INSECTCIDAL PROTEIN COMBINATIONS COMPRISING Cry1AB AND CRY2AA FOR CONTROLLING EUROPEAN CORN BORER, AND METHODS FOR INSECT RESISTANCE MANAGEMENT

The subject invention relates in part to stacking a Cry IAb protein and a Cry2Aa protein to make plants (particularly corn or maize) more durable and less prone to allowing insects to develop that are resistant to the activity of either of these two toxins. These stacks can be used to specifically target European cornborer.

1. A transgenic plant comprising DNA encoding a Cry1Ab insecticidal protein and DNA encoding a Cry2Aa insecticidal protein.

2. The transgenic plant of claim 1, said plant further comprising DNA encoding a third insecticidal protein, said third protein being selected from the group consisting of Cry1Fa, Cry1Be, Cry1I, and DIG-3

3. The transgenic plant of claim 2, wherein said third protein is selected from the group consisting of Cry1Fa and Cry1Be, said plant further comprising DNA encoding fourth and fifth insecticidal proteins selected from the group consisting of Cry1Ca, Cry1Da, Cry1E, and Vip3Ab.

4. Seed of a plant according to claim 1, wherein said seed comprises said DNA.

5. A field of plants comprising non-Bt refuge plants and a plurality of plants according to claim 1, wherein said refuge plants comprise less than 40% of all crop plants in said field.

6. The field of plants of claim 5, wherein said refuge plants comprise less than 30% of all the crop plants in said field.

7. The field of plants of claim 5, wherein said refuge plants comprise less than 20% of all the crop plants in said field.

8. The field of plants of claim 5, wherein said refuge plants comprise less than 10% of all the crop plants in said field.

9. The field of plants of claim 5, wherein said refuge plants comprise less than 5% of all the crop plants in said field.

10. The field of plants of claim 5, wherein said refuge plants are in blocks or strips.

11. A mixture of seeds comprising refuge seeds from non-Bt refuge plants, and a plurality of seeds of claim 4, wherein said refuge seeds comprise less than 40% of all the seeds in the mixture.

12. The mixture of seeds of claim 11, wherein said refuge seeds comprise less than 30% of all the seeds in the mixture.

13. The mixture of seeds of claim 11, wherein said refuge seeds comprise less than 20% of all the seeds in the mixture.

14. The mixture of seeds of claim 11, wherein said refuge seeds comprise less than 10% of all the seeds in the mixture.

15. The mixture of seeds of claim 11, wherein said refuge seeds comprise less than 5% of all the seeds in the mixture.

16. A method of managing development of resistance to a Cry protein by an insect, said method comprising planting seeds to produce a field of plants of claim 5.

17. The field of claim 5, wherein said plants occupy more than 10 acres.

18. The plant of claim 1, wherein said plant is selected from the group consisting of corn, soybeans, and cotton.

19. The plant of claim 18, wherein said plant is a maize plant.

20. A plant cell of a plant of claim 1, wherein said plant cell comprises said DNA encoding said Cry1Ab insecticidal protein and said DNA encoding said Cry2Aa insecticidal protein, wherein said Cry1Ab insecticidal protein is at least 99% identical with SEQ ID NO:1, and said Cry2Aa insecticidal protein is at least 99% identical with SEQ ID NO:2.

21. The plant of claim 1, wherein said Cry1Ab insecticidal protein comprises SEQ ID NO:1, and said Cry2Aa insecticidal protein comprises SEQ ID NO:2.

22. A method of producing the plant cell of claim 20.

23. A method of controlling a European cornborer insect by contacting said insect with a Cry1Ab insecticidal protein and a Cry2Aa insecticidal protein.

Humans grow corn for food and energy applications. Insects eat and damage corn plants and thereby undermine these human efforts.

