proportion of 3FTx proteins after size exclusion chromatography (SEC) separation was used in the experiments. The structural response of cell membranes was described on the basis of single-component and multi-component Langmuir monolayers that mimicked the native membranes. The results show that the mechanism of protein-lipid interactions depends on both the presence of lipid polar parts (especially zwitterionic type of lipids) and the degree of membrane saturation (the greatest-for unsaturated lipids). The biochemical indicators reflecting the tested cells (MDA, LDH, cell survival, induction of inflammation, LD50) proved the results that were obtained for the model.

Stepwise Insertion of Cobra Cardiotoxin CT2 into a Lipid Bilayer Occurs as an Interplay of Protein and Membrane "Dynamic Molecular Portraits"

For many peripheral membrane-binding polypeptides(MBPs), especially β-structural ones, the precise molecular mechanisms of membrane insertion remain unclear. In most cases, only the terminal water-soluble and membrane-bound states have been elucidated, whereas potential functionally important intermediate stages are still not understood in sufficient detail. In this study, we present one of the first successful attempts to describe step-by-step embedding of the MBP cardiotoxin 2 (CT2) from cobra Naja oxiana venom into a lipid bilayer at the atomistic level. CT2 possesses a highly conservative and rigid β-structured three-finger fold shared by many other exogenous and endogenous proteins performing a wide variety of functions. The incorporation of CT2 into the lipid bilayer was analyzed via a 2 μs all-atom molecular dynamics (MD) simulation without restraints. This process was shown to occur over a number of distinct steps, while the geometry of initial membrane attachment drastically differs from that of the final equilibrated state. In the latter one, the hydrophobic platform ("bottom") formed by the tips of the three loops is deeply buried into the lipid bilayer. This agrees well with the NMR data obtained earlier for CT2 in detergent micelles. However, the bottom is too bulky to insert itself into the membrane at once. Instead, the gradual immersion of CT2 initiated by the loop-1 was observed. This initial binding stage was also demonstrated in a series of MD runs with varying starting orientations of the toxin with respect to the bilayer surface. Apart from the nonspecific long-range electrostatic attraction and hydrophobic match/mismatch factor, several specific lipid-binding sites were identified in CT2. They were shown to promote membrane insertion by engaging in strong interactions with lipid head groups, fine-tuning the toxin-membrane accommodation. We therefore propose that the toxin insertion relies on the interplay of nonspecific and specific interactions, which are determined by the "dynamic molecular portraits" of the two players, the protein and the membrane. The proposed model does not require protein oligomerization for membrane insertion and can be further employed to design MBPs with predetermined properties with regard to particular membrane targets.

Cytotoxicity of Snake Venoms and Cytotoxins From Two Southeast Asian Cobras (Naja sumatrana, Naja kaouthia): Exploration of Anticancer Potential, Selectivity, and Cell Death Mechanism

Venoms of cobras (Naja spp.) contain high abundances of cytotoxins, which contribute to tissue necrosis in cobra envenomation. The tissue-necrotizing activity of cobra cytotoxins, nevertheless, indicates anticancer potentials. This study set to explore the anticancer properties of the venoms and cytotoxins from Naja sumatrana (equatorial spitting cobra) and Naja kaouthia (monocled cobra), two highly venomous species in Southeast Asia. The cytotoxicity, selectivity, and cell death mechanisms of their venoms and cytotoxins (NS-CTX from N. sumatrana: NS-CTX; N. kaouthia: NK-CTX) were elucidated in human lung (A549), prostate (PC-3), and breast (MCF-7) cancer cell lines. Cytotoxins were purified through a sequential fractionation approach using cation-exchange chromatography, followed by C₁₈ reverse-phase high-performance liquid chromatography (HPLC) to homogeneity validated with sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and identified by liquid chromatography-tandem mass spectrometry (LCMS/MS). The cobra venoms and their respective cytotoxins exhibited concentration-dependent growth inhibitory effects in all cell lines tested, with the cytotoxins being more potent compared to the corresponding whole venoms. NS-CTX and NK-CTX are, respectively, P-type and S-type isoforms of cytotoxin, based on the amino acid sequences as per LCMS/MS analysis. Both cytotoxins exhibited differential cytotoxic effects in the cell lines tested, with NS-CTX (P-type cytotoxin) being significantly more potent in inhibiting the growth of the cancer cells. Both cytotoxins demonstrated promising selectivity only for the A549 lung cancer cell line (selectivity index = 2.17 and 2.26, respectively) but not in prostate (PC-3) and breast (MCF-7) cancer cell lines (selectivity index < 1). Flow cytometry revealed that the A549 lung cancer cells treated with NS-CTX and NK-CTX underwent necrosis predominantly. Meanwhile, the cytotoxins induced mainly caspase-independent late apoptosis in the prostate (PC-3) and breast (MCF-7) cancer cells lines but lacked selectivity. The findings revealed the limitations and challenges that could be faced during the development of new cancer therapy from cobra cytotoxins, notwithstanding their potent anticancer effects. Further studies should aim to overcome these impediments to unleash the anticancer potentials of the cytotoxins.

