Insecticidal peptides from the theraposid spider Brachypelma albiceps: an NMR-based model of Ba2

Soluble venom and purified fractions of the theraposid spider Brachypelma albiceps were screened for insecticidal peptides based on toxicity to crickets. Two insecticidal peptides, named Ba1 and Ba2, were obtained after the soluble venom was separated by high performance liquid chromatography and cation exchange chromatography. The two insecticidal peptides contain 39 amino acid residues and three disulfide bonds, and based on their amino acid sequence, they are highly identical to the insecticidal peptides from the theraposid spiders Aphonopelma sp. from the USA and Haplopelma huwenum from China indicating a relationship among these genera. Although Ba1 and Ba2 were not able to modify currents in insect and vertebrate cloned voltage-gated sodium ion channels, they have noteworthy insecticidal activities compared to classical arachnid insecticidal toxins indicating that they might target unknown receptors in insect species. The most abundant insecticidal peptide Ba2 was submitted to NMR spectroscopy to determine its 3-D structure; a remarkable characteristic of Ba2 is a cluster of basic residues, which might be important for receptor recognition.

Induction of morphological changes in model lipid membranes and the mechanism of membrane disruption by a large scorpion-derived pore-forming peptide

The membrane disruption mechanism of pandinin 1 (pin1), an antimicrobial peptide isolated from the venom of the African scorpion, was studied using 31P, 13C, 1H solid-state and multidimensional solution-state NMR spectroscopy. A high-resolution NMR solution structure of pin1 showed that the two distinct alpha-helical regions move around the central hinge region, which contains Pro19. 31P NMR spectra of lipid membrane in the presence of pin1, at various temperatures, showed that pin1 induces various lipid phase behaviors depending on the acyl chain length and charge of phospholipids. Notably, it was found that pin1 induced formation of the cubic phase in shorter lipid membranes above Tm. Further, the 13C NMR spectra of pin1 labeled at Leu28 under magic angle spinning (MAS) indicated that the motion of pin1 bound to the lipid bilayer was very slow, with a correlation time of the order of 10(-3) s. 31P NMR spectra of dispersions of four saturated phosphatidyl-cholines in the presence of three types of pin1 derivatives, [W4A, W6A, W15A]-pin1, pin1(1-18), and pin1(20-44), at various temperatures demonstrated that all three pin1 derivatives have a reduced ability to trigger the cubic phase. 13C chemical shift values for pin1(1-18) labeled at Val3, Ala10, or Ala11 under static or slow MAS conditions indicate that pin1(1-18) rapidly rotates around the average helical axis, and the helical rods are inclined at approximately 30 degrees to the lipid long axis. 13C chemical shift values for pin1(20-44) labeled at Gly25, Leu28, or Ala31 under static conditions indicate that pin1(20-44) may be isotropically tumbling. 1H MAS chemical shift measurements suggest that pin1 is located at the membrane-water interface approximately parallel to the bilayer surface. Solid-state NMR results correlated well with the observed biological activity of pin1 in red blood cells and bacteria.

Solution structure of two insect-specific spider toxins and their pharmacological interaction with the insect voltage-gated Na+ channel

Delta-paluIT1 and delta-paluIT2 are toxins purified from the venom of the spider Paracoelotes luctuosus. Similar in sequence to mu-agatoxins from Agelenopsis aperta, their pharmacological target is the voltage-gated insect sodium channel, of which they alter the inactivation properties in a way similar to alpha-scorpion toxins, but they bind on site 4 in a way similar to beta-scorpion toxins. We determined the solution structure of the two toxins by use of two-dimensional nuclear magnetic resonance (NMR) techniques followed by distance geometry and molecular dynamics. The structures of delta-paluIT1 and delta-paluIT2 belong to the inhibitory cystine knot structural family, i.e. a compact disulfide-bonded core from which four loops emerge. Delta-paluIT1 and delta-paluIT2 contain respectively two- and three-stranded anti-parallel beta-sheets as unique secondary structure. We compare the structure and the electrostatic anisotropy of those peptides to other sodium and calcium channel toxins, analyze the topological juxtaposition of key functional residues, and conclude that the recognition of insect voltage-gated sodium channels by these toxins involves the beta-sheet, in addition to loops I and IV. Besides the position of culprit residues on the molecular surface, difference in dipolar moment orientation is another determinant of receptor binding and biological activity differences. We also demonstrate by electrophysiological experiments on the cloned insect voltage-gated sodium channel, para, heterologuously co-expressed with the tipE subunit in Xenopus laevis oocytes, that delta-paluIT1 and delta-paluIT2 procure an increase of Na+ current. delta-PaluIT1-OH seems to have less effect when the same concentrations are used.

Selective Inhibition of Trypsins by Insect Peptides: Role of P6−P10 Loop

PMP-D2 and HI, two peptides from Locusta migratoria, were shown to belong to the family of tight-binding protease inhibitors. However, they interact weakly with bovine trypsin (K(i) around 100 nM) despite a trypsin-specific Arg at the primary specificity site P1. Here we demonstrate that they are potent inhibitors of midgut trypsins isolated from the same insect and of a fungal trypsin from Fusarium oxysporum (K(i) <or= 0.02 nM). Therefore, they display a selectivity not existing for the parent chymotrypsin inhibitor PMP-C. By NMR, we demonstrate that HI possesses a highly rigid structure similar to the crystal structure of a variant of PMP-D2 in complex with bovine alpha-chymotrypsin. The main difference with PMP-C is located in the region from residues 20 to 24 (positions P6-P10) that interacts with the loop containing Gly173 in chymotrypsin. The corresponding residue in mammalian trypsins is always a proline that may generate a steric clash with the inhibitor. The residues thought to confer selectivity were mutated with PMP-C as a model. The resulting analogue PMP-D2(K10W,P21A,W25A) loses some activity toward insect and fungal trypsins but is a more potent inhibitor of mammalian trypsins, corresponding to a decrease of selectivity. This work represents a first attempt in tuning the selectivity of natural peptidic serine protease inhibitors by mutating residues out of the reactive loop (P3-P'3).

Evolutionary origin of inhibitor cystine knot peptides

The inhibitor cystine knot (ICK) fold is an evolutionarily conserved structural motif shared by a large group of polypeptides with diverse sequences and bioactivities. Although found in different phyla (animal, plant, and fungus), ICK peptides appear to be most prominent in venoms of cone snail and spider. Recently, two scorpion toxins activating a calcium release channel have been found to adopt an ICK fold. We have isolated and identified both cDNA and genomic clones for this family of ICK peptides from the scorpion Opistophthalmus carinatus. The gene characterized by three well-delineated exons respectively coding for three structural and functional domains in the toxin precursors illustrates the correlation between exon and module as suggested by the "exon theory of genes." Based on the analysis of precursor organization and gene structure combined with the 3-D fold and functional data, our results highlight a common evolutionary origin for ICK peptides from animals. In contrast, ICK peptides from plant and fungus might be independently evolved from another ancestor.

