Functional duality and structural uniqueness of depressant insect-selective neurotoxins

Cloning and characterization of cDNA sequences encoding two novel alpha-like-toxin precursors from the Chinese scorpion Buthus martensii Karsch

According to a relative conserved fragment of alpha-scorpion toxins, a degenerate primer was designed and synthesized. Two full-length cDNAs encoding the precursors of two novel putative alpha-like-toxins were then amplified from the total cDNAs of venomous glands of the Chinese scorpion Buthusmartensi Karsch using 3' and 5' RACE (rapid amplification of cDNA ends). The precursors were both composed of 85 amino acid residues, including a putative signal peptide of 19 residues and a mature toxin of 66 residues, respectively. The predicted amino acid sequences of these two toxins show a homology of 82% with each other, and of 55-70% with other BmK-originated alpha-like-toxins. Interestingly, it is rarely seen in other alpha or alpha-like-toxins that: (1) Met residue but not a basic amino acid residue (Arg or Lys) is located on position 58 for BmKalpha2; (2) both toxins are ended with double Gly in the C-terminus.

Purification, characterization, and primary structure of four depressant insect-selective neurotoxin analogs from scorpion (Buthus sindicus) venom

Four depressant insect-selective neurotoxin analogs (termed Bs-dprIT1 to 4) from the venom of the scorpion Buthus sindicus were purified to homogeneity in a single step using reverse-phase HPLC. The molecular masses of the purified toxins were 6820.9, 6892.4, 6714.7, and 6657.1 Da, respectively, as determined by mass spectrometry. These long-chain neurotoxins were potent against insects with half lethal dose values of 67, 81, 103, and 78 ng/100 mg larva and 138, 160, 163, and 142 ng/100 mg cockroach, respectively, but were not lethal to mice even at the highest applied dose of 10 microg/20 g mouse. When injected into blowfly larvae (Sarcophaga falculata), Bs-dprIT1 to 4 induced classical manifestations of depressant toxins, i.e., a slow depressant flaccid paralysis. The primary structures of Bs-dprIT 1 to 4 revealed high sequence homology (60-75%) with other depressant insect toxins isolated from scorpion venoms. Despite the high sequence conservation, Bs-dprIT1 to 4 showed some remarkable features such as (i) the presence of methionine (Met(6) in Bs-dprIT1 and Met(24) in Bs-dprIT2 to 4) and histidine (His(53) and His(57) in Bs-dprIT1) residues, i.e., amino acid residues that are uncommon to this type of toxin; (ii) the substitution of two highly conserved tryptophan residues (Trp43 --> Ala and Trp53 --> His) in the sequence of Bs-dprIT1; and (iii) the occurrence of more positively charged amino acid residues at the C-terminal end than in other depressant insect toxins. Multiple sequence alignment, sequence analysis, sequence-based structure prediction, and 3D homology modeling studies revealed a protein fold and secondary structural elements similar to those of other scorpion toxins affecting sodium channel activation. The electrostatic potential calculated on the surface of the predicted 3D model of Bs-dprIT1 revealed a significant positive patch in the region of the toxin that is supposed to bind to the sodium channel.

Molecular characterization of an anti-epilepsy peptide from the scorpion Buthus martensi Karsch

For a long time Asian scorpion Buthus martensi Karsch (BmK) has been used in Chinese traditional medicine to cure many diseases of nervous system. Here we report the purification and characterization of a pharmacologically active neurotoxin from the scorpion BmK. This toxin had little toxicity in mice and insects but was found to have an anti-epilepsy effect in rats, and is thus named as BmK anti-epilepsy peptide (BmK AEP). Its amino-acid sequence was determined by lysylendopeptidase digestion, Edman degradation and mass spectrographic analysis. Based on the determined sequence, the gene coding for this peptide was also cloned and sequenced by the 3' and 5' RACE methods. It encodes a precursor of 85 amino-acid residues including a signal peptide of 21 residues, a mature peptide of 61 residues and three additional residues Gly-Lys-Lys at the C-terminus. The additional Gly sometimes followed by one or two basic residues is prerequisite for the amidation of its C-terminus. C-terminal amidation was also verified by the molecular-mass determination of BmK AEP. This anti-epilepsy peptide toxin shares homology with other depressant insect toxins. The remarkable difference between them was mainly focused at residues 6, 7 and 39; these residues might relate to the unique action of BmK AEP.

Prokaryotically expressed Buthus martensii Karsch insect depressant toxin has insecticidal effects

An insect depressant toxin Buthus martensii Karsch insect toxin 4 (BmK IT(4)) cDNA was cloned into the prokaryotic expression vector pSW202 and expressed in HB101 host cells. The authenticity of this in vitro expressed peptide was confirmed by Western blotting, mass spectrometry and N-terminal peptide sequencing. Bioassays using growth media supplemented with BmK IT(4) demonstrated that, at the end of the 5-day experimental period, about 77% of the testing cotton bollworm larvae were killed. Furthermore, the average weight picked up by larvae grown on BmK IT(4) containing media amounts only to 0.7% of that of control group. Our results indicate that BmK IT(4) may be used for biological control of insect damages in place of other traditional insecticides.

The binding of BmK IT2, a depressant insect-selective scorpion toxin on mammal and insect sodium channels

Binding assay of (125)I-BmK IT2, a depressant insect-selective scorpion toxin showed two non-interacting binding sites on insect neuronal membranes: a high affinity (K(d(1))=0.65+/-0.20 nM) and low capacity (B(max(1))=0.46+/-0.13 pmol/mg protein) binding site, as well as a low-affinity (K(d(2))=78.7+/-16.4 nM) and high capacity (B(max(2))=33.1+/-8.5 pmol/mg protein) binding site. BmK IT2 could associate with and dissociate from its binding sites on insect neuronal membranes in quick manner (k(1)=5.4 x 10(5) S(-1) M(-1) and k(2)=3.2 x 10(4) S(-1) M(-1); k(-1)=7.4 x 10(-4) S(-1) and k(-2)=5.3 x 10(-3) S(-1)). The binding of (125)I-BmK IT2 to insect synaptosomes could be significantly inhibited by native BmK IT2, BmK AS and BmK AS-1 in a dose-dependent manner, and partially by BmK I, but not modified by depolarization of membrane potential and veratridine, In addition, specific binding of (125)I-BmK IT2 seem to be undetectable on rat brain synaptosomes even at high concentration. Whole cell patch-clamping recording found that BmK IT2 could partially inhibit total sodium currents of rat DRG neurons, the inhibitory effects were reversible. The results suggest that the receptor binding site of BmK IT2 on insect sodium channels might be similar to that on sodium channels of mammal peripheral nervous system, but different from that of mammal central nervous system.

