Protein folding determinants: structural features determining alternative disulfide pairing in alpha- and chi/lambda-conotoxins

Oxidative Folding Catalysts of Conotoxins Derived from the Venom Duct Transcriptome of C. frigidus and C. amadis

Two novel redox conopeptides with proline residues outside and within the active site disulfide loop were derived from the venom duct transcriptome of the marine cone snails Conus frigidus and Conus amadis. Mature peptides with possible post-translational modification of 4-trans-hydroxylation of proline, namely, Fr874, Fr890[P1O], Fr890[P2O], Fr906, Am1038, and Am1054, have been chemically synthesized and characterized using mass spectrometry. The estimated reduction potential of cysteine disulfides of synthetic peptides varied from -298 to -328 mV, similar to the active site cysteine disulfide motifs of the redox family of proteins. Fr906/Am1054 exhibited pronounced catalytic activity and assisted in improving the yields of natively folded globular form α-conotoxin ImI. Three-dimensional (3D) structures of the redox conopeptides were optimized using computational methods and verified by 2D-ROESY NMR spectroscopy: C. frigidus peptides adopt an N-terminal helical fold and C. amadis peptides adopt distinct structures based on the Phe4-Pro/Hyp5 peptide bond configuration. The shift in the cis-trans configuration of the Phe4-Pro/Hyp5 peptide bond of Am1038/Am1054 was observed between reduced free thiol and oxidized disulfide forms of the optimized peptides. The report confirms the position-specific effect of hydroxyproline on the oxidative folding of conotoxins and sequence diversity of redox conopeptides in the venom duct of cone snails.

Changes in Potency and Subtype Selectivity of Bivalent NaV Toxins are Knot-Specific

Disulfide-rich peptide toxins have long been studied for their ability to inhibit voltage-gated sodium channel subtype Na_V1.7, a validated target for the treatment of pain. In this study, we sought to combine the pore blocking activity of conotoxins with the gating modifier activity of spider toxins to design new bivalent inhibitors of Na_V1.7 with improved potency and selectivity. To do this, we created an array of heterodimeric toxins designed to target human Na_V1.7 by ligating a conotoxin to a spider toxin and assessed the potency and selectivity of the resulting bivalent toxins. A series of spider-derived gating modifier toxins (GpTx-1, ProTx-II, gHwTx-IV, JzTx-V, CcoTx-1, and Pn3a) and two pore-blocker μ-conotoxins, SxIIIC and KIIIA, were used for this study. We employed either enzymatic ligation with sortase A for C- to N-terminal ligation or click chemistry for N- to N-terminal ligation. The bivalent peptide resulting from ligation of ProTx-II and SxIIIC (Pro[LPATG₆]Sx) was shown to be the best combination as native ProTx-II potency at hNa_V1.7 was conserved following ligation. At hNa_V1.4, a synergistic effect between the pore blocker and gating modifier toxin moieties was observed, resulting in altered sodium channel subtype selectivity compared to the parent peptides. Further studies including mutant bivalent peptides and mutant hNa_V1.7 channels suggested that gating modifier toxins have a greater contribution to the potency of the bivalent peptides than pore blockers. This study delineated potential benefits and drawbacks of designing pharmacological hybrid peptides targeting hNa_V1.7.

Characterization of the Native Disulfide Isomers of the Novel χ-Conotoxin PnID: Implications for Further Increasing Conotoxin Diversity

χ-Conotoxins are known for their ability to selectively inhibit norepinephrine transporters, an ability that makes them potential leads for treating various neurological disorders, including neuropathic pain. PnID, a peptide isolated from the venom of Conus pennaceus, shares high sequence homology with previously characterized χ-conotoxins. Whereas previously reported χ-conotoxins seem to only have a single native disulfide bonding pattern, PnID has three native isomers due to the formation of different disulfide bond patterns during its maturation in the venom duct. In this study, the disulfide connectivity and three-dimensional structure of these disulfide isomers were explored using regioselective synthesis, chromatographic coelution, and solution-state nuclear magnetic resonance spectroscopy. Of the native isomers, only the isomer with a ribbon disulfide configuration showed pharmacological activity similar to other χ-conotoxins. This isomer inhibited the rat norepinephrine transporter (IC50 = 10 ± 2 µM) and has the most structural similarity to previously characterized χ-conotoxins. In contrast, the globular isoform of PnID showed more than ten times less activity against this transporter and the beaded isoform did not display any measurable biological activity. This study is the first report of the pharmacological and structural characterization of an χ-conotoxin from a species other than Conus marmoreus and is the first report of the existence of natively-formed conotoxin isomers.

