Conus Peptides: Phylogenetic Range of Biological Activity

Prey specificity of predatory venoms

Venom represents a key adaptation of many venomous predators, allowing them to immobilise prey quickly through chemical rather than physical warfare. Evolutionary arms races between prey and a predator are believed to be the main factor influencing the potency and composition of predatory venoms. Predators with narrowly restricted diets are expected to evolve specifically potent venom towards their focal prey, with lower efficacy on alternative prey. Here, we evaluate hypotheses on the evolution of prey-specific venom, focusing on the effect of restricted diet, prey defences, and prey resistance. Prey specificity as a potential evolutionary dead end is also discussed. We then provide an overview of the current knowledge on venom prey specificity, with emphasis on snakes, cone snails, and spiders. As the current evidence for venom prey specificity is still quite limited, we also overview the best approaches and methods for its investigation and provide a brief summary of potential model groups. Finally, possible applications of prey-specific toxins are discussed.

Structural analysis of a U-superfamily conotoxin containing a mini-granulin fold: Insights into key features that distinguish between the ICK and granulin folds

We are entering an exciting time in structural biology where artificial intelligence can be used to predict protein structures with greater accuracy than ever before. Extending this level of accuracy to the predictions of disulfide-rich peptide structures is likely to be more challenging, at least in the short term, given the tight packing of cysteine residues and the numerous ways that the disulfide bonds can potentially be linked. It has been previously shown in many cases that several disulfide bond connectivities can be accommodated by a single set of NMR-derived structural data without significant violations. Disulfide-rich peptides are prevalent throughout nature, and arguably the most well-known are those present in venoms from organisms such as cone snails. Here, we have determined the first three-dimensional structure and disulfide connectivity of a U-superfamily cone snail venom peptide, TxVIIB. TxVIIB has a VI/VII cysteine framework that is generally associated with an inhibitor cystine knot (ICK) fold; however, AlphaFold predicted that the peptide adopts a mini-granulin fold with a granulin disulfide connectivity. Our experimental studies using NMR spectroscopy and orthogonal protection of cysteine residues indicate that TxVIIB indeed adopts a mini-granulin fold but with the ICK disulfide connectivity. Our findings provide structural insight into the underlying features that govern formation of the mini-granulin fold rather than the ICK fold and will provide fundamental information for prediction algorithms, as the subtle complexity of disulfide isomers may be not adequately addressed by the current prediction algorithms.

Cytotoxicity and apoptosis induction of the marine Conus flavidus venom in HepG2 cancer cell line

Cancer is still an area of continuous research for finding more effective and selective agents, so our study aimed to explore new anticancer medicines from Cone snails' venoms as marine natural products with promising biological activities. Venoms from seven cone snails collected from two locations on the Red Sea coast (Marsa Alam (Ma) and Hurghada (Hu)) were extracted and subjected to SDS for protein concentrations. The venoms of C. vexillum (Ma), C. vexillum (Hu), and C. flavidus were found to have the highest protein concentrations (2.66, 2.618, and 2.611 mg/mL, respectively). The venom of C. vexillum (Ma) was found to be cytotoxic against the lung cancer cell line A549 (IC50 = 4.511 ± 0.03 µg/mL). On the other hand, the venom of C. flavidus showed a strong cytotoxic effect on both liver and lung cancer cell lines (IC50 = 1.593 ± 0.05 and 7.836 ± 0.4 µg/mL, respectively) when compared to their normal cell lines. Investigating the apoptotic cell death of C. flavidus venom on HepG2 cell lines, it showed total apoptotic cell death by 22.42-fold compared to untreated control and arresting the cell cycle at G2/M phase. Furthermore, its apoptotic cell death in HepG2 cells was confirmed through the upregulation of pro-apoptotic markers and down-regulation of Bcl-2 in both gene and protein expression levels. These findings confirmed the cytotoxic activity of C. flavidus venom through apoptotic cell death in HepG2 cells. So, a detailed study highlighting its structure and molecular target for developing new anticancer agents from natural sources is required.Communicated by Ramaswamy H. Sarma.

Prospecting for candidate molecules from Conus virgo toxins to develop new biopharmaceuticals

Background

A combination of pharmacological and biomedical assays was applied in this study to examine the bioactivity of Conus virgo crude venom in order to determine the potential pharmacological benefit of this venom, and its in vivo mechanism of action.