Solution structure of BmP01 from the venom of scorpion Buthus martensii Karsch

From the venom of scorpion Buthus martensii Karsch,a short peptide (BmP01, 29 amino acid residues) was isolated and characterized as previously reported (Lebren, R. R., et al. (1997) Eur. J. Biochem. 245, 457-464). It was shown to reduce 33% outward K(+) channel (hippocampal neurons) currents at 10 microM. The solution structure of BmP01 was determined by 2D (1)H NMR spectroscopy. The NOEs, coupling constants, and H-D exchange obtained from NMR spectroscopy were used in structural calculations. The conformation of BmP01 is composed of a short alpha-helix (Cys 3-Thr 12) and a two-stranded antiparallel beta-sheet (Ala 15-Asp 20 and Lys 23-Pro 28). There are three disulfide bridges (Cys 3-Cys 19, Cys 6-Cys 24 and Cys 10-Cys 26) connecting the alpha-helix and beta-sheet. Asp 20 to Lys 23 form a type II turn linking the two strands. Structural and electrostatic potential comparison between BmP01 and its analogues are also presented.

AaIT: from neurotoxin to insecticide

AaIT is a single chain neurotoxic polypeptide derived from the venom of the Buthid scorpion Androctonus australis Hector, composed of 70 amino acids cross-linked by four disulfide bridges. Its strict selectivity for insects has been documented by toxicity, electrophysiological and ligand receptor binding assays. These last have shown that various insect neuronal membranes possess a single class of non-interacting AaIT binding sites of high affinity (K(D) = 1-3(n)M) and low capacity (0.5-2.0 pmol/mg prot.). The fast excitatory paralysis induced by AaIT is a result of a presynaptic effect, namely the induction of a repetitive firing in the terminal branches of the insect's motor nerves resulting in a massive and uncoordinated stimulation of the respective skeletal muscles. The neuronal repetitive activity is attributed to an exclusive and specific perturbation of sodium conductance as a consequence of toxin binding to external loops of the insect voltage-dependent sodium channel and modification of its gating mechanism. From a strictly agrotechnical point of view AaIT involvement in plant protection has taken the following two complementary forms: firstly, as a factor for the genetic engineering of insect infective baculoviruses resulting in potent and selective bio-insecticides. The efficacy of the AaIT-expressing, recombinant baculovirus is attributed mainly to its ability to continuously provide and translocate the gene of the expressed toxin to the insect central nervous system; secondly, based on the pharmacological flexibility of the voltage-gated sodium channel, as a device for insecticide resistance management. Channel mutations conferring resistance to a given class of insecticidal agents (such as the KDR phenomenon) may greatly increase susceptibility to the AaIT expressing bioinsecticides. Thus the AaIT is a pharmacological tool for the study of insect neuronal excitability and chemical ecology and the development of new approaches to insect control.

Purification, structure determination and synthesis of covalitoxin-II, a short insect-specific neurotoxic peptide from the venom of the Coremiocnemis validus (Singapore tarantula)

Spider venoms contain toxins that specifically immobilize and kill insects. We report the purification and characterization of a new insect-specific toxin named covalitoxin-II (Cvtx-II; mass, 3406. 24+/-0.64), from Coremiocnemis validus (Singapore tarantula) venom. The complete 31 amino acid sequence of Cvtx-II has been determined and it shows less than 40% identity with spider toxins. However, Cvtx-II has conserved cystine motif analogous to other spider and omega-conotoxins. Cvtx-II was chemically synthesized and identified with the native Cvtx-II. Synthetic Cvtx-II induced insect-specific non-lethal excitatory activity when injected into crickets, but not in cockroaches and mice.

Characterization of four distinct monoclonal antibodies specific to BmK AS-1, a novel scorpion bioactive polypeptide

Four monoclonal antibodies designed as 2#, 3#, 4# and 5# have been raised against a novel bioactive polypeptide BmK AS-1 purified from the Chinese scorpion Buthus martensi Karsch. All of these antibodies exhibited specific affinity with antigen by ELISA and Biosensor assay. Western blot analysis showed that 3# and 4# were able to recognize the denatured antigen, but not 2# and 5#. These antibodies could cross-react with BmK AS, but not with other types of BmK neurotoxins such as BmK I (an alpha-like toxin) and BmK IT (an excitatory insect-selective toxin), and in which only 5# can partially react with BmK IT2 (a depressant insect-selective toxin). Immunocytochemical staining demonstrated that 3#, 4# and 5# antibodies can visualize the antigen bound to the membrane of SK-N-SH neuroblast cells, with the exception of 2#. This suggests that either conformation alteration of receptor binding might be prone to nonvisualization or the epitope recognized by antibody 2# might be overlapped with receptor binding sites of antigen. The antibodies developed in the study should provide powerful new tools for investigating the structure/function relationship and pharmacological mechanism of scorpion neurotoxins.

Sodium channels in central neurons of the tobacco budworm, Heliothis virescens: basic properties and modification by scorpion toxins

Voltage-activated sodium channels in central neurons of larval and adult Heliothis virescens were characterized using whole-cell patch clamp techniques. Macroscopic currents showing rapid activation and inactivation kinetics were uniformly sensitive to tetrodotoxin (IC(50)=1.9nM). Currents began to activate at voltage steps to -45mV and reached half maximal at -30mV. Fast inactivation was evident at voltages as negative as -75mV and reached half maximal at -50mV. Full recovery from inactivation occurred within 1 to 2ms. Currents in larval neurons exhibited similar properties to those of adult neurons, except for the rate of fast inactivation (t(1)), which was significantly slower in larval neurons. The biophysical properties of sodium channels remained unchanged for up to 3days in culture. Two insecticidal neurotoxins, LqhalphaIT and AaIT, produced distinctly different modifications of H. virescens sodium channels. LqhalphaIT slowed channel inactivation, while AaIT specifically shifted voltage-dependent activation to more negative potentials. Overall, the results indicate that sodium channels in H. virescens neurons exhibit biophysical characteristics similar to those of vertebrates, yet possess pharmacological uniqueness with respect to scorpion toxin modification.

