ShK, from the sea anemone Stichodactyla helianthus, is a 35-residue disulfide-rich peptide that blocks the voltage-gated potassium channel Kv1.3 at ca. 10 pM and the related channel Kv1.1 at ca. 16 pM. We developed an analog of this peptide, ShK-186, which is currently in Phase 1b-2a clinical trials for the treatment of autoimmune diseases such as multiple sclerosis and rheumatoid arthritis. While ShK-186 displays a >100-fold improvement in selectivity for Kv1.3 over Kv1.1 compared with ShK, there is considerable interest in developing peptides with an even greater selectivity ratio. In this report, we describe several variants of ShK that incorporate p-phophono-phenylalanine at the N-terminus coupled with internal substitutions at Gln16 and Met21. In addition, we also explored the combinatorial effects of these internal substitutions with an alanine extension at the C-terminus. Their selectivity was determined by patch-clamp electrophysiology on Kv1.3 and Kv1.1 channels stably expressed in mouse fibroblasts. The peptides with an alanine extension blocked Kv1.3 at low pM concentrations and exhibited up to 2250-fold selectivity for Kv1.3 over Kv1.1. Analogs that incorporates p-phosphono-phenylalanine at the N-terminus blocked Kv1.3 with IC50s in the low pM range and did not affect Kv1.1 at concentrations up to 100 nM, displaying a selectivity enhancement of >10,000-fold for Kv1.3 over Kv1.1. Other potentially important Kv channels such as Kv1.4 and Kv1.6 were only partially blocked at 100 nM concentrations of each of the ShK analogs.
immunomodulator; T lymphocyte; potassium channel; disulfide-rich peptide; sea anemone toxin; K+ channel blocker
Conotoxins are the peptidic components of the venoms of marine cone snails (genus Conus). They are remarkably diverse in terms of structure and function. Unique potency and selectivity profiles for a range of neuronal targets have made several conotoxins valuable as research tools, drug leads and even therapeutics, and has resulted in a concerted and increasing drive to identify and characterise new conotoxins. Conotoxins are translated from mRNA as peptide precursors, and cDNA sequencing is now the primary method for identification of new conotoxin sequences. As a result, gene superfamily, a classification based on precursor signal peptide identity, has become the most convenient method of conotoxin classification. Here we review each of the described conotoxin gene superfamilies, with a focus on the structural and functional diversity present in each. This review is intended to serve as a practical guide to conotoxin superfamilies and to facilitate interpretation of the increasing number of conotoxin precursor sequences being identified by targeted-cDNA sequencing and more recently high-throughput transcriptome sequencing.
conotoxin; gene superfamily; conopeptide; Conus; venom; toxin
Apical membrane antigen 1 (AMA1) of the human malaria parasite Plasmodium falciparum has been implicated in invasion of the host erythrocyte. It interacts with malarial rhoptry neck (RON) proteins in the moving junction that forms between the host cell and the invading parasite. Agents that block this interaction inhibit invasion and may serve as promising leads for anti-malarial drug development. The invasion-inhibitory peptide R1 binds to a hydrophobic cleft on AMA1, which is an attractive target site for small molecules that block parasite invasion. In this work, truncation and mutational analyses show that Phe5-Phe9, Phe12 and Arg15 in R1 are the most important residues for high affinity binding to AMA1. These residues interact with two well-defined binding hot spots on AMA1. Computational solvent mapping reveals that one of these hot spots is suitable for small molecule targeting. We also confirm that R1 in solution binds to AMA1 with 1∶1 stoichiometry and adopts a secondary structure consistent with the major form of R1 observed in the crystal structure of the complex. Our results provide a basis for designing high affinity inhibitors of the AMA1-RON2 interaction.