Methods

Two doses (1/5 and 1/10 of LC₅₀, 9.14 and 4.57 mg/kg) of the venom were used in pharmacological assays (central and peripheral analgesic, anti-inflammatory and antipyretic), while 1/2 of LC₅₀ (22.85 mg/kg) was used in cytotoxic assays on experimental animals at different time intervals, and then compared with control and reference drug groups.

Results

The tail immersion time was significantly increased in venom-treated mice compared with the control group. Also, a significant reduction in writhing movement was recorded after injection of both venom doses compared with the control group. In addition, only the high venom concentration has a mild anti-inflammatory effect at the late inflammation stage. The induced pyrexia was also decreased significantly after treatment with both venom doses. On the other hand, significant increases were observed in lipid peroxidation (after 4 hours) and reduced glutathione contents and glutathione peroxidase activity, while contents of lipid peroxidation and nitric oxide (after 24 hours) and catalase activity were depleted significantly after venom administration.

Conclusion

These results indicated that the crude venom of Conus virgo probably contain bioactive components that have pharmacological activities with low cytotoxic effects. Therefore, it may comprise a potential lead compound for the development of drugs that would control pain and pyrexia.

Anti-Ovarian Cancer Conotoxins Identified from Conus Venom

Conotoxins constitute a treasury of drug resources and have attracted widespread attention. In order to explore biological candidates from the marine cone snail, we isolated and identified three novel conopeptides named as Vi14b, Vi002, Vi003, three conotoxin variants named as Mr3d.1, Mr3e.1, Tx3a.1, and three known conotoxins (Vi15a, Mr3.8 and TCP) from crude venoms of Conus virgo, Conus marmoreus and Conus texile. Mr3.8 (I-V, II-VI, III-IV) and Tx3a.1 (I-III, II-VI, IV-V) both showed a novel pattern of disulfide connectivity, different from that previously established for the µ- and ψ-conotoxins. Concerning the effect on voltage-gated sodium channels, Mr3e.1, Mr3.8, Tx3a.1, TCP inhibited Na_v1.4 or Na_v1.8 by 21.51~24.32% of currents at semi-activated state (TP2) at 10 μmol/L. Certain anti-ovarian cancer effects on ID-8 cells were exhibited by Tx3a.1, Mr3e.1 and Vi14b with IC₅₀ values of 24.29 µM, 54.97 µM and 111.6 µM, respectively. This work highlights the role of conotoxin libraries in subsequent drug discovery for ovarian cancer treatment.

A Combined Transcriptomics and Proteomics Approach Reveals the Differences in the Predatory and Defensive Venoms of the Molluscivorous Cone Snail Cylinder ammiralis (Caenogastropoda: Conidae)

Venoms are complex mixtures of proteins that have evolved repeatedly in the animal kingdom. Cone snail venoms represent one of the best studied venom systems. In nature, this venom can be dynamically adjusted depending on its final purpose, whether to deter predators or hunt prey. Here, the transcriptome of the venom gland and the proteomes of the predation-evoked and defensive venoms of the molluscivorous cone snail Cylinder ammiralis were catalogued. A total of 242 venom-related transcripts were annotated. The conotoxin superfamilies presenting more different peptides were O1, O2, T, and M, which also showed high expression levels (except T). The three precursors of the J superfamily were also highly expressed. The predation-evoked and defensive venoms showed a markedly distinct profile. A total of 217 different peptides were identified, with half of them being unique to one venom. A total of 59 peptides ascribed to 23 different protein families were found to be exclusive to the predatory venom, including the cono-insulin, which was, for the first time, identified in an injected venom. A total of 43 peptides from 20 protein families were exclusive to the defensive venom. Finally, comparisons of the relative abundance (in terms of number of peptides) of the different conotoxin precursor superfamilies showed that most of them present similar abundance regardless of the diet.

Human health and snails

Snails can provide a considerable variety of bioactive compounds for cosmetic and pharmaceutical industries, useful for the development of new formulations with less toxicity and post effects compared to regular compounds used for the purpose. Compounds from crude extract, mucus, slime consist of glycans, polypeptides, proteins, etc., and can be used for curing diseases like viral lesions, warts, and different dermal problems. Some particular uses of snails involve treating post-traumatic stress. Micro RNA of Lymnaea stagnalis, was known to be responsible for the development of long-term memory and treatment of Alzheimer's and Dementia like diseases. This review explores the application of various bioactive compounds from snails with its potential as new translational medicinal and cosmetic applications. Snail bioactive compounds like ω-MVIIA, μ-SIIIA, μO-MrVIB, Xen2174, δ-EVIA, α-Vc1.1, σ-GVIIA, Conantokin-G, and Contulakin-G, conopeptides can be used for the development of anti-cancer drugs. These compounds target the innate immunity and improve the defense system of humans and provide protection against these life-threatening health concerns.AbbreviationsFDA: Food and Drug Administration; UTI: urinal tract infection; nAChRs: nicotinic acetylcholine receptors; NMDA: N-methyl-D-aspartate; CNS: central nervous system; CAR T: chimeric antigen receptors therapy; Micro RNA: micro ribonucleic acid.

