Synthesis of a gene coding for an insect-specific scorpion neurotoxin and attempts to express it using baculovirus vectors

Insect Biotechnology

For the purpose of this work, insect biotechnology, which is also known as yellow biotechnology, is the use of insects as well as insect-derived cells or molecules in medical (red biotechnology), agricultural (green biotechnology), and industrial (white) biotechnology. It is based on the application of biotechnological techniques on insects or their cells to develop products or services for human use. Such products are then applied in agriculture, medicine, and industrial biotechnology. Insect biotechnology has proven to be a useful resource in diverse industries, especially for the production of industrial enzymes including chitinases and cellulases, pharmaceuticals, microbial insecticides, insect genes, and many other substances. Insect cells (ICs), and particularly lepidopteran cells, constitute a competitive strategy to mammalian cells for the manufacturing of biotechnology products. Among the wide range of methods and expression hosts available for the production of biotech products, ICs are ideal for the production of complex proteins requiring extensive posttranslational modification. The progress so far made in insect biotechnology essentially derives from scientific breakthroughs in molecular biology, especially with the advances in techniques that allow genetic manipulation of organisms and cells. Insect biotechnology has grown tremendously in the last 30 years.

Baculovirus insecticides in Latin America: historical overview, current status and future perspectives

Baculoviruses are known to regulate many insect populations in nature. Their host-specificity is very high, usually restricted to a single or a few closely related insect species. They are amongst the safest pesticides, with no or negligible effects on non-target organisms, including beneficial insects, vertebrates and plants. Baculovirus-based pesticides are compatible with integrated pest management strategies and the expansion of their application will significantly reduce the risks associated with the use of synthetic chemical insecticides. Several successful baculovirus-based pest control programs have taken place in Latin American countries. Sustainable agriculture (a trend promoted by state authorities in most Latin American countries) will benefit from the wider use of registered viral pesticides and new viral products that are in the process of registration and others in the applied research pipeline. The success of baculovirus-based control programs depends upon collaborative efforts among government and research institutions, growers associations, and private companies, which realize the importance of using strategies that protect human health and the environment at large. Initiatives to develop new regulations that promote the use of this type of ecological alternatives tailored to different local conditions and farming systems are underway.

Expression, delivery and function of insecticidal proteins expressed by recombinant baculoviruses

Since the development of methods for inserting and expressing genes in baculoviruses, a line of research has focused on developing recombinant baculoviruses that express insecticidal peptides and proteins. These recombinant viruses have been engineered with the goal of improving their pesticidal potential by shortening the time required for infection to kill or incapacitate insect pests and reducing the quantity of crop damage as a consequence. A wide variety of neurotoxic peptides, proteins that regulate insect physiology, degradative enzymes, and other potentially insecticidal proteins have been evaluated for their capacity to reduce the survival time of baculovirus-infected lepidopteran host larvae. Researchers have investigated the factors involved in the efficient expression and delivery of baculovirus-encoded insecticidal peptides and proteins, with much effort dedicated to identifying ideal promoters for driving transcription and signal peptides that mediate secretion of the expressed target protein. Other factors, particularly translational efficiency of transcripts derived from recombinant insecticidal genes and post-translational folding and processing of insecticidal proteins, remain relatively unexplored. The discovery of RNA interference as a gene-specific regulation mechanism offers a new approach for improvement of baculovirus biopesticidal efficacy through genetic modification.

A new theraphosid spider toxin causes early insect cell death by necrosis when expressed in vitro during recombinant baculovirus infection