Expansion of Hydra actinoporin-like toxin (HALT) gene family: Expression divergence and functional convergence evolved through gene duplication

Hydra actinoporin-like toxin 1 (HALT-1) was previously shown to cause cytolysis and haemolysis in a number of human cells and has similar functional properties to the actinoporins equinatoxin and sticholysin. In addition to HALT-1, five other HALTs (HALTs 2, 3, 4, 6 and 7) were also isolated from Hydra magnipapillata and expressed as recombinant proteins in this study. We demonstrated that recombinant HALTs have cytolytic activity on HeLa cells but each exhibited a different range of toxicity. All six recombinant HALTs bound to sulfatide, while rHALT-1 and rHALT-3 bound to two additional sphingolipids, lysophosphatidic acid and sphingosine-1-phosphate as indicated by the protein-lipid overlay assay. When either tryptophan¹³³ or tyrosine¹²⁹ of HALT-1 was mutated, the mutant protein lost binding to sulfatide, lysophosphatidic acid and sphingosine-1-phosphate. As further verification of HALTs' binding to sulfatide, we performed ELISA for each HALT. To determine the cell-type specific gene expression of seven HALTs in Hydra, we searched for individual HALT expression in the single-cell RNA-seq data set of Single Cell Portal. The results showed that HALT-1, 4 and 7 were expressed in differentiating stenoteles. HALT-1 and HALT-6 were expressed in the female germline during oogenesis. HALT-2 was strongly expressed in the gland and mucous cells in the endoderm. Information on HALT-3 and HALT-5 could not be found in the single-cell data set. Our findings show that subfunctionalisation of gene expression following duplication enabled HALTs to become specialized in various cell types of the interstitial cell lineage.

Naja atra venom peptide reduces pain by selectively blocking the voltage-gated sodium channel Nav1.8

The voltage-gated sodium channel Nav1.8 is preferentially expressed in peripheral nociceptive neurons and contributes to inflammatory and neuropathic pain. Therefore, Nav1.8 has emerged as one of the most promising analgesic targets for pain relief. Using large-scale screening of various animal-derived toxins and venoms for Nav1.8 inhibitors, here we identified μ-EPTX-Na1a, a 62-residue three-finger peptide from the venom of the Chinese cobra (Naja atra), as a potent inhibitor of Nav1.8, exhibiting high selectivity over other voltage-gated sodium channel subtypes. Using whole-cell voltage-clamp recordings, we observed that purified μ-EPTX-Na1a blocked the Nav1.8 current. This blockade was associated with a depolarizing shift of activation and repolarizing shift of inactivation, a mechanism distinct from that of any other gating modifier toxin identified to date. In rodent models of inflammatory and neuropathic pain, μ-EPTX-Na1a alleviated nociceptive behaviors more potently than did morphine, indicating that μ-EPTX-Na1a has a potent analgesic effect. μ-EPTX-Na1a displayed no evident cytotoxicity and cardiotoxicity and produced no obvious adverse responses in mice even at a dose 30-fold higher than that producing a significant analgesic effect. Our study establishes μ-EPTX-Na1a as a promising lead for the development of Nav1.8-targeting analgesics to manage pain.

A Turn-on Fluorescence Sensor for Heparin Detection Based on a Release of Taiwan Cobra Cardiotoxin from a DNA Aptamer or Adenosine-Based Molecular Beacon

This study presents two sensitive fluorescent assays for sensing heparin on the basis of the electrostatic interaction between heparin and Naja naja atra cardiotoxin 3 (CTX3). Owing to CTX3-induced folded structure of an adenosine-based molecular beacon (MB) or a DNA aptamer against CTX3, a reduction in the fluorescent signal of the aptamer or MB 5'-end labeled with carboxyfluorescein (FAM) and 3'-end labeled with 4-([4-(dimethylamino)phenyl]azo)-benzoic acid (DABCYL) was observed upon the addition of CTX3. The presence of heparin and formation of the CTX3-heparin complex caused CTX3 detachment from the MB or aptamer, and restoration of FAM fluorescence of the 5'-FAM-and-3'-DABCYL-labeled MB and aptamer was subsequently noted. Moreover, the detection of heparin with these CTX3-aptamer and CTX3-MB sensors showed high sensitivity and selectivity toward heparin over chondroitin sulfate and hyaluronic acid regardless of the presence of plasma. The limit of detection for heparin in plasma was determined to be 16 ng/mL and 15 ng/mL, respectively, at a signal-to-noise ratio of 3. This study validates the practical utility of the CTX3-aptamer and CTX3-MB systems for determining the concentration of heparin in a biological matrix.

Synthetic peptide antigens derived from long-chain alpha-neurotoxins: Immunogenicity effect against elapid venoms

Three-finger toxins (3FTXs), especially α-neurotoxins, are the most poorly neutralized elapid snake toxins by current antivenoms. In this work, the conserved structural similarity and motif arrangements of long-chain α-neurotoxins led us to design peptides with consensus sequences. Eight long-chain α-neurotoxins (also known as Type II) were used to generate a consensus sequence from which two peptides were chemically synthesized, LCP1 and LCP2. Rabbit sera raised against them were able to generate partially-neutralizing antibodies, which delayed mice mortality in neutralization assays against Naja haje, Dendrospis polylepis and Ophiophagus hannah venoms.