Synthesis, characterization, and application of cy-dye- and alexa-dye-labeled hongotoxin(1) analogues. The first high affinity fluorescence probes for voltage-gated K+ channels

Hongotoxin(1) (HgTX(1)), a 39-residue peptide recently isolated from the venom of Centruroides limbatus, blocks the voltage-gated K+ channels K(v)1.1, K(v)1.2, and K(v)1.3 at picomolar toxin concentrations (Koschak, A., Bugianesi, R. M., Mitterdorfer, J., Kaczorowski, G. J., Garcia, M. L., and Knaus, H. G. (1998) J. Biol. Chem. 273, 2639-2644). In this report, we determine the three-dimensional structure of HgTX(1) using NMR spectroscopy (PDB-code: 1HLY). HgTX(1) was found to possess a structure similar to previously characterized K+ channel toxins (e.g. margatoxin) consisting of a three-stranded antiparallel beta-sheet (residues 2-4, 26-30, and 33-37) and a helical conformation (part 3(10) helix and part alpha helix; residues 10-20). Due to the importance of residue Lys-28 for high-affinity interaction with the respective channels, lysine-reactive fluorescence dyes cannot be used to label wild-type HgTX(1). On the basis of previous studies (see above) and our NMR data, a HgTX(1) mutant (HgTX(1)-A19C) was engineered, expressed, and purified. HgTX(1)-A19C-SH was labeled using sulfhydryl-reactive Cy3-, Cy5-, and Alexa-dyes. Pharmacological characterization of fluorescently labeled HgTX(1)-A19C in radioligand binding studies indicated that these hongotoxin(1) analogues retain high-affinity for voltage-gated K+ channels and a respective pharmacological profile. Cy3- and Alexa-dye-labeled hongotoxin(1) analogues were used to investigate the localization of K+ channels in brain sections. The distribution of toxin binding closely follows the distribution of K(v)1.2 immunoreactivity with the highest expression levels in the cerebellar Purkinje cell layer. Taken together, these results demonstrate that fluorescently labeled HgTX(1) analogues comprise novel probes to characterize a subset of voltage-gated K+ channels.

The location of the ligand-binding site of carbohydrate-binding modules that have evolved from a common sequence is not conserved

Polysaccharide-degrading enzymes are generally modular proteins that contain non-catalytic carbohydrate-binding modules (CBMs), which potentiate the activity of the catalytic module. CBMs have been grouped into sequence-based families, and three-dimensional structural data are available for half of these families. Clostridium thermocellum xylanase 11A is a modular enzyme that contains a CBM from family 6 (CBM6), for which no structural data are available. We have determined the crystal structure of this module to a resolution of 2.1 A. The protein is a beta-sandwich that contains two potential ligand-binding clefts designated cleft A and B. The CBM interacts primarily with xylan, and NMR spectroscopy coupled with site-directed mutagenesis identified cleft A, containing Trp-92, Tyr-34, and Asn-120, as the ligand-binding site. The overall fold of CBM6 is similar to proteins in CBM families 4 and 22, although surprisingly the ligand-binding site in CBM4 and CBM22 is equivalent to cleft B in CBM6. These structural data define a superfamily of CBMs, comprising CBM4, CBM6, and CBM22, and demonstrate that, although CBMs have evolved from a relatively small number of ancestors, the structural elements involved in ligand recognition have been assembled at different locations on the ancestral scaffold.

Solution structure of Ptu1, a toxin from the assassin bug Peirates turpis that blocks the voltage-sensitive calcium channel N-type

Ptu1 is a toxin from the assassin bug Peirates turpis which has been demonstrated to bind reversibly the N-type calcium channels and to have lower affinity than the omega-conotoxin MVIIA. We have determined the solution structure of Ptu1 by use of conventional two-dimensional NMR techniques followed by distance-geometry and molecular dynamics. The calculated structure of Ptu1 belongs to the inhibitory cystin knot structural family (ICK) that consists of a compact disulfide-bonded core from which four loops emerge. Analysis of the 25 converged solutions indicates that the molecular structure of Ptu1 contains a 2-stranded antiparallel beta-sheet (residues 24-27 and 31-34) as the only secondary structure. The loop 2 that has been described to be critical for the binding of the toxin on the channel is similar in Ptu1 and MVIIA. In this loop, the critical residue, Tyr13, in MVIIA is retrieved in Ptu1 as Phe13, but the presence of an acidic residue (Asp16) in Ptu1 could disturb the binding of Ptu1 on the channel and could explain the lower affinity of Ptu1 toward the N-type calcium channel compared to the one of MVIIA. Analysis of the electrostatic charge's repartition gives some insights about the importance of the basic residues, which could interact with acidic residues of the channel and then provide a stabilization of the toxin on the channel.

Structural basis for alpha-K toxin specificity for K+ channels revealed through the solution 1H NMR structures of two noxiustoxin-iberiotoxin chimeras

Noxiustoxin (NxTX) and iberiotoxin (IbTX) exhibit extraordinary differences in their ability to inhibit current through the large-conductance calcium-activated potassium (maxi-K) and voltage-gated potassium (Kv1.3) channels. The three-dimensional structures of NxTX and IbTX display differences in their alpha/beta turn and in the length of the alpha-carbon backbone. To understand the role of these differences in defining specificity, we constructed two NxTX mutants, NxTX-IbTX I and NxTX-IbTX II, and solved their solution structures by 1H NMR spectroscopy. For NxTX-IbTX I, seven amino acids comprising the alpha/beta turn in NxTX are replaced with six amino acids from the corresponding alpha/beta turn in IbTX (NxTX-YGSSAGA21-27FGVDRF21-26). In addition, NxTX-IbTX II contained the S14W mutation and deletion of the N- and C-terminal residues. Both NxTX-IbTX I and NxTX-IbTX II exhibit an alpha/beta scaffold structure typical of the alpha-K channel toxins. A helix is present from residues 10 to 19 in NxTX-IbTX I and from residues 13 to 19 in NxTX-IbTX II. The beta-sheet, defined by three antiparallel strands, is one residue longer in NxTX-IbTX I relative to NxTX-IbTX II. The two toxins also differ in the structure of the alpha/beta turn with NxTX-IbTX I resembling that of IbTX and with NxTX-IbTX II resembling that of NxTX. These differences in the beta-sheet and alpha/beta turn alter the dimensions of the toxin-channel interaction surface and provide insight into how these NxTX mutations alter K+ channel specificity for the maxi-K and Kv1.3 channels.

Chemosensory protein from the moth Mamestra brassicae. Expression and secondary structure from 1H and 15N NMR

A group of ubiquitous small proteins (average 13 kDa) has been isolated from several sensory organs of a wide range of insect species. They are believed to be involved in chemical communication and perception (olfaction or taste) and have therefore been called chemo-sensory proteins (CSPs). Several CSPs have been identified in the antennae and proboscis of the moth Mamestra brassicae. We have expressed one of the antennal proteins (CSPMbraA6) in large quantities as a soluble recombinant protein in Escherichia coli periplasm. This 112-residue protein is a highly soluble monomer of 13 072 Da with a pI of 5.5. NMR data (1H and 15N) indicate that CSPMbraA6 is well folded and contains seven alpha helices (59 amino acids) and two short extended structures (12 amino acids) from positions 5 to 10 and from 107 to 112. Thirty-seven amino acids are involved in beta turns and coiled segments and four amino acids are not assigned in the NMR spectra (the N-terminus and the residue 52 in the loop 48-53), probably due to their mobility. This is the first report on the expression and structural characterization of a recombinant CSP.