Baculoviruses are the most studied insect viruses in the world and are used for biological control of agricultural and forest insect pests. They are also used as versatile vectors for expression of heterologous proteins. One of the major problems of their use as biopesticides is their slow speed to kill insects. Thus, to address this shortcoming, insect-specific neurotoxins from arachnids have been introduced into the baculovirus genome solely aiming to improve its virulence. In this work, an insecticide-like toxin gene was obtained from a cDNA derived from the venom glands of the theraphosid spider Brachypelma albiceps. The mature form of the peptide toxin (called Ba3) has a high content of basic amino acid residues, potential for three possible disulfide bonds, and a predicted three-stranded β-sheetDifferent constructions of the gene were engineered for recombinant baculovirus Autographa californica multiple nuclepolyhedrovirus (AcMNPV) expression. Five different forms of Ba3 were assessed; (1) the full-length sequence, (2) the pro-peptide and mature region, (3) only the mature region, and the mature region fused to an (4) insect or a (5) virus-derived signal peptide were inserted separately into the genome of the baculovirus. All the recombinant viruses induced cell death by necrosis earlier in infection relative to a control virus lacking the toxin gene. However, the recombinant virus containing the mature portion of the toxin gene induced a faster cell death than the other recombinants. We found that the toxin construct with the signal peptide and/or pro-peptide regions delayed the necrosis phenotype. When infected cells were subjected to ultrastructural analysis, the cells showed loss of plasma membrane integrity and structural changes in mitochondria before death. Our results suggest this use of baculovirus is a potential tool to help understand or to identify the effect of insect-specific toxic peptides when produced during infection of insect cells.

Scorpion and spider venom peptides: gene cloning and peptide expression

This communication reviews most of the important findings related to venom components isolated from scorpions and spiders, mainly by means of gene cloning and expression. Rather than revising results obtained by classical biochemical studies that report structure and function of venom components, here the emphasis is placed on cloning and identification of genes present in the venomous glands of these arachnids. Aspects related to cDNA library construction, specific or random ESTs cloning, transcriptome analysis, high-throughput screening, heterologous expression and folding are briefly discussed, showing some numbers of species and components already identified, but also shortly mentioning limitations and perspectives of research for the future in this field.

The establishment of a controllable expression system in baculovirus: stimulated overexpression of polyhedrin promoter by LEF-2

Previously, controllable gene expression in baculovirus was not possible using an insect system. We found that this was due to a high background activation of minimal promoter by the viral polyhedrin upstream (pu) sequence. Here, by truncation of the pu sequence, regulatory gene expression was established through the tetracycline regulatory expression system. This novel system was used to test the stimulatory function of the polyhedrin promoter by the controlled expression of the late expression factor-2 (lef-2). To efficiently trace lef-2 expression and analyze suppression of this gene, the coding sequences of lef-2 and enhanced green fluorescent protein (egfp) were ligated together to generate a fusion protein, and an approximately 100-fold suppression of egfp-lef-2 expression was achieved by doxycycline treatment. A very low level expression of lef-2 was found to be sufficient for proper expression of polyhedrin promoter; however, progressively higher levels of lef-2 expression could stimulate much higher-than-original polyhedrin promoter expression in the viral genome. This system was found to exhibit significantly better suppression than the double-stranded RNA (dsRNA) strategy, and would be useful for expression of foreign or viral genes whose functions require the interaction of multiple and/or unknown baculovirus gene products.

Delivery methods for peptide and protein toxins in insect control

Since the introduction of DDT in the 1940s, arthropod pest control has relied heavily upon chemical insecticides. However, the development of insect resistance, an increased awareness of the real and perceived environmental and health impacts of these chemicals, and the need for systems with a smaller environmental footprint has stimulated the search for new insecticidal compounds, novel molecular targets, and alternative control methods. In recent decades a variety of biocontrol methods employing peptidic or proteinaceous insect-specific toxins derived from microbes, plants and animals have been examined in the laboratory and field with varying results. Among the many interdependent factors involved with the production of a cost-effective pesticide--production expense, kill efficiency, environmental persistence, pest-specificity, pest resistance-development, public perception and ease of delivery--sprayable biopesticides have not yet found equal competitive footing with chemical counterparts. However, while protein/peptide-based biopesticides continue to have limitations, advances in the technology, particularly of genetically modified organisms as biopesticidal delivery systems, has continually progressed. This review highlights the varieties of delivery methods currently practiced, examining the strengths and weaknesses of each method.

Milestones leading to the genetic engineering of baculoviruses as expression vector systems and viral pesticides

The baculovirus expression vector system (BEVS) is widely established as a highly useful and effective eukaryotic expression system. Thousands of soluble and membrane proteins that, in general, are correctly folded, modified, sorted and assembled to produce highly authentic recombinant proteins have been cloned and expressed. This historical chronology and perspective will focus on the original, peer-reviewed discoveries that were pioneering and seminal to the development of the BEVS and that provided the basis for subsequent and more recent developments and applications.

