Rational design of alpha-conotoxin analogues targeting alpha7 nicotinic acetylcholine receptors: improved antagonistic activity by incorporation of proline derivatives

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that belong to the superfamily of Cys loop receptors. Valuable insight into the orthosteric ligand binding to nAChRs in recent years has been obtained from the crystal structures of acetylcholine-binding proteins (AChBPs) that share significant sequence homology with the amino-terminal domains of the nAChRs. alpha-Conotoxins, which are isolated from the venom of carnivorous marine snails, selectively inhibit the signaling of neuronal nAChR subtypes. Co-crystal structures of alpha-conotoxins in complex with AChBP show that the side chain of a highly conserved proline residue in these toxins is oriented toward the hydrophobic binding pocket in the AChBP but does not have direct interactions with this pocket. In this study, we have designed and synthesized analogues of alpha-conotoxins ImI and PnIA[A10L], by introducing a range of substituents on the Pro(6) residue in these toxins to probe the importance of this residue for their binding to the nAChRs. Pharmacological characterization of the toxin analogues at the alpha(7) nAChR shows that although polar and charged groups on Pro(6) result in analogues with significantly reduced antagonistic activities, analogues with aromatic and hydrophobic substituents in the Pro(6) position exhibit moderate activity at the receptor. Interestingly, introduction of a 5-(R)-phenyl substituent at Pro(6) in alpha-conotoxin ImI gives rise to a conotoxin analogue with a significantly higher binding affinity and antagonistic activity at the alpha(7) nAChR than those exhibited by the native conotoxin.

Structural characterization of five post-translationally modified isomorphs of a novel putative delta-conotoxin from the vermivorous snail Conus delessertii from the Mexican Caribbean Sea

A novel peptide, de7b, was isolated from the venom of Conus delessertii, a worm-hunting species collected in the Caribbean Sea off the Yucatan Peninsula. Its primary structure was determined by automated Edman degradation and confirmed by mass spectrometry: it contains 28 amino acids, including six Cys residues. Peptide de7b is the second, O-conotoxin-like peptide isolated from the venom of this species, and it exists in different post-translationally modified isomorphs, some of which contain gamma-carboxy-glutamate (gamma) and/or 4-hydroxy-proline (O) at positions 4, 7, and/or 14. Its primary structure is DCI(P/O)GG(E/gamma)NCDVFR(O/P)YRCCSGYCILLLCA, with molecular masses varying from 3078.6 to 3154.6Da, depending on the number and kind of modified amino acid residues. Peptide de7b shows significant sequence identity with several O-conotoxins purified and biologically characterized from molluscivorous and piscivorous cone snails of the Indo-Pacific region, the tropical Atlantic and Eastern Pacific Oceans, especially with the delta-conotoxins but also with the omega-conotoxins from molluscivorous species, which suggests that it might affect voltage-gated Na(+) or Ca(2+)channels. Peptide de7b has 32% sequence identity with putative gamma-conotoxin de7a, previously characterized from the same species; both peptides contain the same number of amino acid residues and of non-Cys residues between the pairs of consecutive Cys residues. However, these peptides have charge differences at seven positions within the N-terminal half indicating that they might have distinct molecular targets that remain to be identified.

alpha4/7-conotoxin Lp1.1 is a novel antagonist of neuronal nicotinic acetylcholine receptors

Cone snails comprise approximately 700 species of venomous molluscs which have evolved the ability to generate multiple toxins with varied and exquisite selectivity. alpha-Conotoxin is a powerful tool for defining the composition and function of nicotinic acetylcholine receptors which play a crucial role in excitatory neurotransmission and are important targets for drugs and insecticides. An alpha4/7 conotoxin, Lp1.1, originally identified by cDNA and genomic DNA cloning from Conus leopardus, was found devoid of the highly conserved Pro residue in the first intercysteine loop. To further study this toxin, alpha-Lp1.1 was chemically synthesized and refolded into its globular disulfide isomer. The synthetic Lp1.1 induced seizure and paralysis on freshwater goldfish and selectively reversibly inhibited ACh-evoked currents in Xenopus oocytes expressing rat alpha3beta2 and alpha6alpha3beta2 nAChRs. Comparing the distinct primary structure with other functionally related alpha-conotoxins could indicate structural features in Lp1.1 that may be associated with its unique receptor recognition profile.

NR2B-selective conantokin peptide inhibitors of the NMDA receptor display enhanced antinociceptive properties compared to non-selective conantokins

NR2B-selective inhibitors show lower side-effects in preclinical pain models than non-selective NMDA receptor (NMDAR) antagonists, but it is unclear whether the improved safety of NR2B-selective inhibitors is due to their subtype selectivity or to a unique mode of inhibition of the receptor. In this study, the analgesic effects of intracerebral bolus injections of conantokin peptides with different NMDAR subunit selectivity were determined in mice by the standard hot-plate test, and following stimuli with acetic acid, formalin and complete Freund's adjuvant (CFA). In the standard hot-plate model, con-G[S16Y], a NR2B-selective inhibitor, showed the highest analgesic activity among conantokin peptides tested. In the acetic acid- and CFA-induced pain models, con-G[S16Y] and, to a lesser extent, con-G exhibited higher analgesic activity compared to non-selective inhibitors, such as con-R[1-17]. In the formalin test, while all conantokin peptides could partially suppress the first phase response, only con-G[S16Y] and con-G significantly inhibited the second phase response and suppressed paw edema. Our results suggest that the antinociceptive action of the conantokins may be related to their NR2B-selectivity and that these peptides may be useful as both neurobiological tools for probing mechanisms of nociception and as therapeutic agents for pain relief.

Differentiation of venoms of predatory marine gastropods: divergence of orthologous toxin genes of closely related Conus species with different dietary specializations

Venoms of Conus are remarkably diverse among species and the genes that encode conotoxins show high rates of evolution. Yet no prior studies have specifically explored how conotoxin gene evolution contributes to the differentiation of venoms of closely related Conus species. Previous investigations of four-loop conotoxin expression patterns of six closely related Conus species identified 12 sets of putative orthologous loci from these species, including eight pairs of loci that are coexpressed by two of these six species, C. abbreviatus and C. miliaris. Here I analyze the molecular evolution of orthologous conotoxin loci of these species and specifically examine the divergence of the eight orthologous counterparts of C. abbreviatus and C. miliaris. Tree and maximum likelihood-based analyses of these sequences reveal that positive selection promotes the divergence of orthologous genes among species and that the evolution of orthologues of C. abbreviatus and C. miliaris is asymmetric among species. The asymmetric evolution of conotoxin loci among species may result from lineage-specific dietary shifts or interspecific differences in the impact of selection from predator-prey interactions on conotoxin loci.

Identification of a novel class of conotoxins defined as V-conotoxins with a unique cysteine pattern and signal peptide sequence

Cone snails are predatory gastropod mollusks distributed in all tropical marine habitats with a highly sophisticated defense strategy using small peptides in their venoms. Here, we report the discovery and initial characterization of the V-superfamily conotoxins. A novel conotoxin vi15a was purified from the venom of a worm-hunting species Conus virgo. The sequence of vi15a was determined to have a unique arrangement of cysteine residues (C-C-CC-C-C-C-C), which defines the new V-superfamily conotoxins. The cDNA of vi15a was cloned with RACE method. Its unique signal peptide sequence led to the cloning of another V-superfamily conotoxin, Vt15.1, from Conus vitulinus. These results, as well as the existence of Lt15.1 from Conus litteratus and ca15a from Conus caracteristicus with the same cysteine pattern, suggest that V-superfamily might be a large and diverse group of peptides widely distributed in different Conus species. Like other eight Cys-containing toxins, V-superfamily conotoxins might also adopt an "ICK+1" disulfide bond connectivity. The identification of this novel class of conotoxins will certainly improve our understanding of the structure diversity of disulfide rich toxins.