Streptomyces matensis laminaripentaose hydrolase is an 'inverting' beta-1,3-glucanase

The laminaripentaose-producing beta-1,3-glucanase of Streptomyces matensis is a member of the glycoside hydrolase family GH-64. We have constructed and purified a recombinant hexahistidine-tagged form of the enzyme for characterisation. The enzyme, which exists as a monomer in solution, hydrolyses beta-1,3-glucan by a mechanism leading to overall inversion of the anomeric configuration. This is the first determination of the mechanism prevailing in glycoside hydrolase family GH-64 and this is the first characterisation of an 'inverting' beta-1,3-glucanase.

Disulfide bridge reorganization induced by proline mutations in maurotoxin

Maurotoxin (MTX) is a 34-residue toxin that has been isolated from the venom of the chactidae scorpion Scorpio maurus palmatus, and characterized. Together with Pi1 and HsTx1, MTX belongs to a family of short-chain four-disulfide-bridged scorpion toxins acting on potassium channels. However, contrary to other members of this family, MTX exhibits an uncommon disulfide bridge organization of the type C1-C5, C2-C6, C3-C4 and C7-C8, versus C1-C5, C2-C6, C3-C7 and C4-C8 for both Pi1 and HsTx1. Here, we report that the substitution of MTX proline residues located at positions 12 and/or 20, adjacent to C3 (Cys(13)) and C4 (Cys(19)), results in conventional Pi1- and HsTx1-like arrangement of the half-cystine pairings. In this case, this novel disulfide bridge arrangement is without obvious incidence on the overall three-dimensional structure of the toxin. Pharmacological assays of this structural analog, [A(12),A(20)]MTX, reveal that the blocking activities on Shaker B and rat Kv1.2 channels remain potent whereas the peptide becomes inactive on rat Kv1.3. These data indicate, for the first time, that discrete point mutations in MTX can result in a marked reorganization of the half-cystine pairings, accompanied with a novel pharmacological profile for the analog.

Thermal unfolding of a llama antibody fragment: a two-state reversible process

Camelids produce functional "heavy chain" antibodies which are devoid of light chains and CH1 domains [Hamers-Casterman, C., et al. (1993) Nature 363, 446-448]. It has been shown that the variable domains of these heavy chain antibodies (the V(HH) fragments) are functional at or after exposure to high temperatures, in contrast to conventional antibodies [Linden van der, R. H. J., et al. (1999) Biochim. Biophys. Acta 1431, 37-44]. For a detailed understanding of the higher thermostability of these V(HH) fragments, knowledge of their structure and conformational dynamics is required. As a first step toward this goal, we report here the essentially complete (1)H and (15)N NMR backbone resonance assignments of a llama V(HH) antibody fragment, and an extensive analysis of the structure at higher temperatures. The H-D exchange NMR data at 300 K indicate that the framework of the llama V(HH) fragment is highly protected with a DeltaG(ex) of >5.4 kcal/mol, while more flexibility is observed for surface residues, particularly in the loops and the two outer strands (residues 4-7, 10-13, and 58-60) of the beta-sheet. The CD data indicate a reversible, two-state unfolding mechanism with a melting transition at 333 K and a DeltaH(m) of 56 kcal/mol. H-D exchange studies using NMR and ESI-MS show that below 313 K exchange occurs through local unfolding events whereas above 333 K exchange mainly occurs through global unfolding. The lack of a stable core at high temperatures, observed for V(HH) fragments, has also been observed for conventional antibody fragments. The main distinction between the llama V(HH) fragment and conventional antibody fragments is the reversibility of the thermal unfolding process, explaining its retained functionality after exposure to high temperatures.

Maurotoxin versus Pi1/HsTx1 scorpion toxins. Toward new insights in the understanding of their distinct disulfide bridge patterns

Maurotoxin (MTX) is a scorpion toxin acting on several K(+) channel subtypes. It is a 34-residue peptide cross-linked by four disulfide bridges that are in an "uncommon" arrangement of the type C1-C5, C2-C6, C3-C4, and C7-C8 (versus C1-C5, C2-C6, C3-C7, and C4-C8 for Pi1 or HsTx1, two MTX-related scorpion toxins). We report here that a single mutation in MTX, in either position 15 or 33, resulted in a shift from the MTX toward the Pi1/HsTx1 disulfide bridge pattern. This shift is accompanied by structural and pharmacological changes of the peptide without altering the general alpha/beta scaffold of scorpion toxins.

Solution structure of the module X2 1 of unknown function of the cellulosomal scaffolding protein CipC of Clostridium cellulolyticum

Multidimensional, homo- and heteronuclear magnetic resonance spectroscopy combined with dynamical annealing has been used to determine the structure of a 94 residue module (X2 1) of the scaffolding protein CipC from the anaerobic bacterium Clostridium cellulolyticum. An experimental data set comprising 1647 nuclear Overhauser effect-derived restraints, 105 hydrogen bond restraints and 66 phi torsion angle restraints was used to calculate 20 converging final solutions. The calculated structures have an average rmsd about the mean structure of 0.55(+/-0.11) A for backbone atoms and 1.40(+/-0.11) A for all heavy atoms when fitted over the secondary structural elements. The X2 1 module has an immunoglobulin-like fold with two beta-sheets packed against each other. One sheet contains three strands, the second contains four strands. An additional strand is intercalated between the beta-sandwich, as well as two turns of a 3(.10) helix. X2 1 has a surprising conformational stability and may act as a conformational linker and solubility enhancer within the scaffolding protein. The fold of X2 1 is very similar to that of telokin, titin Ig domain, hemolin D2 domain, twitchin immunoglobulin domain and the first four domains of the IgSF portion of transmembrane cell adhesion molecule. As a consequence, the X2 1 module is the first prokaryotic member assigned to the I set of the immunoglobulin superfamily even though no sequence similarity with any member of this superfamily could be detected.

Solution structure of hpTX2, a toxin from Heteropoda venatoria spider that blocks Kv4.2 potassium channel

HpTX2 is a toxin from the venom of Heteropoda venatoria spider that has been demonstrated to bind on Kv4.2 potassium channel. We have determined the solution structure of recombinant HpTX2 by use of conventional two-dimensional NMR techniques followed by distance-geometry and molecular dynamics. The calculated structure belongs to the Inhibitory Cystin Knot structural family that consists in a compact disulfide-bonded core, from which four loops emerge. A poorly defined two-stranded antiparallel beta-sheet (residues 20-23 and 25-28) is detected. Analysis of the electrostatic charge anisotropy allows us to propose a functional map of HpTX2 different from the one described for kappa-conotoxin PVIIA, but strongly related to the one of charybdotoxin. The orientation of the dipole moment of HpTX2 emerges through K27 which could therefore be the critical lysine residue. Close to this lysine are a second basic residue, R23, an aromatic cluster (F7, W25, W30) and an hydrophobic side chain (L24). The high density in aromatic side chains of the putative functional surface as well as the lack of an asparagine is proposed to be the structural basis of the specificity of HpTX2 toward Kv4.2 channel.