Ringhalexin from Hemachatus haemachatus: A novel inhibitor of extrinsic tenase complex

Anticoagulant therapy is used for the prevention and treatment of thromboembolic disorders. Blood coagulation is initiated by the interaction of factor VIIa (FVIIa) with membrane-bound tissue factor (TF) to form the extrinsic tenase complex which activates FX to FXa. Thus, it is an important target for the development of novel anticoagulants. Here, we report the isolation and characterization of a novel anticoagulant ringhalexin from the venom of Hemachatus haemachatus (African Ringhals Cobra). Amino acid sequence of the protein indicates that it belongs to the three-finger toxin family and exhibits 94% identity to an uncharacterized Neurotoxin-like protein NTL2 from Naja atra. Ringhalexin inhibited FX activation by extrinsic tenase complex with an IC50 of 123.8 ± 9.54 nM. It is a mixed-type inhibitor with the kinetic constants, Ki and Ki' of 84.25 ± 3.53 nM and 152.5 ± 11.32 nM, respectively. Ringhalexin also exhibits a weak, irreversible neurotoxicity on chick biventer cervicis muscle preparations. Subsequently, the three-dimensional structure of ringhalexin was determined at 2.95 Å resolution. This study for the first time reports the structure of an anticoagulant three-finger toxin. Thus, ringhalexin is a potent inhibitor of the FX activation by extrinsic tenase complex and a weak, irreversible neurotoxin.

Privileged frameworks from snake venom

Venom as a form of chemical prey capture is a key innovation that has underpinned the explosive radiation of the advanced snakes (Caenophidia). Small venom proteins are often rich in disulfide bonds thus facilitating stable molecular scaffolds that present key functional residues on the protein surface. New toxin types are initially developed through the venom gland over-expression of normal body proteins, their subsequent gene duplication and diversification that leads to neofunctionalisation as random mutations modify their structure and function. This process has led to preferentially selected (privileged) cysteine-rich scaffolds that enable the snake to build arrays of toxins many of which may lead to therapeutic products and research tools. This review focuses on cysteine-rich small proteins and peptides found in snake venoms spanning natriuretic peptides to phospholipase enzymes, while highlighting their three-dimensional structures and biological functions as well as their potential as therapeutic agents or research tools.

Endocytotic routes of cobra cardiotoxins depend on spatial distribution of positively charged and hydrophobic domains to target distinct types of sulfated glycoconjugates on cell surface

Cobra cardiotoxins (CTX) are a family of three-fingered basic polypeptides known to interact with diverse targets such as heparan sulfates, sulfatides, and integrins on cell surfaces. After CTX bind to the membrane surface, they are internalized to intracellular space and exert their cytotoxicity via an unknown mechanism. By the combined in vitro kinetic binding, three-dimensional x-ray structure determination, and cell biology studies on the naturally abundant CTX homologues from the Taiwanese cobra, we showed that slight variations on the spatial distribution of positively charged or hydrophobic domains among CTX A2, A3, and A4 could lead to significant changes in their endocytotic pathways and action mechanisms via distinct sulfated glycoconjugate-mediated processes. The intracellular locations of these structurally similar CTX after internalization are shown to vary between the mitochondria and lysosomes via either dynamin2-dependent or -independent processes with distinct membrane cholesterol sensitivity. Evidence is presented to suggest that the shifting between the sulfated glycoconjugates as distinct targets of CTX A2, A3, and A4 might play roles in the co-evolutionary arms race between venomous snake toxins to cope with different membrane repair mechanisms at the cellular levels. The sensitivity of endocytotic routes to the spatial distribution of positively charged or hydrophobic domains may provide an explanation for the diverse endocytosis pathways of other cell-penetrating basic polypeptides.

The Anticancer Effect of Cytotoxin 1 from Naja atra Cantor Venom is Mediated by a Lysosomal Cell Death Pathway Involving Lysosomal Membrane Permeabilization and Cathepsin B Release

The cytotoxin family of cobra venom proteins, also called cardiotoxins, can activate both necrotic and apoptotic cell death pathways in cancer cells. Cytotoxin 1 (CTX1)from Naja atra Cantor venom is a 60 amino acid, 6698 Da protein with as yet untested anticancer efficacy and cell selectivity. We tested the toxicity of CTX1 on a number of cancer cell lines (MCF-7, P388, K562, and H22) and on one normal human cell line (16HBE). The rank order of cytotoxicity was MCF-7 > P388 ≈ K562 >H22 ≈ 16HBE, indicating that the effect of CTX1 on certain cancer cell types was relatively selective.Treatment with CTX1 greatly prolonged the survival of P388 ascites tumors bearing KM mice compared to cyclophosphamide treatment. Cell viability, apoptosis, and lysosomal permeability assays all demonstrated that CTX1 induced dose- and time-dependent cell death, with most cells exhibiting the morphological and biochemical features of late apoptosis and necrosis. Mitochondrial membrane potential was lost in CTX1-treated P388 cells. In addition, CTX1 induced an increase in both lysosomal membrane permeability and cathepsin B protease activity. These analyses reveal that CTX1 possesses significant and selective anticancer activity, likely by inducing programmed cell death through mitochondrial and/or lysosomal pathways.