Purification and cDNA cloning of an insecticidal protein from the venom of the scorpion Orthochirus scrobiculosus

Injection of crude venom from the scorpion Orthochirus scrobiculosus into larvae of Heliothis virescens (Lepidoptera: Noctuidae) caused trembling and uncoordinated movement before development of a progressive and prolonged flaccid paralysis. The isolation of the toxin (OsI-1) responsible for this effect of O. scrobiclosus venom is described. The molecular mass of OsI-1 toxin was 6994 Da, as determined by desorption mass spectroscopy. The complete primary structure of OsI-1 was deduced from the sequence of cDNA clones obtained by rapid amplification of cDNA ends (RACE) PCR. Comparison of the deduced amino acid sequence of OsI-1 with those of other insecticidal scorpion toxins indicates that it is a sodium (Na+) channel active depressant insect-selective toxin. The analysis of amino acid sequence of the toxin in conjunction with mass spectroscopy data indicates post-translational modification in maturation with the removal of 3 C-terminal amino acids and amidation of the C-terminus.

Caenorhabditis elegans degenerins and vertebrate ENaC ion channels contain an extracellular domain related to venom neurotoxins

The DEG/ENaC (DEGenerin/Epithelial Na+ Channel) superfamily includes closely related ion channel subunits from divergent species ranging from the simple nematode Caenorhabditis elegans to humans. Members of this protein group play roles in several important processes including transduction of mechanical stimuli, sodium re-absorption and blood pressure regulation. Structure/function relationships in members of this superfamily are just beginning to be elaborated. Using a bio-informatics approach, we identified a novel structural element in the extracellular region of DEG/ENaC proteins that exhibits significant similarity to venom neurotoxins. Since venom neurotoxins bind to sodium channels at high affinity, we suggest that the related domain embedded in DEG/ENaC channels may interact with other regions of the channel or channel complex to modulate channel function.

Scorpion toxins specific for Na+-channels

Na+-channel specific scorpion toxins are peptides of 60-76 amino acid residues in length, tightly bound by four disulfide bridges. The complete amino acid sequence of 85 distinct peptides are presently known. For some toxins, the three-dimensional structure has been solved by X-ray diffraction and NMR spectroscopy. A constant structural motif has been found in all of them, consisting of one or two short segments of alpha-helix plus a triple-stranded beta-sheet, connected by variable regions forming loops (turns). Physiological experiments have shown that these toxins are modifiers of the gating mechanism of the Na+-channel function, affecting either the inactivation (alpha-toxins) or the activation (beta-toxins) kinetics of the channels. Many functional variations of these peptides have been demonstrated, which include not only the classical alpha- and beta-types, but also the species specificity of their action. There are peptides that bind or affect the function of Na+-channels from different species (mammals, insects or crustaceans) or are toxic to more than one group of animals. Based on functional and structural features of the known toxins, a classification containing 10 different groups of toxins is proposed in this review. Attempts have been made to correlate the presence of certain amino acid residues or 'active sites' of these peptides with Na+-channel functions. Segments containing positively charged residues in special locations, such as the five-residue turn, the turn between the second and the third beta-strands, the C-terminal residues and a segment of the N-terminal region from residues 2-11, seems to be implicated in the activity of these toxins. However, the uncertainty, and the limited success obtained in the search for the site through which these peptides bind to the channels, are mainly due to the lack of an easy method for expression of cloned genes to produce a well-folded, active peptide. Many scorpion toxin coding genes have been obtained from cDNA libraries and from polymerase chain reactions using fragments of scorpion DNAs, as templates. The presence of an intron at the DNA level, situated in the middle of the signal peptide, has been demonstrated.

Crystal structure of neurotoxin Ts1 from Tityus serrulatus provides insights into the specificity and toxicity of scorpion toxins

The crystal structure of neurotoxin Ts1, a major component of the venom of the Brazilian scorpion Tityus serrulatus, has been determined at 1.7 A resolution. It is the first X-ray structure of a highly toxic anti-mammalian beta-toxin. The folding of the polypeptide chain of Ts1 is similar to that of other scorpion toxins. A cysteine-stabilised alpha-helix/beta-sheet motif forms the core of the flattened molecule. All residues identified as functionally important by chemical modification and site-directed mutagenesis are located on one side of the molecule, which is therefore considered as the Na+channel recognition site. The distribution of charged and non-polar residues over this surface determines the specificity of the toxin-channel interaction. Comparison to other scorpion toxins shows that positively charged groups at positions 1 and 12 as well as a negative charge at position 2 are likely determinants of the specificity of beta-toxins. In contrast, the contribution of the conserved aromatic cluster to the interaction might be relatively small. Comparison of Ts1 to weak beta-toxins from Centruroides sculpturatus Ewing reveals that a number of basic amino acid residues located on the face of the molecule opposite to the binding surface may account for the high toxicity of Ts1.

Covalent structures of BmK AS and BmK AS-1, two novel bioactive polypeptides purified from Chinese scorpion Buthus martensi Karsch

Complete amino acid sequences of two novel bioactive polypeptides, each containing 66 amino acid residues, BmK AS and BmK AS-1 purified from the venom of Chinese scorpion Buthus martensi Karsch, have been determined by Edman sequencing and mass spectrometry on native proteins, reduced and S-carboxymethylated proteins and their peptides obtained after cleavage with proteolytic enzymes. Sequence analysis showed 86.4% structural identity between BmK AS and BmK AS-1 and also a high sequence similarity between BmK ASs and AaH IT4, a unique anti-insect toxin and a ligand of Na+ channels obtained from Sahara scorpion A. australis Hector, but poor sequence homology between BmK ASs and those of the known alpha-, beta-type and long-chain insect-selective type scorpion neurotoxins. The positions of four disulfide bridges in BmK AS-1 were established as Cys-12 and Cys-62, Cys-16 and Cys-37, Cys-23 and Cys-44, and Cys-27 and Cys-46, which are the same as those in alpha- and beta-scorpion neurotoxins. These results suggest that BmK ASs and AaH IT4 may form a new group sharing similar structural and functional properties in the family of scorpion neurotoxic polypeptides.