A new fold in the scorpion toxin family, associated with an activity on a ryanodine-sensitive calcium channel

We determined the structure in solution by (1)H two-dimensional NMR of Maurocalcine from the venom of Scorpio maurus. This toxin has been demonstrated to be a potent effector of ryanodyne-sensitive calcium channel from skeletal muscles. This is the first description of a scorpion toxin which folds following the Inhibitor Cystine Knot fold (ICK) already described for numerous toxic and inhibitory peptides, as well as for various protease inhibitors. Its three dimensional structure consists of a compact disulfide-bonded core from which emerge loops and the N-terminus. A double-stranded antiparallel beta-sheet comprises residues 20-23 and 30-33. A third extended strand (residues 9-11) is perpendicular to the beta-sheet. Maurocalcine structure mimics the activating segment of the dihydropyridine receptor II-III loop and is therefore potentially useful for dihydropyridine receptor/ryanodine receptor interaction studies. Proteins 2000;40:436-442.

[Animal toxins and ion channels]

Animal venoms contain various toxins which act on ion-channels, responsible for either sodium, potassium, calcium or chloride permeation. Structure determination of these toxins demonstrate that they are organised around two different structural motifs: potassium and sodium channel effectors are organised around an alpha-helix connected by two disulfide bridges to a two- or three-stranded beta sheet whereas calcium channels effectors are structured around an "Inhibitory Cystine Knot" motif made of a dense disulfide-rich core from which emerge several loops. Analysis of local structural modifications allows us to understand the structural basis of the selectivity of these effectors towards the various ion channels. This is the first step in the design of new synthetic molecules which are potent therapeutic drugs for diseases involving ion channel dysfunctioning.

Generating a high affinity scorpion toxin receptor in KcsA-Kv1.3 chimeric potassium channels

The crystal structure of the bacterial K(+) channel, KcsA (Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M., Cohen, S. L., Chait, B. T., and MacKinnon, R. (1998) Science 280, 69-77), and subsequent mutagenesis have revealed a high structural conservation from bacteria to human (MacKinnon, R., Cohen, S. L., Kuo, A., Lee, A., and Chait, B. T. (1998) Science 280, 106-109). We have explored this conservation by swapping subregions of the M1-M2 linker of KcsA with those of the S5-S6 linker of the human Kv-channel Kv1.3. The chimeric K(+) channel constructs were expressed in Escherichia coli, and their multimeric state was analyzed after purification. We used two scorpion toxins, kaliotoxin and hongotoxin 1, which bind specifically to Kv1.3, to analyze the pharmacological properties of the KcsA-Kv1.3 chimeras. The results demonstrate that the high affinity scorpion toxin receptor of Kv1.3 could be transferred to KcsA. Our biochemical studies with purified KcsA-Kv1.3 chimeras provide direct chemical evidence that a tetrameric channel structure is necessary for forming a functional scorpion toxin receptor. We have obtained KcsA-Kv1.3 chimeras with kaliotoxin affinities (IC(50) values of approximately 4 pm) like native Kv1.3 channels. Furthermore, we show that a subregion of the S5-S6 linker may be an important determinant of the pharmacological profile of K(+) channels. Using available structural information on KcsA and kaliotoxin, we have developed a structural model for the complex between KcsA-Kv1.3 chimeras and kaliotoxin to aid future pharmacological studies of K(+) channels.

Synthesis, 1H NMR structure, and activity of a three-disulfide-bridged maurotoxin analog designed to restore the consensus motif of scorpion toxins

Maurotoxin (MTX) is a 34-residue toxin that has been isolated from the venom of the chactidae scorpion Scorpio maurus palmatus. The toxin displays an exceptionally wide range of pharmacological activity since it binds onto small conductance Ca(2+)-activated K(+) channels and also blocks Kv channels (Shaker, Kv1.2 and Kv1.3). MTX possesses 53-68% sequence identity with HsTx1 and Pi1, two other K(+) channel short chain scorpion toxins cross-linked by four disulfide bridges. These three toxins differ from other K(+)/Cl(-)/Na(+) channel scorpion toxins cross-linked by either three or four disulfide bridges by the presence of an extra half-cystine residue in the middle of a consensus sequence generally associated with the formation of an alpha/beta scaffold (an alpha-helix connected to an antiparallel beta-sheet by two disulfide bridges). Because MTX exhibits an uncommon disulfide bridge organization among known scorpion toxins (C1-C5, C2-C6, C3-C4, and C7-C8 instead of C1-C4, C2-C5, and C3-C6 for three-disulfide-bridged toxins or C1-C5, C2-C6, C3-C7, and C4-C8 for four-disulfide-bridged toxins), we designed and chemically synthesized an MTX analog with three instead of four disulfide bridges ([Abu(19),Abu(34)]MTX) and in which the entire consensus motif of scorpion toxins was restored by the substitution of the two half-cystines in positions 19 and 34 (corresponding to C4 and C8) by two isosteric alpha-aminobutyrate (Abu) derivatives. The three-dimensional structure of [Abu(19), Abu(34)]MTX in solution was solved by (1)H NMR. This analog adopts the alpha/beta scaffold with now conventional half-cystine pairings connecting C1-C5, C2-C6, and C3-C7 (with C4 and C8 replaced by Abu derivatives). This novel arrangement in half-cystine pairings that concerns the last disulfide bridge results mainly in a reorientation of the alpha-helix regarding the beta-sheet structure. In vivo, [Abu(19),Abu(34)]MTX remains lethal in mice as assessed by intracerebroventricular injection of the peptide (LD(50) value of 0. 25 microg/mouse). The structural variations are also accompanied by changes in the pharmacological selectivity of the peptide, suggesting that the organization pattern of disulfide bridges should affect the three-dimensional presentation of certain key residues critical to the blockage of K(+) channel subtypes.

Maurotoxin and the Kv1.1 channel: voltage-dependent binding upon enantiomerization of the scorpion toxin disulfide bridge Cys31-Cys34

Maurotoxin (MTX) is a 34-amino acid polypeptide cross-linked by four disulfide bridges that has been isolated from the venom of the scorpion Scorpio maurus palmatus and characterized. Maurotoxin competed with radiolabeled apamin and kaliotoxin for binding to rat brain synaptosomes and blocked K+ currents from Kv1 channel subtypes expressed in Xenopus oocytes. Structural characterization of the synthetic toxin identified half-cystine pairings at Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34 This disulfide bridge pattern is unique among known scorpion toxins, particularly the existence of a C-terminal '14-membered disulfide ring' (i.e. cyclic domain 31-34), We therefore studied structure-activity relationships by investigating the structure and pharmacological properties of synthetic MTX peptides either modified at the C-terminus ¿i.e. MTX(1-29), [Abu31,34]-MTX and [Cys31,34, Tyr32]D-MTX) or mimicking the cyclic C-terminal domain [i.e. MTX(31-34)]. Unexpectedly, the absence of a disulfide bridge Cys31-Cys34 in [Abu 31,34]-MTX and MTX(1-29) resulted in MTX-unrelated half-cystine pairings of the three remaining disulfide bridges for the two analogs, which is likely to be responsible for their inactivity against Kv1 channel subtypes. Cyclic MTX(31-34) was also biologically inactive. [Cys31,34, Tyr32]D-MTX, which had a 'native', MTX-related, disulfide bridge organization, but a D-residue-induced reorientation of the C-terminal disulfide bridge, was potent at blocking the Kv1.1 channel. This peptide-induced Kv1.1 blockage was voltage-dependent (a property not observed for MTX), maximal in the low depolarization range and associated with on-rate changes in ligand binding. Thus, the cyclic C-terminal domain of MTX seems to be crucial for recognition of Kv1.3, and to a lesser extent, Kv1.2 channels and it may contribute to the stabilization and strength of the interaction between the toxin and the Kv1.1 channel.