Current in-plant transgenic control of these pests is achieved through plant expression of a crystal (Cry) delta endotoxin gene coding for the Cry1Fa protein from Bacillus thuringiensis. Cry1Fa is the protein toxin currently in the Herculex™ brand of Dow AgroSciences transgenic corn seeds (Herculex, Herculex-Extra, and Herculex-RW) that are resistant to FAW and ECB insect pests. This protein works by binding to specific receptor(s) located in the midgut of insects, and forms pores within the gut cells. The formation of these pores prevents insects from regulating osmotic balance which results in their death.

However, there exists some concern that insects might be able to develop resistance to the action of Cry1Fa through genetic alterations of the receptors within their gut that bind Cry1Fa. Insects that produce receptors with a reduced ability to bind Cry1Fa can be resistant to the activity of Cry1Fa, and thus survive on plants that express this protein.

With a single Cry toxin continuously present in the plant during growth conditions, there is concern that insects could develop resistance to the activity of this protein through genetic alterations of the receptor that binds Cry1Fa toxin in the insect gut. Reductions in toxin binding due to these alterations in the receptor would lead to reduced toxicity of the Cry1Fa possibly leading to eventual decreased effectiveness of the protein when expressed in a crop.

The subject invention relates in part to stacking a Cry1Ab protein and a Cry2Aa protein to make plants (particularly corn or maize) more durable and less prone to allowing insects to develop that are resistant to the activity of either of these two toxins. These stacks can be used to specifically target European corn borer (ECB).

The subject invention relates in part to stacking a Cry1Ab insecticidal protein and a Cry2Aa insecticidal protein to make plants (particularly corn or maize) more durable and less prone to allowing insects to develop that are resistant to the activity of either of these two toxins. These stacks can be used to specifically target European corn borer (ECB; Ostrinia nubilalis).

The subject invention also relates in part to triple stacks or “pyramids” of three (or more) protein toxins, with a Cry1Ab protein and a Cry2Aa protein being the base pair. (By “separate sites of action,” it is meant that any of the given proteins do not cause cross-resistance with each other.) Adding a third protein that targets ECB can provide a protein with a third site of action against ECB. In some preferred embodiments, the third protein can be selected from the group consisting of DIG-3 (see US 2010-00269223), Cry1I, Cry1Be, Cry2Aa, and Cry1Fa. See e.g. U.S. Ser. No. 61/284,278, filed Dec. 16, 2009. See also US 2008-0311096.

Thus, in some preferred pyramid embodiments, the selected toxins have three separate sites of action against ECB. Again, preferred pyramid combinations are the subject pair of proteins plus a third IRM protein.

The subject pairs and/or tripe stacks (active against ECB) can also be combined with additional proteins—for targeting fall armyworm (FAW), for example. Such proteins can include Vip3, Cry1C, Cry1D, and/or Cry1E, for example. Cry1Be and/or Cry1Fa can also be used to target FAW and ECB.

GENBANK can be used to obtain the sequences for any of the genes and proteins disclosed or mentioned herein. See Appendix A.

The subject invention also relates to three insecticidal proteins (Cry proteins in some preferred embodiments) that are active against a single target pest but that do not result in cross-resistance against each other.

Plants (and acreage planted with such plants) that produce these three (at least) toxins are included within the scope of the subject invention. Additional toxins/genes can also be added, but these particular triple stacks would, according to the subject invention, advantageously and surprisingly provide three sites of action against ECB.

Pairs or triple stacks (and/or combinations of additional proteins) of the subject invention can help to reduce or eliminate the requirement for refuge acreage (e.g., less than 40%, less than 20%, less than 10%, less than 5%, or even 0% refuge). A field thus planted of over 10 acres is thus included within the subject invention. The subject polynucleotide(s) are preferably in a genetic construct under control of a non-Bacillus-thuringiensis promoter(s). The subject polynucleotides can comprise codon usage for enhanced expression in a plant.

To counteract the ability of insects to develop resistance to a Cry protein, we identified Cry toxins that non-competitively bind to protein receptors in the ECB gut. It was discovered that Cry1Ab does not to displace Cry2Aa binding to receptors located in the insect gut of ECB larvae.