Aah VI, a novel, N-glycosylated anti-insect toxin from Androctonus australis hector scorpion venom: isolation, characterisation, and glycan structure determination

Aah VI was isolated from the venom of the North African scorpion, Androctonus australis hector. It is the first glycosylated neurotoxin from scorpion venom to be described. It was not toxic to mice, when injected intracerebroventricularly at a dose of 1.2 microg per animal. However, it had typical activity in Blatella germanica cockroaches resulting in gradual paralysis and very low toxicity (LD50 = 8.5 microg/g of animal). It consists of 66 amino acid residues and is heterogeneously N-glycosylated at a single site, on asparagine 9, of the Asn-Gly-Thr sequence. The potential N-glycosylation site was deduced from automatic Edman degradation and amino acid analysis, and glycan heterogeneity was evidenced by ESMS. Determination of the N-glycan structures (dHex, Hex and HexNAc) was assessed by nanoESMS/MS with picomolar amounts of sample. Current knowledge of N-glycan structure and composition suggests that the glycan structures are derived from a common core.

Purification and partial characterization of a 'short' insectotoxin-like peptide from the venom of the scorpion Parabuthus schlechteri

A disulfide-rich, low-molecular-mass toxin-like peptide has been isolated from Parabuthus schlechteri venom using gel filtration, ion exchange, and reversed phase chromatography. Partial characterization of this peptide reveals a relationship with four-disulfide bridge proteins belonging to the family of 'short' insectotoxins (44% residue identity). In recognition hereof, the peptide was named PBITx1 (sITx10). Our work also reports on the deduced sequences of two other 'short' insectotoxins from Buthus eupeus, I3 and I4, and it provides a consensus sequence and nomenclature for all known 'short' insectotoxins. Finally, sequence similarities with K+ channel blockers (charybdotoxin, kappa-conotoxin), and a Cl- channel blocker (chlorotoxin) are highlighted.

New toxins acting on sodium channels from the scorpion Leiurus quinquestriatus hebraeus suggest a clue to mammalian vs insect selectivity

Two new toxins were purified from Leiurus quinquestriatus hebraeus (Lqh) scorpion venom, Lqh II and Lqh III. Lqh II sequence reveals only two substitutions, as compared to AaH II, the most active scorpion alpha-toxin on mammals from Androctounus australis Hector. Lqh III shares 80% sequence identity with the alpha-like toxin Bom III from Buthus occitanus mardochei. Using bioassays on mice and cockroach coupled with competitive binding studies with 125I-labeled scorpion alpha-toxins on rat brain and cockroach synaptosomes, the animal selectivity was examined. Lqh II has comparable activity to mammals as AaH II, but reveals significantly higher activity to insects attributed to its C-terminal substitution, and competes at low concentration for binding on both mammalian and cockroach sodium channels. Lqh II thus binds to receptor site 3 on sodium channels. Lqh III is active on both insects and mammals but competes for binding only on cockroach. The latter indicates that Lqh III binds to a distinct receptor site. Thus, Lqh II and Lqh III represent two different scorpion toxin groups, the alpha- and alpha-like toxins, respectively, according to the structural and pharmacological criteria. These new toxins may serve as a lead for clarification of the structural basis for insect vs mammal selectivity of scorpion toxins.

Rapid purification and molecular modeling of AaIT peptides from venom of Androctonus australis

As recombinant viruses expressing scorpion toxins are moving closer toward the market, it is important to obtain large amounts of pure toxin for biochemical characterization and the evaluation of biological activity in nontarget organisms. In the past, we purified a large amount of Androctonus australis anti-insect toxin (AaIT) present in the venom of A. australis with an analytical reversed-phase column by repeated runs of crude sample. We now report 20 times improved efficiency and speed of the purification by employing a preparative reversed-phase column. In just two consecutive HPLC steps, almost 1 mg of AaIT was obtained from 70 mg crude venom. Furthermore, additional AaIT was obtained from side fractions in a second HPLC run. Recently discovered insect selective toxin, AaIT5, was isolated simultaneously from the same venom batch. It shows different biological toxicity symptoms than the known excitatory and depressant insect toxins. AaIT5 gave 100% mortality with a dose of less than 1.3 micrograms against fourth-instar tobacco budworms Heliothis virescens 24 h after injection. During the purification process, we implemented mass spectrometry in addition to bioassays to monitor the presence of AaIT and AaIT5 in the HPLC fractions. Mass spectrometric screening can unambiguously follow the purification process and can greatly facilitate and expedite the downstream purification of AaIT and AaIT5 eliminating the number of bioassays required. Further, electrospray ionization was compared with matrix-assisted desorption/ionization and evaluated as a method of choice for mass spectrometric characterization of fractions from the venom purification for it provided higher mass accuracy and relative quantitation capability. Molecular models were built for AaIT5, excitatory toxin AaIT4, and depressant toxin LqhIT2. Three-dimensional structure of AaIT5 was compared with structures of the other two toxins, suggesting that AaIT5 is similar to depressant toxins.

Novel insecticidal peptides from Tegenaria agrestis spider venom may have a direct effect on the insect central nervous system

Fractionation of venom from an agelenid spider, Tegenaria agrestis, resulted in the isolation of a family of three peptides with potent insecticidal activity. These peptide toxins, TaITX-1, -2, and -3, whose sequences were revealed from cloned cDNAs, each consist of 50 amino acid residues, six of which are cysteines. They appear to be amidated at their C-termini and exhibit greater than 90% sequence identity. Unlike other reported spider toxins, the TaI toxins are processed from precursors containing no propeptide sequences. In lepidopteran larvae and corn rootworm beetles, the insecticidal Tegenaria toxins caused an unusual excitatory symptomatology with 50% paralytic doses ranging from 0.23 to 2.6 nmol/g. In a series of electrophysiological experiments performed in house fly larvae, these toxins caused an elevated rate of firing from central nervous system neurons. No significant effects were found when any peripheral sensory or motor systems were examined. Thus, it appears that the TaI toxins may act in a fashion not previously reported for insecticidal peptide toxins; they may act directly on the insect central nervous system.