Solution structure of BmKTX, a K+ blocker toxin from the Chinese scorpion Buthus Martensi

BmKTX is a toxin recently purified from the venom of Buthus Martensi, which belongs to the kaliotoxin family. We have determined its solution structure by use of conventional two-dimensional NMR techniques followed by distance-geometry and energy minimization. The calculated structure is composed of a short alpha-helix (residues 14 to 20) connected by a tight turn to a two-stranded antiparallel beta-sheet (sequences 25-27 and 32-34). The beta-turn connecting these strands belongs to type I. The N-terminal segment (sequence 1 to 8) runs parallel to the beta-sheet although it cannot be considered as a third strand. Comparison of the conformation of BmKTX and toxins of the kaliotoxin family clearly demonstrates that they are highly related. Therefore, analysis of the residues belonging to the interacting surface of those toxins allows us to propose a functional map of BmKTX slightly different from the one of KTX and AgTX2, which may explain the variations in affinities of these toxins towards the Kv1.3 channels.

Chemical synthesis and structure-activity relationships of Ts kappa, a novel scorpion toxin acting on apamin-sensitive SK channel

Tityus kappa (Ts kappa), a novel toxin from the venom of the scorpion Tityus serrulatus, is a 35-residue polypeptide cross-linked by three disulphide bridges and acts on small-conductance calcium-activated potassium channels (SK channels). Ts K was chemically synthesized using the solid-phase method and characterized. The synthetic product, sTs kappa, was indistinguishable from the natural toxin when tested in vitro in competition assay with radiolabelled apamin for binding to rat brain synaptosomes (IC50 = 3 nM). The sTs kappa was further tested in vivo for lethal activity to mice following intracerebroventricular inoculation (LD50 = 70 ng per mouse). The half-cystine pairings were formerly established by enzyme-based cleavage of sTs kappa; they were between Cys7-Cys28, Cys13-CyS33 and Cys17-Cys35, which is a disulphide bridge pattern similar to that of other short scorpion toxins. According to previous studies on SK channel-acting toxins, the putative influence of certain basic residues of Ts kappa (i.e. Arg6, Arg9, Lys18, Lys19) in its pharmacological activity was investigated using synthetic point-mutated analogues of the toxin with an Ala substitution at these positions. Data from binding assay, together with conformational analysis of the synthetic analogues by 1H-NMR, suggest that Arg6, and to a lesser extent Arg9, are important residues for an high-affinity interaction of this toxin with SK channels; interestingly these residues are located outside the alpha-helical structure, whereas the pharmacologically important basic residues from other SK channel-specific toxins had been located inside the alpha-helix.

Solution structure of potassium channel-inhibiting scorpion toxin Lq2

Lq2 is a unique scorpion toxin. Acting from the extracellular side, Lq2 blocks the ion conduction pore in not only the voltage- and Ca2+ -activated channels, but also the inward-rectifier K+ channels. This finding argues that the three-dimensional structures of the pores in these K+ channels are similar. However, the amino acid sequences that form the external part of the pore are minimally conserved among the various classes of K+ channels. Because Lq2 can bind to all the three classes of K+ channels, we can use Lq2 as a structural probe to examine how the non-conserved pore-forming sequences are arranged in space to form similar pore structures. In the present study, we determined the three-dimensional structure of Lq2 using nuclear magnetic resonance (NMR) techniques. Lq2 consists of an alpha-helix (residues S10 to L20) and a beta-sheet, connected by an alphabeta3 loop (residues N22 to N24). The beta-sheet has two well-defined anti-parallel strands (residues G26 to M29 and residues K32 to C35), which are connected by a type I' beta-turn centered between residues N30 and K31. The N-terminal segment (residues Z1 to T8) appears to form a quasi-third strand of the beta-sheet.

Solution structure of two new toxins from the venom of the Chinese scorpion Buthus martensi Karsch blockers of potassium channels

The solution structure of BmTX2 purified from the venom of the Chinese Buthid Buthus martensi has been determined by 2D NMR spectroscopy techniques which led to the description of its 3D conformation. The structure consists of a triple-stranded beta-sheet connected to a helical structure. This helix encompasses 10 residues, from 11 to 20, begins with a turn of 310 helix, and ends with an alpha helix. The three strands of beta sheet comprise residues 2-6, with a bulge covering residues 4 and 5, 26-29, and 32-35, with a type I' beta turn centered on residues 30-31. We also characterized the solution structure of BmTX1. The two toxins which are potent blockers of both large-conductance calcium-activated potassium channels (BKCa channels) and voltage-gated potassium channels (Kv1. 3) are highly superimposable and possess the same structural characteristics. Analysis of these structures allows us to hypothesize that, besides the main surface of interaction described by the functional map of charybdotoxin, one can expect that the binding of scorpion toxins on BKCa channels may involve residues on the edge of this surface.

Retroviral envelope glycoprotein processing: structural investigation of the cleavage site

Proteolytic activation of retroviral envelope glycoprotein precursors occurs at the carboxyl side of a consensus motif consisting of the amino acid sequence (Arg/Lys)-Xaa-(Arg/Lys)-Arg. Synthetic peptides spanning the processing sites of HIV-1/2 and SIV glycoprotein precursors were examined for their ability to be cleaved by the subtilisin-like endoproteases kexin and furin. To determine the potential role of secondary structure on proteolytic activation, we examined the secondary structure of synthetic peptides by circular dichroism and NMR spectroscopy. The results indicate that (i) the peptides were correctly cleaved by kexin and furin and therefore could be used as specific substrates for the purification and characterization of the lymphocyte endoprotease(s) responsible for proteolytic processing of precursors; (ii) the regions surrounding the cleavage sites could be characterized by their flexibility in aqueous solutions. However, a loop has been shown to be a determinant for the specificity of the interaction between the enzyme and its substrate as determined by molecular modeling. Furthermore, we determine and propose a possible structure of the cleavage site which fits to the active site of the modeled furin.

Solution structure of maurotoxin, a scorpion toxin from Scorpio maurus, with high affinity for voltage-gated potassium channels

Maurotoxin (MTX), purified from the scorpionid Scorpio maurus is a potent ligand for potassium channels. It shows a broad specificity as being active on Kv1.1 (Kd = 37 nM), Kv1.2 (Kd = 0.8 nM), Kv1.3 (Kd = 150 nM) voltage-gated potassium channels, as well as on small-conductance calcium-activated potassium channels. It has a unique disulfide pairing among the scorpion toxins family. The solution structure of MTX has been determined by 2D-NMR techniques, which led to the full description of its 3D conformation: a bended helix from residues 6 to 16 connected by a loop to a two-stranded antiparallel beta sheet (residues 23 to 26 and 28 to 31). The interaction of MTX with the pore region of the Kv1.2 potassium channel has been modeled according to their charge anisotropy. The structure of MTX is similar to other short scorpion toxins despite its peculiar disulfide pairing. Its interaction with the Kv1.2 channel involves a dipole moment, which guides and orients the toxin onto the pore, toward the binding site, and which thus is responsible for the specificity.