We found that Cry2Aa and Cry1Ab are toxic to ECB larvae, yet they do not fully interact with the same receptor site(s); this shows that their toxicity will not be subject to cross-resistance in ECB.

Thus insects having developed resistance to Cry1Ab would still be susceptible to the toxicity of Cry2Aa proteins, for example, which bind alternative receptor sites. We have obtained biochemical data that supports this. Having combinations of these proteins expressed in transgenic plants thus provides a useful and valuable mechanism to reduce the probability for the development of insect resistance in the field and thus lead towards a reduction in the requirement for refugia. The data herein described below shows the Cry2Aa protein interacting at separate target site(s) within the insect gut compared to Cry1Ab and thus would make excellent stacking partners.

If resistance were to occur through alterations in the affinity of the insect gut receptors that bind to the Cry toxins, the alteration would have to occur in at least two different receptors simultaneously to allow the insects to survive on plants expressing the multiple proteins. The probability of this occurring is extremely remote, thus increasing the durability of the transgenic product to ward of insects being able to develop tolerance to the proteins.

We radio-iodinated the Cry1Ab protein and used radioreceptor binding assay techniques to measure their binding interaction with putative receptor proteins located within the insect gut membranes. The gut membranes were prepared as brush border membrane vesicles (BBMV) by the method of Wolfersberger. Iodination of the toxins were conducted using either iodo beads or iodogen treated tubes from Pierce Chemicals. Specific activity of the radiolabeled toxin was approximately 1-4 μCi/μg protein. Binding studies were carried out essentially by the procedures of Liang (1995).

The data presented herein shows the toxins interacting at separate target site within the insect gut compared to Cry1Ab and thus would make excellent stacking partners.

The subject invention can be used with a variety of plants. Examples include corn (maize), soybeans, and cotton.

Genes and toxins useful according to the subject invention include not only the full length sequences disclosed but also fragments of these sequences, variants, mutants, and fusion proteins which retain the characteristic pesticidal activity of the toxins specifically exemplified herein. As used herein, the terms “variants” or “variations” of genes refer to nucleotide sequences which encode the same toxins or which encode equivalent toxins having pesticidal activity. As used herein, the term “equivalent toxins” refers to toxins having the same or essentially the same biological activity against the target pests as the claimed toxins.

As used therein, the boundaries represent approximately 95% (e.g. Cry1Ab's and Cry2Aa's), 78% (e.g. Cry1A's and Cry2A's), and 45% (Cry1's and Cry2's) sequence identity, per “Revision of the Nomenclature for the Bacillus thuringiensis Pesticidal Crystal Proteins,” N. Crickmore, D. R. Zeigler, J. Feitelson, E. Schnepf, J. Van Rie, D. Lereclus, J. Baum, and D. H. Dean. Microbiology and Molecular Biology Reviews (1998) Vol 62: 807-813. These cut offs can also be applied to the core proteins only.

Fragments and equivalents which retain the pesticidal activity of the exemplified toxins would be within the scope of the subject invention. Also, because of the redundancy of the genetic code, a variety of different DNA sequences can encode the amino acid sequences disclosed herein. It is well within the skill of a person trained in the art to create these alternative DNA sequences encoding the same, or essentially the same, toxins. These variant DNA sequences are within the scope of the subject invention. As used herein, reference to “essentially the same” sequence refers to sequences which have amino acid substitutions, deletions, additions, or insertions which do not materially affect pesticidal activity. Fragments of genes encoding proteins that retain pesticidal activity are also included in this definition.