Genetically modified baculoviruses: a historical overview and future outlook

The concept of using genetic engineering to improve the natural insecticidal activity of baculoviruses emerged during the 1980s. Both academic and industrial laboratories have since invested a great deal of effort to generate genetically modified (GM) or recombinant baculoviruses with dramatically improved speeds of kill. Optimal production methodologies and formulations have also been developed, and the safety and ecology of the recombinant baculoviruses have been thoroughly investigated. Unfortunately, the initial excitement that was generated by these technologies was tempered when industry made a critical decision to not complete the registration process of GM baculoviruses for pest insect control. In this chapter, we summarize the developments in the field from a historical perspective and provide our opinions as to the current status and future potential of the technology. We will argue that GM baculoviruses are valuable and viable tools for pest insect control both alone and in combination with wild-type viruses. We believe that these highly effective biopesticides still have a bright future in modern agriculture as public awareness and acceptance of GM organisms, including GM baculoviruses, increases.

Polyclonal Antibody Against a Recombinant Chlorotoxin-like Peptide from the Chinese Scorpion and Detection of its Putative Receptors in Human Glioma Cells

The nucleotide sequence of a type of chlorotoxin-like peptide, an inhibitor of small-conductance Cl(-) channels, from the scorpion, Buthus martensii Karsch, was synthesized (named rBmK CTa) according to the sequence optimized for codon usage in E. coli. It was over-expressed using a pExSecI expression system and purified to homogeneity. Polycolonal antibodies to the purified protein were raised in rats. Overlay assay and pull-down assay showed that this toxin specially binds to two proteins in the glioma cells with corresponding molecular weights of about 80 and 35 kDa. They may serve as candidate receptors or alternative cellular component for interaction with rBmK CTa.

Enhancing insecticidal efficacy of baculovirus by early expressing an insect neurotoxin, LqhIT2, in infected Trichoplusia ni larvae

LqhIT(2), an insect specific neurotoxin from the venom of Leiurus quinquestriatus hebraeus, has been demonstrated to improve insecticidal efficacy of Autographa californica nuclear polyhedrosis virus (AcMNPV). A polyhedrin-positive recombinant AcMNPVvAcP(hsp70)EGFP/P(pag90)IT(2) was engineered for larvae to express the enhanced green fluorescence protein (EGFP) and LqhIT(2) under the control of P(hsp70) and P(pag90) promoters, respectively. This would allow a visual observation of the viral infection and an improvement of the insecticidal efficacy. The insecticidal activity of this recombinant baculovirus, a wild type AcMNPV and four other recombinant baculoviruses, was evaluated and compared in terms of mortality, body weight, median lethal time (LT(50)), and median lethal concentration (LC(50)). Insecticidal efficacy was unaltered when treated with vAcP(hsp70)EGFP, moderately improved when infected by vAcP(10)IT(2) (a P(10)-promoted LqhIT ( 2 ) gene), and significantly elevated when treated with vAcP(pag90)IT(2) or vAcP(hsp70)EGFP/P(pag90)IT(2). No apparent difference was observed in insecticidal efficacy when additional EGFP was expressed as a visible marker. These results suggest that recombinant AcMNPV vAcP(hsp70)EGFP/P(pag90)IT(2) may be used as an effective insecticide against Trichoplusia ni and other lepidopterous insect pests.

Baculoviruses-- re-emerging biopesticides

Biological control of agricultural pests has gained importance in recent years due to increased pressure to reduce the use of agrochemicals and their residues in the environment and food. Viruses of a few families are known to infect insects but only those belonging to the highly specialized family Baculoviridae have been used as biopesticides. They are safe to people and wildlife, their specificity is very narrow. Their application as bioinsecticides was limited until recently because of their slow killing action and technical difficulties for in vitro commercial production. Two approaches for the wider application of baculoviruses as biopesticides will be implemented in future. In countries where use of genetically modified organisms is restricted, the improvements will be mainly at the level of diagnostics, in vitro production and changes in biopesticide formulations. In the second approach, the killing activity of baculoviruses may be augmented by genetic modifications of the baculovirus genome with genes of another natural pathogen. It is expected that the baculoviruses improved by genetic modifications will be gradually introduced in countries which have fewer concerns towards genetically modified organisms.