Solution structure of TsKapa, a charybdotoxin-like scorpion toxin from Tityus serrulatus with high affinity for apamin-sensitive Ca(2+)-activated K+ channels

TsKapa (TsK), purified from the Buthidae Tityus serrulatus is a very high potent ligand for small-conductance apamin-sensitive calcium-activated potassium channels (SK). It is able to efficiently compete with apamin for binding on this channel (K0.5 = 0.3 nM) [Legros, C. et al., FEBS Lett. 390:81-84, 1996]. The solution structure of TsK has been determined by 2D-NMR techniques, which led to the full description of its 3D conformation: a short alpha helix from residues 14 to 20 and a three-stranded antiparallel beta sheet (residues 2-3, 27-29, and 32-34). The interaction of TsK with the SK potassium channel has been modeled according to the charge anisotropy of the ligand. The resulting dipole moment orientates TsK so that it presents toward the receptor, a surface, mainly basic, encompassing residues K18 and K19 on one side and R9 and Y8 on the other. Despite its three-dimensional structure that is related with scorpion toxins active on voltage-gated potassium channels such as charybdotoxin, the pharmacological activity and specificity of TsK is related with shorter scorpion toxins (i.e., possessing an only two-stranded beta sheet) such as scyllatoxin (also named leiurotoxin I) or P05.

1H-NMR-derived secondary structure and overall fold of a natural anatoxin from the scorpion Androctonus australis hector

The venom of the scorpion Androctonus australis hector contains several protein neurotoxins of which structure and structure/activity relationships have been extensively studied. It also contains polypeptides such as Aah STR1, which are not toxic, while having highly similar sequences to fully active toxins. We have determined the solution structure of Aah STR1 by use of conventional two-dimensional NMR techniques followed by distance-geometry and energy minimization. We have demonstrated that, despite its lack of toxicity, Aah STR1 is structurally highly related to anti-mammal scorpion toxins specific for Na+ channels. The calculated structure is composed of a short alpha-helix (residues 26-33) connected by a tight turn to a three-stranded antiparallel beta-sheet (sequences 3-6, 38-41 and 44-48). This beta-sheet is right-handed twisted as usual for such secondary structures. The beta-turn connecting the strands 38-41 and 44-48 belongs to type II'. The overall fold of Aah STR1 is typical of beta-type scorpion toxins. This is, however, the first example of such a fold in Old World scorpion toxins. Either the absence of a basic residue in position 63 or the high mobility of loops, compared to active beta-type neurotoxins, may explain the lack of activity of this protein.

Differential involvement of disulfide bridges on the folding of a scorpion toxin

Leiurotoxin I is a neurotoxin, blocker of Ca(2+)-activated apamin-sensitive K+ channel, purified from the venom of the scorpion Leiurus quinquestriatus hebraeus. It is a 31-residue polypeptide reticulated by three disulfide bridges, i.e. Cys3-Cys21, Cys8-Cys26 and Cys12-Cys28. To investigate the role of these disulfide bridges in the folding of this toxin, analogs lacking one disulfide bridge were synthesized. The structures of two analogs in which two half-cystines were placed by alpha-aminobutyrate residues to suppress one disulfide bridge, were analyzed by 1H NMR. The NMR studies reveal a three-dimensional structure identical with the native toxin for the analog lacking disulfide bridge Cys3-Cys21 and a loss of organized structure for another analog lacking disulfide bridge Cys12-Cys28. These analogs are, respectively, fully active and weakly active (2% of the residual activity) when tested in vitro for their ability to interact with their receptor channel and in vivo for their neurotoxic activity in mice. This suggest that disulfide bridge Cys12-Cys28 is essential for the folding process. In contrast, the lack of disulfide bridge Cys3-Cys21 does not affect the folding and the maintenance of bioactive conformation of Leiurotoxin I.

Determination of the three-dimensional structure of CC chemokine monocyte chemoattractant protein 3 by 1H two-dimensional NMR spectroscopy

MCP-3 is a beta chemokine consisting of 76 amino acid residues. It has been described to be involved in the activation of all leukocytic cells, activation mediated by the presence of multiple binding sites on the target cells. Its three-dimensional structure has been studied by making use of two-dimensional 1H NMR spectroscopy. MCP-3 exhibits the same monomeric structure as the other chemokines, i.e., a three-stranded antiparallel beta sheet covered on one face by an alpha helix. Although it belongs to the same subfamily as RANTES (Chung et al., 1995; Faitbrother et al., 1994) and hMIP-1beta (Lodi et al., 1994), the MCP-3 dimer is folded like IL-8 with the so-called alphabeta sandwich structural motif. Structural and sequence analysis gives clear indications suggesting that the other MCP chemokines may have the same quaternary structure, contrary to the other beta chemokines.

Chemical synthesis and characterization of maurotoxin, a short scorpion toxin with four disulfide bridges that acts on K+ channels

Maurotoxin is a toxin isolated from the venom of the Tunisian chactoid scorpion Scorpio maurus. It is a 34-amino-acid peptide cross-linked by four disulfide bridges. Maurotoxin competes with radiolabeled apamin and kaliotoxin for binding to rat-brain synaptosomes. Due to its very low concentration in venom (0.6% of the proteins), maurotoxin was chemically synthesized by means of an optimized solid-phase technique. The synthetic maurotoxin was characterized. It was lethal to mice following intracerebroventricular injection (LD50, 80 ng/mouse). The synthetic maurotoxin competed with 125I-apamin and 125I-kaliotoxin for binding to rat-brain synaptosomes with half-maximal effects at concentrations of 5 nM and 0.2 nM, respectively. Synthetic maurotoxin was tested on K+ channels and was found to block the Kv1.1, Kv1.2, and Kv1.3 currents with half-maximal blockage (IC50) at 37, 0.8 and 150 nM, respectively. Thus, maurotoxin is a scorpion toxin with four disulfide bridges that acts on K+ channels. The half-cystine pairings of synthetic maurotoxin were identified by enzymatic cleavage. The pairings were Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34. This disulfide organization is unique among known scorpion toxins. The physicochemical and pharmacological properties of synthetic maurotoxin were indistinguishable from those of natural maurotoxin, which suggests that natural maurotoxin adopts the same half-cystine pairing pattern. The conformation of synthetic maurotoxin was investigated by means of circular dichroism spectroscopy and molecular modeling. In spite of its unusual half-cystine pairings, the synthetic-maurotoxin conformation appears to be similar to that of other short scorpion toxins.

Characterization of PO1, a new peptide ligand of the apamin-sensitive Ca2+ activated K+ channel

A new peptide ligand of the small conductance Ca2+ activated K+ channels has been purified from the venom (obtained by manual rather than electrical stimulation of the scorpion Androctonus mauretanicus mauretanicus), by following the inhibition of the 125I-apamin binding to its receptor on rat brain synaptosomes. Only one step on a C18 reversed-phase high-performance liquid chromatography column was necessary to obtain PO1. Its K0.5 for the apamin binding site was 100 nM. The amino acid sequence of PO1 is different from those of leiurotoxin and PO5. For the first time the same peptide was also purified from the venoms of two other species of North African scorpions, Androctonus australis and Buthus occitanus tunetanus. PO1 was chemically synthesized by the solid-phase technique and fully characterized. A model of PO1 was constructed by amino acid replacement using PO5 nuclear magnetic resonance studies as the starting model. Structure-activity relationships between these toxins and their receptor are discussed.