A further method for identifying the genes encoding the toxins and gene portions useful according to the subject invention is through the use of oligonucleotide probes. These probes are detectable nucleotide sequences. These sequences may be detectable by virtue of an appropriate label or may be made inherently fluorescent as described in International Application No. WO93/16094. As is well known in the art, if the probe molecule and nucleic acid sample hybridize by forming a strong bond between the two molecules, it can be reasonably assumed that the probe and sample have substantial homology. Preferably, hybridization is conducted under stringent conditions by techniques well-known in the art, as described, for example, in Keller, G. H., M. M. Manak (1987) DNA Probes, Stockton Press, New York, N.Y., pp. 169-170. Some examples of salt concentrations and temperature combinations are as follows (in order of increasing stringency): 2×SSPE or SSC at room temperature; 1×SSPE or SSC at 42° C.; 0.1×SSPE or SSC at 42° C.; 0.1×SSPE or SSC at 65° C. Detection of the probe provides a means for determining in a known manner whether hybridization has occurred. Such a probe analysis provides a rapid method for identifying toxin-encoding genes of the subject invention. The nucleotide segments which are used as probes according to the invention can be synthesized using a DNA synthesizer and standard procedures. These nucleotide sequences can also be used as PCR primers to amplify genes of the subject invention.

Certain proteins of the subject invention have been specifically exemplified herein. Since these proteins are merely exemplary of the proteins of the subject invention, it should be readily apparent that the subject invention comprises variant or equivalent proteins (and nucleotide sequences coding for equivalent proteins) having the same or similar pesticidal activity of the exemplified protein. Equivalent proteins will have amino acid homology with an exemplified protein. This amino acid identity will typically be greater than 75%, greater than 90%, and could be greater than 91, 92, 93, 94, 95, 96, 97, 98, or 99%. The amino acid identity will be highest in critical regions of the protein which account for biological activity or are involved in the determination of three-dimensional configuration which ultimately is responsible for the biological activity. In this regard, certain amino acid substitutions are acceptable and can be expected if these substitutions are in regions which are not critical to activity or are conservative amino acid substitutions which do not affect the three-dimensional configuration of the molecule. For example, amino acids may be placed in the following classes: non-polar, uncharged polar, basic, and acidic. Conservative substitutions whereby an amino acid of one class is replaced with another amino acid of the same type fall within the scope of the subject invention so long as the substitution does not materially alter the biological activity of the compound. Following is a listing of examples of amino acids belonging to each class. In some instances, non-conservative substitutions can also be made. The critical factor is that these substitutions must not significantly detract from the biological activity of the protein.

Plant transformation. A preferred recombinant host for production of the insecticidal proteins of the subject invention is a transformed plant. Genes encoding Bt toxin proteins, as disclosed herein, can be inserted into plant cells using a variety of techniques which are well known in the art. For example, a large number of cloning vectors comprising a replication system in Escherichia coli and a marker that permits selection of the transformed cells are available for preparation for the insertion of foreign genes into higher plants. The vectors comprise, for example, pBR322, pUC series, M13 mp series, pACYC184, inter alia. Accordingly, the DNA fragment having the sequence encoding the Bt toxin protein can be inserted into the vector at a suitable restriction site. The resulting plasmid is used for transformation into E. coli. The E. coli cells are cultivated in a suitable nutrient medium, then harvested and lysed. The plasmid is recovered. Sequence analysis, restriction analysis, electrophoresis, and other biochemical-molecular biological methods are generally carried out as methods of analysis. After each manipulation, the DNA sequence used can be cleaved and joined to the next DNA sequence. Each plasmid sequence can be cloned in the same or other plasmids. Depending on the method of inserting desired genes into the plant, other DNA sequences may be necessary. If, for example, the Ti or Ri plasmid is used for the transformation of the plant cell, then at least the right border, but often the right and the left border of the Ti or Ri plasmid T-DNA, has to be joined as the flanking region of the genes to be inserted. The use of T-DNA for the transformation of plant cells has been intensively researched and sufficiently described in EP 120 516, Lee and Gelvin (2008), Hoekema (1985), Fraley et al., (1986), and An et al., (1985), and is well established in the art.

Once the inserted DNA has been integrated in the plant genome, it is relatively stable. The transformation vector normally contains a selectable marker that confers on the transformed plant cells resistance to a biocide or an antibiotic, such as Bialaphos, Kanamycin, G418, Bleomycin, or Hygromycin, inter alia. The individually employed marker should accordingly permit the selection of transformed cells rather than cells that do not contain the inserted DNA.

