The leading biorational pesticide, Bacillus thuringiensis, is a ubiquitous gram-positive, spore-forming bacterium that forms a parasporal crystal during the stationary phase of its growth cycle. B. thuringiensis was initially characterized as an insect pathogen, and its insecticidal activity was attributed largely or completely (depending on the insect) to the parasporal crystals. This observation led to the development of bioinsecticides based on B. thuringiensis for the control of certain insect species among the orders Lepidoptera, Diptera, and Coleoptera (for a review, see reference 33). There are more recent reports of B. thuringiensis isolates active against other insect orders (Hymenoptera, Homoptera, Orthoptera, and Mallophaga) and against nematodes, mites, and protozoa (109, 110). B. thuringiensis is already a useful alternative or supplement to synthetic chemical pesticide application in commercial agriculture, forest management, and mosquito control. It is also a key source of genes for transgenic expression to provide pest resistance in plants.

In 1989, Höfte and Whiteley reviewed the known cry genes and proposed a systematic nomenclature for them (164). Since then, the number of sequenced crystal protein genes (encoding Cry and Cyt proteins) has grown from 14 to well more than 100. In our accompanying work (79), we propose a revised nomenclature to accommodate this wealth of new sequence data. The present work reviews the extensive progress during the past decade in determining the gene expression, structure, and mechanism of action for these classes of proteins. The proposed revised nomenclature will be used throughout.

B. thuringiensis seems to be indigenous to many environments (36, 65, 255). Strains have been isolated worldwide from many habitats, including soil (59, 88, 154, 255, 354), insects (59), stored-product dust (54, 65, 87, 267), and deciduous and coniferous leaves (175, 354). Isolation typically involves heat treatment to select for spores, sometimes with an acetate enrichment step (382) or antibiotic selection (89). The diversity in flagellar H-antigen agglutination reactions is one indication of the enormous genetic diversity among B. thuringiensis isolates. The Pasteur Institute has catalogued 55 different flagellar serotypes and eight nonflagellated biotypes (202, 205).

There is considerable evidence that B. thuringiensis and Bacillus cereus should be considered a single species. Classical biochemical and morphological methods of classifying bacteria have consistently failed to distinguish B. thuringiensis from B. cereus (31, 139, 177, 229, 305). Modern molecular methodsincluding chromosomal DNA hybridization (179), phospholipid and fatty acid analysis (40, 178), 16S rRNA sequence comparison (20, 318), amplified fragment length polymorphism analysis (181), and genomic restriction digest analysis (56, 57)likewise support the single-species hypothesis. An attempt to distinguish B. thuringiensis isolates from B. cereus by analysis of a 16S rRNA variable region largely failed, yielding as many false positives and negatives as accurate identifications (373). The production of the parasporal crystal, the defining quality of B. thuringiensis, is too narrow a criterion for taxonomic purposes (237). Indeed, some B. cereus strains hybridize to cry1A-specific probes (56). Although we will employ the official nomenclature with two species names for these organisms, it is perhaps best to think of them as members of B. cereus sensu lato.

The remarkable diversity of B. thuringiensis strains and toxins is due at least in part to a high degree of genetic plasticity. Most B. thuringiensis toxin genes appear to reside on plasmids (138), often as parts of composite structures that include mobile genetic elements (195, 218). Many cry gene-containing plasmids appear to be conjugative in nature (137).

B. thuringiensis has developed a fascinating array of molecular mechanisms to produce large amounts of pesticidal toxins during the stationary phase of growth (8, 30). One can only speculate about the ecological value to the bacterium of using several cry gene expression systems. However, coexpression of multiple toxins is likely to increase the host range of a given strain or of a population exchanging toxin genes. One report has suggested plasmid transfer between different B. thuringiensis strains during growth within an insect (170). We are not aware of any critical experiments directed towards understanding bacterial toxin gene expression within the gut of a susceptible pest.

Persistence of B. thuringiensis spores in the laboratory, greenhouse, and field or forest environment has been reasonably well studied (299, 403, 405). B. thuringiensis spores can survive for several years after spray applications (6), although rapid declines in population and toxicity have been noted. Methods of detection have generally been limited to spore counts.