Identification and structural characterization of a new three-finger toxin hemachatoxin from Hemachatus haemachatus venom

Snake venoms are rich sources of biologically active proteins and polypeptides. Three-finger toxins are non-enzymatic proteins present in elapid (cobras, kraits, mambas and sea snakes) and colubrid venoms. These proteins contain four conserved disulfide bonds in the core to maintain the three-finger folds. Although all three-finger toxins have similar fold, their biological activities are different. A new three-finger toxin (hemachatoxin) was isolated from Hemachatus haemachatus (Ringhals cobra) venom. Its amino acid sequence was elucidated, and crystal structure was determined at 2.43 Å resolution. The overall fold is similar to other three-finger toxins. The structure and sequence analysis revealed that the fold is maintained by four highly conserved disulfide bonds. It exhibited highest similarity to particularly P-type cardiotoxins that are known to associate and perturb the membrane surface with their lipid binding sites. Also, the increased B value of hemachotoxin loop II suggests that loop II is flexible and may remain flexible until its interaction with membrane phospholipids. Based on the analysis, we predict hemachatoxin to be cardiotoxic/cytotoxic and our future experiments will be directed to characterize the activity of hemachatoxin.

Structure, sulfatide binding properties, and inhibition of platelet aggregation by a disabled-2 protein-derived peptide

Disabled-2 (Dab2) targets membranes and triggers a wide range of biological events, including endocytosis and platelet aggregation. Dab2, through its phosphotyrosine-binding (PTB) domain, inhibits platelet aggregation by competing with fibrinogen for α(IIb)β(3) integrin receptor binding. We have recently shown that the N-terminal region, including the PTB domain (N-PTB), drives Dab2 to the platelet membrane surface by binding to sulfatides through two sulfatide-binding motifs, modulating the extent of platelet aggregation. The three-dimensional structure of a Dab2-derived peptide encompassing the sulfatide-binding motifs has been determined in dodecylphosphocholine micelles using NMR spectroscopy. Dab2 sulfatide-binding motif contains two helices when embedded in micelles, reversibly binds to sulfatides with moderate affinity, lies parallel to the micelle surface, and when added to a platelet mixture, reduces the number and size of sulfatide-induced aggregates. Overall, our findings identify and structurally characterize a minimal region in Dab2 that modulates platelet homotypic interactions, all of which provide the foundation for rational design of a new generation of anti-aggregatory low-molecular mass molecules for therapeutic purposes.

The role of sulfatide lipid domains in the membrane pore-forming activity of cobra cardiotoxin

Cobra CTX A3, the major cardiotoxin (CTX) from Naja atra, is a cytotoxic, basic β-sheet polypeptide that is known to induce a transient membrane leakage of cardiomyocytes through a sulfatide-dependent CTX membrane pore formation and internalization mechanism. The molecular specificity of CTX A3-sulfatide interaction at atomic levels has also been shown by both nuclear magnetic resonance (NMR) and X-ray diffraction techniques to reveal a role of CTX-induced sulfatide conformational changes for CTX A3 binding and dimer formation. In this study, we investigate the role of sulfatide lipid domains in CTX pore formation by various biophysical methods, including fluorescence imaging and atomic force microscopy, and suggest an important role of liquid-disordered (ld) and solid-ordered (so) phase boundary in lipid domains to facilitate the process. Fluorescence spectroscopic studies on the kinetics of membrane leakage and CTX oligomerization further reveal that, although most CTXs can oligomerize on membranes, only a small fraction of CTXs oligomerizations form leakage pores. We therefore suggest that CTX binding at the boundary between the so and so/ld phase coexistence sulfatide lipid domains could form effective pores to significantly enhance the CTX-induced membrane leakage of sulfatide-containing phosphatidylcholine vesicles. The model is consistent with our earlier observations that CTX may penetrate and lyse the bilayers into small aggregates at a lipid/protein molar ratio of about 20 in the ripple P(β)' phase of phosphatidylcholine bilayers and suggest a novel mechanism for the synergistic action of cobra secretary phospholipase A2 and CTXs.

Membrane-damaging activity of Taiwan cobra cardiotoxin 3 is responsible for its bactericidal activity

This study investigates the causal relationship between membrane-damaging activity and bactericidal activity of Naja naja atra (Taiwan cobra) cardiotoxin 3 (CTX3). CTX3 showed greater inhibitory activity for the growth of Staphylococcus aureus (Gram-positive bacteria) relative to that of Escherichia coli (Gram-negative bacteria). The CTX3 antibacterial activity is positively correlated with the increase in membrane permeability of bacterial cells. Morphological examination showed that CTX3 disrupted bacterial membrane integrity.CTX3 showed similar binding capability with lipopolysaccharide (LPS) and lipoteichoic acid (LTA), and destabilization of LPS layer and inhibition of LTA biosynthesis on cell wall increased the CTX3 bactericidal effect on E. coli. and S. aureus, respectively. Compared with that of E. coli, CTX3 notably permeabilized model membrane of S. aureus. CTX3 membrane-damaging activity was inhibited by LPS and LTA, while increasing the CTX3 concentration counteracted the inhibitory action of LPS and LTA. Oxidation of Met residues on loop II of CTX3 simultaneously reduced the membrane-permeabilizing activity and bactericidal effect of CTX3. Taken together, our data indicate that CTX3 bactericidal activity depends highly on its ability to induce membrane permeability.