A new approach to insect-pest control--combination of neurotoxins interacting with voltage sensitive sodium channels to increase selectivity and specificity

Voltage-sensitive sodium channels are responsible for the generation of electrical signals in most excitable tissues and serve as specific targets for many neurotoxins. At least seven distinct classes of neurotoxins have been designated on the basis of physiological activity and competitive binding studies. Although the characterization of the neurotoxin receptor sites was predominantly performed using vertebrate excitable preparations, insect neuronal membranes were shown to possess similar receptor sites. We have demonstrated that the two mutually competing anti-insect excitatory and depressant scorpion toxins, previously suggested to occupy the same receptor site, bind to two distinct receptors on insect sodium channels. The latter provides a new approach to their combined use in insect control strategy. Although the sodium channel receptor sites are topologically separated, there are strong allosteric interactions among them. We have shown that the lipid-soluble sodium channel activators, veratridine and brevetoxin, reveal divergent allosteric modulation on scorpion alpha-toxins binding at homologous receptor sites on mammalian and insect sodium channels. The differences suggest a functionally important structural distinction between these channel subtypes. The differential allosteric modulation may provide a new approach to increase selective activity of pesticides on target organisms by simultaneous application of allosterically interacting drugs, designed on the basis of the selective toxins. Thus, a comparative study of neurotoxin receptor sites on mammalian and invertebrate sodium channels may elucidate the structural features involved in the binding and activity of the various neurotoxins, and may offer new targets and approaches to the development of highly selective pesticides.

Anti-insect toxin 5 (AaIT5) from Androctonus australis

An insect-selective scorpion toxin (AaIT5) was purified from the venom of the North African scorpion Androctonus australis, and its amino acid sequence was determined by a combination of automated Edman degradation, electrospray-ionization mass spectrometry, and sequence alignment. This insect toxin is very potent against the tobacco budworm, Heliothis virescens (100% lethal dose < 1.8 microg/100 mg body mass) and shows a distinct insect specificity and various symptoms. It is not toxic to mice after subcutaneous injection. The molecular mass of this toxin is 6882 Da and the amino acid sequence is similar to those of Androctonus australis anti-insect toxin 4 (AaIT4), Leiurus quinquestriatus depressant anti-insect toxins (LqhIT2, LqqIT2), and Buthotus judaicus depressant anti-insect toxin (BjIT2).

In vitro folding and functional analysis of an anti-insect selective scorpion depressant neurotoxin produced in Escherichia coli

The selective toxicity of depressant scorpion neurotoxins to insects is useful in studying insect sodium channel gating and has an applied potential. In order to establish a genetic system enabling a structure-activity approach, the functional expression of such polypeptides is required. By engineering the cDNA encoding the depressant scorpion neurotoxin, LahIT2, behind the T7 promoter, large amounts of recombinant insoluble and nonactive toxin were obtained in Escherichia coli. Following denaturation and reduction, the recombinant protein, constructed with an additional N-terminal methionine residue, was subjected to renaturation. Optimal conditions for reconstitution of a functional toxin, having a dominant fold over many other possible isoforms, were established. The recombinant active toxin was purified by RP-HPLC and characterized. Toxicity (ED50) to insects, binding affinity (IC50) to an insect receptor site, and electrophysiological effect on an insect axonal preparation were found to be similar to those of the native toxin. Substitution of the C-terminal glycine by a Gly-Lys-Lys triplet did not abolish folding but affected toxicity (3.5-fold decrease) of LqhIT2. Apparently, this efficient bacterial expression system (500 micrograms HPLC-purified toxin/1 liter E. coli culture) provides the means for studying structure/ activity relationship and the molecular basis for the phylogenetic selectivity of scorpion depressant neurotoxins.

Biochemical and pharmacological characterization of a depressant insect toxin from the venom of the scorpion Buthacus arenicola

A depressant toxin active on insects, Buthacus arenicola IT2, was isolated from the venom of the North African scorpion B. arenicola and its structural and pharmacological properties were investigated. B. arenicola IT2 is a single polypeptide of 61 amino acid residues, including 8 half-cystines but no methionine and histidine, with a molecular mass of 6835 Da. Its amino acid sequence is 79-95% identical to other depressant toxins from scorpions. When injected into the cockroach Blatella germanica, B. arenicola IT2 induced a slow depressant flaccid paralysis with a LD50 of 175 ng. B. arenicola IT2 has two non-interacting binding sites in cockroach neuronal membranes: one of high affinity (Kd1 = 0.11 +/- 0.04 nM) and low capacity (Bmax1 = 2.2 +/- 0.6 pmol/mg), and one of low affinity (Kd2 = 24 +/- 7 nM) and high capacity (Bmax2 = 226 +/- 92 pmol/mg). Its binding to these two sites was completely inhibited by Leiurus quinquestriatus quinquestriatus IT2, a depressant toxin from L. quinquestriatus quinquestriatus. Reciprocal-binding experiments between B. arenicola IT2 and the excitatory insect-toxin A. australis Hector IT revealed competition between the two toxins for the high-affinity sites of B. arenicola IT2. B. arenicola IT2 has a higher affinity than L. quinquestriatus hebraeus IT2, a depressant toxin from L. quinquestriatus hebraeus. Thus, B. arenicola IT2 represents an interesting tool to study the receptor site for depressant toxins on insect sodium channels.

Refined electrophysiological analysis suggests that a depressant toxin is a sodium channel opener rather than a blocker

The effects of a recombinant depressant insect toxin from Leiurus quinquestriatus hebraeus, Lqh IT2-r, have been studied in current and voltage-clamp conditions on the isolated axonal and DUM neuron preparations of the cockroach Periplaneta americana. Lqh IT2-r depolarizes the axon, blocks the evoked action potentials, and modifies the amplitude and the kinetics of the sodium current. The inward transient peak current is greatly decreased and is followed by a maintained slow activating-deactivating sodium current. The slow component develops at membrane potentials more negative than the control, and has a time constant of activation of several tens of milliseconds. The flaccid properties of Lqh IT2-r do not correspond to a blockage of the Na+ channels, but may be attributed to modified Na+ channels which open at more negative potential, activate slowly and do not inactivate normally.