Incorporating NOE-Derived Distances in Conformer Generation of Cyclic Peptides with Distance Geometry

Nuclear magnetic resonance (NMR) data from NOESY (nuclear Overhauser enhancement spectroscopy) and ROESY (rotating frame Overhauser enhancement spectroscopy) experiments can easily be combined with distance geometry (DG) based conformer generators by modifying the molecular distance bounds matrix. In this work, we extend the modern DG based conformer generator ETKDG, which has been shown to reproduce experimental crystal structures from small molecules to large macrocycles well, to include NOE-derived interproton distances. In noeETKDG, the experimentally derived interproton distances are incorporated into the distance bounds matrix as loose upper (or lower) bounds to generate large conformer sets. Various subselection techniques can subsequently be applied to yield a conformer bundle that best reproduces the NOE data. The approach is benchmarked using a set of 24 (mostly) cyclic peptides for which NOE-derived distances as well as reference solution structures obtained by other software are available. With respect to other packages currently available, the advantages of noeETKDG are its speed and that no prior force-field parametrization is required, which is especially useful for peptides with unnatural amino acids. The resulting conformer bundles can be further processed with the use of structural refinement techniques to improve the modeling of the intramolecular nonbonded interactions. The noeETKDG code is released as a fully open-source software package available at www.github.com/rinikerlab/customETKDG.

The Redox-Active Conopeptide Derived from the Venom Duct Transcriptome of Conus lividus Assists in the Oxidative Folding of Conotoxin

The tetrapeptides Li504 and Li520, differing in the modification of the 4-trans-hydroxylation of proline, are novel conopeptides derived from the venom duct transcriptome of the marine cone snail Conus lividus. These predicted mature peptides are homologous to the active site motif of oxidoreductases that catalyze the oxidation, reduction, and rearrangement of disulfide bonds in peptides and proteins. The estimated reduction potential of the disulfide of Li504 and Li520 is within the range of disulfide reduction potentials of oxidoreductases, indicating that they may catalyze the oxidative folding of conotoxins. Conformational features of Li504 and Li520 include the trans configuration of the Cys1-Pro2/Hyp2 peptide bond with a type 1 turn that is similar to the active site motif of glutaredoxin that regulates the oxidation of cysteine thiols to disulfides. Li504- and Li520-assisted oxidative folding of α-conotoxin ImI confirms that Li520 improves the yield of the natively folded peptide by concomitantly decreasing the yield of the non-native disulfide isomer and thus acts as a miniature disulfide isomerase. The geometry of the Cys1-Hyp2 peptide bond of Li520 shifts between the trans and cis configurations in the disulfide form and thiol/thiolate form, which regulates the deprotonation of the N-terminal cysteine residue. Hydrogen bonding of the hydroxyl group of 4-trans-hydroxyproline with the interpeptide chain unit in the mixed disulfide form may play a vital role in shifting the geometry of the Cys1-Hyp2 peptide bond from cis to trans configuration. The Li520 conopeptide together with similar peptides derived from other species may constitute a new family of "redox-active" conopeptides that are integral components of the oxidative folding machinery of conotoxins.

α-Conotoxin Peptidomimetics: Probing the Minimal Binding Motif for Effective Analgesia

Several analgesic α-conotoxins have been isolated from marine cone snails. Structural modification of native peptides has provided potent and selective analogues for two of its known biological targets-nicotinic acetylcholine and γ-aminobutyric acid (GABA) G protein-coupled (GABAB) receptors. Both of these molecular targets are implicated in pain pathways. Despite their small size, an incomplete understanding of the structure-activity relationship of α-conotoxins at each of these targets has hampered the development of therapeutic leads. This review scrutinises the N-terminal domain of the α-conotoxin family of peptides, a region defined by an invariant disulfide bridge, a turn-inducing proline residue and multiple polar sidechain residues, and focusses on structural features that provide analgesia through inhibition of high-voltage-activated Ca2+ channels. Elucidating the bioactive conformation of this region of these peptides may hold the key to discovering potent drugs for the unmet management of debilitating chronic pain associated with a wide range of medical conditions.