The Venom Repertoire of Conus gloriamaris (Chemnitz, 1777), the Glory of the Sea

The marine cone snail Conus gloriamaris is an iconic species. For over two centuries, its shell was one of the most prized and valuable natural history objects in the world. Today, cone snails have attracted attention for their remarkable venom components. Many conotoxins are proving valuable as research tools, drug leads, and drugs. In this article, we present the venom gland transcriptome of C. gloriamaris, revealing this species' conotoxin repertoire. More than 100 conotoxin sequences were identified, representing a valuable resource for future drug discovery efforts.

Novel Conopeptides of Largely Unexplored Indo Pacific Conus sp

Cone snails are predatory creatures using venom as a weapon for prey capture and defense. Since this venom is neurotoxic, the venom gland is considered as an enormous collection of pharmacologically interesting compounds having a broad spectrum of targets. As such, cone snail peptides represent an interesting treasure for drug development. Here, we report five novel peptides isolated from the venom of Conus longurionis, Conus asiaticus and Conus australis. Lo6/7a and Lo6/7b were retrieved from C. longurionis and have a cysteine framework VI/VII. Lo6/7b has an exceptional amino acid sequence because no similar conopeptide has been described to date (similarity percentage <50%). A third peptide, Asi3a from C. asiaticus, has a typical framework III Cys arrangement, classifying the peptide in the M-superfamily. Asi14a, another peptide of C. asiaticus, belongs to framework XIV peptides and has a unique amino acid sequence. Finally, AusB is a novel conopeptide from C. australis. The peptide has only one disulfide bond, but is structurally very different as compared to other disulfide-poor peptides. The peptides were screened on nAChRs, Na_V and K_V channels depending on their cysteine framework and proposed classification. No targets could be attributed to the peptides, pointing to novel functionalities. Moreover, in the quest of identifying novel pharmacological targets, the peptides were tested for antagonistic activity against a broad panel of Gram-negative and Gram-positive bacteria, as well as two yeast strains.

Conotoxin gene superfamilies

Conotoxins are the peptidic components of the venoms of marine cone snails (genus Conus). They are remarkably diverse in terms of structure and function. Unique potency and selectivity profiles for a range of neuronal targets have made several conotoxins valuable as research tools, drug leads and even therapeutics, and has resulted in a concerted and increasing drive to identify and characterise new conotoxins. Conotoxins are translated from mRNA as peptide precursors, and cDNA sequencing is now the primary method for identification of new conotoxin sequences. As a result, gene superfamily, a classification based on precursor signal peptide identity, has become the most convenient method of conotoxin classification. Here we review each of the described conotoxin gene superfamilies, with a focus on the structural and functional diversity present in each. This review is intended to serve as a practical guide to conotoxin superfamilies and to facilitate interpretation of the increasing number of conotoxin precursor sequences being identified by targeted-cDNA sequencing and more recently high-throughput transcriptome sequencing.

Diversity of conotoxin gene superfamilies in the venomous snail, Conus victoriae

Animal venoms represent a vast library of bioactive peptides and proteins with proven potential, not only as research tools but also as drug leads and therapeutics. This is illustrated clearly by marine cone snails (genus Conus), whose venoms consist of mixtures of hundreds of peptides (conotoxins) with a diverse array of molecular targets, including voltage- and ligand-gated ion channels, G-protein coupled receptors and neurotransmitter transporters. Several conotoxins have found applications as research tools, with some being used or developed as therapeutics. The primary objective of this study was the large-scale discovery of conotoxin sequences from the venom gland of an Australian cone snail species, Conus victoriae. Using cDNA library normalization, high-throughput 454 sequencing, de novo transcriptome assembly and annotation with BLASTX and profile hidden Markov models, we discovered over 100 unique conotoxin sequences from 20 gene superfamilies, the highest diversity of conotoxins so far reported in a single study. Many of the sequences identified are new members of known conotoxin superfamilies, some help to redefine these superfamilies and others represent altogether new classes of conotoxins. In addition, we have demonstrated an efficient combination of methods to mine an animal venom gland and generate a library of sequences encoding bioactive peptides.