µ-SIIIA, a novel µ-conotoxin from Conus striatus, appeared to be a selective blocker of tetrodotoxin-sensitive sodium channels in frog preparations. It also exhibited potent analgesic activity in mice, although its selectivity profile against mammalian sodium channels remained unknown. We have determined the structure of µ-SIIIA in aqueous solution and characterized its backbone dynamics by NMR and its functional properties electrophysiologically. Consistent with the absence of hydroxyprolines, µ-SIIIA adopts a single conformation with all peptide bonds in the trans conformation. The C-terminal region contains a well-defined helix encompassing residues 11–16, while residues 3–5 in the N-terminal region form a helix-like turn resembling 310 helix. The Trp12 and His16 side chains are in close proximity, as in the related conotoxin µ-SmIIIA, but Asn2 is further away. Dynamics measurements show that the N-terminus and Ser9 have larger magnitude motions on the sub-ns timescale, while the C-terminus is more rigid. Cys4, Trp12 and Cys13 undergo significant conformational exchange on µs - ms timescales. µ-SIIIA is a potent, nearly irreversible blocker of NaV1.2, but also blocks NaV1.4 and NaV1.6 with submicromolar potency. The selectivity profile of µ-SIIIA, including poor activity against the cardiac sodium channel, NaV1.5, is similar to that of the closely related µ-KIIIA, suggesting that the C-terminal regions of both are critical for blocking neuronal NaV1.2. The structural and functional characterization described in this paper of an analgesic µ-conotoxin that targets neuronal subtypes of mammalian sodium channels provides a basis for the design of novel analogues with an improved selectivity profile.
Among the μ-conotoxins that block vertebrate voltage-gated sodium channels (VGSCs), some have been shown to be potent analgesics following systemic administration in mice. We have determined the solution structure of a new representative of this family, μ-BuIIIB, and established its disulfide connectivities by direct mass spectrometric collision induced dissociation fragmentation of the peptide with disulfides intact. The major oxidative folding product adopts a 1-4/2-5/3-6 pattern with the following disulfide bridges: Cys5-Cys17, Cys6-Cys23 and Cys13-Cys24. The solution structure reveals that the unique N-terminal extension in μ-BuIIIB, which is also present in μ-BuIIIA and μ-BuIIIC but absent in other μ-conotoxins, forms part of a short α-helix encompassing Glu3 to Asn8. This helix is packed against the rest of the toxin and stabilized by the Cys5-Cys17 and Cys6-Cys23 disulfide bonds. As such, the side chain of Val1 is located close to the aromatic rings of Trp16 and His20, which are located on the canonical helix that displays several residues found to be essential for VGSC blockade in related μ-conotoxins. Mutations of residues 2 and 3 in the N-terminal extension enhanced the potency of μ-BuIIIB for NaV1.3. One analog, [d-Ala2]BuIIIB, showed a 40-fold increase, making it the most potent peptide blocker of this channel characterized to date and thus a useful new tool with which to characterize this channel. Based on previous results for related μ-conotoxins, the dramatic effects of mutations at the N-terminus were unanticipated, and suggest that further gains in potency might be achieved by additional modifications of this region.
The μ-conotoxin μ-KIIIA, from Conus kinoshitai, blocks mammalian neuronal voltage-gated sodium channels (VGSCs) and is a potent analgesic following systemic administration in mice. We have determined its solution structure using NMR spectroscopy. Key residues identified previously as being important for activity against VGSCs (Lys7, Trp8, Arg10, Asp11, His12 and Arg14) all reside on an α-helix with the exception of Arg14. To further probe structure-activity relationships of this toxin against VGSC subtypes, we have characterised the analogue μ-KIIIA[C1A,C9A], in which the Cys residues involved in one of the three disulfides in μ-KIIIA were replaced with Ala. Its structure is quite similar to that of μ-KIIIA, indicating that the Cys1-Cys9 disulfide bond could be removed without any significant distortion of the α-helix bearing the key residues. Consistent with this, μ-KIIIA[C1A,C9A] retained activity against VGSCs, with its rank order of potency being essentially the same as that of μ-KIIIA, namely, NaV1.2 > NaV1.4 > NaV1.7 ≥ NaV1.1 > NaV1.3 > NaV1.5. Kinetics of block were obtained for NaV1.2, NaV1.4 and NaV1.7, and in each case both kon and koff values of μ-KIIIA[C1A,C9A] were larger than those of μ-KIIIA. Our results show that the key residues for VGSC binding lie mostly on an α-helix and that the first disulfide bond can be removed without significantly affecting the structure of this helix, although the modification accelerates the on- and off-rates of the peptide against all tested VGSC subtypes. These findings lay the groundwork for the design of minimized peptides and helical mimetics as novel analgesics.