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.

Cloning and isolation of a Conus cysteine-rich protein homologous to Tex31 but without proteolytic activity

We cloned and isolated a cysteine-rich protein, designated Mr30, from Conus marmoreus. Mr30 belongs to the cysteine-rich secretory protein family that is highly homologous to Tex31 previously obtained from Conus textile and reported as a protease responsible for processing of pro-conotoxins. Mr30, purified by a procedure similar to that of Tex31, indeed showed low proteolytic activity. However, further investigations revealed that the detected protease activity actually resulted from a trace amount of protease(s) contamination rather than from Mr30 itself. This finding led us to rethink the role of conus cysteine-rich secretory proteins: they were probably not responsible for the processing of pro-conotoxins as previously deduced, but their real biological functions remained to be clarified.

Isolation and characterization of a T-superfamily conotoxin from Conus litteratus with targeting tetrodotoxin-sensitive sodium channels

A T-1-conotoxin, lt5d, was purified and characterized from the venom of vermivorous hunting cone snails Conus litteratus. The complete amino acid sequence of lt5d (DCCPAKLLCCNP) has been determined by Edman degradation. With two disulfide bonds, the calculated average mass is 1274.57 Da, which is confirmed by MALDI-TOF mass spectrometry (average mass 1274.8778). Under whole cell patch-clamp mode, lt5d inhibits tetrodotoxin-sensitive sodium currents on adult rat dorsal root ganglion neurons, but has no effects on tetrodotoxin-resistant sodium currents. The inhibition of TTX-sensitive sodium currents by lt5d was found to be concentration-dependent with the IC(50) value of 156.16 nM. Thus, this is the first T-superfamily conotoxin identified to block TTX-sensitive sodium channels.

Identification of six novel T-1 conotoxins from Conus pulicarius by molecular cloning

Cone snails are a group of ancient marine gastropods with highly sophisticated defense and prey strategies using conotoxins in their venom. Conotoxins are a diverse array of small peptides, mostly with multiple disulfide bridges. Using a 3' RACE approach, we identified six novel peptides from the venom ducts of a worm-hunting cone snail Conus pulicarius. These peptides are named Pu5.1-Pu5.6 as their primary structures show the typical pattern of T-1 conotoxin family, a large and diverse group of peptides widely distributed in venom ducts of all major feeding types of Conus. Except for the conserved signal peptide sequences in the precursors and unique arrangement of Cys residues (CC-CC) in mature domains, the six novel T-1 conotoxins show remarkable sequence diversity in their pro and mature regions and are, thus, likely to be functionally diversified. Here, we present a simple and fast strategy of gaining novel disulfide-rich conotoxins via molecular cloning and our detailed sequence analysis will pave the way for the future functional characterization of toxin-receptor interaction.

Structure/Function Characterization of μ-Conotoxin KIIIA, an Analgesic, Nearly Irreversible Blocker of Mammalian Neuronal Sodium Channels

Peptide neurotoxins from cone snails continue to supply compounds with therapeutic potential. Although several analgesic conotoxins have already reached human clinical trials, a continuing need exists for the discovery and development of novel non-opioid analgesics, such as subtype-selective sodium channel blockers. Micro-conotoxin KIIIA is representative of micro-conopeptides previously characterized as inhibitors of tetrodotoxin (TTX)-resistant sodium channels in amphibian dorsal root ganglion neurons. Here, we show that KIIIA has potent analgesic activity in the mouse pain model. Surprisingly, KIIIA was found to block most (>80%) of the TTX-sensitive, but only approximately 20% of the TTX-resistant, sodium current in mouse dorsal root ganglion neurons. KIIIA was tested on cloned mammalian channels expressed in Xenopus oocytes. Both Na(V)1.2 and Na(V)1.6 were strongly blocked; within experimental wash times of 40-60 min, block was reversed very little for Na(V)1.2 and only partially for Na(V)1.6. Other isoforms were blocked reversibly: Na(V)1.3 (IC50 8 microM), Na(V)1.5 (IC50 284 microM), and Na(V)1.4 (IC50 80 nM). "Alanine-walk" and related analogs were synthesized and tested against both Na(V)1.2 and Na(V)1.4; replacement of Trp-8 resulted in reversible block of Na(V)1.2, whereas replacement of Lys-7, Trp-8, or Asp-11 yielded a more profound effect on the block of Na(V)1.4 than of Na(V)1.2. Taken together, these data suggest that KIIIA is an effective tool to study structure and function of Na(V)1.2 and that further engineering of micro-conopeptides belonging to the KIIIA group may provide subtype-selective pharmacological compounds for mammalian neuronal sodium channels and potential therapeutics for the treatment of pain.

Conotoxins of the O-superfamily affecting voltage-gated sodium channels

The venoms of predatory cone snails harbor a rich repertoire of peptide toxins that are valuable research tools, but recently have also proven to be useful drugs. Among the conotoxins with several disulfide bridges, the O-superfamily toxins are characterized by a conserved cysteine knot pattern: C-C-CC-C-C. While omega-conotoxins and kappa-conotoxins block Ca(2+) and K(+) channels, respectively, the closely related delta- and microO-conotoxins affect voltage-gated Na(+) channels (Na(v) channels). delta-conotoxins mainly remove the fast inactivation of Na(v) channels and, thus, functionally resemble long-chain scorpion alpha-toxins. microO-conotoxins are functionally similar to micro-conotoxins, since they inhibit the ion flow through Na(v) channels. Recent results from functional and structural assays have gained insight into the underlying molecular mechanisms. Both types of toxins are voltage-sensor toxins interfering with the voltage-sensor elements of Na(v) channels.

AChBP-targeted alpha-conotoxin correlates distinct binding orientations with nAChR subtype selectivity

Neuronal nAChRs are a diverse family of pentameric ion channels with wide distribution throughout cells of the nervous and immune systems. However, the role of specific subtypes in normal and pathological states remains poorly understood due to the lack of selective probes. Here, we used a binding assay based on acetylcholine-binding protein (AChBP), a homolog of the nicotinic acetylcholine ligand-binding domain, to discover a novel alpha-conotoxin (alpha-TxIA) in the venom of Conus textile. Alpha-TxIA bound with high affinity to AChBPs from different species and selectively targeted the alpha(3)beta(2) nAChR subtype. A co-crystal structure of Ac-AChBP with the enhanced potency analog TxIA(A10L), revealed a 20 degrees backbone tilt compared to other AChBP-conotoxin complexes. This reorientation was coordinated by a key salt bridge formed between Arg5 (TxIA) and Asp195 (Ac-AChBP). Mutagenesis studies, biochemical assays and electrophysiological recordings directly correlated the interactions observed in the co-crystal structure to binding affinity at AChBP and different nAChR subtypes. Together, these results establish a new pharmacophore for the design of novel subtype-selective ligands with therapeutic potential in nAChR-related diseases.