Snake cytotoxins bind to membranes via interactions with phosphatidylserine head groups of lipids

The major representatives of Elapidae snake venom, cytotoxins (CTs), share similar three-fingered fold and exert diverse range of biological activities against various cell types. CT-induced cell death starts from the membrane recognition process, whose molecular details remain unclear. It is known, however, that the presence of anionic lipids in cell membranes is one of the important factors determining CT-membrane binding. In this work, we therefore investigated specific interactions between one of the most abundant of such lipids, phosphatidylserine (PS), and CT 4 of Naja kaouthia using a combined, experimental and modeling, approach. It was shown that incorporation of PS into zwitterionic liposomes greatly increased the membrane-damaging activity of CT 4 measured by the release of the liposome-entrapped calcein fluorescent dye. The CT-induced leakage rate depends on the PS concentration with a maximum at approximately 20% PS. Interestingly, the effects observed for PS were much more pronounced than those measured for another anionic lipid, sulfatide. To delineate the potential PS binding sites on CT 4 and estimate their relative affinities, a series of computer simulations was performed for the systems containing the head group of PS and different spatial models of CT 4 in aqueous solution and in an implicit membrane. This was done using an original hybrid computational protocol implementing docking, Monte Carlo and molecular dynamics simulations. As a result, at least three putative PS-binding sites with different affinities to PS molecule were delineated. Being located in different parts of the CT molecule, these anion-binding sites can potentially facilitate and modulate the multi-step process of the toxin insertion into lipid bilayers. This feature together with the diverse binding affinities of the sites to a wide variety of anionic targets on the membrane surface appears to be functionally meaningful and may adjust CT action against different types of cells.

Reactive oxygen species and p38 mitogen-activated protein kinase induce apoptotic death of U937 cells in response to Naja nigricollis toxin-gamma

The aim of the present study is to elucidate the signalling components related to Naja nigricollis toxin--induced apoptosis in human leukaemia U937 cells. It was found that toxin--induced apoptotic cell death was attributed mainly to activation of p38 mitogen-activated protein kinase (MAPK), reactive oxygen species (ROS) generation and loss of mitochondrial membrane potential (deltapsim). Subsequent modulation of Bcl-2 family member and cytochrome c release accompanied with activation of caspase-9 and -3 were involved in the death of U937 cells. SB202190 (p38 MAPK inhibitor) and N-acetylcysteine (antioxidant) significantly attenuated toxin--induced cell death and loss of deltapsim, and completely abolished the production of ROS. In contrast to N-acetylcysteine, degradation of Bcl-2/Bcl-XL and mitochondrial localization of Bax were notably decreased by SB202190. Inhibitors of electron transport (rotenone and antimycin A) or inhibitor of mitochondrial permeability transition pore (cyclosporine A) reduced the effect of toxin- on ROS generation, loss of deltapsim and cytochrome c release. Noticeably, pre-treatment with N-acetylcysteine or rotenone eliminated markedly ROS accompanied with reduction in p38 MAPK activation. Taken together, these results suggest that the cytotoxicity of toxin- is initiated by p38-MAPK-mediated mitochondrial dysfunction followed by ROS production and activation of caspases, and that ROS further augments p38 MAPK activation and mitochondrial alteration.

Interaction of Naja naja atra cardiotoxin 3 with H-trisaccharide modulates its hemolytic activity and membrane-damaging activity

To address whether saccharide moieties of blood groups A, B and O antigens modulate hemolytic activity of Naja naja atra cardiotoxins (CTXs), the present study was carried out. Unlike other CTX isotoxins, hemolytic activity of CTX3 toward blood group O cholesterol-depleted red blood cells (RBCs) was notably lower than that of blood groups A and B cholesterol-depleted RBCs. Conversion of blood group B RBCs into blood group O RBCs by alpha-galactosidase treatment attenuated the susceptibility for hemolytic activity of CTX3, suggesting that H-antigen affected hemolytic potency of CTX3. Pre-incubation with H-trisaccharide reduced hemolytic activity and membrane-damaging activity of CTX3. Moreover, CTX3 showed a higher binding capability with H-trisaccharide than other CTXs did. CD spectra showed that the binding with H-trisaccharide induced changes in gross conformation of CTX3. Self-quenching studies revealed that oligomerization of CTX3 was affected in the presence of H-trisaccharide. Taken together, our data suggest that the binding of CTX3 with H-antigen alters its membrane-bound mode, thus reducing its hemolytic activity toward blood group O cholesterol-depleted RBCs.

Role of heparan sulfates and glycosphingolipids in the pore formation of basic polypeptides of cobra cardiotoxin

Cobra venom contains cardiotoxins (CTXs) that induce tissue necrosis and systolic heart arrest in bitten victims. CTX-induced membrane pore formation is one of the major mechanisms responsible for the venom's designated cytotoxicity. This chapter examines how glycoconjugates such as heparan sulfates (HS) and glycosphingolipids, located respectively in the extracellular matrix and lipid bilayers of the cell membranes, facilitate CTX pore formation. Evidences for HS-facilitated cell surface retention and glycosphingolipid-facilitated membrane bilayer insertion of CTX are reviewed. We suggest that similar physical steps could play a role in the mediation of other pore forming toxins (PFT). The membrane pores formed by PFT are expected to have limited lifetime on biological cell surface as a result of membrane dynamics during endocytosis and/or rearrangement of lipid rafts.