In the preparation of synthetic conotoxins containing multiple disulfide bonds, oxidative folding can produce numerous permutations of disulfide bond connectivities. Establishing the native disulfide connectivities thus presents a significant challenge when the venom-derived peptide is not available, as is increasingly the case when conotoxins are identified from cDNA sequences. Here, we investigate the disulfide connectivity of μ-conotoxin KIIIA, which was predicted originally to have a [C1-C9,C2-C15,C4-C16] disulfide pattern based on homology with closely-related μ-conotoxins. The two major isomers of synthetic μ-KIIIA formed during oxidative folding were purified and their disulfide connectivities mapped by direct mass spectrometric CID fragmentation of the disulfide-bonded polypeptides. Our results show that the major oxidative folding product adopts a [C1-C15,C2-C9,C4-C16] disulfide connectivity, while the minor product adopts a [C1-C16,C2-C9,C4-C15] connectivity. Both of these peptides were potent blockers of NaV1.2 (Kd 5 and 230 nM, respectively). The solution structure for μ-KIIIA based on NMR data was recalculated with the [C1-C15,C2-C9,C4-C16] disulfide pattern; its structure was very similar to the μ-KIIIA structure calculated with the incorrect [C1-C9,C2-C15,C4-C16] disulfide pattern, with an α-helix spanning residues 7–12. In addition, the major folding isomers of μ-KIIIB, an N-terminally extended isoform of μ-KIIIA identified from its cDNA sequence, were isolated. These folding products had the same disulfide connectivities as for μ-KIIIA, and both blocked NaV1.2 (Kd 470 and 26 nM, respectively). Our results establish that the preferred disulfide pattern of synthetic μ-KIIIA/μ-KIIIB folded in vitro is 1-5/2-4/3-6 but that other disulfide isomers are also potent sodium channel blockers. These findings raise questions about the disulfide pattern(s) of μ-KIIIA in the venom of Conus kinoshitai; indeed, the presence of multiple disulfide isomers in the venom could provide a means to further expand the snail's repertoire of active peptides.
HsTX1 toxin, from the scorpion Heterometrus spinnifer, is a 34-residue, C-terminally amidated peptide cross-linked by four disulfide bridges. Here we describe new HsTX1 analogues with an Ala, Phe, Val or Abu substitution at position 14. Complexes of HsTX1 with the voltage-gated potassium channels Kv1.3 and Kv1.1 were created using docking and molecular dynamics simulations, then umbrella sampling simulations were performed to construct the potential of mean force (PMF) of the ligand and calculate the corresponding binding free energy for the most stable configuration. The PMF method predicted that the R14A mutation in HsTX1 would yield a > 2 kcal/mol gain for the Kv1.3/Kv1.1 selectivity free energy relative to the wild-type peptide. Functional assays confirmed the predicted selectivity gain for HsTX1[R14A] and HsTX1[R14Abu], with an affinity for Kv1.3 in the low picomolar range and a selectivity of more than 2,000-fold for Kv1.3 over Kv1.1. This remarkable potency and selectivity for Kv1.3, which is significantly up-regulated in activated effector memory cells in humans, suggest that these analogues represent valuable leads in the development of therapeutics for autoimmune diseases.