Role of electrostatic interactions in defining the potency of neurotoxin B-IV from Cerebratulus lacteus

Chemical modification implicates arginine residues of the Cerebratulus lacteus neurotoxin B-IV in biological activity. In the present study, we used site-directed mutagenesis to assess the functional contributions of each of these residues. Panels of mutants at each site have been constructed by polymerase chain reaction and recombinant proteins expressed and purified to homogeneity using an Escherichia coli expression system developed in this laboratory. All substitutions for Arg-17 (Gln, Ala, or Lys) yield proteins having undetectable levels of activity, while charge neutralizing replacement of Arg-25 (R25Q) causes a 400-fold reduction in specific toxicity. However, the R25K mutein is almost as active as natural toxin. Circular dichroism spectroscopy indicates that there are no major conformational changes in any of these muteins. These results therefore demonstrate the requirement for a guanidinium group at position 17, and a positive charge at position 25. NMR analyses (Hansen, P. E., Kem, W. R., Bieber, A. L., and Norton, R. S. (1992) Eur. J. Biochem. 210, 231-240) reveal neurotoxin B-IV to contain two antiparallel alpha-helices, which together include 57% of the sequence. Both Arg-17 and Arg-25 lie on the same face of the N-terminal helix (residues 13-26), as do the carboxyl groups of Glu-13 and Asp-21. However, charge neutralizing mutations of the latter two sites have no effects on biological activity. Arg-34, situated near the N terminus of helix 2 (residues 33-49) is also important for activity, as its replacement by Gln or Ala diminishes activity by 20- and 80-fold, respectively. However, unlike Arg-17 and Arg-25, thermal denaturation experiments suggest that R34Q may be structurally destabilized relative to wild-type B-IV.

A new scorpion venom toxin paralytic to insects that affects Na+ channel activation. Purification, structure, antigenicity and mode of action

A new toxin, BotIT2, with a unique mode of action on the isolated giant axon of the cockroach Periplaneta americana and DUM (dorsal unpaired median) neurons, has been purified from the venom of the scorpion Buthus occitanus tunetanus. Its structural, antigenic and pharmacological properties are compared to those of three other groups of neurotoxins found in Buthidae scorpion venoms. Like excitatory, depressant and alpha-type insect-selective neurotoxins, BotIT2 is toxic to insects, but shows the following common and distinctive characteristics. (a) As alpha-type toxins, BotIT2 lack strict selectivity to insects; they have measurable but low toxicity to mice. (b) As depressant toxins and unlike alpha-type toxins, BotIT2 is able to displace iodinated AaHIT from its binding sites in insect neuronal membranes. This indicates that the binding site for BotIT2 is identical, contiguous or in allosteric interaction with that of AaHIT and depressant toxins. (c) The BotIT2 amino acid sequence shows strong similarity to depressant toxins. However, unexpectedly, despite this high sequence similarity, BotIT2 shares moderate cross-antigenic reactivity with depressant toxins. (d) Voltage and current-clamp studies show that BotIT2 induces limited depolarization concomitantly with the development of depolarizing after potential, repetitive activity and later plateau potentials terminated by bursts. Under voltage-clamp conditions, BotIT2 specifically acts on Na+ channels by decreasing the peak Na+ current and by simultaneously inducing a new current with very slow activation/deactivation kinetics. The voltage dependence of this slow current is not significantly different from that of the control current. These observations indicate that BotIT2 chiefly modifies the kinetics of axonal and DUM neuronal membrane Na(+)-channel activation.

A recombinant insect-specific alpha-toxin of Buthus occitanus tunetanus scorpion confers protection against homologous mammal toxins

We have constructed a cDNA library from venom glands of the scorpion Buthus occitanus tunetanus and cloned a DNA sequence that encodes an alpha-toxin. This clone was efficiently expressed in Escherichia coli as a fusion protein with two Ig-binding (Z) domains of protein A from Staphylococcus aureus. After CNBr treatment of the fusion protein and HPLC purification, we obtained approximately 1 mg recombinant apha-toxin/l bacterial culture. The toxin, called Bot XIV, displays no toxicity towards mammals but is active towards insects as shown by its paralytic activity against Blatella germanica cockroach and by electrophysiological studies on Periplaneta americana cockroaches. The Bot XIV protein fused to two Z domains is highly immunogenic in mice and induces production of antisera that specifically recognize and neutralize highly toxic components that had been injected into mice. This fusion protein could be very useful for development of potent protective antisera against scorpion venoms.

High performance liquid chromatography purification and amino acid sequence of toxins from the muscarinic fraction of Tityus discrepans scorpion venom

Tityus discrepans venom was fractionated by gel filtration on Sephadex G-50 column. The peptides in fraction II from Sephadex were further purified by high performance liquid chromatography, through a C4 reverse-phase column. Lethality of purified peptides was determined by injection into mice and crabs, and their effects were verified electrophysiologically on frog (Hyla crepitans) sartorius neuromuscular junction. Toxins having retention times between 39.6 and 40.7 min depolarized the muscle membrane and caused acetylcholine release at the endplate. The toxin eluted at 42.67 min increased the frequency of miniature endplate potentials without depolarizing muscle fibres. The four most active toxins were reduced, carboxymethylated and sequenced by automatic Edman degradation and named TdII-1 to II-4. Toxin gamma from Tityus serrulatus venom and the toxins from T. discrepans venom were found to be structurally distinct. TdII-1 to II-4 lack the pancreatic effects of T. serrulatus' toxin gamma; yet, the five toxins act on Na+ channels.

Scorpion toxins affecting sodium current inactivation bind to distinct homologous receptor sites on rat brain and insect sodium channels

Sodium channels posses receptor sites for many neurotoxins, of which several groups were shown to inhibit sodium current inactivation. Receptor sites that bind alpha- and alpha-like scorpion toxins are of particular interest since neurotoxin binding at these extracellular regions can affect the inactivation process at intramembranal segments of the channel. We examined, for the first time, the interaction of different scorpion neurotoxins, all affecting sodium current inactivation and toxic to mammals, with alpha-scorpion toxin receptor sites on both mammalian and insect sodium channels. As specific probes for rat and insect sodium channels, we used the radiolabeled alpha-scorpion toxins AaH II and LqhalphaIT, the most active alpha-toxins on mammals and insect, respectively. We demonstrate that the different scorpion toxins may be classified to several groups, according to their in vivo and in vitro activity on mammalian and insect sodium channels. Analysis of competitive binding interaction reveal that each group may occupy a distinct receptor site on sodium channels. The alpha-mammal scorpion toxins and the anti-insect Lqh alphaIT bind to homologous but not identical receptor sites on both rat brain and insect sodium channels. Sea anemone toxin ATX II, previously considered to share receptor site 3 with alpha-scorpion toxins, is suggested to bind to a partially overlapping receptor site with both AaH II and Lqh alphaIT. Competitive binding interactions with other scorpion toxins suggest the presence of a putative additional receptor site on sodium channels, which may bind a unique group of these scorpion toxins (Bom III and IV), active on both mammals and insects. We suggest the presence of a cluster of receptor sites for scorpion toxins that inhibit sodium current inactivation, which is very similar on insect and rat brain sodium channels, in spite of the structural and pharmacological differences between them. The sea anemone toxin ATX II is also suggested to bind within this cluster.