Toxinology provides multidirectional and multidimensional opportunities: A personal perspective

In nature, toxins have evolved as weapons to capture and subdue the prey or to counter predators or competitors. When they are inadvertently injected into humans, they cause symptoms ranging from mild discomfort to debilitation and death. Toxinology is the science of studying venoms and toxins that are produced by a wide variety of organisms. In the past, the structure, function and mechanisms of most abundant and/or most toxic components were characterized to understand and to develop strategies to neutralize their toxicity. With recent technical advances, we are able to evaluate and determine the toxin profiles using transcriptomes of venom glands and proteomes of tiny amounts of venom. Enormous amounts of data from these studies have opened tremendous opportunities in many directions of basic and applied research. The lower costs for profiling venoms will further fuel the expansion of toxin database, which in turn will provide greater exciting and bright opportunities in toxin research.

Structural and Functional Analyses of Cone Snail Toxins

Cone snails are marine gastropod mollusks with one of the most powerful venoms in nature. The toxins, named conotoxins, must act quickly on the cone snails´ prey due to the fact that snails are extremely slow, reducing their hunting capability. Therefore, the characteristics of conotoxins have become the object of investigation, and as a result medicines have been developed or are in the trialing process. Conotoxins interact with transmembrane proteins, showing specificity and potency. They target ion channels and ionotropic receptors with greater regularity, and when interaction occurs, there is immediate physiological decompensation. In this review we aimed to evaluate the structural features of conotoxins and the relationship with their target types.

Ordered and Isomerically Stable Bicyclic Peptide Scaffolds Constrained through Cystine Bridges and Proline Turns

Bicyclic peptides are attractive scaffolds for the design of potent protein binders and new therapeutics. However, peptide bicycles constrained through disulfide bonds are rarely stable or tolerant to sequence manipulation owing to disulfide isomerization, especially for peptides lacking a regular secondary structure. Herein, we report the discovery and identification of a class of bicyclic peptide scaffolds with ordered but irregular secondary structures. These peptides have a conserved cysteine/proline framework for directing the oxidative folding into a fused bicyclic structure that consists of four irregular turns and a 3₁₀ helix (characterized by NMR spectroscopy). This work shows that bicyclic peptides can be stabilized into ordered structures by manipulating both the disulfide bonds and proline-stabilized turns. In turn, this could inspire the design and engineering of multicyclic peptides with new structures and benefit the development of novel protein binders and therapeutics.

Role of the disulfide bond on the structure and activity of μ-conotoxin PIIIA in the inhibition of NaV1.4

μ-Conotoxin PIIIA, a peptide toxin isolated from Conus purpurascens, blocks the skeletal muscle voltage-gated sodium channel NaV1.4 with significant potency. PIIIA has three disulfide bonds, which contribute largely to its highly constrained and stable structure. In this study, a combination of experimental studies and computational modeling were performed to assess the effects of deletion of the disulfide bonds on the structure and activity of PIIIA. The final results indicate that the three disulfide bonds of PIIIA are required to produce the effective inhibition of NaV1.4, and the removal of any one of the disulfide bonds significantly reduces its binding affinity owing to secondary structure variation, among which the Cys11-Cys22 is the most important for sustaining the structure and activity of PIIIA.

Peptide ligands for targeting the extracellular domain of EGFR: Comparison between linear and cyclic peptides