Synthesis and characterization of leiurotoxin I analogs lacking one disulfide bridge: evidence that disulfide pairing 3-21 is not required for full toxin activity

Leiurotoxin I (Lei-NH2), a toxin isolated from the venom of the scorpion Leiurus quinquestriatus hebraeus, is a blocker of the apamin-sensitive Ca(2+)-activated K+ channels. It is a 31-residue polypeptide cross-linked by three disulfide bridges which are presumably between Cys3-Cys21, Cys8-Cys26, and Cys12-Cys28. To investigate the role of these disulfides, analogs of Lei-NH2 lacking one disulfide bridge (i.e., [Abu3,21]Lei-NH2, [Abu8,26]Lei-NH2, and [Abu12,28]Lei-NH2) were chemically synthesized by selective replacement of each pair of half-cystines forming a bridge by two alpha-aminobutyrate (Abu) residues. The two disulfide pairings of the main folded form of the synthetic analogs were established by enzymatic proteolysis. They were as expected between Cys8-Cys26 and Cys12-Cys28 for [Abu3,21]Lei-NH2 but were unexpectedly between Cys3-Cys12 and Cys21-Cys28 for [Abu8,26]Lei-NH2 and between Cys3-Cys8 and Cys21-Cys26 for [Abu12,28]Lei-NH2. The synthetic peptides were tested in vitro for their capacity to compete with the binding of [125I]apamin to rat brain synaptosomes and in vivo for their neurotoxicity in mice. In both assays, [Abu3,21]Lei-NH2 exhibited full Lei-NH2-like activity whereas [Abu8,26]Lei-NH2 and [Abu12,28]-Lei-NH2 possessed only residual activities (< 2% native toxin activity). This suggests that disulfide bridge Cys3-Cys21 is not essential per se for high toxin activity. Circular dichroism (CD) spectroscopy of the three analogs showed that only [Abu3,21]Lei-NH2 exhibited a CD spectrum similar to that of Lei-NH2, suggesting they both adopt closely related conformations, in agreement with the pharmacological data. Structural models of the analogs were constructed on the basis of the disulfide pairing assignment and compared with that of Lei-NH2.

Characterization of a new peptide from Tityus serrulatus scorpion venom which is a ligand of the apamin-binding site

A new ligand (Ts kappa) of the apamin binding site on rat brain synaptosomes (K0.5 = 300 pM) was purified and characterized from the venom of Tityus serrulatus. It is a polypeptide toxin of 35 amino acid residues, with three disulfide bridges. Its cDNA was amplified from a venom gland cDNA library and the nucleotide sequence determined. A model of Ts kappa was constructed by amino acid replacement using charybdotoxin structure as determined by 1H nuclear magnetic resonance as starting model.

Solution structure of P01, a natural scorpion peptide structurally analogous to scorpion toxins specific for apamin-sensitive potassium channel

The venom of the North African scorpion Androctonus mauretanicus mauretanicus possesses numerous highly active neurotoxins that specifically bind to various ion channels. One of these, P05, has been found to bind specifically to calcium-activated potassium channels and also to compete with apamin, a toxin extracted from bee venom. Besides the highly potent ones, several of these peptides (including that of P01) have been purified and been found to possess only a very weak, although significant, activity in competition with apamin. The amino acid sequence of P01 shows that it is shorter than P05 by two residues. This deletion occurs within an alpha-helix stretch (residues 5-12). This alpha-helix has been shown to be involved in the interaction of P05 with its receptor via two arginine residues. These two arginines are absent in the P01 sequence. Furthermore, a proline residue in position 7 of the P01 sequence may act as an alpha-helix breaker. We have determined the solution structure of P01 by conventional two-dimensional 1H nuclear magnetic resonance and show that 1) the proline residue does not disturb the alpha-helix running from residues 5 to 12; 2) the two arginines are topologically replaced by two acidic residues, which explains the drop in activity; 3) the residual binding activity may be due to the histidine residue in position 9; and 4) the overall secondary structure is conserved, i.e., an alpha-helix running from residues 5 to 12, two antiparallel stretches of beta-sheet (residues 15-20 and 23-27) connected by a type I' beta-turn, and three disulfide bridges connecting the alpha-helix to the beta-sheet.

Structure-activity relationship study of a scorpion toxin with high affinity for apamin-sensitive potassium channels by means of the solution structure of analogues

Scorpion venoms contain numerous toxic polypeptides displaying various pharmacological activities. These toxins interact with ion channels of excitable membranes. Long toxins (60-70 amino acids) are known to interact with sodium channels, whereas most of the short toxins (31-37 amino acids) found their toxicity in modifying the potassium channel functions. A family of short scorpion toxins are known to interact specifically with apamin-sensitive calcium-activated potassium channels. Structure-activity relationship studies of these toxins have demonstrated that a short region located on the solvent-exposed side of an alpha-helix is involved in the interaction with their receptor. Two positions, i.e. residues 6 and 7 in the sequence, are essential for the full activity of these molecules. We have synthesized analogues of these toxins and demonstrated that the three-dimensional structure is not affected by these mutations, and thus that the observed variations of activity are only due to the chemical function carried by the side chain. This interaction between the toxins and their receptor is thus purely electrostatic.

Molecular cloning, cDNA analysis, and localization of a monomer of the N-acetylglucosamine-specific receptor of the thyroid, NAGR1, to chromosome 19p13.3-13.2

We have proposed that the GlcNAc thyroid receptor triggers selective recycling of immature GlcNAc-bearing thyroglobulin molecules through the Golgi back to the apical membrane for further processing until maturation is achieved. This process, which we call "receptor-mediated exocytosis," prevents lysosomal degradation of thyroid prohormones. In the present study, we report cloning of the cDNA encoding the (or one of the) monomer(s) constituting the human GlcNAc thyroid receptor. This novel gene, called NAGR1, was assigned by in situ hybridization to subbands p13.3-p13.2 of chromosome 19. Northern blot analysis showed that the mRNA encoding NAGR1 was present as a single transcript of 2.1 kb in the thyroid, but not in the heart, brain, placenta, lung, liver, skeletal muscle, kidney, and pancreas. The deduced amino acid sequence comprised a 51-kDa type I membrane protein with a single spanning region and a short intracytoplasmic domain. Sequence analysis showed that NAGR1 is a glycine-, tryptophan-, and methionine-rich protein with no cysteine residues or glycosylation site. No sequence homology with any known cDNA or protein was noted. The extracellular domain is composed of 420 amino acids and contains a region of 204 residues showing 15 repeats of 4 amino acids, each 1 having an acidic amino acid presumably involved in calcium coordination. The intracellular domain contained what appeared to be a tyrosine internalization signal. The usefulness of this clone in glycobiology, cell biology, and thyroid pathology studies is discussed.