Effect of Leiurus quinquestriatus hebreus venom on calcium and deoxyglucose uptake in cultured cardiac cells

The effects of scorpion venom Leiurus quinquestriatus hebreus were studied on cardiac cells grown in culture. The venom (30 micrograms/ml) increased significantly (P < 0.05) Ca2+ uptake into intact cardiocytes and to sarcoplasmic reticulum of skinned cells. [3H]Deoxyglucose uptake was also increased significantly (P < 0.05) in venom treated cardiocytes. It was found that fractions I and III of the venom, separated by gel filtration and ion exchange chromatography, are responsible for the increased Ca2+ uptake by the sarcoplasmic reticulum, whereas fraction IIb, III and IV are responsible for the accelerated rate of uptake of 45Ca and [3H]deoxyglucose by intact cells. Ca channel blockers prevented these effects and similar results were obtained by propranolol. Thus, it is concluded that the venom exerts its effect through activation of beta-adrenoceptors which causes the opening of L-type Ca channels.

A microinjection technique using Drosophila melanogaster for bioassay-guided isolation of neurotoxins in arthropod venoms

Modern analytical techniques permit isolation and structural determination of neurotoxins at the picomole level. However, bioassay-guided fractionation of the sample often relies on simple injection assays using insects, vertebrates or crustaceans of a fairly large size, thus consuming quite a large amount of the samples being investigated. In order to investigate samples of very small size, we have devised an insect microinjection method using glass micropipettes and Drosophila melanogaster adults as test insects. The validity of the method was tested with a series of six buthoid scorpion venoms (Androctonus australis, Buthotus judaicus, Buthus tamulus, Centruroides sculpturatus, Leiurus quinquestriatus hebraeus, Tityus serrulatus) and one chactoid scorpion (Scorpio maurus palmatus) as standards. The LD50S of the venoms were determined using both the microinjection method and a classical injection assay with crickets (Gryllus bimaculatus) as test insects. Results demonstrated that the new method can successfully be applied to the study of insect neurotoxic activity in arthropod venoms. The Gryllus:Drosophila ratio in amount of sample utilized is 100. However, for all Buthoid venoms tested, except L. quinquestriatus, Drosophila showed less sensitivity, thus reducing the gain by a factor of 2-10. Drosophila were several times more sensitive to the only chactoid venom tested. These results clearly demonstrate the advantage of using this microtechnique, when limited amounts of material are available for both chemical and biological work.

Positive cooperativity among insecticidal scorpion neurotoxins

The insecticidal activity of scorpion neurotoxic polypeptides increased 5-10-fold with no apparent increase in mammalian toxicity when a combination of two toxins was injected. Synergistic combinations could be predicted from binding studies and competitive displacement assays. Our results indicate that simultaneous expression in baculovirus or other transgenic organisms of the synergistic combinations of insecticidal toxins may result in more potent insect-selective biopesticides.

Alpha-scorpion toxins binding on rat brain and insect sodium channels reveal divergent allosteric modulations by brevetoxin and veratridine

At least six topologically separated neurotoxin receptor sites have been identified on sodium channels that reveal strong allosteric interactions among them. We have studied the allosteric modulation induced by veratridine, binding to receptor site 2, and brevetoxin PbTx-1, occupying receptor site 5, on the binding of alpha-scorpion toxins at receptor site 3, on three different neuronal sodium channels: rat brain, locust, and cockroach synaptosomes. We used 125I-AaH II, the most active alpha-scorpion toxin on vertebrates, and 125I-Lqh alpha IT, shown to have high activity on insects, as specific probes for receptor site 3 in rat brain and insect sodium channels. Our results reveal that brevetoxin PbTx-1 generates three types of effects at receptor site 3:1) negative allosteric modulation in rat brain sodium channels, 2) positive modulation in locust sodium channels, and 3) no effect on cockroach sodium channel. However, PbTx-1 activates sodium channels in cockroach axon similarly to its activity in other preparation. Veratridine positively modulates both rat brain and locust sodium channels but had no effect on alpha-toxin binding in cockroach. The dramatic differences in allosteric modulations in each sodium channel subtype suggest structural differences in receptor sites for PbTx-1 and/or at the coupling regions with alpha-scorpion toxin receptor sites in the different sodium channels, which can be detected by combined application of specific channel modifiers and may elucidate the dynamic gating activity and the mechanism of allosteric interactions among various neurotoxin receptors.

Isolation and characterization of a novel toxin from the venom of the scorpion Centruroides limpidus limpidus Karsch

A novel peptide, toxic to mice, was purified from the venom of the Mexican scorpion Centruroides limpidus limpidus, by means of gel filtration and ion exchange chromatography, followed by high performance liquid chromatography (HPLC). The complete amino acid sequence was determined by automatic Edman degradation of reduced and alkylated toxin, and by overlapping sequences of fragments of the toxin, generated by cleavage with proteinase V8 separated by HPLC. This toxin is composed of 66 amino acid residues, contains eight half-cystine residues, and is highly similar (91%) to the amino acid sequence deduced for toxin 1 of C. limpidus tecomanus and toxin 4 from C. noxius venom (89%). This peptide displaces the binding of radiolabeled toxin 2 of C. noxius from synaptosomal membranes of rat brain with superimposable kinetics, supporting the conclusion that it belongs to the beta-scorpion toxin class. Further characterization of C. l. limpidus toxin 1, as we have named it, was performed by means of competition experiments with monoclonal antibodies and various purified scorpion toxins, using an ELISA assay. A panel of six distinct monoclonal antibodies (mAB) against toxin 2 and 3 of C. noxius was used. From these, only three clones, originally named BCF1, BCF8 and BCF9, were able to recognize toxin 1 from C. l. limpidus.

Binding of an alpha scorpion toxin to insect sodium channels is not dependent on membrane potential

The insect-specific Lqh alpha IT toxin resembles alpha scorpion toxins affecting mammals by its amino acid sequence and effects on sodium conductance. The present study reveals that Lqh alpha IT does not bind to rat brain membranes and possesses in locust neuronal membranes a single class of high affinity (Kd = 1.06 +/- 0.15 nM) and low capacity (Bmax = 0.7 +/- 0.19 pmol/mg protein) binding sites. The latter are: (1) distinct from binding sites of other sodium channel neurotoxins; (2) inhibited by sea anemone toxin II; (3) cooperatively interacting with veratridine; (4) not dependent on membrane potential, in contrast to the binding sites of alpha toxins in vertebrate systems. These data suggest the occurrence of (a) conformational-structural differences between insect and mammal sodium channels and (b) the animal group specificity and pharmacological importance of the alpha scorpion toxins.