Animal venoms represent a vast library of bioactive peptides and proteins with proven potential, not only as research tools but also as drug leads and therapeutics. This is illustrated clearly by marine cone snails (genus Conus), whose venoms consist of mixtures of hundreds of peptides (conotoxins) with a diverse array of molecular targets, including voltage- and ligand-gated ion channels, G-protein coupled receptors and neurotransmitter transporters. Several conotoxins have found applications as research tools, with some being used or developed as therapeutics. The primary objective of this study was the large-scale discovery of conotoxin sequences from the venom gland of an Australian cone snail species, Conus victoriae. Using cDNA library normalization, high-throughput 454 sequencing, de novo transcriptome assembly and annotation with BLASTX and profile hidden Markov models, we discovered over 100 unique conotoxin sequences from 20 gene superfamilies, the highest diversity of conotoxins so far reported in a single study. Many of the sequences identified are new members of known conotoxin superfamilies, some help to redefine these superfamilies and others represent altogether new classes of conotoxins. In addition, we have demonstrated an efficient combination of methods to mine an animal venom gland and generate a library of sequences encoding bioactive peptides.
ShK, a 35-residue peptide from a sea anemone, is a potent blocker of potassium channels. Here we describe a new ShK analogue with an additional C-terminus Lys residue and amide. ShK-K-amide is a potent blocker of Kv1.3 and, in contrast to ShK and ShK-amide, is selective for Kv1.3. To understand this selectivity, we created complexes of ShK-K-amide with Kv1.3 and Kv1.1 using docking and molecular dynamics simulations, then performed umbrella sampling simulations to construct the potential of mean force of the ligand and calculate the corresponding binding free energy for the most stable configuration. The results agree well with experimental data.
ShK; C-terminal amide; potassium channel; electrophysiology; potential of mean force; umbrella sampling
The voltage-gated potassium channel Kv1.3 is a well-established target for treatment of autoimmune diseases. ShK peptide from a sea anemone is one of the most potent blockers of Kv1.3 but its application as a therapeutic agent for autoimmune diseases is limited by its lack of selectivity against other Kv channels, in particular Kv1.1. Accurate models of Kv1.x-ShK complexes suggest that specific charge mutations on ShK could considerably enhance its specificity for Kv1.3. Here we evaluate the K18A mutation on ShK, and calculate the change in binding free energy associated with this mutation using the path-independent free energy perturbation and thermodynamic integration methods, with a novel implementation that avoids convergence problems. To check the accuracy of the results, the binding free energy differences were also determined from path-dependent potential of mean force calculations. The two methods yield consistent results for the K18A mutation in ShK and predict a 2 kcal/mol gain in Kv1.3/Kv1.1 selectivity free energy relative to wild-type peptide. Functional assays confirm the predicted selectivity gain for ShK[K18A] and suggest that it will be a valuable lead in the development of therapeutics for autoimmune diseases.
The polypeptide toxin ShK is a potent blocker of Kv1.3 potassium channels, which play a crucial role in the activation of human effector memory T-cells (TEM). Selective blockers constitute valuable therapeutic leads for the treatment of autoimmune diseases mediated by TEM cells, such as multiple sclerosis, rheumatoid arthritis, and type-1 diabetes. We have established a recombinant peptide expression system in order to generate isotopically-labelled ShK and various ShK analogues for in-depth biophysical and pharmacological studies. ShK was expressed as a thioredoxin fusion protein in Escherichia coli BL21 (DE3) cells and purified initially by Ni2+ iminodiacetic acid affinity chromatography. The fusion protein was cleaved with enterokinase and purified to homogeneity by reverse-phase HPLC. NMR spectra of 15N-labelled ShK were similar to those reported previously for the unlabelled synthetic peptide, confirming that recombinant ShK was correctly folded. Recombinant ShK blocked Kv1.3 channels with a Kd of 25 pM and inhibited the proliferation of human and rat T lymphocytes with a preference for TEM cells, with similar potency to synthetic ShK in all assays. This expression system also enables the efficient production of 15N-labelled ShK for NMR studies of peptide dynamics and of the interaction of ShK with Kv1.3 channels.