Synthesis, Expression and Purification of a Type of Chlorotoxin-like Peptide from the Scorpion, Buthus martensii Karsch, and its Acute Toxicity Analysis

A gene, rBmK Cta, encoding a chlorotoxin-like peptide from the scorpion, Buthus martensii Karsch, was synthesized according to the sequence optimized for codon usage in Escherichia coli and was expressed in E. coli BL21 (DE3) using a pExSecI expression system in which the IgG-binding domain-ZZ of protein A is fused to the N-terminal of rBmK CTa. The fusion protein, ZZ-rBmK CTa, was expressed in soluble form (7.8 mg l(-1)) and was purified to give a single band on SDS-PAGE. The domain-ZZ of fusion protein ZZ-rBmK CTa was removed by cleavage of an Asn-Gly peptide bond with hydroxylamine. The rBmK CTa was separated from the IgG-binding moiety by a second passage through the IgG affinity column. Western blot analysis demonstrated that this protein was rBmK CTa. Acute toxicity assay in mice demonstrated that the rBmK CTa had an LD(50) value of 4.3 mg kg(-1).

Data Analysis for a Dual-Channel Virus Counter

A simple algorithm is presented for quantitative analysis of simultaneous events on a dual-channel flow cytometer designed specifically for virus counting. The algorithm, which is based on matrix analysis of burst lag times, was evaluated using baculovirus samples that had previously been quantified by the plaque titer method. The results indicated statistical reliability for the algorithm, with three of six samples yielding the same value, within error, for viruses per unit volume as the plaque titer. The other three samples yielded values within a factor approximately 2, which was deemed acceptable given the limitations of the plaque titer method.

Expression and purification of the BmK Mm2 neurotoxin from the scorpion Buthus martensii Karsch and its biological activity test

The gene encoding neurotoxin (BmK Mm2) from the scorpion Buthus martensii Karsch was expressed in Escherichia coli BL21 (DE3) at a level of 1.6 mg/L using expression plasmid pExSecI system. SDS-PAGE analysis of the cell lysate confirmed that gene BmK Mm2 was expressed in soluble form and the expressed production was secreted into Luria-Bertani (LB) culture medium from Escherichia coli. According to the characters of pExSecI expression system, the IgG binding domain-ZZ of Protein A is fused to the N-terminal of BmK Mm2. Recombinant BmK Mm2 (ZZ-BmK Mm2, pI 6.81, 22.007 kDa) was purified rapidly and efficiently by IgG-Sepharose 6 Fast Flow and Superdex-75 gel filtration chromatography, produced a single band on SDS-PAGE. Western blot analysis demonstrated that this protein was recombinant BmK Mm2. The results of MTT assay, morphological observation of nucleus and single cell gel electrophoresis showed that the expressed recombinant BmK Mm2 was toxic for glial cells of mice, which indicate that it has biological activity.

Poneratoxin, a neurotoxin from ant venom

Poneratoxin is a small neuropeptide found in the venom of the ant Paraponera clavata. It is stored in the venom reservoir as an inactive 25-residue peptide. Here we describe both chemically synthesized poneratoxin and poneratoxin obtained by expression in insect cells. When expressed in insect cells, poneratoxin was observed attached to cell membranes. Both synthetic and recombinant ponerotoxins were soluble below pH 4.5. The structure of synthetic poneratoxin was characterized by circular dichroism and solved by nuclear magnetic resonance. In an environment imitating a lipid bilayer, at pH within the range of insect hemolymph, synthetic poneratoxin has a V shape, with two alpha-helices connected by a beta-turn. Insect larvae were paralyzed by injection of either of the purified toxins, with the recombinant one acting faster. The recombinant toxin-producing baculovirus reduced the average survival time of the insect host by 25 h compared with unmodified virus. Mass spectrometry analysis showed that the recombinant toxin has an N-terminal 21-residue extension, possibly improving its stability and/or stabilizing the membrane-bound state. The potential use of poneratoxin for the construction of biological insecticide is discussed.