Diversity of the O-superfamily conotoxins from Conus miles

Conopeptides display prominent features of hypervariability and high selectivity of large gene families that mediate interactions between organisms. Remarkable sequence diversity of O-superfamily conotoxins was found in a worm-hunting cone snail Conus miles. Five novel cDNA sequences encoding O-superfamily precursor peptides were identified in C. miles native to Hainan by RT-PCR and 3'-RACE. They share the common cysteine pattern of the O-superfamily conotoxin (C-C-CC-C-C, with three disulfide bridges). The predicted peptides consist of 27-33 amino acids. We then performed a phylogenetic analysis of the new and published homologue sequences from C. miles and the other Conus species. Sequence divergence (%) and residue substitutions to view evolutionary relationships of the precursors' signal, propeptide, and mature toxin regions were analyzed. Percentage divergence of the amino acid sequences of the prepro region exhibited high conservation, whereas the sequences of the mature peptides ranged from almost identical with to highly divergent from inter- and intra-species. Despite the O-superfamily being a large and diverse group of peptides, widely distributed in the venom ducts of all major feeding types of Conus and discovered in several Conus species, it was for the first time that the newly found five O-superfamily peptides in this research came from the vermivorous C. miles. So far, conotoxins of the O-superfamily whose properties have been characterized are from piscivorous and molluscivorous Conus species, and their amino acid sequences and mode of action have been discussed in detail. The elucidated cDNAs of the five toxins are new and of importance and should attract the interest of researchers in the field, which would pave the way for a better understanding of the relationship of their structure and function.

chi-Conotoxin and tricyclic antidepressant interactions at the norepinephrine transporter define a new transporter model

Monoamine neurotransmitter transporters for norepinephrine (NE), dopamine and serotonin are important targets for antidepressants and analgesics. The conopeptide chi-MrIA is a noncompetitive and highly selective inhibitor of the NE transporter (NET) and is being developed as a novel intrathecal analgesic. We used site-directed mutagenesis to generate a suite of mutated transporters to identify two amino acids (Leu(469) and Glu(382)) that affected the affinity of chi-MrIA to inhibit [(3)H]NE uptake through human NET. Residues that increased the K(d) of a tricyclic antidepressant (nisoxetine) were also identified (Phe(207), Ser(225), His(296), Thr(381), and Asp(473)). Phe(207), Ser(225), His(296), and Thr(381) also affected the rate of NE transport without affecting NE K(m). In a new model of NET constructed from the bLeuT crystal structure, chi-MrIA-interacting residues were located at the mouth of the transporter near residues affecting the binding of small molecule inhibitors.

Albumin is a redox-active crowding agent that promotes oxidative folding of cysteine-rich peptides

Oxidative folding that occurs in a crowded cellular milieu is characterized by multifaceted interactions that occur among nascent polypeptides and resident components of the endoplasmic reticulum (ER) lumen. Macromolecular crowding has been considered an essential factor in the folding of polypeptides, but the excluded volume effect has not been evaluated for small, disulfide-rich peptides. In the research presented, we examined how macromolecular crowding agents, such as albumin, ovalbumin, and polysaccharides, influenced the kinetics and thermodynamics of forming disulfide bonds in four model peptides of varying molecular size from 13 residues (1.4 kDa) to 58-residues (6.5 kDa): conotoxins: GI, PVIIA, r11a, and bovine pancreatic trypsin inhibitor. Our results indicate that the excluded volume effect does not significantly alter the folding rates nor equilibria for these peptides. In stark contrast, folding reactions were dramatically accelerated, when protein-based crowding agents were present at concentrations lower than those predicted to provide the excluded volume effect. Submillimolar albumin alone was as effective as glutathione in promoting the oxidative folding of GI conotoxin at concentrations typically found in the ER. To the best of our knowledge, this is the first report and quantitative characterization of oxidative folding of peptides mediated by other than thioredoxin-based protein disulfide bonds. Our work raises a possibility that concurrent secretory and ER-resident proteins may influence the oxidative folding of small, cysteine-rich peptides not as crowding agents, but as redox-active factors.

From the identification of gene organization of alpha conotoxins to the cloning of novel toxins

In the venoms of cone snails, alpha conotoxins are competitive antagonists of nicotinic acetylcholine receptors. Eleven novel cDNA and eight partial gene sequences (including two pseudogenes) of alpha conotoxins were identified from five species of cone snail. As expected, every cDNA encodes a precursor of prepropeptide. In all the partial genes of alpha conotoxins identified, there is a long intron inserted at a fixed position in the pro-region, dividing the encoding region into two exons. The mutation rate in exon I (encoding the signal peptide and a part of pro-region) is much lower than that in exon II (encoding the other part of pro-region, the mature peptide and 3' untranslational region). Interestingly, the sequences at the 5' and 3' end of introns are highly conserved. In addition, in the identified introns exist long dinucleotide (e.g. "GT", "CA") or trinucleotide ("CAT") repeats. In the special case of Pu 1.1, there are five almost identical repeats of a 150 bp sequence in the long intron. Taking advantage of the conserved 3' end sequence of intron, 16 alpha conotoxins, as well as a pseudogene and three kappa A conotoxins, were identified from their genomic DNAs. Based on the comparison of these cDNA and gene sequences, a hypothesis of the alpha conotoxin evolution was proposed.

A novel alpha conotoxin (alpha-PIB) isolated from C. purpurascens is selective for skeletal muscle nicotinic acetylcholine receptors

The alpha-conotoxin family is comprised of peptides that share the following arrangement of cysteine residues in the primary amino acid sequence: -CC-C-C-, where each dash represents a variable number of amino acids. The number of amino acids between cysteine residues has been used to group the alpha-conotoxins into distinct subfamilies. These subfamilies include the alpha 4/7-, alpha 4/3- and alpha 3/5-conotoxins, so named for the number of amino acids between 2nd/3rd and 3rd/4th cysteine residues, respectively. The alpha 3/5-conotoxins antagonize vertebrate-muscle nicotinic acetylcholine receptors (nAChRs), while the alpha 4/7- and alpha 4/3-conotoxins primarily inhibit vertebrate neuronal nAChRs. To date, these three subfamilies are the most extensively characterized of the alpha-conotoxin family. Here we report the purification and characterization of an unusual alpha 4/4-conotoxin, alpha-conotoxin PIB (alpha-PIB), from the venom of Conus purpurascens, with the following amino-acid sequence: ZSOGCCWNPACVKNRC (Z=pyroglutamate, O=hydroxyproline). This peptide demonstrates high affinity inhibition of vertebrate-muscle nAChRs, and paralytic effects when injected in vivo. Testing of alpha-PIB against other receptors indicated that the inhibitory effect is specific for skeletal muscle nAChRs. alpha-PIB shares the key biochemical and pharmacological characteristics of the alpha-conotoxin family.

Predicting conotoxin superfamily and family by using pseudo amino acid composition and modified Mahalanobis discriminant

The conotoxin proteins are disulfide rich small peptides that target ion channels and G protein coupled receptors. And they provide promising application in treating some chronic pain, epilepsy, cardiovascular diseases, and so on. Conotoxins may be classified into 11 superfamilies: A, D, I1, I2, J, L, M, O, P, S, and T according to the disulfide connectivity, highly conserved N-terminal precursor sequence and similar mode of actions. Successful prediction mature conotoxin superfamily peptide has important signification for the biological and pharmacological functions of the toxins. In this study, a new algorithm of increment of diversity combined with modified Mahalanobis discriminant is presented to predict five superfamilies by using the pseudo amino acid composition. The results of jackknife cross-validation test show that the overall prediction sensitivity and specificity are 88% and 91%, respectively. The predictive algorithm is also used to predict three O-conotoxin families. The 72% sensitivity and 78% specificity are obtained. These results indicate that the conotoxin superfamily peptides correlate with their amino acid compositions.

Identification and molecular diversity of T-superfamily conotoxins from Conus lividus and Conus litteratus

The T-superfamily conotoxins comprise a large and diverse group of biologically active peptides and are widely distributed in venom ducts of all major feeding types of Conus. Six novel T-superfamily peptides from the two worm-hunting cone snail species of Conus lividus andConus. litteratus native to Hainan were identified and determined to share a common signal sequence as well as a conserved arrangement of cysteine residues (CC-CC). The predicted mature peptides consist of 11-15 amino acids only. Phylogenetic analyses of new conotoxins from C. lividus andC. litteratus in present study and published homologue T-superfamily sequences from the other Conus species was systematically performed. Phylogenetic trees, residue substitutions to view evolutionary relationships of the precursors' signal, propeptide, and mature toxin regions were explored, as well as residue frequency component and cystine codon usage. Percent divergence of the amino acid sequences of the signal-region exhibited high conservation, whereas the sequences of the mature peptides ranged from high similarity to high divergence between inter- and intro-species. Notably, diversity of pro-peptide region was also high with intermediate percent divergence between that observed in signal and toxin-regions. Consensus hydrophobic residues Leu, Val, Ala, Ile and Pro of signal regions were abundant, whereas among propeptides, basic residues Arg and Lys and acidic residue Asp, addition of hydrophilic residues Thr and Ser were abundant. Residue frequency components were hypervariable in mature toxin region except for highly conservative cystine frame residues. The T-superfamily conotoxins have been previously found mainly in piscivorous and molluscivorous cone snails. The newly identified six T-superfamily peptides described in this investigation exemplify the first to be found from vermivorousC. lividus andC. litteratus. The elucidated cDNAs of the six toxins will facilitate a better understanding of the relationship between structure and function as well as provide a framework for their further research and development.