Roles of lysine residues and N-terminal alpha-amino group in membrane-damaging activity of Taiwan cobra cardiotoxin 3 toward anionic and zwitterionic phospholipid vesicles

In contrast to a slight increase in activity toward phosphatidylcholine (EYPC)/dimyristoyl phosphatidic acid (DMPA) vesicles, guanidination of Naja naja atra cardiotoxin 3 (CTX3) and selective trinitrophenylation of N-terminal alpha-amino group enhanced notably membrane-damaging activity on EYPC/egg yolk sphingomyelin (EYSM) vesicles. Chemically modified CTX3 showed a reduction in its hemolytic activity and cytotoxicity. These reflected that membrane-damaging activity of CTX3 was affected by phospholipid compositions. Phospholipid-binding capability and oligomeric assembly upon binding with lipid vesicles did not closely correlate with membrane-damaging potency of native and modified CTX3. Moreover, different topographical contacts and distinctive modes for the binding of CTX3 and its modified derivatives with anionic phospholipid vesicles (EYPC/DMPA) and zwitterionic phospholipid vesicles (EYPC/EYSM) were observed. Compared with in the case of EYPC/DMPA, the interaction between CTX molecules and EYPC/EYSM was drastically reduced by increasing salt concentration and heparin. Taken together, our data indicate that guanidination of Lys residues and trinitrophenylation of alpha-amino group alter differently the interacted modes upon absorption on anionic phospholipid vesicles and zwitterionic phospholipid vesicles. The findings also suggest that positively charged residues of CTX3 play a distinctive role in damaging anionic and zwitterionic phospholipid vesicles.

Membrane-bound conformation of Naja nigricollis toxin gamma affects its membrane-damaging activity

To address whether the conformational events associated with the absorption of Naja nigricollis toxin gamma on water-lipid interface plays a vital role in its membrane-damaging activity, the present study is carried out. Membrane-damaging activity of toxin gamma on 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC)/1, 2-dimyristoyl-phosphatidic acid (DMPA) vesicles was approximately 13-fold of that on 1, 2-dipalmitoyl-phosphatidylcholine (DPPC)/DMPA vesicles, while the binding affinity of toxin gamma for POPC/DMPA was twofold of that for DPPC/DMPA. Time-resolved fluorescence, acrylamide quenching and Fourier transform infrared spectra showed that POPC/DMPA-bound toxin gamma and DPPC/DMPA-bound toxin gamma did not adopt the same conformation. Moreover, geometrical arrangement of toxin gamma in contact with POPC/DMPA vesicles was different from that with DPPC/DMPA vesicles as evidenced by N-(fluorescein-5-thiocarbamoyl)-1,2-dihexadecanoyl-phosphatidylcholine fluorescence enhancement and cross-linking of membrane-bound toxin gamma. Taken together, our data show that different membrane packing densities arising from phospholipid acyl chain affect membrane-bound conformation of toxin gamma, thus changing its membrane-damaging activity.

Accelerated exchange of exon segments in Viperid three-finger toxin genes (Sistrurus catenatus edwardsii; Desert Massasauga)

Background

Snake venoms consist primarily of proteins and peptides showing a myriad of potent biological activities which have been shaped by both adaptive and neutral selective forces. Venom proteins are encoded by multigene families that have evolved through a process of gene duplication followed by accelerated evolution in the protein coding region.

Results

Here we report five gene structures of three-finger toxins from a viperid snake, Sistrurus catenatus edwardsii. These toxin genes are structured similarly to elapid and hydrophiid three-finger toxin genes, with two introns and three exons. Both introns and exons show distinct patterns of segmentation, and the insertion/deletion of segments may define their evolutionary history. The segments in introns, when present, are highly similar to their corresponding segments in other members of the gene family. In contrast, some segments in the exons show high similarity, while others are often distinctly different among corresponding regions of the isoforms.

Conclusion

Ordered, conserved exon structure strongly suggests that segments in corresponding regions in exons have been exchanged with distinctly different ones during the evolution of these genes. Such a "switching" of segments in exons may result in drastically altering the molecular surface topology and charge, and hence the molecular targets of these three-finger toxins. Thus the phenomenon of accelerated segment switch in exons to alter targeting (ASSET) may play an important role in the evolution of three-finger toxins, resulting in a family of toxins with a highly conserved structural fold but widely varying biological activities.