ShK; potassium channels; protein expression; isotopic labelling; NMR
Suppressor of Cytokine Signaling (SOCS)5 is thought to act as a tumour suppressor through negative regulation of JAK/STAT and epidermal growth factor (EGF) signaling. However, the mechanism/s by which SOCS5 acts on these two distinct pathways is unclear. We show for the first time that SOCS5 can interact directly with JAK via a unique, conserved region in its N-terminus, which we have termed the JAK interaction region (JIR). Co-expression of SOCS5 was able to specifically reduce JAK1 and JAK2 (but not JAK3 or TYK2) autophosphorylation and this function required both the conserved JIR and additional sequences within the long SOCS5 N-terminal region. We further demonstrate that SOCS5 can directly inhibit JAK1 kinase activity, although its mechanism of action appears distinct from that of SOCS1 and SOCS3. In addition, we identify phosphoTyr317 in Shc-1 as a high-affinity substrate for the SOCS5-SH2 domain and suggest that SOCS5 may negatively regulate EGF and growth factor-driven Shc-1 signaling by binding to this site. These findings suggest that different domains in SOCS5 contribute to two distinct mechanisms for regulation of cytokine and growth factor signaling.
Peptide toxins typically bind to their target ion channels or receptors with high potency and selectivity, making them attractive leads for therapeutic development. In some cases the native peptide as it is found in the venom from which it originates can be used directly, but in many instances it is desirable to truncate and/or stabilize the peptide to improve its therapeutic properties. A complementary strategy is to display the key residues that make up the pharmacophore of the peptide toxin on a non-peptidic scaffold, thereby creating a peptidomimetic. This review exemplifies these approaches with peptide toxins from marine organisms, with a particular focus on conotoxins.
peptide toxin; peptidomimetic; ion channel; pain; cone snail
Merozoite surface protein 2 (MSP2) is an abundant glycosylphosphatidylinositol (GPI)-anchored protein of Plasmodium falciparum, which is a potential component of a malaria vaccine. As all forms of MSP2 can be categorized into two allelic families, a vaccine containing two representative forms of MSP2 may overcome the problem of diversity in this highly polymorphic protein. Monomeric recombinant MSP2 is an intrinsically unstructured protein, but its conformational properties on the merozoite surface are unknown. This question is addressed here by analyzing the 3D7 and FC27 forms of recombinant and parasite MSP2 using a panel of monoclonal antibodies raised against recombinant MSP2. The epitopes of all antibodies, mapped using both a peptide array and by nuclear magnetic resonance (NMR) spectroscopy on full-length recombinant MSP2, were shown to be linear. The antibodies revealed antigenic differences, which indicate that the conserved N- and C-terminal regions, but not the central variable region, are less accessible in the parasite antigen. This appears to be an intrinsic property of parasite MSP2 and is not dependent on interactions with other merozoite surface proteins as the loss of some conserved-region epitopes seen using the immunofluorescence assay (IFA) on parasite smears was also seen on Western blot analyses of parasite lysates. Further studies of the structural basis of these antigenic differences are required in order to optimize recombinant MSP2 constructs being evaluated as potential vaccine components.
Suppressor of cytokine signalling 3 (SOCS3) is responsible for regulating the cellular response to a variety of cytokines, including interleukin 6 and leukaemia inhibitory factor. Identification of the SOCS box domain led to the hypothesis that SOCS3 can associate with functional E3 ubiquitin ligases and thereby induce the degradation of bound signalling proteins. This model relies upon an interaction between the SOCS box, elonginBC and a cullin protein that forms the E3 ligase scaffold. We have investigated this interaction in vitro using purified components and show that SOCS3 binds to elonginBC and cullin5 with high affinity. The SOCS3–elonginBC interaction was further characterised by determining the solution structure of the SOCS box–elonginBC ternary complex and by deletion and alanine scanning mutagenesis of the SOCS box. These studies revealed that conformational flexibility is a key feature of the SOCS–elonginBC interaction. In particular, the SOCS box is disordered in isolation and only becomes structured upon elonginBC association. The interaction depends upon the first 12 residues of the SOCS box domain and particularly on a deeply buried, conserved leucine. The SOCS box, when bound to elonginBC, binds tightly to cullin5 with 100 nM affinity. Domains upstream of the SOCS box are not required for elonginBC or cullin5 association, indicating that the SOCS box acts as an independent binding domain capable of recruiting elonginBC and cullin5 to promote E3 ligase formation.