Sequence diversity of O-superfamily conopetides from Conus marmoreus native to Hainan

The full-length cDNAs of six new O-superfamily conotoxins (CTX) were cloned and sequenced from Conus marmoreus native to Hainan in China South Sea using RT-PCR and 3'-RACE. Six novel conotoxin precursors encoded by these cDNAs consist of three typical regions of signal, pro-peptide and mature peptide. All the six toxin regions share a common O-superfamily cysteine pattern (C-C-CC-C-C, with three disulfide bridges). The predicted precursors are composed of 73-88 amino acids, and the predicted mature peptides consist of 26-34 amino acids. Phylogenetic analysis of new conotoxins from C. marmoreus from the present study and published homologue T-superfamily sequences from other Conus species was performed systematically. Patterns of sequence divergence for three regions of signal, pro-region and mature peptides, as well as Cys codon usage define the major O-superfamily branches and suggest how these separate branches arose. Percent identities of the amino acid sequences of the signal region exhibited high conservation, whereas the sequences of the mature peptides ranged from almost identical to highly divergent between inter- and intra-species. Notably, the diversity of the pro-region was also high with intermediate divergence between that observed in signal and toxin regions. Amino acid sequences and their mode of action (target) of previously identified conotoxins from molluscivorous C. marmoreus for the known conotoxins classes are discussed in detail. The data presented are new and should pave the way for chemical synthesis of these unique conotoxins for to allow determination of the molecular targets of these peptides, and also to provide clues for a better understanding of the phylogeny of these peptides.

Characterization of novel M-superfamily conotoxins with new disulfide linkage

The M-superfamily with the typical Cys framework (-CC-C-C-CC-) is one of the seven major superfamilies of conotoxins found in the venom of cone snails. Based on the number of residues in the last Cys loop (between C4 and C5), M-superfamily conotoxins can be provisionally categorized into four branches (M-1, M-2, M-3, M-4) [Corpuz GP, Jacobsen RB, Jimenez EC, Watkins M, Walker C, Colledge C, Garrett JE, McDougal O, Li W, Gray WR, et al. (2005) Biochemistry44, 8176-8186]. Here we report the purification of seven M-superfamily conotoxins from Conus marmoreus (five are novel and two are known as mr3a and mr3b) and one known M-1 toxin tx3a from Conus textile. In addition, six novel cDNA sequences of M-superfamily conotoxins have been identified from C. marmoreus, Conus leopardus and Conus quercinus. Most of the above novel conotoxins belong to M-1 and M-2 and only one to M-3. The disulfide analyses of two M-1 conotoxins, mr3e and tx3a, revealed that they possess a new disulfide bond arrangement (C1-C5, C2-C4, C3-C6) which is different from those of the M-4 branch (C1-C4, C2-C5, C3-C6) and M-2 branch (C1-C6, C2-C4, C3-C5). This newly characterized disulfide connectivity was confirmed by comparing the HPLC profiles of native mr3e and its two regioselectively folded isoforms. This is the first report of three different patterns of disulfide connectivity in conotoxins with the same cysteine framework.

Novel O-superfamily Conotoxins Identified by cDNA Cloning From Three Vermivorous Conus Species

The O-superfamily of conotoxins includes several subfamilies with different pharmacological targets, all of which are voltage-gated ion channels and distributed widely in varied Conus species. The venom components from any Conus species are quite distinct from those of other species. Seven novel O-superfamily peptides were identified by cDNA cloning from the three vermivorous Conus species of C. betulinus, C. lividus and C. caracteristicus native to Hainan. They share three common signal sequences, and a conserved arrangement of cysteine residues (C-C-CC-C-C). Phylogenetic analysis of newly found conotoxins in this study and known homologue O-superfamily sequences from the other Conus species was performed systematically. Divergence and percentage identity of the amino acid sequences of the signal regions suggest that the novel conotoxins described in this investigation belong to the three broad clades: MSGL, ME-QK and MKLT, each of which has its own characteristic signature signal sequence and cysteine codon conservation. Relative to this work, it is noted that O-superfamily conotoxins are not well represented from vermivorous species. The elucidated cDNAs of these newly found vermivorous toxins would facilitate a better understanding for basic research and drug discovery.

Direct cDNA cloning of novel conotoxins of the T-superfamily from Conus textile

The T-superfamily is a large and diverse group of peptides, widely distributed in venom ducts of all major feeding types of Conus. These peptides are likely to be functionally diverse. A directed PCR-based approach using primers based on the conserved signal sequence was applied to investigate new conotoxins of the T-superfamily from Conus textile native to Hainan. Using RT-PCR and 3'-RACE, four novel cDNA sequences encoding precursor peptides were identified in C. textile. They share a common T-superfamily cysteine pattern (CC-CC, with two disulfide bridges). The predicted peptides are small (9-12 amino acids). TeAr193 composed of nine amino acid residues is one of the shortest T-superfamily conotoxins ever found. Patterns of sequence divergence and Cys codon usage define the major T-superfamily branches and suggest how these separate branches arose. The sequences of the signal regions exhibited highest conservation, whereas the sequences of the mature peptides were either almost identical or highly divergent; and conservation of the pro-region was intermediate between that observed in signal and toxin regions. The elucidated cDNAs of the four toxins will facilitate a better understanding of the relationship between structure and function.

Pseudo amino acid composition and multi-class support vector machines approach for conotoxin superfamily classification

Conotoxins are disulfide rich small peptides that target a broad spectrum of ion-channels and neuronal receptors. They offer promising avenues in the treatment of chronic pain, epilepsy and cardiovascular diseases. Assignment of newly sequenced mature conotoxins into appropriate superfamilies using a computational approach could provide valuable preliminary information on the biological and pharmacological functions of the toxins. However, creation of protein sequence patterns for the reliable identification and classification of new conotoxin sequences may not be effective due to the hypervariability of mature toxins. With the aim of formulating an in silico approach for the classification of conotoxins into superfamilies, we have incorporated the concept of pseudo-amino acid composition to represent a peptide in a mathematical framework that includes the sequence-order effect along with conventional amino acid composition. The polarity index attribute, which encodes information such as residue surface buriability, polarity, and hydropathy, was used to store the sequence-order effect. Several methods like BLAST, ISort (Intimate Sorting) predictor, least Hamming distance algorithm, least Euclidean distance algorithm and multi-class support vector machines (SVMs), were explored for superfamily identification. The SVMs outperform other methods providing an overall accuracy of 88.1% for all correct predictions with generalized squared correlation of 0.75 using jackknife cross-validation test for A, M, O and T superfamilies and a negative set consisting of short cysteine rich sequences from different eukaryotes having diverse functions. The computed sensitivity and specificity for the superfamilies were found to be in the range of 84.0-94.1% and 80.0-95.5%, respectively, attesting to the efficacy of multi-class SVMs for the successful in silico classification of the conotoxins into their superfamilies.