Cellular uptake mechanisms and potential therapeutic utility of peptidic cell delivery vectors: progress 2001-2006

Cell delivery vectors (CDVs) are short amphipathic and cationic peptides and peptide derivatives, usually containing multiple lysine and arginine residues. They possess inherent membrane activity and can be conjugated or complexed with large impermeable macromolecules and even microscopic particles to facilitate cell entry. Various mechanisms have been proposed but it is now becoming clear that the main port of entry into cells of such CDV constructs involves adsorptive-mediated endocytosis rather than direct penetration of the plasma membrane. It is still unclear, however, how and to what extent CDV constructs are capable of exiting endosomal compartments and reaching their intended cellular site of action, usually the cytosol or the nucleus. Furthermore, although many CDVs can mediate cellular uptake of their cargo and appear comparatively non-toxic to cells in tissue culture, the utility of CDVs for in vivo applications remains poorly understood. Whatever the mechanisms of cell entry and disposition, the overriding question as far as potential pharmacological application of CDV conjugates is concerned is whether or not a therapeutic margin can be achieved by their administration. Such a margin will only result if the intracellular concentration in the target tissues necessary to elicit the biological effect of the CDV cargo can be achieved at systemic CDV exposure levels that are non-toxic to both target and bystander cells. It is proposed that the focus of CDV research now be shifted from mechanistic in vitro studies with labeled but otherwise unconjugated CDVs to in vivo pharmacological and toxicological studies using CDV-derivatized and other cationized forms of inherently non-permeable macromolecules of true therapeutic interest.

Sulfatide binding properties of murine and human antiganglioside antibodies

Antiganglioside antibodies form an important component of the innate and adaptive B cell repertoire, where they provide antimicrobial activity through binding encapsulated bacterial glycans. In an aberrant role, they target peripheral nerve gangliosides to induce autoimmune nerve injury. An important characteristic of antiganglioside antibodies is their ability to selectively recognize highly defined glycan structures. Since sialylated and sulfated glycans often share lectin recognition patterns, we here explored the possibility that certain antiganglioside antibodies might also bind 3-O-sulfo-beta-D-galactosylceramide (sulfatide), an abundant constituent of plasma and peripheral nerve myelin, that could thereby influence any immunoregulatory or autoimmune properties. Out of 25 antiganglioside antibodies screened in solid phase assays, 20 also bound sulfatide (10(-5) to 10(-6) M range) in addition to their favored ganglioside glycan epitope ( approximately 10(-7) M range). Solution inhibition studies demonstrated competition between ganglioside and sulfatide, indicating close proximity or sharing of the antigen binding variable region domain. Sulfatide and 3-O-sulfo-beta-D-galactose were unique in having this property amongst a wide range of sulfated glycans screened, including 4- and 6-O-sulfo-beta-D-galactose analogues. Antiganglioside antibody binding to 3-O-sulfo-beta-D-galactose was highly dependent upon the spatial presentation of the ligand, being completely inhibited by conjugation to protein or polyacrylamide (PAA) matrices. Binding was also absent when sulfatide was incorporated into plasma membranes, including myelin, under conditions in which antibody binding to ganglioside was retained. These data demonstrate that sulfatide binding is a common property of antiganglioside antibodies that may provide functional insights into, and consequences for this component of the innate immune repertoire.

Plasmodium falciparum biosynthesizes sulfoglycosphingolipids

Sulfated glycosphingolipids are present on the surface of a variety of cells. They are active participants in adhesion processes in many systems and appear to be involved in the regulation of cell proliferation, differentiation and other developmental cellular events. However, the body of knowledge about synthesis, structure, and function of glycolipids in parasitic protozoa is very limited so far. In this work, we show by metabolic incorporation of [(14)C]palmitic acid, [(14)C]glucose and Na(2)(35)SO(4) that sulfoglycosphingolipids are biosynthesized in the three intraerythrocytic stages of Plasmodium falciparum. After saponification, purification of the labelled acidic components was achieved and two components named SPf1 and SPf2 were characterized. Chemical degradations and TLC analysis pointed out to sulfolipidic structures. Analysis by UV-MALDI-TOF mass spectrometry in the negative ion mode using nor-harmane as matrix showed for SPf2 a structure consisting in a disulfated hexose linked to a 20:1 sphingosine acylated with C18:0 fatty acid. Interestingly, parasite treatment with low concentrations of d,l-threo-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) caused an arrest on parasite development associated to the inhibition of sulfoglycolipid biosynthesis. Taking into account that sulfoglycolipidic structures are currently involved in adhesion processes, our findings open the possibility to study the participation of this type of structures in the described aggregation phenomena in severe malaria and may contribute to clarify the pathogenesis of the disease. This report shows for the first time the synthesis of sulfoglycolipids in Apicomplexa.

The mechanism of cytotoxicity by Naja naja atra cardiotoxin 3 is physically distant from its membrane-damaging effect

In order to dissect out whether multiple activities of cardiotoxins (CTXs) are connected, to some extent, with each other, studies on reduced and S-carboxyamidomethylated (Rcam) Naja naja atra CTX3 were carried out in the present study. Although both CTX3 and Rcam-CTX3 induced apoptotic death of PC-3 cells as evidenced by propodium iodide/annexin V double staining, degradation of procaspases and DNA fragmentation, the cytotoxicity of Rcam-CTX3 was mostly 100-fold lower than that noted with native toxin. However, Rcam-CTX3 retained approximately 38% of the membrane-damaging activity of native toxin as revealed by the decrease in calcein self-quenching from phospholipid vesicles. These results are likely to reflect that the mechanism of cytotoxicity by CTX3 is not heavily dependent on its membrane-perturbing effect, and suggest that the structural elements within CTX3 responsible for the two activities are probably separated.