Expression of an antitumor-analgesic peptide from the venom of Chinese scorpion Buthus martensii karsch in Escherichia coli

The gene encoding a putative mature antitumor-analgesic peptide (AGAP) from the venom of the Chinese scorpion Buthus martensii Karsch was obtained by polymerase chain reaction (PCR) according to its cDNA sequence and expressed in Escherichia coli. While most of the recombinant AGAP was expressed in the form of insoluble inclusion body. The recombinant AGAP was purified to homogeneity by metal chelating affinity chromatography. Pharmaceutical tests showed that the recombinant AGAP has both analgesic and antitumor activities on mice.

Purification, characterization and biosynthesis of parabutoxin 3, a component of Parabuthus transvaalicus venom

A novel peptidyl inhibitor of voltage-gated K+ channels, named parabutoxin 3 (PBTx3), has been purified to homogeneity from the venom of Parabuthus transvaalicus. This scorpion toxin contains 37 residues, has a mass of 4274 Da and displays 41% identity with charybdotoxin (ChTx, also called 'alpha-KTx1.1'). PBTx3 is the tenth member (called 'alpha-KTx1.10') of subfamily 1 of K+ channel-blocking peptides known thus far. Electrophysiological experiments using Xenopus laevis oocytes indicate that PBTx3 is an inhibitor of Kv1 channels (Kv1.1, Kv1.2, Kv1.3), but has no detectable effects on Kir-type and ERG-type channels. The dissociation constants (Kd) for Kv1.1, Kv1.2 and Kv1.3 channels are, respectively, 79 microm, 547 nm and 492 nm. A synthetic gene encoding a PBTx3 homologue was designed and expressed as a fusion protein with the maltose-binding protein (MBP) in Escherichia coli. The recombinant protein was purified from the bacterial periplasm compartment using an amylose affinity resin column, followed by a gel filtration purification step and cleavage by factor Xa (fXa) to release the recombinant toxin peptide (rPBTx3). After final purification and refolding, rPBTx3 was shown to be identical to the native PBTx3 with respect to HPLC retention time, mass spectrometric analysis and functional properties. The three-dimensional structure of PBTx3 is proposed by homology modelling to contain a double-stranded antiparallel beta sheet and a single alpha-helix, connected by three disulfide bridges. The scaffold of PBTx3 is homologous to most other alpha-KTx scorpion toxins.

Recombinant baculoviruses for insect control

Baculoviruses are double-stranded DNA viruses which are highly selective for several insect groups. They are valuable natural control agents, but their utility in many agricultural applications has been limited by their slow speed of kill and narrow host specificity. Baculoviruses have been genetically modified to express foreign genes under powerful promoters in order to accelerate their speed of kill. In our and other laboratories, the expression of genes coding for insect juvenile hormone esterases and various peptide neurotoxins has resulted in recombinant baculoviruses with promise as biological insecticides. These viruses are efficacious in the laboratory, greenhouse and field and dramatically reduce damage caused by insect feeding. The recombinant viruses synergize and are synergized by classical pesticides such as pyrethroids. Since they are highly selective for pest insects, they can be used without disrupting biological control. Because the recombinant virus produces fewer progeny in infected larvae than the wild-type virus, they are rapidly out-competed in the ecosystem. The viruses can be used effectively with crops expressing endotoxins of Bacillus thuringiensis. They can be produced industrially but also by village industries, indicating that they have the potential to deliver sustainable pest control in developing countries. It remains to be seen, however, whether the current generation of recombinant baculoviruses will be competitive with the new generation of synthetic chemical pesticides. Current research clearly indicates, though, that the use of biological vectors of genes for insect control will find a place in agriculture. Baculoviruses will also prove valuable in testing the potential utility of proteins and peptides for insect control.

Expression of anti-neuroexcitation peptide (ANEP) of scorpion Buthus martensii Karsch in Escherichia coli

According to the cDNA sequence of anti-neuroexcitation peptide of scorpion Buthus martensii Karsch, the putative mature anti-neuroexcitation peptide (ANEP) encoding DNA fragment was obtained by a PCR method, then was cloned into expression plasmid pET28a, fused with His tag at its 3' end. When expressed in E. coli BL21 (DE3), the expression of recombinant ANEP was 15% of total cellular proteins, while most recombinant ANEP products existed in the form of insoluble inclusion bodies. Coexpression of molecular chaperones or protein disulfide isomerase could not improve its solubility. The recombinant ANEP in the cell lysate was purified to homogeneity by metal chelating affinity chromatography and Superdex 30 chromatography. In bioassay with convulsive mice model induced by thiosemicarbazide, recombinant ANEP could apparently delay the convulsion seizure of model animals by 18% and showed anti-neuroexcitatory activity.