SOCS; cytokine signalling; ubiquitin ligase; elongin; cullin
Janus kinases (JAKs) are key effectors in controlling immune responses and maintaining hematopoiesis. SOCS3 (Suppressor of Cytokine Signaling-3) is a major regulator of JAK signaling and here we investigate the molecular basis of its mechanism of action. We found that SOCS3 bound and directly inhibited the catalytic domains of JAK1, JAK2 and TYK2, but not JAK3 via an evolutionarily conserved motif unique to JAKs. Mutation of this motif led to the formation of an active kinase that could not be inhibited by SOCS3. Surprisingly, we found that SOCS3 simultaneously bound JAK and the cytokine receptor to which it is attached, revealing how specificity is generated in SOCS action and explaining why SOCS3 inhibits only a subset of cytokines. Importantly, SOCS3 inhibited JAKs via a non-competitive mechanism, making it a template for the development of specific and effective inhibitors to treat JAK-based immune and proliferative diseases.
Conotoxins (CTxs) selectively target a range of ion channels and receptors, making them widely used tools for probing nervous system function. Conotoxins have been previously grouped into superfamilies according to signal sequence and into families based on their cysteine framework and biological target. Here we describe the cloning and characterization of a new conotoxin, from Conus vexillum, named αB-conotoxin VxXXIVA. The peptide does not belong to any previously described conotoxin superfamily and its arrangement of Cys residues is unique among conopeptides. Moreover, in contrast to previously characterized conopeptide toxins, which are expressed initially as prepropeptide precursors with a signal sequence, a ‘‘pro’’ region, and the toxin-encoding region, the precursor sequence of αB-VxXXIVA lacks a ‘‘pro’’ region. The predicted 40-residue mature peptide, which contains four Cys, was synthesized in each of the three possible disulfide arrangements. Investigation of the mechanism of action of αB-VxXXIVA revealed that the peptide is a nicotinic acetylcholine receptor (nAChR) antagonist with greatest potency against the α9α10 subtype. 1H nuclear magnetic resonance (NMR) spectra indicated that all three αB-VxXXIVA isomers were poorly structured in aqueous solution. This was consistent with circular dichroism (CD) results which showed that the peptides were unstructured in buffer, but adopted partially helical conformations in aqueous trifluoroethanol (TFE) solution. The α9α10 nAChR is an important target for the development of analgesics and cancer chemotherapeutics, and αB-VxXXIVA represents a novel ligand with which to probe the structure and function of this protein.
μ-Conotoxin KIIIA (μ-KIIIA) blocks mammalian voltage-gated sodium channels (VGSCs) and is a potent analgesic following systemic administration in mice. Previous structure-activity studies of μ-KIIIA identified a helical pharmacophore for VGSC blockade. This suggested a route for designing truncated analogues of μ-KIIIA by incorporating the key residues into an α-helical scaffold. As (i, i+4) lactam bridges constitute a proven approach for stabilizing α-helices, we designed and synthesized six truncated analogues of μ-KIIIA containing single lactam bridges at various locations. The helicity of these lactam analogues was analysed by NMR spectroscopy, and their activities were tested against mammalian VGSC subtypes NaV1.1 through 1.7. Two of the analogues, Ac-cyclo9/13[Asp9,Lys13]KIIIA7–14 and Ac-cyclo9/13[Lys9,Asp13]KIIIA7–14, displayed µM activity against VGSC subtypes NaV1.2 and NaV1.6; importantly, the subtype selectivity profile for these peptides matched that of μ-KIIIA. Our study highlights structure-activity relationships within these helical mimetics and provides a basis for the design of additional truncated peptides as potential analgesics.