Colorectal cancer (CRC) is the third most common solid internal malignancy among cancers. Early detection of cancer is key to increasing the survival rate of colorectal cancer patients. Overexpression of the EGFR protein is associated with CRC. We have designed a series of peptides that are highly specific for the extracellular domain of EGFR, based on our earlier studies on linear peptides. The previously reported linear peptide LARLLT, known to bind to EGFR, was modified with the goals of increasing its stability and its specificity toward EGFR. Peptide modifications, including D-amino acid substitution, cyclization, and chain reversal, were investigated. In addition, to facilitate labeling of the peptide with a fluorescent dye, an additional lysine residue was introduced onto the linear (KLARLLT) and cyclic peptides cyclo(KLARLLT) (Cyclo.L1). The lysine residue was also converted into an azide group in both a linear and reversed cyclic peptide sequences cyclo(K(N3)larllt) (Cyclo.L1.1) to allow for subsequent "click" conjugation. The cyclic peptides showed enhanced binding to EGFR by SPR. NMR and molecular modeling studies suggest that the peptides acquire a β-turn structure in solution. In vitro stability studies in human serum show that the cyclic peptide is more stable than the linear peptide.

In vivo and in vitro testing of native α-conotoxins from the injected venom of Conus purpurascens

α-Conotoxins inhibit nicotinic acetylcholine receptors (nAChRs) and are used as probes to study cholinergic pathways in vertebrates. Model organisms, such as Drosophila melanogaster, express nAChRs in their CNS that are suitable to investigate the neuropharmacology of α-conotoxins in vivo. Here we report the paired nanoinjection of native α-conotoxin PIA and two novel α-conotoxins, PIC and PIC[O7], from the injected venom of Conus purpurascens and electrophysiological recordings of their effects on the giant fiber system (GFS) of D. melanogaster and heterologously expressed nAChRs in Xenopus oocytes. α-PIA caused disruption of the function of giant fiber dorsal longitudinal muscle (GF-DLM) pathway by inhibiting the Dα7 nAChR a homolog to the vertebrate α7 nAChR, whereas PIC and PIC[O7] did not. PIC and PIC[O7] reversibly inhibited ACh-evoked currents mediated by vertebrate rodent (r)α1β1δγ, rα1β1δε and human (h)α3β2, but not hα7 nAChR subtypes expressed in Xenopus oocytes with the following selectivity: rα1β1δε > rα1β1δγ ≈ hα3β2 >> hα7. Our study emphasizes the importance of loop size and α-conotoxin sequence specificity for receptor binding. These studies can be used for the evaluation of the neuropharmacology of novel α-conotoxins that can be utilized as molecular probes for diseases such as, Alzheimer's, Parkinson's, and cancer. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

The Role of Individual Disulfide Bonds of μ-Conotoxin GIIIA in the Inhibition of NaV1.4

μ-Conotoxin GIIIA, a peptide toxin isolated from Conus geographus, preferentially blocks the skeletal muscle sodium channel Na_V1.4. GIIIA folds compactly to a pyramidal structure stabilized by three disulfide bonds. To assess the contributions of individual disulfide bonds of GIIIA to the blockade of Na_V1.4, seven disulfide-deficient analogues were prepared and characterized, each with one, two, or three pairs of disulfide-bonded Cys residues replaced with Ala. The inhibitory potency of the analogues against Na_V1.4 was assayed by whole cell patch-clamp on rNa_V1.4, heterologously expressed in HEK293 cells. The corresponding IC₅₀ values were 0.069 ± 0.005 μM for GIIIA, 2.1 ± 0.3 μM for GIIIA-1, 3.3 ± 0.2 μM for GIIIA-2, and 15.8 ± 0.8 μM for GIIIA-3 (-1, -2 and -3 represent the removal of disulfide bridges Cys3–Cys15, Cys4–Cys20 and Cys10–Cys21, respectively). Other analogues were not active enough for IC₅₀ measurement. Our results indicate that all three disulfide bonds of GIIIA are required to produce effective inhibition of Na_V1.4, and the removal of any one significantly lowers its sodium channel binding affinity. Cys10–Cys21 is the most important for the Na_V1.4 potency.

Effect of two synthetic disulfide bond variants of a 13-mer toxin from Conus californicus on the transcription of pro-inflammatory cytokines induced by LPS

This study presents the effects of two synthetic disulfide bond variants of cal16b, a 13-mer Ca²⁺ channel blocker conotoxin, on pro- and anti-inflammatory cytokine gene transcription in the murine macrophage-like cell line J774A.1 stimulated with LPS. The globular form (cal16b_1) acted as an anti-inflammatory agent; in contrast, the ribbon disulfide variant (cal16b_2) had a pro-inflammatory effect. Our results suggest that the pro- and anti-inflammatory effects are mediated by the three-dimensional structure.