Expression and purification of the BmK M1 neurotoxin from the scorpion Buthus martensii Karsch

The gene encoding a neurotoxin (BmK M1) from the scorpion Buthus martensii Karsch was expressed in Saccharomyces cerevisiae at a high level with the alcohol dehydrogenase promoter. SDS-PAGE of the culture confirmed expression and showed secretion into medium from yeast. Recombinant BmK M1 was purified rapidly and efficiently by ion exchange and gel filtration chromatography to homogeneity, produced a single band on tricine-SDS-PAGE, and processed the homologous N-terminus. Amino acid analysis and N-terminal sequencing demonstrated that the recombinant toxin was processed correctly from the alpha-mating factor leader sequence and was chemically identical to the native form. The expressed recombinant BmK M1 was toxic for mice, which indicated that it was biologically active. Quantitative estimation showed that recombinant BmK M1 had an LD(50) similar to that of the native toxin.

The characterization and phylogenetic relationship of the Trichoplusia ni single capsid nuclear polyhedrosis virus polyhedrin gene

The polyhedrin gene (polh) was identified from the Trichoplusia ni (Tni) single capsid nuclear polyhedrosis virus (SNPV). An EcoRI fragment containing the truncated polyhedrin gene was detected by hybridization with an AcMNPV expression vector probe; the remaining portion of the gene was amplified by reverse PCR. An open reading frame (ORF) of 741 nucleotides (nt), encoding a putative protein of 246 amino acids (a.a) with Mr 28,780 Da was identified. The 5'-noncoding region contained the putative late (TAAG) transcription initiation motif. The 3' end, downstream of the translation stop codon, lacked an obvious putative poly (A) signal. Nucleotide and amino acid homology are greater than 80% to that of Mamestra brassicae polyhedrin sequences. Results suggest that T. niSNPV is a member of the group II nuclear polyhedrosis viruses.

A field release of genetically engineered gypsy moth (Lymantria dispar L.) nuclear polyhedrosis virus (LdNPV)

The gypsy moth (Lymantria dispar L.) nuclear polyhedrosis virus was genetically engineered for nonpersistence by removal of the gene coding for polyhedrin production and stabilized using a coocclusion process. A beta-galactosidase marker gene was inserted into the genetically engineered virus (LdGEV) so that infected larvae could be tested for its presence using a colorimetric assay. In 1993, LdGEV-infected gypsy moths were released in a forested plot in Massachusetts to test for spread and persistence. A similar forested plot 2 km away served as a control. For 3 years (1993-1995), gypsy moths were established in the two plots in Massachusetts to serve as test and control populations. Each week, larvae were collected from both plots. These field-collected larvae were reared individually, checked for mortality, and then tested for the presence of beta-galactosidase. Other gypsy moth larvae were confined on LdGEV-contaminated foliage for 1 week and then treated as the field-collected larvae. The LdGEV was sought in bark, litter, and soil samples collected from each plot. To verify the presence of the LdGEV, polymerase chain reaction, slot blot DNA hybridization, and restriction enzyme analysis were also used on larval samples. Field-collected larvae infected with the engineered virus were recovered in the release plot in 1993, but not in subsequent years; no field-collected larvae from the control plot contained the engineered virus. Larvae confined on LdGEV-contaminated foliage were killed by the virus. No LdGEV was recovered from bark, litter, or soil samples from either of the plots.

Ecological considerations for the environmental impact evaluation of recombinant baculovirus insecticides

The history of baculoviruses in insect control and the current status of recombinant baculovirus (recBV) insecticides in the laboratory and the field are briefly outlined. A conceptual model for impact evaluation is described that distinguishes between scientific impact evaluation and regulatory risk assessment. Its components are identified and reviewed in the light of existing ecological theory and experimental study under the categories of impact identification, exposure identification, and impact evaluation. Impact identification aims to identify species and populations sensitive to direct or indirect impacts by a recBV. Exposure identification examines how susceptible populations may be exposed to a recBV. Impact evaluation combines these data to predict the potential for recBV impacts in the environment.

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.