Merozoite surface protein 2 (MSP2), one of the most abundant proteins on the surface of Plasmodium falciparum merozoites, is a promising malaria vaccine candidate. MSP2 is intrinsically unstructured and forms amyloid-like fibrils in solution. As this propensity of MSP2 to form fibrils in solution has the potential to impede its development as a vaccine candidate, finding an inhibitor that inhibits fibrillogenesis may enhance vaccine development. We have shown previously that EGCG inhibits the formation of MSP2 fibrils. Here we show that EGCG can alter the β-sheet-like structure of the fibril and disaggregate pre-formed fibrils of MSP2 into soluble oligomers. The fibril remodelling effects of EGCG and other flavonoids were characterized using Thioflavin T fluorescence assays, electron microscopy and other biophysical methods.
malaria; vaccine; MSP2; amyloid; flavonoid; EGCG
CCR7− effector memory T (TEM) lymphocytes are involved in autoimmune diseases such as multiple sclerosis, type 1 diabetes mellitus and rheumatoid arthritis. These cells express Kv1.3 potassium channels that play a major role in their activation. Blocking these channels preferentially inhibits the activation of CCR7− TEM cells, with little or no effects on CCR7+ naïve and central memory T cells. Blockers of lymphocyte Kv1.3 channels therefore show considerable potential as therapeutics for autoimmune diseases. ShK, a 35-residue polypeptide isolated from the Caribbean sea anemone Stichodactyla helianthus, blocks Kv1.3 channels at picomolar concentrations. Although ShK was effective in treating rats with delayed type hypersensitivity and a model of multiple sclerosis, it lacks selectivity for Kv1.3 channels over closely-related Kv1 channels. Extensive mutagenesis studies combined with elucidation of the structure of ShK led to models of ShK docked with the channel. This knowledge was valuable in the development of new ShK analogs with improved selectivity and increasing stability, which have proven efficacious in preventing and/or treating animal models of delayed type hypersensitivity, type 1 diabetes, rheumatoid arthritis, and multiple sclerosis without inducing generalized immunosuppression. They are currently undergoing further evaluation as potential immunomodulators for the treatment of autoimmune diseases.
immunomodulator; autoimmunity; venom peptide; Kv1.3 channel; lymphocyte
Electrophysiological and pharmacological studies coupled with molecular identification have revealed a unique network of ion channels—Kv1.3, KCa3.1, CRAC (Orai1 + Stim1), TRPM7, Clswell—in lymphocytes that initiates and maintains the calcium signaling cascade required for activation. The expression pattern of these channels changes during lymphocyte activation and differentiation, allowing the functional network to adapt during an immune response. The Kv1.3 channel is of interest because it plays a critical role in subsets of T and B lymphocytes implicated in autoimmune disorders. The ShK toxin from the sea anemone Stichodactyla helianthus is a potent blocker of Kv1.3. ShK-186, a synthetic analog of ShK, is being developed as a therapeutic for autoimmune diseases, and is scheduled to begin first-in-man phase-1 trials in 2011. This review describes the journey that has led to the development of ShK-186.