A 'conovenomic' analysis of the milked venom from the mollusk-hunting cone snail Conus textile--the pharmacological importance of post-translational modifications

Cone snail venoms provide a largely untapped source of novel peptide drug leads. To enhance the discovery phase, a detailed comparative proteomic analysis was undertaken on milked venom from the mollusk-hunting cone snail, Conus textile, from three different geographic locations (Hawai'i, American Samoa and Australia's Great Barrier Reef). A novel milked venom conopeptide rich in post-translational modifications was discovered, characterized and named α-conotoxin TxIC. We assign this conopeptide to the 4/7 α-conotoxin family based on the peptide's sequence homology and cDNA pre-propeptide alignment. Pharmacologically, α-conotoxin TxIC demonstrates minimal activity on human acetylcholine receptor models (100 μM, <5% inhibition), compared to its high paralytic potency in invertebrates, PD50 = 34.2 nMol kg(-1). The non-post-translationally modified form, [Pro](2,8)[Glu](16)α-conotoxin TxIC, demonstrates differential selectivity for the α3β2 isoform of the nicotinic acetylcholine receptor with maximal inhibition of 96% and an observed IC50 of 5.4 ± 0.5 μM. Interestingly its comparative PD50 (3.6 μMol kg(-1)) in invertebrates was ~100 fold more than that of the native peptide. Differentiating α-conotoxin TxIC from other α-conotoxins is the high degree of post-translational modification (44% of residues). This includes the incorporation of γ-carboxyglutamic acid, two moieties of 4-trans hydroxyproline, two disulfide bond linkages, and C-terminal amidation. These findings expand upon the known chemical diversity of α-conotoxins and illustrate a potential driver of toxin phyla-selectivity within Conus.

Conus vexillum venom induces oxidative stress in Ehrlich's ascites carcinoma cells: an insight into the mechanism of induction

BACKGROUND

It is estimated that venoms of marine cone snails (genus Conus) contain more than 100,000 different small peptides with a wide range of pharmacological and biological actions. Some of these peptides were developed into potential therapeutic agents and as molecular tools to understand biological functions of nervous and cardiovascular systems. In this study we examined the cytotoxic and anticancer properties of the marine vermivorous cone snail Conus vexillum (collected from Hurgada and Sharm El-Shaikh, Red Sea, Egypt) and suggest the possible mechanisms involved. The in vitro cytotoxic effects of Conus venom were assessed against Ehrlich's ascites carcinoma (EAC) cells.

RESULTS

Conus venom treatment resulted in concentration-dependent cytotoxicity as indicated by a lactate dehydrogenase leakage assay. Apoptotic effects were measured in vivo by measuring levels of reactive oxygen species and oxidative defense agents in albino mice injected with EAC cells. Conus venom (1.25 mg/kg) induced a significant increase (p < 0.05) in several oxidative stress biomarkers (lipid peroxidation, protein carbonyl content and reactive nitrogen intermediates) of EAC cells after 3, 6, 9 and 12 hours of venom injection. Conus venom significantly reduced (p < 0.05) the activities of oxidative defense enzymes (catalase and superoxide dismutase) as well as the total antioxidant capacity of EAC cells, as evidenced by lowered levels of reduced glutathione.

CONCLUSIONS

These results demonstrate the cytotoxic potential of C. vexillum venom by inducing oxidative stress mediated mechanisms in tumor cells and suggest that the venom contains novel molecules with potential anticancer activity.

Intraspecific variation in the venom of the vermivorous cone snail Conus vexillum

A combination of proteomic and biochemical assays was used to examine variations in the venom of Conus vexillum taken from two locations (Hurgada and Sharm El-Shaikh) in the Red Sea, Egypt. Using MALDI/TOF-MS, a remarkable degree of intra-species variation between venom samples from both locations was identified. To evaluate variability in the cytotoxic effects of Conus venom, mice were injected with the same dose from each location. The oxidative stress biomarkers [malondialdehyde (MDA), protein carbonyl content (PCC)], antioxidants [glutathione (GSH), superoxide dismutase (SOD), catalase (CAT)], total antioxidant capacity (TAC) and nitric oxide (NO), were measured 3, 6, 9 and 12h post venom injection. The venoms induced a significant increase in the levels of PCC, MDA, NO, GSH and CAT. The venoms significantly inhibited the activity of SOD and reduced the TAC. Toxicological data showed that the venom obtained from Hurgada was more potent than that obtained from Sharm El-Shaikh. It can be concluded that: (1) the venom of the same Conus species from different regions is highly diversified (2) the venoms from different locations reflect clear differences in venom potency and (3) the cytotoxic effects of C. vexillum venom can be attributed to its ability to induce oxidative stress.