Structural determinants in protein folding: a single conserved hydrophobic residue determines folding of EGF domains

The epidermal growth factor (EGF) domain is evolutionarily conserved despite hypervariability in amino acid sequences. They fold into a three-looped conformation with a disulfide pairing of C(1)-C(3), C(2)-C(4) ,and C(5)-C(6). To elucidate the structural determinants that dictate the EGF fold, we selected the fourth and fifth EGF domains of thrombomodulin (TM) as models; the former domain folds into the canonical conformation, while the latter domain folds with alternate disulfide pairing of C(1)-C(2), C(3)-C(4), and C(5)-C(6). Since their third disulfide (C(5)-C(6)) is conserved, we examined the folding tendencies of synthetic peptides corresponding to truncated domain four (t-TMEGF4) and five (t-TMEGF5), encompassing the segment C(1) to C(4). These peptides fold into their respective disulfide isoforms indicating that they contain all the required structural determinants. On the basis of the folding tendencies of these peptides in the absence and presence of 6 M Gn·HCl or 0.5 M NaCl, we determined that hydrophobic interactions are needed for the canonical EGF fold but not for the noncanonical fold. Sequence alignment of extant EGF domains and examination of their three-dimensional structures allowed us to identify a highly conserved hydrophobic residue in intercysteine loop 3 as the key contributor, which nucleates the hydrophobic core and acts as the lynch pin. When this hydrophobic residue (Tyr25) was substituted with a more hydrophilic Thr, the hydrophobic interactions were disrupted, and t-TMEGF4-Y25T folds similar to t-TMEGF5. Taken together, our results for the first time demonstrate that a single conserved hydrophobic residue acts as the key determinant in the folding of EGF domains.

Oxidative folding and preparation of α-conotoxins for use in high-throughput structure-activity relationship studies

α-Conotoxins are peptide neurotoxins that selectively inhibit various subtypes of nicotinic acetylcholine receptors. They are important research tools for studying numerous pharmacological disorders, with profound potential for developing drug leads for treating pain, tobacco addiction, and other conditions. They are characterized by the presence of two disulfide bonds connected in a globular arrangement, which stabilizes a bioactive helical conformation. Despite extensive structure-activity relationship studies that have produced α-conotoxin analogs with increased potency and selectivity towards specific nicotinic acetylcholine receptor subtypes, the efficient production of diversity-oriented α-conotoxin combinatorial libraries has been limited by inefficient folding and purification procedures. We have investigated the optimized conditions for the reliable folding of α-conotoxins using simplified oxidation procedures for use in the accelerated production of synthetic combinatorial libraries of α-conotoxins. To this end, the effect of co-solvent, redox reagents, pH, and temperature on the proportion of disulfide bond isomers was determined for α-conotoxins exhibiting commonly known Cys loop spacing frameworks. In addition, we have developed high-throughput 'semi-purification' methods for the quick and efficient parallel preparation of α-conotoxin libraries for use in accelerated structure-activity relationship studies. Our simplified procedures represent an effective strategy for the preparation of large arrays of correctly folded α-conotoxin analogs and permit the rapid identification of active hits directly from high-throughput pharmacological screening assays.

Modulation of conotoxin structure and function is achieved through a multienzyme complex in the venom glands of cone snails

The oxidative folding of large polypeptides has been investigated in detail; however, comparatively little is known about the enzyme-assisted folding of small, disulfide-containing peptide substrates. To investigate the concerted effect of multiple enzymes on the folding of small disulfide-rich peptides, we sequenced and expressed protein-disulfide isomerase (PDI), peptidyl-prolyl cis-trans isomerase, and immunoglobulin-binding protein (BiP) from Conus venom glands. Conus PDI was shown to catalyze the oxidation and reduction of disulfide bonds in two conotoxins, α-GI and α-ImI. Oxidative folding rates were further increased in the presence of Conus PPI with the maximum effect observed in the presence of both enzymes. In contrast, Conus BiP was only observed to assist folding in the presence of microsomes, suggesting that additional co-factors were involved. The identification of a complex between BiP, PDI, and nascent conotoxins further suggests that the folding and assembly of conotoxins is a highly regulated multienzyme-assisted process. Unexpectedly, all three enzymes contributed to the folding of the ribbon isomer of α-ImI. Here, we identify this alternative disulfide-linked species in the venom of Conus imperialis, providing the first evidence for the existence of a "non-native" peptide isomer in the venom of cone snails. Thus, ER-resident enzymes act in concert to accelerate the oxidative folding of conotoxins and modulate their conformation and function by reconfiguring disulfide connectivities. This study has evaluated the role of a number of ER-resident enzymes in the folding of conotoxins, providing novel insights into the enzyme-guided assembly of these small, disulfide-rich peptides.