Novel gamma-carboxyglutamic acid-containing peptides from the venom of Conus textile

The cone snail is the only invertebrate system in which the vitamin K-dependent carboxylase (or gamma-carboxylase) and its product gamma-carboxyglutamic acid (Gla) have been identified. It remains the sole source of structural information of invertebrate gamma-carboxylase substrates. Four novel Gla-containing peptides were purified from the venom of Conus textile and characterized using biochemical methods and mass spectrometry. The peptides Gla(1)-TxVI, Gla(2)-TxVI/A, Gla(2)-TxVI/B and Gla(3)-TxVI each have six Cys residues and belong to the O-superfamily of conotoxins. All four conopeptides contain 4-trans-hydroxyproline and the unusual amino acid 6-l-bromotryptophan. Gla(2)-TxVI/A and Gla(2)-TxVI/B are isoforms with an amidated C-terminus that differ at positions +1 and +13. Three isoforms of Gla(3)-TxVI were observed that differ at position +7: Gla(3)-TxVI, Glu7-Gla(3)-TxVI and Asp7-Gla(3)-TxVI. The cDNAs encoding the precursors of the four peptides were cloned. The predicted signal sequences (amino acids -46 to -27) were nearly identical and highly hydrophobic. The predicted propeptide region (-20 to -1) that contains the gamma-carboxylation recognition site (gamma-CRS) is very similar in Gla(2)-TxVI/A, Gla(2)-TxVI/B and Gla(3)-TxVI, but is more divergent for Gla(1)-TxVI. Kinetic studies utilizing the Conusgamma-carboxylase and synthetic peptide substrates localized the gamma-CRS of Gla(1)-TxVI to the region -14 to -1 of the polypeptide precursor: the Km was reduced from 1.8 mm for Gla (1)-TxVI lacking a propeptide to 24 microm when a 14-residue propeptide was attached to the substrate. Similarly, addition of an 18-residue propeptide to Gla(2)-TxVI/B reduced the Km value tenfold.

Diversity and evolution of conotoxins based on gene expression profiling of Conus litteratus

Cone snails are attracting increasing scientific attention due to their unprecedented diversity of invaluable channel-targeted peptides. As arguably the largest and most successful evolutionary genus of invertebrates, Conus also may become the model system to study the evolution of multigene families and biodiversity. Here, a set of 897 expressed sequence tags (ESTs) derived from a Conus litteratus venom duct was analyzed to illuminate the diversity and evolution mechanism of conotoxins. Nearly half of these ESTs represent the coding sequences of conotoxins, which were grouped into 42 novel conotoxin cDNA sequences (seven superfamilies), with T-superfamily conotoxins being the dominant component. The gene expression profile of conotoxin revealed that transcripts are expressed with order-of-magnitude differences, sequence divergence within a superfamily increases from the N to the C terminus of the open reading frame, and even multiple scaffold-different mature peptides exist in a conotoxin gene superfamily. Most excitingly, we identified a novel conotoxin superfamily and three novel cysteine scaffolds. These results give an initial insight into the C. litteratus transcriptome that will contribute to a better understanding of conotoxin evolution and the study of the cone snail genome in the near future.

Therapeutic applications of conotoxins that target the neuronal nicotinic acetylcholine receptor

Pain therapeutics discovered by molecular mining of the expressed genome of Australian predatory cone snails are providing lead compounds for the treatment of neurological diseases such as multiple sclerosis, shingles, diabetic neuropathy and other painful neurological conditions. The high specificity exhibited by these novel compounds for neuronal receptors and ion channels in the brain and nervous system indicates the high degree of selectivity that this class of neuropeptides can be expected to show when used therapeutically in humans. A lead compound, ACV1 (conotoxin Vc1.1 from Conus victoriae), has entered Phase II clinical trials and is being developed for the treatment for neuropathic pain. ACV1 will be targeted initially for the treatment of sciatica, shingles and diabetic neuropathy. The compound is a 16 amino acid peptide [Sandall et al., 2003. A novel alpha-conotoxin identified by gene sequencing is active in suppressing the vascular response to selective stimulation of sensory nerves in vivo. Biochemistry 42, 6904-6911], an antagonist of neuronal nicotinic acetylcholine receptors. It has potent analgesic activity following subcutaneous or intramuscular administration in several preclinical animal models of human neuropathic pain [Satkunanathan et al., 2005. Alpha conotoxin Vc1.1 alleviates neuropathic pain and accelerates functional recovery of injured neurons. Brain. Res. 1059, 149-158]. ACV1 may act as an analgesic by decreasing ectopic excitation in sensory nerves. In addition ACV1 appears to accelerate the recovery of injured nerves and tissues.

Structural and functional diversities among mu-conotoxins targeting TTX-resistant sodium channels

mu-Conotoxins are peptides that block sodium channels. Molecular cloning was used to identify four novel mu-conotoxins: CnIIIA, CnIIIB, CIIIA, and MIIIA from Conus consors, C. catus and C. magus. A comparison of their sequences with those of previously characterized mu-conotoxins suggested that the new mu-conotoxins were likely to target tetrodotoxin-resistant (TTX-r) sodium channels. The four peptides were chemically synthesized, and their biological activities were characterized. The new conotoxins all blocked, albeit with varying potencies, TTX-r sodium currents in frog dorsal-root-ganglion (DRG) neurons. The more potent of the four new mu-conotoxins, CnIIIA and CIIIA, exhibited a strikingly different selectivity profile in blocking TTX-r versus TTX-sensitive channels, as determined by their ability to block extracellularly recorded action potentials in three preparations from frog: skeletal muscle, cardiac muscle and TTX-treated C-fibers. CnIIIA was highly specific for TTX-r sodium channels, whereas CIIIA was nonselective. Both peptides appeared significantly less potent in blocking TTX-r sodium currents in rat and mouse DRG neurons. When CnIIIA and CIIIA were injected intracranially into mice, both induced seizures, but only CIIIA caused paralysis. This is the most comprehensive characterization to date of the structural and functional diversities of an emerging group of mu-conotoxins targeting TTX-r sodium channels.

Alpha-RgIA: a novel conotoxin that specifically and potently blocks the alpha9alpha10 nAChR

The alpha9 and alpha10 nicotinic acetylcholine receptor (nAChR) subunits assemble to form the alpha9alpha10 nAChR subtype. This receptor is believed to mediate cholinergic synaptic transmission between efferent olivocochlear fibers and the hair cells of the cochlea. In addition alpha9 and/or alpha10 expression has been described in dorsal root ganglion neurons, lymphocytes, skin keratinocytes, and the pars tuberalis of the pituitary. Specific antagonists that selectively block the alpha9alpha10 channel could be valuable tools for elucidating its role in these diverse tissues. This study describes a novel alpha-conotoxin from the Western Atlantic species Conus regius, alpha-conotoxin RgIA (alpha-RgIA), that is a subtype specific blocker of the alpha9alpha10 nAChR. alpha-RgIA belongs to the alpha4/3 subfamily of the alpha-conotoxin family; sequence and subtype specificity comparisons between alpha-RgIA and previously characterized alpha4/3 toxins indicate that the amino acids in the C-terminal half of alpha-RgIA are responsible for its preferential inhibition of the alpha9alpha10 nAChR subtype.