A large number of techniques are available for inserting DNA into a plant host cell. Those techniques include transformation with T-DNA using Agrobacterium tumefaciens or Agrobacterium rhizogenes as transformation agent, fusion, injection, biolistics (microparticle bombardment), or electroporation as well as other possible methods. If Agrobacteria are used for the transformation, the DNA to be inserted has to be cloned into special plasmids, namely either into an intermediate vector or into a binary vector. The intermediate vectors can be integrated into the Ti or Ri plasmid by homologous recombination owing to sequences that are homologous to sequences in the T-DNA. The Ti or Ri plasmid also comprises the vir region necessary for the transfer of the T-DNA. Intermediate vectors cannot replicate themselves in Agrobacteria. The intermediate vector can be transferred into Agrobacterium tumefaciens by means of a helper plasmid (conjugation). Binary vectors can replicate themselves both in E. coli and in Agrobacteria. They comprise a selection marker gene and a linker or polylinker which are framed by the Right and Left T-DNA border regions. They can be transformed directly into Agrobacteria (Holsters et al., 1978). The Agrobacterium used as host cell is to comprise a plasmid carrying a vir region. The vir region is necessary for the transfer of the T-DNA into the plant cell. Additional T-DNA may be contained. The bacterium so transformed is used for the transformation of plant cells. Plant explants can advantageously be cultivated with Agrobacterium tumefaciens or Agrobacterium rhizogenes for the transfer of the DNA into the plant cell. Whole plants can then be regenerated from the infected plant material (for example, pieces of leaf, segments of stalk, roots, but also protoplasts or suspension-cultivated cells) in a suitable medium, which may contain antibiotics or biocides for selection. The plants so obtained can then be tested for the presence of the inserted DNA. No special demands are made of the plasmids in the case of injection and electroporation. It is possible to use ordinary plasmids, such as, for example, pUC derivatives.

The transformed cells grow inside the plants in the usual manner. They can form germ cells and transmit the transformed trait(s) to progeny plants. Such plants can be grown in the normal manner and crossed with plants that have the same transformed hereditary factors or other hereditary factors. The resulting hybrid individuals have the corresponding phenotypic properties.

In a preferred embodiment of the subject invention, plants will be transformed with genes wherein the codon usage has been optimized for plants. See, for example, U.S. Pat. No. 5,380,831, which is hereby incorporated by reference. While some truncated toxins are exemplified herein, it is well-known in the Bt art that 130 kDa-type (full-length) toxins have an N-terminal half that is the core toxin, and a C-terminal half that is the protoxin “tail.” Thus, appropriate “tails” can be used with truncated/core toxins of the subject invention. See e.g. U.S. Pat. No. 6,218,188 and U.S. Pat. No. 6,673,990. In addition, methods for creating synthetic Bt genes for use in plants are known in the art (Stewart and Burgin, 2007). One non-limiting example of a preferred transformed plant is a fertile maize plant comprising a plant expressible gene encoding a Cry1Da protein, and further comprising a second plant expressible gene encoding a Cry1Be protein.

Transfer (or introgression) of the Cry1Da- and Cry1Be-determined trait(s) into inbred maize lines can be achieved by recurrent selection breeding, for example by backcrossing. In this case, a desired recurrent parent is first crossed to a donor inbred (the non-recurrent parent) that carries the appropriate gene(s) for the Cry1D- and Cry1C-determined traits. The progeny of this cross is then mated back to the recurrent parent followed by selection in the resultant progeny for the desired trait(s) to be transferred from the non-recurrent parent. After three, preferably four, more preferably five or more generations of backcrosses with the recurrent parent with selection for the desired trait(s), the progeny will be heterozygous for loci controlling the trait(s) being transferred, but will be like the recurrent parent for most or almost all other genes (see, for example, Poehlman & Sleper (1995) Breeding Field Crops, 4th Ed., 172-175; Fehr (1987) Principles of Cultivar Development, Vol. 1: Theory and Technique, 360-376).