Enhanced Bioactivity of Recombinant Baculoviruses Expressing Insect-Specific Spider Toxins in Lepidopteran Crop Pests

Two genetically enhanced isolates of the Autographa californica nuclear polyhedrosis virus (AcMNPV) expressing insect-specific neurotoxin genes from the spiders Diguetia canities and Tegenaria agrestis were evaluated for their commercial potential. Since prevention of feeding damage is of primary importance in assessing agronomic efficacy, a method for estimating the median time to cessation of feeding (FT50) was developed. Neonate droplet feeding assays with preoccluded virus samples were conducted to compare the FT50s and median survival times (ST50s) of larvae infected by the toxin-expressing recombinant viruses with those of larvae infected by wild-type AcMNPV and the appropriate polyhedrin-minus control viruses. Low dosages were used to minimize the effect of dosage on the response times, and the time to molting of noninfected larvae was used to audit variability among batches of larvae within and between tests. Appropriate statistics are discussed. To evaluate host spectrum, response times were compared in three lepidopteran insect pests, Trichoplusia ni Hubner, Spodoptera exigua (Hubner), and Heliothis virescens (Fabricius). The recombinant viruses expressing insect-specific toxin genes from T. agrestis and D. canities, designated vAcTalTX-1 and vAcDTX9.2, respectively, significantly reduced both FT50 and ST50 in all three lepidopteran pests. Reductions in feeding times compared to the wild-type virus ranged from 16 to 39% with vAcTalTX-1 and 30 to 40% with vAcDTX9.2. Reductions in survi val time were lower, ranging from 18 to 33% with vAcTalTX-1 and 9 to 24% with vAcDTX9.2. While vAcTalTX-1 tended to kill faster than vAcDTX9.2, vAcDTX9.2 stopped feeding faster than vAcTalTX-1, suggesting that it would be more effective in reducing crop damage.

Detection and identification of multiple baculoviruses using the polymerase chain reaction (PCR) and restriction endonuclease analysis

A technique using the polymerase chain reaction (PCR) and restriction analysis was developed for the simultaneous detection of eight baculoviruses. The baculoviruses detected by this technique were Autographa californica multiple-embedded nuclear polyhedrosis virus (MNPV). Anticarsia gemmatalis MNPV, Bombyx mori MNPV, Orgyia pseudotsugata MNPV. Spodoptera frugiperda MNPV, S. exigua MNPV, Anagrapha falcifera MNPV, Heliothis zea single-embedded nuclear polyhedrosis virus (SNPV). A highly conserved DNA sequence within the coding region of the polyhedrin gene was targeted for amplification. One pair of degenerate primers was designed, and PCR conditions were optimized to produce 575 base pair fragments for all eight baculoviruses. Restriction analysis of the PCR products resulted in distinct profiles for each virus. This technique would be useful in monitoring the release of wild type as well as genetically engineered baculoviruses.

Enhancement of oocyte growth in the cockroach Blaberus craniifer by a scorpion toxin, charybdotoxin

We investigated the ovarian response of Blaberus craniifer to charybdotoxin in both imaginal molt headless females and isolated fed females, at three criteria. 1--Vitellogenesis onset, detected by immunocytochemical localisation of sites binding anti-eggs antibodies in the basal oocyte in headless females: 60% of treated females present a positive response. 2--Oocyte length at D4 during vitellogenesis of isolated fed females which was enhanced by 0.1 - 0.2 microg toxin; doses higher than 0.5 microg/female decrease ovarian growth. 3--Time of oviposition of paired females which was shortened by 24 hrs by the toxin. These results suggest that low doses charybdotoxin enhance vitellogenesis, possibly via the nervous system by means of a substance conditioning both protein synthesis by the fat body and ovarian uptake.

A baculovirus vector derived from immediately early gene promoter of Autographa californica nuclear polyhedrosis virus

A transfer vector was constructed in which the neomycin resistance (neo) gene was under the control of a copy of Autographa californica nuclear polyhedrosis virus (AcMNPV) IE1 gene promoter at the p10 locus. After cotransfection of Spodoptera frugiperda (Sf9) cells with the transfer vector and infectious AcMNPV DNA, the recombinant virus-containing neo gene was selected by serial passage of the mixed progenies from cotransfection. This was done at low MOI in the presence of G418 in growth medium and was followed by limited dilution. RNA dot hybridization showed that the neo gene was transcribed from immediately early phase to very late phase, in infected Sf9 cells. These results demonstrate that a new baculovirus vector system had been constructed in infected cells. Furthermore, a new method for selection of positive recombinant baculovirus had been developed.