alpha-Conotoxin GI benzoylphenylalanine derivatives. 1H-NMR structures and photoaffinity labeling of the Torpedo californica nicotinic acetylcholine receptor

alpha-Conotoxins are small peptides from cone snail venoms that function as nicotinic acetylcholine receptor (nAChR)-competitive antagonists differentiating between nAChR subtypes. Current understanding about the mechanism of these selective interactions is based largely on mutational analyses, which identify amino acids in the toxin and nAChR that determine the energetics of ligand binding. To identify regions of the nAChR involved in alpha-conotoxin binding by use of photoactivated cross-linking, two benzoylphenylalanine (Bpa) analogs of alpha-conotoxin GI, GI(Bpa12) and GI(Bpa4), were synthesized by replacing the respective residues with Bpa, and their (1)H-NMR structures were determined. Both analogs preserved the GI conformation, but only GI(Bpa12) displaced (125)I-labeled GI from the Torpedo californica nAChR. (125)I-labeled GI(Bpa12) bound to two sites on the receptor (K(d) 13 and 1800 nM), and on UV irradiation specifically photolabeled the alpha, gamma and delta subunits. Photolabeling sites were mapped by selective proteolysis and enzymatic deglycosylation, combined with SDS/PAGE, HPLC and Edman degradation. In the alpha subunit, cobratoxin-inhibited incorporation was limited to the 22-kDa fragment beginning at alphaSer173 and containing the agonist-binding site segment C. In the gamma subunit, radioactivity was localized to two distinct peptides containing agonist-binding site segments F and D: nonglycosylated 24-kDa and glycosylated 13-kDa fragments starting at gammaAla167 and gammaAla49, respectively. The labeling of these fragments is discussed in terms of a model of GI(Bpa12) bound to the extracellular domain of the Torpedo nAChR homology model derived from the cryo-electron microscopy structure of Torpedo marmorata nAChR and X-ray crystal structures of snail acetylcholine-binding protein complexes with agonists and antagonists.

cDNA cloning of two novel T-superfamily conotoxins from Conus leopardus

The full-length cDNAs of two novel T-superfamily conotoxins, Lp5.1 and Lp5.2, were cloned from a vermivorous cone snail Conus leopardus using 3'/5'-rapid amplification of cDNA ends. The cDNA of Lp5.1 encodes a precursor of 65 residues, including a 22-residue signal peptide, a 28-residue propeptide and a 15-residue mature peptide. Lp5.1 is processed at the common signal site-X-Arg- immediately before the mature peptide sequences. In the case of Lp5.2, the precursor includes a 25-residue signal peptide and a 43-residue sequence comprising the propeptide and mature peptide, which is probably cleaved to yield a 29-residue propeptide and a 14-residue mature toxin. Although these two conotoxins share a similar signal sequence and a conserved disulfide pattern with the known T-superfamily, the pro-region and mature peptides are of low identity, especially Lp5.2 with an identity as low as 10.7% compared with the reference Mr5.1a. The elucidated cDNAs of these two toxins will facilitate a better understanding of the species distribution, the sequence diversity of T-superfamily conotoxins, the special gene structure and the evolution of these peptides.

A novel M-superfamily conotoxin with a unique motif from Conus vexillum

Cone snails are tropical marine mollusks that envenomate prey with a complex mixture of neuropharmacologically active compounds for the purpose of feeding and defence, each evolved to act in a highly specific manner on different parts of the nervous system. Here, we report the peptide purification, molecular cloning, chemical synthesis, and functional characterization of a structurally unique toxin isolated from the venom of Conus vexillum. The novel peptide, designated Vx2, was composed of 21 amino acid residues cross-linked by 3 disulfide bonds (WIDPSHYCCCGGGCTDDCVNC). Intriguingly, its mature peptide sequence shows low level of similarity with other identified conotoxins, and its unique motif (-CCCGGGC-) was not reported in other Conus peptides. However, its signal peptide sequence shares high similarity with those of the M-superfamily conotoxins. Hence, Vx2 could be classified into a new family of the M-superfamily.

Screening for post-translational modifications in conotoxins using liquid chromatography/mass spectrometry: an important component of conotoxin discovery

Mass spectrometry has emerged as an important technique for conotoxin analysis due to its capacity for selective, sensitive, information-rich analyses. Using liquid chromatography/mass spectrometry, Conus venom can be fractionated and the peptides surveyed for specific post-translational modifications, indicating those toxin components likely to have an important biological function. With Conus striatus and Conus victoriae venom as models, bromination, carboxylation and glycosylation modifications are identified through characteristics such as isotopic distribution and labile losses observed during mass spectrometric analysis. This modification screening approach enables the identification of a C. victoriae bromo-carboxy-conotoxin, designated vc5c, as a candidate for detailed mass spectrometric analysis. Using a cDNA sequence coupled with liquid chromatography/mass spectrometry and nanoelectrospray ionization-ion trap-mass spectrometry, the sequence of vc5c is determined to be ICCYPNXWCCD, where W is 6-bromotryptophan, X is gamma-carboxy glutamate and C is disulfide-linked cysteine. This represents the ninth T-superfamily (-CC-CC- scaffold) toxin that has been isolated from venom and characterized.

A novel conotoxin framework with a helix-loop-helix (Cs alpha/alpha) fold

Venomous predatory animals, such as snakes, spiders, scorpions, sea anemones, and cone snails, produce a variety of highly stable cystine-constrained peptide scaffolds as part of their neurochemical strategy for capturing prey. Here we report a new family of four-cystine, three-loop conotoxins (designated framework 14). Three peptides of this family (flf14a-c) were isolated from the venom of Conus floridanus floridensis, and one (vil14a) was isolated from the venom of Conus villepinii, two worm-hunting Western Atlantic cone snail species. The primary structure for these peptides was determined using Edman degradation sequencing, and their cystine pairing was assessed by limited hydrolysis with a combination of CNBr and chymotrypsin under nonreducing, nonalkylating conditions in combination with MALDI-TOF MS analysis of the resulting peptidic fragments. CD spectra and nanoNMR spectroscopy of these conotoxins directly isolated from the cone snails revealed a highly helical secondary structure for the four conotoxins. Sequence-specific nanoNMR analysis at room temperature revealed a well-defined helix-loop-helix tertiary structure that resembles that of the Cs alpha/alpha scorpion toxins kappa-hefutoxin, kappa-KTx1.3, and Om-toxins, which adopt a stable three-dimensional fold where the two alpha-helices are linked by the two disulfide bridges. One of these conotoxins (vil14a) has a Lys/Tyr dyad, separated by approximately 6A, which is a conserved structural feature in K(+) channel blockers. The presence of this framework in scorpions and in cone snails indicates a common molecular imprint in the venom of apparently unrelated predatory animals and suggests a common ancestral genetic origin.

Genes expressed in a turrid venom duct: divergence and similarity to conotoxins

The toxoglossate mollusks are a large group of venomous animals (>10,000 species) conventionally divided into three groups, the cone snails, the auger snails, and the turrid snails; turrids account for >90% of the biodiversity of toxoglossans. Only the venoms of cone snails have been intensively investigated, with little work focused on turrids. We report the first broad characterization of genes expressed in venom ducts of any turrid species. Twenty-three different cDNA clones encoding putative toxins were characterized from the venom duct of the turrine species Lophiotoma olangoensis Olivera 2002 and belong to 16 different gene families. Of the 16 different Lophiotoma olangoensis gene families that encode putative toxins, for only 1 was there clear evidence of sequence similarity with any conotoxin gene family. The I-like gene family of Lophiotoma olangoensis was found to be related to the K channel-targeted I(2) conotoxin superfamily. Most putative Lophiotoma toxins are cysteine-rich polypeptides, with a significant fraction much larger (>80 amino acids) than the toxins from cone snails. A small number were not cysteine-rich but had hydrophobic amino acid clusters interspersed with arginine residues. This is only 1 of >10,000 different turrid venoms that needs to be characterized. From this study, a common origin with Conus for one family of putative turrid toxins is indicated.