Insect Resistance Management (IRM) Strategies.

Roush et al., for example, outlines two-toxin strategies, also called “pyramiding” or “stacking,” for management of insecticidal transgenic crops. (The Royal Society. Phil. Trans. R. Soc. Lond. B. (1998) 353, 1777-1786).

On their website, the United States Environmental Protection Agency (epa.gov/oppbppd1/biopesticides/pips/bt_corn_refuge_—2006.htm) publishes the following requirements for providing non-transgenic (i.e., non-B.t.) refuges (a section of non-Bt crops/corn) for use with transgenic crops producing a single Bt protein active against target pests.

• • “The specific structured requirements for corn borer-protected Bt (Cry1Ab or Cry1F) corn products are as follows:
  • Structured refuges: 20% non-Lepidopteran Bt corn refuge in Corn Belt;
    • 50% non-Lepidopteran Bt refuge in Cotton Belt
  • Blocks
  • Internal (i.e., within the Bt field)
  • External (i.e., separate fields within ½ mile (¼ mile if possible) of the Bt field to maximize random mating)

In-Field Strips

• • Strips must be at least 4 rows wide (preferably 6 rows) to reduce the effects of larval movement”

In addition, the National Corn Growers Association, on their website:

• • (ncga.com/insect-resistance-management-fact-sheet-bt-corn) also provides similar guidance regarding the refuge requirements. For example:

“Requirements of the Corn Borer IRM:

• • Plant at least 20% of your corn acres to refuge hybrids
  • In cotton producing regions, refuge must be 50%
  • Must be planted within ½ mile of the refuge hybrids
  • Refuge can be planted as strips within the Bt field; the refuge strips must be at least 4 rows wide
  • Refuge may be treated with conventional pesticides only if economic thresholds are reached for target insect
  • Bt-based sprayable insecticides cannot be used on the refuge corn
  • Appropriate refuge must be planted on every farm with Bt corn”

As stated by Roush et al. (on pages 1780 and 1784 right column, for example), stacking or pyramiding of two different proteins each effective against the target pests and with little or no cross-resistance can allow for use of a smaller refuge. Roush suggests that for a successful stack, a refuge size of less than 10% refuge, can provide comparable resistance management to about 50% refuge for a single (non-pyramided) trait. For currently available pyramided Bt corn products, the U.S. Environmental Protection Agency requires significantly less (generally 5%) structured refuge of non-Bt corn be planted than for single trait products (generally 20%).

There are various ways of providing the IRM effects of a refuge, including various geometric planting patterns in the fields (as mentioned above) and in-bag seed mixtures, as discussed further by Roush et al. (supra), and U.S. Pat. No. 6,551,962.

The above percentages, or similar refuge ratios, can be used for the subject double or triple stacks or pyramids. For triple stacks with three sites of action against a single target pest, a goal would be zero refuge (or less than 5% refuge, for example). This is particularly true for commercial acreage—of over 10 acres for example.

All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety to the extent they are not inconsistent with the explicit teachings of this specification.

Unless specifically indicated or implied, the terms “a”, “an”, and “the” signify “at least one” as used herein.

Following are examples that illustrate procedures for practicing the invention. These examples should not be construed as limiting. All percentages are by weight and all solvent mixture proportions are by volume unless otherwise noted. All temperatures are in degrees Celsius.