Proteomic analysis provides insights on venom processing in Conus textile

Conus species of marine snails deliver a potent collection of toxins from the venom duct via a long proboscis attached to a harpoon tooth. Conotoxins are known to possess powerful neurological effects and some have been developed for therapeutic uses. Using mass-spectrometry based proteomics, qualitative and quantitative differences in conotoxin components were found in the proximal, central and distal sections of the Conus textile venom duct suggesting specialization of duct sections for biosynthesis of particular conotoxins. Reversed phase HPLC followed by Orbitrap mass spectrometry and data analysis using SEQUEST and ProLuCID identified 31 conotoxin sequences and 25 post-translational modification (PTM) variants with King-Kong 2 peptide being the most abundant. Several previously unreported variants of known conopeptides were found and this is the first time that HyVal is reported for a disulfide rich Conus peptide. Differential expression along the venom duct, production of PTM variants, alternative proteolytic cleavage sites, and venom processing enroute to the proboscis all appear to contribute to enriching the combinatorial pool of conopeptides and producing the appropriate formulation for a particular hunting situation. The complementary tools of mass spectrometry-based proteomics and molecular biology can greatly accelerate the discovery of Conus peptides and provide insights on envenomation and other biological strategies of cone snails.

Two new 4-Cys conotoxins (framework 14) of the vermivorous snail Conus austini from the Gulf of Mexico with activity in the central nervous system of mice

As part of continuing studies of the venom components present in Conus austini (syn.: Conus cancellatus), a vermivorous cone snail collected in the western Gulf of Mexico, Mexico, two major peptides, as14a and as14b, were purified and characterized. Their amino acid sequences were determined by automatic Edman sequencing after reduction and alkylation. Their molecular masses, established by matrix-assisted laser desorption ionization time-of-flight mass spectrometry, confirmed the chemical analyses and indicated that as14a and as14b have free C-termini. Each peptide contains 4-Cys residues arranged in a pattern (C-C-C-C, framework 14). The primary structure of as14a is GGVGRCIYNCMNSGGGLNFIQCKTMCY (experimental monoisotopic mass 2883.92Da; calculated monoisotopic mass 2884.20Da), whereas that of as14b is RWDVDQCIYYCLNGVVGYSYTECQTMCT (experimental monoisotopic mass 3308.63Da; calculated monoisotopic mass 3308.34Da). Both purified peptides elicited scratching and grooming activity in mice, and as14b also caused body and rear limb extension and tail curling immediately upon injection. The high sequence similarity of peptide as14a with peptide vil14a from the vermivorous C. villepinii suggests that the former might block K+ channels.

Clinico-toxinological characterization of the acute effects of the venom of the marine snail, Conus loroisii

The venom of the marine snail, Conus loroisii, was studied to assess its risk and lethal factors in regard of human welfare. The lethality of the crude venom (LD50-5.0 mg/kg via i.p.) in mice was associated with reduced motor activity, asphyxiation, followed by respiratory failure. The effects on vital tissues revealed vascular congestion and inflammatory cell infiltration around the portal triad of the liver, spongiosis of the brain, hemorrhages/congested blood vessels in lung and endothelial cells of the renal tubule. Repeated measures of hematological profiles indicated that the venom significantly reduced erythrocytes (P<0.001, GLM repeated measures), followed associated with depletion of hemoglobin, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin and platelet count. Serum enzymes such as, glutamic-oxaloacetic transaminase, glutamic-pyruvic transaminase, lactate dehydrogenase and alkaline and acid phosphatases were altered significantly (P<0.05, Friedman test), which in turn confirmed the damage of vital organ tissues. Dual effect of the venom on the activity of mouse brain acetylcholinesterase stand for concentration specific, whereas maximal inhibition (60.41%, P<0.05, Wilcoxon signed rank test) in erythrocyte acetylcholinesterase did not show the dual activity observed in brain. The Ciphergen ProteinChip analysis of the envenomed serum further revealed that the venom causes changes in definite molecules involved in inflammatory process and ionic transport. In all, the venom of C. loroisii is potentially lethal to mammals, through its rapid action on the central and peripheral nervous systems by blocking neurotransmission with selective interference of ionic channels/receptors.