Blockade of neuronal α7-nAChR by α-conotoxin ImI explained by computational scanning and energy calculations

α-Conotoxins potently inhibit isoforms of nicotinic acetylcholine receptors (nAChRs), which are essential for neuronal and neuromuscular transmission. They are also used as neurochemical tools to study nAChR physiology and are being evaluated as drug leads to treat various neuronal disorders. A number of experimental studies have been performed to investigate the structure-activity relationships of conotoxin/nAChR complexes. However, the structural determinants of their binding interactions are still ambiguous in the absence of experimental structures of conotoxin-receptor complexes. In this study, the binding modes of α-conotoxin ImI to the α7-nAChR, currently the best-studied system experimentally, were investigated using comparative modeling and molecular dynamics simulations. The structures of more than 30 single point mutants of either the conotoxin or the receptor were modeled and analyzed. The models were used to explain qualitatively the change of affinities measured experimentally, including some nAChR positions located outside the binding site. Mutational energies were calculated using different methods that combine a conformational refinement procedure (minimization with a distance dependent dielectric constant or explicit water, or molecular dynamics using five restraint strategies) and a binding energy function (MM-GB/SA or MM-PB/SA). The protocol using explicit water energy minimization and MM-GB/SA gave the best correlations with experimental binding affinities, with an R² value of 0.74. The van der Waals and non-polar desolvation components were found to be the main driving force for binding of the conotoxin to the nAChR. The electrostatic component was responsible for the selectivity of the various ImI mutants. Overall, this study provides novel insights into the binding mechanism of α-conotoxins to nAChRs and the methodological developments reported here open avenues for computational scanning studies of a rapidly expanding range of wild-type and chemically modified α-conotoxins.

Conotoxins are peptide toxins extracted from the venom of carnivorous marine cone snails. Members of the α-conotoxin subfamily potently block nicotinic acetylcholine receptors (nAChRs), which are involved in signal transmission between two neurons or between neurons and muscle fibers. nAChRs are important pharmacological targets due to their involvement in the transmission of pain stimuli and also in numerous neurone diseases and disorders. Their potency and specificity have led to the development of α-conotoxins as drug leads, and also to their use in the investigation of the role of nAChRs in various physiological processes. The most studied conotoxin/nAChR system, ImI/α7, was modeled in this study, and several computational methods were tested for their ability to explain the perturbations observed experimentally after introducing single point mutations into either ImI or the α7 receptor. The aim of this study was to establish a theoretical basis to rapidly identify new α-conotoxin mutants that might have improved specificity and affinity for a given receptor subtype. Furthermore, hundreds of thousands of conotoxins are predicted to exist, and computational methods are needed to help streamline the discovery of their molecular targets.

Synthetic α-conotoxin mutants as probes for studying nicotinic acetylcholine receptors and in the development of novel drug leads

α-Conotoxins are peptide neurotoxins isolated from venomous marine cone snails that are potent and selective antagonists for different subtypes of nicotinic acetylcholine receptors (nAChRs). As such, they are valuable probes for dissecting the role that nAChRs play in nervous system function. In recent years, extensive insight into the binding mechanisms of α-conotoxins with nAChRs at the molecular level has aided in the design of synthetic analogs with improved pharmacological properties. This review examines the structure-activity relationship studies involving α-conotoxins as research tools for studying nAChRs in the central and peripheral nervous systems and their use towards the development of novel therapeutics.