Leiurotoxin I, a scorpion toxin specific for Ca(2+)-activated K+ channels. Structure-activity analysis using synthetic analogs

Recently, we reported a structure-activity relationship study on P05, a novel leiurotoxin I-like scorpion toxin which is selective for the apamin-sensitive Ca(2+)-activated K+ channel [Sabatier et al. (1993) Biochemistry 32, 2763-2770]. Arg6, Arg7 and C-terminal His31 appeared to be key residues for P05 biological activity. Owing to the high sequence identity between P05 and leiurotoxin I (87%), several analogs of leiurotoxin I (Lei-NH2) with point mutations at these positions were designed and chemically synthesized using an optimized solid-phase technique. The synthesized peptides were [L6]Lei-NH2, [R7]Lei-NH2, Lei-OH and [R7]Lei-OH, as well as fragment [R7,Abu8]N4-S11-NH2. A chimeric analog ([M22,K24,R27]Lei-NH2), which possesses part of the iberiotoxin C-terminus, was also constructed. Circular dichroism analyses of these analogs, in agreement with their structural models obtained by molecular dynamics, showed that the point mutations did not significantly affect the overall secondary structures, as compared to natural Lei-NH2. All the peptides and natural toxins were compared in vitro for their capacity to inhibit binding of [125I]-apamin to rat brain synaptosomes, and in vivo for their specific neurotoxicity in mice. The Arg6 residue was essential for high biological activity of leiurotoxin I. Further, substitution of Met7 in the natural toxin by Arg7, or C-terminal amidation of His31, greatly increased affinity for the apamin receptor but did not significantly affect toxin neurotoxicity. Remarkably, the chimeric analog [M22,K24,R27]Lei-NH2 was found to retain leiurotoxin I-like activity, thus indicating that the negatively charged residues Asp24 and Glu27 (and Ile22) are not directly involved in the high toxin bioactivity. However, the chimeric molecule had no iberiotoxin-like effect on rat muscular maxi-K+ channels incorporated in lipid bilayers.

Solution structure of P05-NH2, a scorpion toxin analog with high affinity for the apamin-sensitive potassium channel

The venom of the scorpion Androctonus mauretanicus mauretanicus contains a toxin--P05--which is structurally and functionally similar to scorpion leiurotoxin I (87% sequence identity), a blocker of the apamin-sensitive Ca(2+)-activated K+ channels. P05, a 31-residue polypeptide cross-linked by three disulfide bridges, also possesses binding and physiological properties similar to those of the bee venom toxin apamin (18 residues, two disulfides). However, the amino acid sequences of these two polypeptides are dissimilar, except for a common Arg-Arg-Cys-Gln motif which is located on an alpha-helix. P05-NH2, a synthetic analog of P05, unlike native P05, was found to bind irreversibly to the apamin receptor. The solution structure of P05-NH2 has been solved by conventional two-dimensional NMR techniques followed by distance geometry and energy minimization. The obtained conformation is composed of two and an half turns of alpha-helix (residues 5-14) connected by a tight turn to a two-stranded antiparallel beta-sheet (sequences 17-22 and 25-29). This beta-sheet has a right-handed twist as usual for such secondary structures. The beta-turn connecting the two strands belongs to type II'. This structure is homologous to all scorpion toxin structures known so far as well as to insect defensins. The three arginines known to be involved in the pharmacological activity, i.e., Arg6, Arg7, and Arg13, are all located on the solvent-exposed side of the helix and form a positively charged surface which includes Gln9. The calculated electrostatic potential is highly asymmetric with the greatest positive potential centered on the Arg-rich alpha-helix side.(ABSTRACT TRUNCATED AT 250 WORDS)

P05, a new leiurotoxin I-like scorpion toxin: synthesis and structure-activity relationships of the alpha-amidated analog, a ligand of Ca(2+)-activated K+ channels with increased affinity

The venom of the scorpion Androctonus mauretanicus mauretanicus contains a toxin, P05, which is structurally and functionally similar to scorpion leiurotoxin I (87% sequence identity), a blocker of the apamin-sensitive Ca(2+)-activated K+ channels. It is a 31-residue polypeptide cross-linked by three disulfide bridges. A C-terminal carboxyl-amidated analog of P05 (sP05-NH2) was chemically synthesized by the solid-phase technique and fully characterized. Toxicity assays in vivo established that sP05-NH2, like native P05, is a potent and lethal neurotoxic agent in mice (LD50 of 20 ng per mouse). Pharmacological assays in vitro however showed that, unlike P05 which has a binding affinity of 2 x 10(-11) M, sP05-NH2 apparently binds irreversibly to the apamin receptor. Iodination at the C-terminal His gave diiodo-sP05-NH2, which had a binding affinity similar to that of native P05. The disulfide bridge pairings were chemically determined for sP05-NH2 and thereby deduced for P05 and leiurotoxin I: linkages were between Cys3 and Cys21, Cys8 and Cys26, and Cys12 and Cys28. Molecular dynamics refinement of P05 also using data from leiurotoxin I suggests that P05 is mainly composed of a double-stranded, antiparallel beta-sheet (from Leu18 to Val29) linked to an alpha-helix (from Arg6 to Gly16) by two disulfides (Cys8-Cys26 and Cys12-Cys28) and to an extended fragment (from Thr1 to Leu5) by the third disulfide (Cys3-Cys21). In agreement with the model, circular dichroism analysis of sP05-NH2 showed that the toxin structure is highly rigid.(ABSTRACT TRUNCATED AT 250 WORDS)

Solution structure of human corticotropin releasing factor by 1H NMR and distance geometry with restrained molecular dynamics

The structure of human corticotropin releasing factor (hCRF) has been determined by proton nuclear magnetic resonance (1H NMR) in a mixed-solvent system of 66% trifluoroethanol/34% H2O at pH 3.8 and 37 degrees C. Nearly complete resonance assignment was achieved by using standard two-dimensional methods. Distance restraints for structure calculations were obtained by qualitative analysis of intra- and inter-residue nuclear Overhauser effects. Structures were obtained from the distance restraints by distance geometry, followed by refinement using molecular dynamics and were completed with amide hydrogen exchange data. The structure of hCRF in this solvent comprises an extended N-terminal tetrapeptide connected to a well-defined alpha-helix between residues 6 and 36. The first half of the alpha-helix (residues 6-20) is clearly amphipathic. The five carboxy-terminal residues are predominantly disordered.

Solution conformation of human neuropeptide Y by 1H nuclear magnetic resonance and restrained molecular dynamics

The solution structure of human neuropeptide Y has been solved by conventional two-dimensional NMR techniques followed by distance-geometry and molecular-dynamics methods. The conformation obtained is composed of two short contiguous alpha-helices comprising residues 15-26 and 28-35, linked by a hinge inducing a 100 degree angle. The first helix (15-26) is connected to a polyproline stretch (residues 1-10) by a tight hairpin (residues 11-14). The helices and the polyproline stretch are packed together by hydrophobic interactions. This structure is related to that of the homologous avian pancreatic polypeptide and bovine pancreatic polypeptide. The C- and N-terminii, known to be involved in the biological activity for respectively the receptor binding and activation, are close together in space. The side chains of residues Arg33, Arg35 and Tyr36 on the one hand, and Tyr1 and Pro2 on the other, form a continuous solvent-exposed surface of 4.9 mm2 which is supposed to interact with the receptor for neuropeptide Y.