Analysis of expressed sequence tags from the venom ducts of Conus striatus: focusing on the expression profile of conotoxins

Cone snails (genus Conus) are predatory marine gastropods that use venom peptides for interacting with prey, predators and competitors. A majority of these peptides, generally known as conotoxins demonstrate striking selectivity in targeting specific subtypes of ion channels and neurotransmitter receptors. So they are not only useful tools in neuroscience to characterize receptors and receptor subtypes, but offer great potential in new drug research and development as well. Here, a cDNA library from the venom ducts of a fish-hunting cone snail species, Conus striatus is described for the generation of expressed sequence tags (ESTs). A total of 429 ESTs were grouped into 137 clusters or singletons. Among these sequences, 221 were toxin sequences, accounting for 52.1% (corresponding to 19 clusters) of all transcripts. A-superfamily (132 ESTs) and O-superfamily conotoxins (80 ESTs) constitute the predominant toxin components. Some non-disulfide-rich Conus peptides were also found. The expression profile of conotoxins also explained to some extent the pharmacological and physiological reactions elicited by this typical piscivorous species. For the first time, a nonstop transcript of conotoxin was identified, which is suggestive that alternative polyadenylation may be a means of post-transcriptional regulation of conotoxin production. A comparison analysis of these conotoxins reveals the different variation and divergence patterns in these two superfamilies. Our investigations indicate that focal hyper-mutation, block substitution and exon shuffling are three main mechanisms leading to the conotoxin diversity in a species. The comprehensive set of Conus gene sequences allowed the identification of the representative classes of conotoxins and related components, which may lay the foundation for further research and development of conotoxins.

Novel α-conotoxins identified by gene sequencing from cone snails native to Hainan, and their sequence diversity

Conotoxins (CTX) from the venom of marine cone snails (genus Conus) represent large families of proteins, which show a similar precursor organization with surprisingly conserved signal sequence of the precursor peptides, but highly diverse pharmacological activities. By using the conserved sequences found within the genes that encode the alpha-conotoxin precursors, a technique based on RT-PCR was used to identify, respectively, two novel peptides (LiC22, LeD2) from the two worm-hunting Conus species Conus lividus, and Conus litteratus, and one novel peptide (TeA21) from the snail-hunting Conus species Conus textile, all native to Hainan in China. The three peptides share an alpha4/7 subfamily alpha-conotoxins common cysteine pattern (CCX(4)CX(7)C, two disulfide bonds), which are competitive antagonists of nicotinic acetylcholine receptor (nAChRs). The cDNA of LiC22N encodes a precursor of 40 residues, including a propeptide of 19 residues and a mature peptide of 21 residues. The cDNA of LeD2N encodes a precursor of 41 residues, including a propeptide of 21 residues and a mature peptide of 16 residues with three additional Gly residues. The cDNA of TeA21N encodes a precursor of 38 residues, including a propeptide of 20 residues and a mature peptide of 17 residues with an additional residue Gly. The additional residue Gly of LeD2N and TeA21N is a prerequisite for the amidation of the preceding C-terminal Cys. All three sequences are processed at the common signal site -X-Arg- immediately before the mature peptide sequences. The properties of the alpha4/7 conotoxins known so far were discussed in detail. Phylogenetic analysis of the new conotoxins in the present study and the published homologue of alpha4/7 conotoxins from the other Conus species were performed systematically. Patterns of sequence divergence for the three regions of signal, proregion, and mature peptides, both nucleotide acids and residue substitutions in DNA and peptide levels, as well as Cys codon usage were analyzed, which suggest how these separate branches originated. Percent identities of the DNA and amino acid sequences of the signal region exhibited high conservation, whereas the sequences of the mature peptides ranged from almost identical to highly divergent between inter- and intra-species. Notably, the diversity of the proregion was also high, with an intermediate percentage of divergence between that observed in the signal and in the toxin regions. The data presented are new and are of importance, and should attract the interest of researchers in this field. The elucidated cDNAs of these toxins will facilitate a better understanding of the relationship of their structure and function, as well as the process of their evolutionary relationships.

Hyperhydroxylation: A New Strategy for Neuronal Targeting by Venomous Marine Molluscs

Venomous marine molluscs belonging to the genus Conus (cone snails) utilize a unique neurochemical strategy to capture their prey. Their venom is composed of a complex mixture of highly modified peptides (conopeptides) that interact with a wide range of neuronal targets. In this chapter, we describe a set of modifications based upon the hydroxylation of polypeptidic chains that are defining within the neurochemical strategy used by cone snails to capture their prey. In particular, we present a differential hydroxylation strategy that affects the neuronal targeting of a new set of a-conotoxins, mini-M conotoxins, conophans, and y-hydroxyconophans. Differential hydroxylation, preferential hydroxylation and hyperhydroxylation have been observed in these conopeptide families as a means of augmenting the venom arsenal used by cone snails for neuronal targeting and prey capture.

Synthesis of linear, branched, and cyclic peptide chimera

Chimeric peptides are unnatural constructs consisting of bioactive compounds from at least two different peptide(s) and/or protein(s) or two sequences from different parts of the same protein. Such multifunctional peptide combinations are prepared to enhance the biological activity or selectivity of their components. New biological effects can also be achieved with the chimera. In this chapter the synthesis of three different types of chimeric peptides will be described. In a linear chimera, two peptide epitopes from different parts of glycoprotein D (gD) of herpes simplex virus (HSV) are combined. A branched chimera, built from linear peptides, consists of tuftsin oligomers with immunostimulatory activity and an epitope peptide of HSV gD. The third compound is a cyclic chimeric molecule, where alpha-conotoxin GI as a host peptide is modified by the incorporation of a core epitope from HSV gD as a guest sequence.

Crystal structure of nicotinic acetylcholine receptor homolog AChBP in complex with an α-conotoxin PnIA variant

Conotoxins (Ctx) form a large family of peptide toxins from cone snail venoms that act on a broad spectrum of ion channels and receptors. The subgroup alpha-Ctx specifically and selectively binds to subtypes of nicotinic acetylcholine receptors (nAChRs), which are targets for treatment of several neurological disorders. Here we present the structure at a resolution of 2.4 A of alpha-Ctx PnIA (A10L D14K), a potent blocker of the alpha(7)-nAChR, bound with high affinity to acetylcholine binding protein (AChBP), the prototype for the ligand-binding domains of the nAChR superfamily. Alpha-Ctx is buried deep within the ligand-binding site and interacts with residues on both faces of adjacent subunits. The toxin itself does not change conformation, but displaces the C loop of AChBP and induces a rigid-body subunit movement. Knowledge of these contacts could facilitate the rational design of drug leads using the Ctx framework and may lead to compounds with increased receptor subtype selectivity.

Precursors of Novel Gla-Containing Conotoxins Contain a Carboxy-Terminal Recognition Site That Directs γ-Carboxylation,

Vitamin K-dependent gamma-glutamyl carboxylase catalyzes the conversion of glutamyl residues to gamma-carboxyglutamate. Its substrates include vertebrate proteins involved in blood coagulation, bone mineralization, and signal transduction and invertebrate ion channel blockers known as conotoxins. Substrate recognition involves a recognition element, the gamma-carboxylation recognition site, typically located within a cleavable propeptide preceding the targeted glutamyl residues. We have purified two novel gamma-carboxyglutamate-containing conotoxins, Gla-TxX and Gla-TxXI, from the venom of Conus textile. Their cDNA-deduced precursors have a signal peptide but no apparent propeptide. Instead, they contain a C-terminal extension that directs gamma-carboxylation but is not found on the mature conotoxin. A synthetic 13-residue "postpeptide" from the Gla-TxXI precursor reduced the K(m) for the reaction of the Conus gamma-carboxylase with peptide substrates, including FLEEL and conantokin-G, by up to 440-fold, regardless of whether it was positioned at the N- or C-terminal end of the mature toxin. Comparison of the postpeptides to propeptides from other conotoxins suggested some common elements, and amino acid substitutions of these residues perturbed gamma-carboxylation of the Gla-TxXI peptide. The demonstration of a functional and transferable C-terminal postpeptide in these conotoxins indicates the presence of the gamma-carboxylation recognition site within the postpeptide and defines a novel precursor structure for vitamin K-dependent polypeptides. It also provides the first formal evidence to prove that gamma-carboxylation occurs as a post-translational rather than a cotranslational process.