Iodination of Cry toxins. Purified truncated Cry toxins were was iodinated using Iodo-Beads or Iodo-gen (Pierce). Briefly, two Iodo-Beads were washed twice with 500 μl of phosphate buffered saline, PBS (20 mM sodium phosphate, 0.15 M NaCl, pH 7.5), and placed into a 1.5 ml centrifuge tube behind lead shielding. To this was added 100 μl of PBS. In a hood and through the use of proper radioactive handling techniques, 0.5 mCi Na¹²⁵I (17.4 Ci/mg, Lot 0114, Amersham) was added to the PBS solution with the Iodo-Bead. The components were allowed to react for 5 minutes at room temperature, then 2-25 μg of highly pure truncated Cry protein was added to the solution and allowed to react for an additional 3-5 minutes. The reaction was terminated by removing the solution from the iodo-beads and applying it to a 0.5 ml desalting Zeba spin column (InVitrogen) equilibrated in PBS. The iodo-bead was washed twice with 10 μl of PBS each and the wash solution also applied to the desalting column. The radioactive solution was eluted through the desalting column by centrifugation at 1,000×g for 2 min. Using this procedure, the cry toxin in 100 mM phosphate buffer (pH 8) was first cleaned of lipopolysaccharides (LPS) by passing it through a small 0.5 ml polymyxin column multiple times. To the iodo-gen tube (Pierce Chem. Co.) was added 20 μg of the LPS-free Cry1Da toxin, then 0.5 mCi of Na¹²⁵I. The reaction mixture was shaken for 15 min at 25° C. The solution was removed from the tube, and 50 μl of 0.2M non-radiolabeled NaI added to quench the reaction. The protein was dialyzed vs PBS with 3 changes of buffer to remove any unbound¹²⁵I.

Radio-purity of the iodinated Cry proteins was determined by SDS-PAGE, phosphorimaging and gamma counting. Briefly, 2 μl of the radioactive protein was separated by SDS-PAGE. After separation, the gels were dried using a BioRad gel drying apparatus following the manufacturer's instructions. The dried gels were imaged by wrapping them in Mylar film (12 μm thick), and exposing them under a Molecular Dynamics storage phosphor screen (35 cm×43 cm), for 1 hour. The plates were developed using a Molecular Dynamics Storm 820 phosphorimager and the imaged analyzed using ImageQuant™ software. The radioactive band along with areas immediately above and below the band were cut from the gel using a razor blade and counted in a gamma counter. Radioactivity was only detected in the Cry protein band and in areas below the band. No radioactivity was detected above the band, indicating that all radioactive contaminants consisted of smaller protein components than the truncated Cry protein. These components most probably represent degradation products.

Preparation and Fractionation of Solubilized BBMV's. Last instar Spodoptera frugiperda, Ostrinia nubilalis, or Heleothis. zea larvae were fasted overnight and then dissected in the morning after chilling on ice for 15 minutes. The midgut tissue was removed from the body cavity, leaving behind the hindgut attached to the integument. The midgut was placed in 9× volume of ice cold homogenization buffer (300 mM mannitol, 5 mM EGTA, 17 mM tris. base, pH 7.5), supplemented with Protease Inhibitor Cocktail¹(Sigma P-2714) diluted as recommended by the supplier. The tissue was homogenized with 15 strokes of a glass tissue homogenizer. BBMV's were prepared by the MgCl₂precipitation method of Wolfersberger (1993). Briefly, an equal volume of a 24 mM MgCl₂solution in 300 mM mannitol was mixed with the midgut homogenate, stirred for 5 minutes and allowed to stand on ice for 15 min. The solution was centrifuged at 2,500×g for 15 min at 4° C. The supernatant was saved and the pellet suspended into the original volume of 0.5-X diluted homogenization buffer and centrifuged again. The two supernatants were combined, centrifuged at 27,000×g for 30 min at 4° C. to form the BBMV fraction. The pellet was suspended into 10 ml homogienization buffer and supplemented to protease inhibitors and centrifuged again at 27,000×g of r30 min at 4° C. to wash the BBMV's. The resulting pellet was suspended into BBMV Storage Buffer (10 mM HEPES, 130 mM KCl, 10% glycerol, pH 7.4) to a concentration of about 3 mg/ml protein. Protein concentration was determined by using the Bradford method (1976) with bovine serum albumin (BSA) as the standard. Alkaline phosphatase determination was made prior to freezing the samples using the Sigma assay following manufacturer's instructions. The specific activity of this marker enzyme in the BBMV fraction typically increased 7-fold compared to that found in the midgut homogenate fraction. The BBMV's were aliquoted into 250 samples, flash frozen in liquid N₂and stored at −80° C.¹Final concentration of cocktail components (in μM) are AEBSF (500), EDTA (250 mM), Bestatin (32), E-64 (0.35), Leupeptin (0.25), and Aprotinin (0.075).