Characterization of a novel alpha4/4-conotoxin, Qc1.2, from vermivorous Conus quercinus

As part of continuing studies of the identification of gene organization and cloning of novel alpha-conotoxins, the first alpha4/4-conotoxin identified in a vermivorous Conus species, designated Qc1.2, was originally obtained by cDNA and genomic DNA cloning from Conus quercinus collected in the South China Sea. The predicted mature toxin of Qc1.2 contains 14 amino acid residues with two disulfide bonds (I-III, II-IV connectivity) in a native globular configuration. The mature peptide of Qc1.2 is supposed to contain an N-terminal post-translationally processed pyroglutamate residue and a free carboxyl C-terminus. This peptide was chemically synthesized and refolded for further characterization of its functional properties. The synthetic Qc1.2 has two interconvertible conformations in aqueous solution, which may be due to the cis-trans isomerization of the two successive Pro residues in its first Cys loop. Using the Xenopus oocyte heterologous expression system, Qc1.2 was shown to selectively inhibit both rat neuronal alpha3beta2 and alpha3beta4 subtypes of nicotinic acetylcholine receptors with low potency. A block of about 63% and 37% of the ACh-evoked currents was observed, respectively, and the toxin dissociated rapidly from the receptors. Compared with other characterized alpha-conotoxin members, the unusual structural features in Qc1.2 that confer to its receptor recognition profile are addressed.

Interaction of alpha-conotoxin ImII and its analogs with nicotinic receptors and acetylcholine-binding proteins: additional binding sites on Torpedo receptor

alpha-Conotoxins interact with nicotinic acetylcholine receptors (nAChRs) and acetylcholine-binding proteins (AChBPs) at the sites for agonists/competitive antagonists. alpha-Conotoxins blocking muscle-type or alpha7 nAChRs compete with alpha-bungarotoxin. However, alpha-conotoxin ImII, a close homolog of the alpha7 nAChR-targeting alpha-conotoxin ImI, blocked alpha7 and muscle nAChRs without displacing alpha-bungarotoxin (Ellison et al. 2003, 2004), suggesting binding at a different site. We synthesized alpha-conotoxin ImII, its ribbon isomer (ImIIiso), 'mutant' ImII(W10Y) and found similar potencies in blocking human alpha7 and muscle nAChRs in Xenopus oocytes. Both isomers displaced [(125)I]-alpha-bungarotoxin from human alpha7 nAChRs in the cell line GH(4)C(1) (IC(50) 17 and 23 microM, respectively) and from Lymnaea stagnalis and Aplysia californica AChBPs (IC(50) 2.0-9.0 microM). According to SPR measurements, both isomers bound to immobilized AChBPs and competed with AChBP for immobilized alpha-bungarotoxin (K(d) and IC(50) 2.5-8.2 microM). On Torpedo nAChR, alpha-conotoxin [(125)I]-ImII(W10Y) revealed specific binding (K(d) 1.5-6.1 microM) and could be displaced by alpha-conotoxin ImII, ImIIiso and ImII(W10Y) with IC(50) 2.7, 2.2 and 3.1 microM, respectively. As alpha-cobratoxin and alpha-conotoxin ImI displaced [(125)I]-ImII(W10Y) only at higher concentrations (IC(50)> or = 90 microM), our results indicate that alpha-conotoxin ImII and its congeners have an additional binding site on Torpedo nAChR distinct from the site for agonists/competitive antagonists.

Structural determinants of protein folding

The last several decades have seen an explosion of knowledge in the field of structural biology. With critical advances in spectroscopic techniques in examining structures of biomacromolecules, in maturation of molecular biology techniques, as well as vast improvements in computation prowess, protein structures are now being elucidated at an unprecedented rate. In spite of all the recent advances, the protein folding puzzle remains as one of the fundamental biochemical challenges. A facet to this empiric problem is the structural determinants of protein folding. What are the driving forces that pivot a polypeptide chain to a specific conformation amongst the vast conformation space? In this review, we shall discuss some of the structural determinants to protein folding that have been identified in the recent decades.

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.