Insect neuropeptides: discovery and application in insect management

New approaches to the development of insect control agents have been revealed through the molecular description of neuropeptides, their biogenesis, action, and degradation. Prerequisite to the exploitation of a neuropeptide as a lead to control agent development is a thorough understanding of the biochemistry of the neuropeptide and appreciation of its physiological impact. Reliable bioassays must be coupled with advanced biochemical and molecular genetic technologies to overcome limitations imposed by the typically low endogenous levels of individual neuropeptides. Purification, amino acid sequencing, and gene cloning provide the molecular tools necessary for studies on neuropeptide synthesis, processing, secretion, receptor binding, and inactivation. Each of these areas consists of a number of amino acid sequence-, and enzyme-dependent steps which may be considered as targets for the development of highly specific control agents. These agents will include antagonist and superagonists, peptidomimetics, recombinant peptides delivered through the baculovirus technology, receptor blockers, and enzyme inhibitors.

Characterization of toxin III of the scorpion Leiurus quinquestriatus quinquestriatus: a new type of alpha-toxin highly toxic both to mammals and insects

The primary structure of toxin III of Leiurus quinquestriatus quinquestriatus (Lqq III) was elucidated by automatic Edman degradation of the reduced and S-carboxymethylated protein and derived tryptic peptides. Like other scorpion toxins that are active on sodium channels, Lqq III, consisting of 64 amino acids, is a 7 kDa single-chain polypeptide crosslinked by four disulfide bridges. It belongs to the alpha-toxin group, as judged by competition experiments with 125I AaH II for binding to rat brain synaptosomes (K0.5 = 7 x 10(-7) M). Lqq III is the first alpha-toxin to be characterized that is highly toxic to mice [LD50 = 50 micrograms (7.1 nmol)/kg body wt], by subcutaneous injection, insects Blatella germanica [LD50 = 60 ng (8.5 pmol)/g body wt.] and Musca domestica [LD50 = 120 ng (17 pmol)/g body wt]. When tested via the intracerebroventricular route, the toxicity for mice [55 micrograms (8 nmol)/kg] was of the same order as that found by subcutaneous injection, indicating that Lqq III has a higher affinity for peripheral sodium channels that for those of the central nervous system. There are three differences between the sequences of Lqq III and Lqh alpha IT, an alpha-toxin isolated from the venom of Leiurus quinquestriatus hebraeus. These substitutions are found at positions 20, 24, and 64 (Ser-->Ala,Asp-->Glu and His-->Arg, respectively). Surprisingly Lqh alpha IT is only weakly active in mice [LD50 = 5 mg (0.7 mumol)/kg], while in insects its toxicity is similar to that of Lqq III [140 ng (20 pmol)/g body wt blowfly larvae]. These observations are relevant to the definition of scorpion toxin structure-activity relationships.

Depressant insect selective neurotoxins from scorpion venom: chemistry, action, and gene cloning

The present study examines the similarity in the symptoms and binding properties between the depressant and excitatory insect-selective neurotoxins, derived from scorpion venom. A comparison of their primary structures and neuromuscular effects is presented. A new depressant toxin (LqhIT2) was purified from the venom of the scorpion Leiurus quinquestriatus hebraeus. The effects of this toxin on a prepupal housefly neuromuscular preparation mimic its effects on the intact insect, i.e, a brief period of repetitive bursts of regular junction potentials (JPs) is followed by reduced amplitude JPs ending with a block of the neuromuscular transmission. "Loose" patch clamp recordings indicate that the repetitive activity has a presynaptic origin (the motor nerve) and resembles the effect of the excitatory toxin AaIT. The final synaptic block is supposed to be the end result of neuronal membrane depolarization. Such an effect is not caused by an excitatory toxin, which induces long "trains" of repetitive firing. The amino acid sequences of three depressant toxins were determined by automatic Edman degradation indicating a high degree of sequence homology. This conservation differs from those of other groups of scorpion toxins. The opposing pharmacological effects of depressant toxins are discussed in light of the above neuromuscular effects and sequence analysis. A genetic approach in the study of the structure-function relationships of the depressant toxins was initiated by isolating cDNA clones encoding the LqhIT2 and BjIT2 toxins. Their sequence analysis revealed the precursor form of these toxins: A 21 amino acid residue signal peptide followed by a 61 amino acid region of the mature toxin, and three additional amino acids at the carboxy terminus.

Development of recombinant viral insecticides by expression of an insect-specific toxin and insect-specific enzyme in nuclear polyhedrosis viruses

As supplements to classical chemical insecticides, two approaches to develop recombinant baculovirus insecticides are described. In one approach an insect-specific toxin is expressed leading to a dramatic reduction in time to death. In the second approach an insect juvenile hormone esterase is expressed which leads to a reduction in feeding. Modifications of the wildtype esterase led to viruses which reduced the time to death as effectively as did the toxin-expressing virus. In both cases existing recombinant viruses are viewed as leads, and approaches to further improvement in the engineered viruses are suggested. Many of these approaches are based on analogy with the development of classical synthetic insecticides. Using these viruses as examples, the potential utility and limitations of recombinant viruses and other biological insecticides are discussed.

Nucleotide sequence and structure analysis of a cDNA encoding an alpha insect toxin from the scorpion Leiurus quinquestriatus hebraeus

A approximately 370 base pair cDNA encoding the alpha insect toxin Lqh alpha IT of the scorpion Leiurus quinquestriatus hebraeus was cloned and sequenced. The deduced amino acid sequence for the putative mature polypeptide is identical to the protein sequence determined chemically (Eitan et al., Biochemistry 29, 5941, 1990). A 19 amino acid signal peptide precedes the 64 amino acid long toxin. Two additional amino acid residues that do not correspond to the purified toxin are found at the COOH-terminus and may imply post-translational modification. The signal peptide region in the present clone differs obviously from that encoding the depressant insect toxin LqhIT2 derived from the same venom, but strongly resembles the leader peptide sequence of an alpha-mammal toxin from the scorpion Androctonus australis.