Binding of¹²⁵I Cry Proteins to BBMV's. To determine the optimal amount of BBMV protein to use in the binding assays, a saturation curve was generated.¹²⁵I radiolabeled Cry protein (0.5 nM) was incubated for 1 hr. at 28° C. with various amounts of BBMV protein, ranging from 0-500 μg/ml in binding buffer (8 mM NaHPO₄, 2 mM KH₂PO₄, 150 mM NaCl, 0.1% bovine serum albumin, pH 7.4). Total volume was 0.5 ml. Bound¹²⁵I Cry protein was separated from unbound by sampling 150 μl of the reaction mixture in triplicate from a 1.5 ml centrifuge tube into a 500 μl centrifuge tube and centrifuging the samples at 14,000×g for 6 minutes at room temperature. The supernatant was gently removed, and the pellet gently washed three times with ice cold binding buffer. The bottom of the centrifuge containing the pellet was cut out and placed into a 13×75-mm glass culture tube. The samples were counted for 5 minutes each in the gamma counter. The counts contained in the sample were subtracted from background counts (reaction without any protein) and was plotted versus BBMV protein concentration. The optimal amount of protein to use was determined to be 0.15 mg/ml of BBMV protein.

To determine the binding kinetics, a saturation curve was generated. Briefly, BBMV's (150 μg/ml) were incubated for 1 hr. at 28° C. with increasing concentrations of¹²⁵I Cry toxin, ranging from 0.01 to 10 nM. Total binding was determined by sampling 150 μl of each concentration in triplicate, centrifugation of the sample and counting as described above. Non-specific binding was determined in the same manner, with the addition of 1,000 nM of the homologous trypsinized non-radioactive Cry toxin added to the reaction mixture to saturate all non-specific receptor binding sites. Specific binding was calculated as the difference between total binding and non-specific binding.

Homologous and heterologous competition binding assays were conducted using 150 μg/ml BBMV protein and 0.5 nM of the¹²⁵I radiolabeled Cry protein. The concentration of the competitive non-radiolabeled Cry toxin added to the reaction mixture ranged from 0.045 to 1,000 nM and were added at the same time as the radioactive ligand, to assure true binding competition. Incubations were carried out for 1 hr. at 28° C. and the amount of¹²⁵I Cry protein bound to its receptor toxin measured as described above with non-specific binding subtracted. One hundred percent total binding was determined in the absence of any competitor ligand. Results were plotted on a semi-logarithmic plot as percent total specific binding versus concentration of competitive ligand added.

FIG. 1 shows percent specific binding of¹²⁵I Cry1Ab (0.5 nM) in BBMV's from ECB versus competition by unlabeled homologous Cry1Ab (♦) and heterologous Cry2Aa (□). The displacement curve for homologous competition by Cry1Ab results in a sigmoidal shaped curve showing 50% displacement of the radioligand at about 3 nM of Cry1Ab. Cry2Aa at a concentration of 1,000 nM (2.000-fold greater than¹²⁵I Cry1Ab being displaced) results in less than 50% displacement. Error bars represent the range of values obtained from triplicate determinations.

• Wolfersberger, M. G., (1993), Preparation and Partial Characterization of Amino Acid Transporting Brush Border Membrane Vesicles from the Larval Midgut of the Gypsy Moth (Lymantria Dispar). Arch. Insect Biochem. Physiol. 24: 139-147.
• Liang, Y., Patel, S. S., and Dean, D. H., (1995), Irreversible Binding Kinetics of Bacillus thuringiensis Cry1A Delta-Endotoxins to Gypsy Moth Brush Border Membrane Vesicles is Directly Correlated to Toxicity. J. Biol. Chem., 270, 24719-24724.