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.

Baculoviruses: high-level expression in insect cells

Baculoviruses continue to serve as workhorse vectors for the high-level expression of eukaryotic genes in insect cells; however, numerous researchers are also finding novel uses for these vectors by taking advantage of the unique nature of the viruses and their host cells. Concurrently, the technology involved in constructing and utilizing these vectors is being improved so that the time and effort required to construct expression vectors are reduced.

A cellular promoter-based expression cassette for generating recombinant baculoviruses directing rapid expression of passenger genes in infected insects

We have developed an expression cassette which allows the generation of recombinant baculoviruses that can express passenger genes under the control of a constitutive cellular promoter derived from the cytoplasmic actin gene of the silkmoth Bombyx mori. Silkmoth tissue culture cells which were infected with a recombinant B. mori nuclear polyhedrosis virus (BmNPV) containing the gene-encoding chloramphenicol acetyltransferase (CAT) under the control of this expression cassette expressed significant CAT activity beginning 5 hr postinfection (p.i.). Cells infected with a recombinant BmNPV containing the cat gene under the control of the polyhedrin gene promoter did not express CAT activity until 20 hr p.i. Silkworm larvae were also infected with the two recombinant viruses by hemocelic injections and all larval tissues examined were found to express the cat gene. While significant actin-cassette-driven CAT expression in vivo was first seen at 24 hr p.i., expression from the polyhedrin promoter was not seen until 48 hr p.i. By 60 hr p.i., tissues of larvae infected with the recombinant virus expressing cat under polyhedrin promoter control were found to exhibit sixfold higher CAT activity than those infected with recombinant virus expressing the cat gene under the control of the actin promoter. The 24-hr temporal advantage in expression of a passenger gene in infected larvae indicates that the actin-promoter-based expression cassette or other analogous cellular promoter-based cassettes could be used for generating recombinant baculovirus insecticides which could incapacitate pest insects more quickly than viruses employing the polyhedrin or other late viral promoters for expressing insect-incapacitating proteins.

Expression of a gene encoding a scorpion insectotoxin peptide in yeast, bacteria and plants

The nucleotide sequence encoding the scorpion insectotoxin I5A was chemically synthesized and expressed in yeast, bacteria and tobacco. The I5A peptides produced in these organisms were purified using an immunoaffinity chromatography procedure. I5A produced using the bacterial secretion system was efficiently secreted and released into the culture medium. In contrast, only a trace amount of I5A was detected in bacterial cytosols when expressed from a direct expression vector, suggesting that I5A was unstable in bacterial cells. I5A secreted from yeast using an alpha-factor signal sequence was shown to have an N-terminal (Glu-Ala)2 extension, indicating incomplete processing of the secreted peptide by dipeptidyl aminopeptidase A. In tobacco, a nonsecreted form of the protein was produced. No measurable insect toxicity was observed when insect larvae were assayed, regardless of whether I5A was produced in yeast, bacteria or tobacco. The lack of toxicity is almost certainly the result of improper folding due to incorrect disulfide bond formation. The inability to produce a biologically active peptide must be overcome before scorpion toxins might be used for the genetic engineering of plants for insect resistance. The yeast and bacterial expression systems described here may be useful for further studies on the problem of expressing a biologically active peptide.

Rapid efficient production of baculovirus expression vectors

Recombinant baculoviruses have been used to produce foreign proteins and have the potential to be safe, efficacious insecticides. Isolation of recombinant virus is usually by plaque phenotype. Typical recombination rates are less than 1%, thus requiring time consuming inspection of hundreds of individual plaques. We describe a method of generating recombinants which requires less time than current protocols and frequently produces recombinants at rates exceeding 30%. This protocol employs liposome-mediated transfection, reduced post-transfection incubation times, linearized parental virus which produces occlusion positive plaques in clones of the parental genotype, and colorimetric detection of recombinants. This protocol allows the initial, and frequently the final, isolation of recombinants in 7 days.