Conotoxins as Research Tools and Drug Leads

The complex mixture of biologically active peptides that constitute the venom of Conus species provides a rich source of ion channel neurotoxins. These peptides, commonly known as conotoxins, exhibit a high degree of selectivity and potency for different ion channels and their subtypes making them invaluable tools for unravelling the secrets of the nervous system. Furthermore, several conotoxin molecules have profound applications in drug discovery, with some examples currently undergoing clinical trials. Despite their relatively easy access by chemical synthesis, rapid access to libraries of conotoxin analogues for use in structure-activity relationship studies still poses a significant limitation. This is exacerbated in conotoxins containing multiple disulfide bonds, which often require synthetic strategies utilising several steps. This review will examine the structure and activity of some of the known classes of conotoxins and will highlight their potential as neuropharmacological tools and as drug leads. Some of the classical and more recent approaches to the chemical synthesis of conotoxins, particularly with respect to the controlled formation of disulfide bonds will be discussed in detail. Finally, some examples of structure-activity relationship studies will be discussed, as well as some novel approaches for designing conotoxin analogues.

Identification of a Novel Pharmacophore for Peptide Toxins Interacting with K+ Channels

KappaM-conotoxin RIIIK blocks TSha1 K+ channels from trout with high affinity by interacting with the ion channel pore. As opposed to many other peptides targeting K+ channels, kappaM-RIIIK does not possess a functional dyad. In this study we combine thermodynamic mutant cycle analysis and docking calculations to derive the binding mode of kappaM-conotoxin RIIIK to the TSha1 channel. The final model reveals a novel pharmacophore, where no positively charged side chain occludes the channel pore. Instead the positive-charged residues of the toxin form a basic ring; kappaM-RIIIK is anchored to the K+ channel via electrostatic interactions of this basic ring with the loop and pore helix residues of the channel. The channel amino acid Glu-354 is likely to be a fundamental determinant of the selectivity of kappaM-RIIIK for the TSha1 channel. The Cgamma-OH of Hyp-15 is in contact with the carbonyls of the selectivity filter, disturbing the charge distribution pattern necessary for the coordination of K+ ions. This novel, experimentally based pharmacophore model proves the existence of diverse binding modes of peptidic toxins to K+ channels and underlines the role of intermolecular electrostatic interactions involving channel loop side chains in determining the selectivity of toxins for specific K+ channel types.

Oxidative folding of conotoxins sharing an identical disulfide bridging framework

Conotoxins are short, disulfide-rich peptide neurotoxins produced in the venom of predatory marine cone snails. It is generally accepted that an estimated 100,000 unique conotoxins fall into only a handful of structural groups, based on their disulfide bridging frameworks. This unique molecular diversity poses a protein folding problem of relationships between hypervariability of amino acid sequences and mechanism(s) of oxidative folding. In this study, we present a comparative analysis of the folding properties of four conotoxins sharing an identical pattern of cysteine residues forming three disulfide bridges, but otherwise differing significantly in their primary amino acid sequence. Oxidative folding properties of M-superfamily conotoxins GIIIA, PIIIA, SmIIIA and RIIIK varied with respect to kinetics and thermodynamics. Based on rates for establishing the steady-state distribution of the folding species, two distinct folding mechanisms could be distinguished: first, rapid-collapse folding characterized by very fast, but low-yield accumulation of the correctly folded form; and second, slow-rearrangement folding resulting in higher accumulation of the properly folded form via the reshuffling of disulfide bonds within folding intermediates. Effects of changing the folding conditions indicated that the rapid-collapse and the slow-rearrangement mechanisms were mainly determined by either repulsive electrostatic or productive noncovalent interactions, respectively. The differences in folding kinetics for these two mechanisms were minimized in the presence of protein disulfide isomerase. Taken together, folding properties of conotoxins from the M-superfamily presented in this work and from the O-superfamily published previously suggest that conotoxin sequence diversity is also reflected in their folding properties, and that sequence information rather than a cysteine pattern determines the in vitro folding mechanisms of conotoxins.

Definition of the M-Conotoxin Superfamily: Characterization of Novel Peptides from Molluscivorous Conus Venoms

Most of the >50,000 different pharmacologically active peptides in Conus venoms belong to a small number of gene superfamilies. In this work, the M-conotoxin superfamily is defined using both biochemical and molecular criteria. Novel excitatory peptides purified from the venoms of the molluscivorous species Conus textile and Conus marmoreus all have a characteristic pattern of Cys residues previously found in the mu-, kappaM-, and psi-conotoxins (CC-C-C-CC). The new peptides are smaller (12-19 amino acids) than the mu-, kappaM-, and psi-conotoxins (22-24 amino acids). One peptide, mr3a, was chemically synthesized in a biologically active form. Analysis of the disulfide bridges of a natural peptide tx3c from C. textile and synthetic peptide mr3a from C. marmoreus showed a novel pattern of disulfide connectivity, different from that previously established for the mu- and psi-conotoxins. Thus, these peptides belong to a new group of structurally and pharmacologically distinct conotoxins that are particularly prominent in the venoms of mollusc-hunting Conus species. Analysis of cDNA clones encoding the novel peptides as well as those encoding mu-, kappaM-, and psi-conotoxins revealed highly conserved amino acid residues in the precursor sequences; this conservation in both amino acid sequence and in the Cys pattern defines a gene superfamily, designated the M-conotoxin superfamily. The peptides characterized can be provisionally assigned to four distinct groups within the M-superfamily based on sequence similarity within and divergence between each group. A notable feature of the superfamily is that two distinct structural frameworks have been generated by changing the disulfide connectivity on an otherwise conserved Cys pattern.

Sequence diversity of T-superfamily conotoxins from Conus marmoreus

Remarkable sequence diversity of T-superfamily conotoxins was found in a mollusk-hunting cone snail Conus marmoreus. The sequence of mr5a purified from the snail venom was determined, while six other sequences of Mr5.1a, Mr5.1b, Mr5.2, Mr5.3, Mr5.4a, and Mr5.4b were deduced from their corresponding cDNA cloned by RACE approach. mr5a of 10 amino acid residues is one of the shortest T-superfamily conotoxins ever found. They all share a typical (-CC-CC-) Cys pattern, a conserved signal peptide and a long 3'-untranslated region. A consensus Glu residue is preceded by the second two adjacent cysteines in all these toxins except in mr5a, whereas Mr5.1a, Mr5.1b, Mr5.4a and Mr5.4b are abundant in Trp residues. The identification of these highly divergent T-superfamily conotoxins will facilitate the understanding the relationship of their structure and function.

Electrostatic Recognition and Induced Fit in the κ-PVIIA Toxin Binding to Shaker Potassium Channel

Brownian dynamics (BD) and molecular dynamics (MD) simulations and electrostatic calculations were performed to study the binding process of kappa-PVIIA to the Shaker potassium channel and the structure of the resulting complex. BD simulations, guided by electrostatic interactions, led to an initial alignment between the toxin and the channel protein. MD simulations were then carried out to allow for rearrangements from this initial structure. After approximately 4 ns, a critical "induced fit" process was observed to last for approximately 2 ns. In this process, the interface was reorganized, and side chains were moved so that favorable atomic contacts were formed or strengthened, while unfavorable contacts were eliminated. The final complex structure was stabilized through electrostatic interactions with the positively charged side chain of Lys7 of kappa-PVIIA deeply inserted into the channel pore and other hydrogen bonds and by hydrophobic interactions involving Phe9 and Phe23 of the toxin. The validity of the predicted structure for the complex was assessed by calculating the effects of mutating charged and polar residues of both the toxin and the channel protein, with the calculated effects correlating reasonably well with experimental data. The present study suggests a general binding mechanism, whereby proteins are pre-aligned in their diffusional encounter by long-range electrostatic attraction, and nanosecond-scale rearrangements within the initial complex then lead to a specifically bound complex.

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.