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.

mr1e, a conotoxin from Conus marmoreus with a novel disulfide pattern

Conotoxins are well known for their highly variable structures and functions. Here we report the identification of a novel conotoxin named mr1e from Conus marmoreus. mr1e is composed of 11 amino acid residues cross-linked by two disulfide bonds (CCHSSWCKHLC). The spacing of intercysteine loops in mr1e is exactly the same as that in alpha4/3 conotoxins. However, the native mr1e peptide co-eluted on reverse-phase HPLC with the regioselectively synthesized ribbon disulfide linkage isomer (C1-C4, C2-C3) but not the globular linkage isomer (C1-C3, C2-C4). Although this peptide has the same disulfide connectivity as the chi-conotoxins, their sequences do not share significant homology. Thus, mr1e could be defined as a novel conotoxin family. By intracranial injection into mice, mr1e showed an excitatory effect. The characterization of mr1e certainly enriches our understanding of conotoxins, and also opens an avenue for further structural and functional investigation.

Folding of conotoxins: formation of the native disulfide bridges during chemical synthesis and biosynthesis of Conus peptides

Conopeptides from >700 species of predatory marine Conus snails provide an impressive molecular diversity of cysteine-rich peptides. Most of the estimated 50,000-100,000 distinct conopeptides range in size from 10 to 50 amino acid residues, often with multiple posttranslational modifications. The great majority contain from two to four disulfide bridges. As the biosynthetic and chemical production of this impressive repertoire of disulfide-rich peptides has been investigated, particularly the formation of native disulfide bridges, differences between in vivo and in vitro oxidative folding have become increasingly evident. In this article, we provide an overview of the molecular diversity of conotoxins with an emphasis on the cysteine patterns and disulfide frameworks. The conotoxin folding studies reviewed include regioselective and direct oxidation strategies, recombinant expression, optimization of folding methods, mechanisms of in vitro folding, and preliminary data on the biosynthesis of conotoxins in venom ducts. Despite these studies, how the cone snails efficiently produce properly folded conotoxins remains unanswered. As chemists continue to master oxidative folding techniques, insights gleaned from how conotoxins are folded in vivo will likely lead to the development of the new folding methods, as well as shed some light on fundamental mechanisms relevant to the protein folding problem.

Structure-guided drug design: conferring selectivity among neuronal nicotinic receptor and acetylcholine-binding protein subtypes

Neuronal nicotinic receptors, encoded by nine genes of the alpha and three of the beta type of subunits, and whose gene products assemble in distinct permutations as pentameric molecules, constitute a fertile area for structure-guided drug design. Design strategies are augmented by a wide variety of peptide, alkaloid and terpenoid toxins from various marine and terrestrial species that interact with nicotinic receptors. Also, acetylcholine-binding proteins from mollusks, as structural surrogates of the receptor that mimic its extracellular domain, provide atomic resolution templates for analysis of structure and response. Herein, we describe a structure-guided approach to nicotinic ligand design that employs crystallography of this protein as the basic template, but also takes into consideration the dynamic properties of the receptor molecules in their biological media. We present the crystallographic structures of several complexes of various agonists and antagonists that associate with the agonist site and can competitively block the action of acetylcholine. In so far as the extracellular domain is involved, we identify additional non-competitive sites at those subunit interfaces where agonists do not preferentially bind. Ligand association at these interface sites may modulate receptor function. Ligand binding is also shown by solution-based spectroscopic and spectrometric methods to affect the dynamics of discrete domains of the receptor molecule. The surrogate receptor molecules can then be employed to design ligands selective for receptor subtype through the novel methods of freeze-frame, click chemistry that uses the very structure of the target molecule as a template for synthesis of the inhibitor.

Chemical modification of conotoxins to improve stability and activity

Conotoxins are small disulfide-rich peptides from the venom of cone snails. Along with other conopeptides, they target a wide range of membrane receptors, ion channels, and transporters, and because of their high potency and selectivity for defined subtypes of these receptors, they have attracted a great deal of attention recently as leads in drug development. However, like most peptides, conopeptides potentially suffer from the disadvantages of poor absorption, poor stability, or short biological half-lives. Recently, various chemical approaches, including residue substitutions, backbone cyclization, and disulfide-bridge modification, have been reported to increase the stability of conopeptides. These manufactured interventions add to the array of post-translational modifications that occur naturally in conopeptides. They enhance the versatility of these peptides as tools in neuroscience and as drug leads.