Involvement of c-jun N-terminal kinase in G2/M arrest and caspase-mediated apoptosis induced by cardiotoxin III (Naja naja atra) in K562 leukemia cells

Cardiotoxin III (CTX III), a basic polypeptide with 60 amino acid residues isolated from Naja naja atra venom, may have a potentiality as a structural template for rational drug design in killing cancer cells. Treatment of K562 cells with 0.3 microM of CTX III resulted in G2/M phase cell cycle arrest that was associated with a marked decline in protein levels of G2/M regulatory proteins including cyclin A, cyclin B1, Cdk2 and Cdc25C. In contrast to no effect on the phosphorylation of ERK, p38 MAPK and Akt, an activation of JNK was noted when K562 cells were exposed to CTX III. CTX III-mediated G2/M phase arrest and apoptosis were reduced by treatment with the JNK-specific inhibitor SP600125, but not by ERK and p38MAPK inhibitors. Further investigation showed that the specific JNK inhibitor, SP600125, reduced the activation of caspase-3, caspase-9, and reversed the decline in the expression of cyclin B1. Taken together, our data show for the first time that JNK, but not ERK, p38MAPK or Akt signaling, plays an important role in CTX III-mediated G2/M arrest and apoptosis in K562 cancer cells.

Modelling study of dimerization in mammalian defensins

BACKGROUND

Defensins are antimicrobial peptides of innate immunity functioning by non-specific binding to anionic phospholipids in bacterial membranes. Their cationicity, amphipathicity and ability to oligomerize are considered key factors for their action. Based on structural information on human beta-defensin 2, we examine homologous defensins from various mammalian species for conserved functional physico-chemical characteristics.

RESULTS

Based on homology greater than 40%, structural models of 8 homologs of HBD-2 were constructed. A conserved pattern of electrostatics and dynamics was observed across 6 of the examined defensins; models backed by energetics suggest that the defensins in these 6 organisms are characterized by dimerization-linked enhanced functional potentials. In contrast, dimerization is not energetically favoured in the sheep, goat and mouse defensins, suggesting that they function efficiently as monomers.

CONCLUSION

Beta-defensin 2 from some mammals may work as monomers while those in others, including humans, work as oligomers. This could potentially be used to design human defensins that may be effective at lower concentrations and hence have therapeutic benefits.

Purification and characterization of Ophiophagus hannah cytotoxin-like proteins

Three cytotoxin-like proteins from the venom of Ophiophagus hannah were isolated by a combination of ion exchange chromatography and reverse phase HPLC. Amino acid sequence analysis revealed that these proteins all consisted of 63 amino acids and shared approximate 50% and 56% sequence identity with Naja naja atra cardiotoxins and cardiotoxin-like basic proteins (CLBPs), respectively. CD spectra revealed that their secondary structure was dominated with beta-sheet as those noted with cardiotoxins and CLBPs. O. hannah cytotoxin-like protein exhibited a cell-lytic activity on SK-N-SH cells, but its activity was more weak than that noted for N. naja atra cardiotoxin 3. Alternatively, apoptotic cell death was induced by the addition of N. naja atra CLBP. Based on the sequence information with the toxin molecules, the functional residues and regions related to the differential activity with O. hannah cytotoxin-like protein, cardiotoxin and CLBP are discussed.

Non-cytotoxic cobra cardiotoxin A5 binds to alpha(v)beta3 integrin and inhibits bone resorption. Identification of cardiotoxins as non-RGD integrin-binding proteins of the Ly-6 family

Severe tissue necrosis with a retarded wound healing process is a major symptom of a cobra snakebite. Cardiotoxins (CTXs) are major components of cobra venoms that belong to the Ly-6 protein family and are implicated in tissue damage. The interaction of the major CTX from Taiwan cobra, i.e. CTX A3, with sulfatides in the cell membrane has recently been shown to induce pore formation and cell internalization and to be responsible for cytotoxicity in cardiomyocytes (Wang, C.-H., Liu, J.-H., Lee, S.-C., Hsiao, C.-D., and Wu, W.-g. (2006) J. Biol. Chem. 281, 656-667). We show here that one of the non-cytotoxic CTXs, i.e. CTX A5 or cardiotoxin-like basic polypeptide, from Taiwan cobra specifically bound to alpha(v)beta3 integrin and inhibited bone resorption activity. We found that both membrane-bound and recombinant soluble alpha(v)beta3 integrins bound specifically to CTX A5 in a dose-dependent manner. Surface plasmon resonance analysis showed that human soluble alpha(v)beta3 bound to CTX A5 with an apparent affinity of approximately 0.3 microM. Calf pulmonary artery endothelial cells, which constitutively express alpha(v)beta3, showed a CTX A5 binding profile similar to that of membrane-bound and soluble alpha(v)beta3 integrins, suggesting that endothelial cells are a potential target for CTX action. We tested whether CTX A5 inhibits osteoclast differentiation and bone resorption, a process known to be involved in alpha(v)beta3 binding and inhibited by RGD-containing peptides. We demonstrate that CTX A5 inhibited both activities at a micromolar range by binding to murine alpha(v)beta3 integrin in osteoclasts and that CTX A5 co-localized with beta3 integrin. Finally, after comparing the integrin binding affinity among CTX homologs, we propose that the amino acid residues near the two loops of CTX A5 are involved in integrin binding. These results identify CTX A5 as a non-RGD integrin-binding protein with therapeutic potential as an integrin antagonist.