Apical membrane antigen 1 (AMA1) is essential for malaria parasite invasion of erythrocytes and is therefore an attractive target for drug development. Peptides that bind AMA1 have been identified from random peptide libraries expressed on the surface of phage. Of these, R1, which binds to a hydrophobic ligand binding site on AMA1, was a particularly potent inhibitor of parasite invasion of erythrocytes in vitro. The solution structure of R1 contains a turn-like conformation between residues 5–10. Here the importance of residues in this turn-like structure for binding to AMA1 was examined by site-directed mutagenesis and NMR spectroscopy. The peptide was expressed as a fusion protein following replacement of Met16 by Leu in order to accommodate cyanogen bromide cleavage. This modified peptide (R2) displayed the same affinity for AMA1 as R1, showing that the identity of the side chain at position 16 was not critical for binding. Substitution of Phe5, Pro7, Leu8, and Phe9 with alanine led to significant (7.5- to > 350-fold) decreases in affinity for AMA1. Comparison of backbone amide and CαH chemical shifts for these R2 analogues with corresponding values for R2 showed no significant changes, with the exception of R2(P7A), where slightly larger differences were observed, particularly for residues flanking position 7. The absence of significant changes in the secondary chemical shifts suggests that these mutations had little effect on the solution conformation of R2. The identification of a non-polar region of these peptides containing residues essential for AMA1 binding establishes a basis for the design of anti-malarial drugs based on R1 mimetics.
apical membrane antigen 1; malaria; Plasmodium falciparum; peptide expression; NMR; structure; dynamics
Insulin-like growth factor binding proteins (IGFBP-1 to -6) bind insulin-like growth factors-I and -II (IGF-I and IGF-II) with high affinity. These binding proteins maintain IGFs in the circulation and direct them to target tissues, where they promote cell growth, proliferation, differentiation, and survival via the type 1 IGF receptor. IGFBPs also interact with many other molecules, which not only influence their modulation of IGF action but also mediate IGF-independent activities that regulate processes such as cell migration and apoptosis by modulating gene transcription. IGFBPs-1 to -6 are structurally similar proteins consisting of three distinct domains, N-terminal, linker, and C-terminal. There have been major advances in our understanding of IGFBP structure in the last decade and a half. While there is still no structure of an intact IGFBP, several structures of individual N- and C-domains have been solved. The structure of a complex of N-BP-4:IGF-I:C-BP-4 has also been solved, providing a detailed picture of the structural features of the IGF binding site and the mechanism of binding. Structural studies have also identified features important for interaction with extracellular matrix components and integrins. This review summarizes structural studies reported so far and highlights features important for binding not only IGF but also other partners. We also highlight future directions in which structural studies will add to our knowledge of the role played by the IGFBP family in normal growth and development, as well as in disease.
insulin-like growth factor; IGF binding protein; protein structure
Merozoite surface protein 2 (MSP2) from the human malaria parasite Plasmodium falciparum is expressed as a GPI-anchored protein on the merozoite surface. MSP2 is assumed to have a role in erythrocyte invasion and is a leading vaccine candidate. Recombinant MSP2 forms amyloid-like fibrils upon storage, as do peptides corresponding to sequences in the conserved N-terminal region, which constitutes the structural core of fibrils formed by full-length MSP2. We have investigated the roles of individual residues in fibril formation and local ordered structure in two peptides, a recombinant 25-residue peptide corresponding to the entire N-terminal domain of mature MSP2 and an 8-residue peptide from the central region of this domain (residues 8–15). Both peptides formed fibrils that were similar to amyloid-like fibrils formed by full-length MSP2. Phe11 and Ile12 have important roles both in stabilising local structure in these peptides and promoting fibril formation; the F11A and I12A mutants of MSP28–15 were essentially unstructured in solution and fibril formation at pH 7.4 and 4.7 was markedly retarded. The T10A mutant showed intermediate behaviour, having a less well-defined structure than wild-type and slower fibril formation at pH 7.4. The mutation of Phe11 and Ile12 in MSP21–25 significantly retarded but did not abolish fibril formation, indicating that these residues also play a key role in fibril formation by the entire N-terminal conserved region. These mutations had little effect on the aggregation of full-length MSP2, however, suggesting that regions outside the conserved N-terminus have unanticipated importance for fibril formation in the full-length protein.
malaria; fibril; amyloid; nuclear magnetic resonance; structure