Biochemical and gene expression analyses of conotoxins in Conus textile venom ducts

Each Conus snail species produces 50-200 unique peptide-based conotoxins, derived from a number of different gene superfamilies. Conotoxins are synthesized and secreted in a long venom duct, but biochemical and molecular aspects of their biosynthesis remain poorly understood. Here, we analyzed expression patterns of conotoxin genes belonging to different superfamilies in Conus textile venom ducts. The results demonstrate that specific gene families are expressed in particular regions of the venom duct. Biochemical analysis using liquid chromatography and mass spectrometry revealed an even more localized accumulation of individual conotoxins. This study demonstrates for the first time that specialization of gene expression, processing, and secretion of conotoxins occurs in different regions of the venom duct.

Conotoxin TVIIA, a novel peptide from the venom of Conus tulipa 1. Isolation, characterization and chemical synthesis

A novel conotoxin belonging to the 'four-loop' structural class has been isolated from the venom of the piscivorous cone snail Conus tulipa. It was identified using a chemical-directed strategy based largely on mass spectrometric techniques. The new toxin, conotoxin TVIIA, consists of 30 amino-acid residues and contains three disulfide bonds. The amino-acid sequence was determined by Edman analysis as SCSGRDSRCOOVCCMGLMCSRGKCVSIYGE where O = 4-transL-hydroxyproline. Two under-hydroxylated analogues, [Pro10]TVIIA and [Pro10,11]TVIIA, were also identified in the venom of C. tulipa. The sequences of TVIIA and [Pro10]TVIIA were further verified by chemical synthesis and coelution studies with native material. Conotoxin TVIIA has a six cysteine/four-loop structural framework common to many peptides from Conus venoms including the omega-, delta- and kappa-conotoxins. However, TVIIA displays little sequence homology with these well-characterized pharmacological classes of peptides, but displays striking sequence homology with conotoxin GS, a peptide from Conus geographus that blocks skeletal muscle sodium channels. These new toxins and GS share several biochemical features and represent a distinct subgroup of the four-loop conotoxins.

Molecular genetics of ecological diversification: duplication and rapid evolution of toxin genes of the venomous gastropod Conus

Predatory snails in the marine gastropod genus Conus stun prey by injecting a complex mixture of peptide neurotoxins. These conotoxins are associated with trophic diversification and block a diverse array of ion channels and neuronal receptors in prey species, but the evolutionary genesis of this functional diversity is unknown. Here we show that conotoxins with little amino acid similarity are in fact products of recently diverged loci that are rapidly evolving by strong positive selection in the vermivorous cone, Conus abbreviatus, and that the rate of conotoxin evolution is higher than that of most other known proteins. Gene duplication and diversifying selection result in the formation of functionally variable conotoxins that are linked to ecological diversification and evolutionary success of this genus.

Determination of the solution structures of conantokin-G and conantokin-T by CD and NMR spectroscopy

Conantokin-G and conantokin-T are two paralytic polypeptide toxins originally isolated from the venom of the fish-hunting cone snails of the genus Conus. Conantokin-G and conantokin-T are the only naturally occurring peptidic compounds which possess N-methyl-D-aspartate receptor antagonist activity, produced by a selective non-competitive antagonism of polyamine responses. They are also structurally unusual in that they contain a disproportionately large number of acid labile post-translational gamma-carboxyglutamic acid (Gla) residues. Although no precise structural information has previously been published for these peptides, early spectroscopic measurements have indicated that both conantokin-G and conantokin-T form alpha-helical structures, although there is some debate whether the presence of calcium ions is required for these peptides to adopt this fold. We now report a detailed structural study of synthetic conantokin-G and conantokin-T in a range of solution conditions using CD and 1H NMR spectroscopy. The three-dimensional structures of conantokin-T and conantokin-G were calculated from 1H NMR-derived distance and dihedral restraints. Both conantokins were found to contain a mixture of alpha- and 310 helix, that give rise to curved and straight helical conformers. Conantokin-G requires the presence of divalent cations (Zn2+, Ca2+, Cu2+, or Mg2+) to form a stable alpha-helix, while conantokin-T adopts a stable alpha-helical structure in aqueous conditions, in the presence or absence of divalent cations (Zn2+, Ca2+, Cu2+, or Mg2+).