Meadows (266) has analyzed three prevailing hypothetical niches of B. thuringiensis in the environment: as an entomopathogen, as a phylloplane inhabitant, and as a soil microorganism. Available data are still insufficient to choose among these and other possibilities, although B. thuringiensis seems to have been more readily isolated from insect cadavers or stored-product dusts than from soil (36, 65). It is also noteworthy that B. thuringiensis and B. cereus are able to multiply in the insect hemocoel and to provoke septicemia (156, 157, 358). Early work recognized the presence of a number of extracellular compounds that might contribute to virulence, including phospholipases (434), other heat-labile toxin activities (reviewed in reference 332), and -exotoxins (221). More recent characterization has shown that proteases (232), chitinases (356), and the secreted vegetative insecticidal proteins (VIPs) (108) (see below) may contribute to virulence. B. cereus and B. thuringiensis also produce antibiotic compounds that have antifungal activity (357); one of these products can act to synergize crystal protein-induced intoxication of certain lepidopterans (253). The Cry toxins are, therefore, the most prominent of a number of virulence factors allowing the development of the bacteria in dead or weakened insect larvae. Such data are at least suggestive that many strains of B. thuringiensis and some strains of B. cereus can be regarded as opportunistic insect pathogens. A more thorough understanding of the true ecological roles of B. thuringiensis would be of great importance, both for improving the reliability of risk assessment and for developing efficient methods for isolating novel B. thuringiensis strains containing useful -endotoxin genes.

A number of pesticidal proteins unrelated to the Cry proteins are produced by some strains of B. thuringiensis during vegetative growth (108, 401). These VIPs do not form parasporal crystal proteins and are apparently secreted from the cell. The VIPs are presently excluded from the Cry protein nomenclature because they are not crystal-forming proteins. The term VIP is a misnomer in the sense that some B. thuringiensis Cry proteins are also produced during vegetative growth as well as during the stationary and sporulation phases, most notably Cry3Aa (see "cry gene expression"). The location of the vip genes in the B. thuringiensis genome has not been reported, although it would not be surprising to find them residing on large plasmids that encode cry genes.

The vip1A gene encodes a 100-kDa protein that is apparently processed from its N terminus to yield an ~80-kDa protein upon secretion. The 80-kDa Vip1A protein is reported to be toxic to western corn rootworm larvae in conjunction with the Vip2A protein, whose coding region is located immediately upstream (401). Interestingly, Vip1A shows sequence similarity to the protective antigen of the tripartite Bacillus anthracis toxin (298).

The vip3A gene encodes an 88-kDa protein that is produced during vegetative growth but is not processed upon secretion. Genes encoding Vip3A-type proteins appear to be common among strains of B. thuringiensis and B. cereus (108). This protein is reported to exhibit toxicity towards a wide variety of lepidopteran insect pests, including Agrotis ipsilon, Spodoptera frugiperda, Spodoptera exigua, and Helicoverpa zea (108). When fed to susceptible insects at lethal concentrations, Vip3A causes gut paralysis and lysis of midgut epithelial cells: the physical manifestations of Vip3A intoxication resemble those of the Cry proteins (431).

GENETICS AND MOLECULAR BIOLOGY

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The B. thuringiensis Genome

B. thuringiensis strains have a genome size of 2.4 to 5.7 million bp (56). Physical maps have been constructed for two B. thuringiensis strains (57, 58). Comparison with B. cereus chromosomal maps suggests that all of these chromosomes have a similar organization in the half near the replication origin while displaying greater variability in the terminal half (57). Most B. thuringiensis isolates have several extrachromosomal elements, some of them circular and others linear (56). It has long been recognized that the proteins comprising the parasporal crystal are generally encoded by large plasmids (138). Sequences hybridizing to cry gene probes occur commonly among B. thuringiensis chromosomes as well (58), although it is unclear to what degree these chromosomal homologs contribute to production of the crystal.

The B. thuringiensis species harbors a large variety of transposable elements, including insertion sequences and transposons. The general characteristics of these elements have been extensively reviewed by Mahillon et al. (248). Here, the B. thuringiensis transposable elements are described with regard to their structural association with the cry genes.

The first studies on the structural organization of the cry1A gene environment showed that genes of this type were flanked by two sets of inverted repeated sequences (195, 218). Nucleotide sequence analysis revealed that these repetitive elements were insertion sequences that have been designated IS231 and IS232 (219, 237). IS231 belongs to the IS4 family of insertion sequences (315), and IS232 belongs to the IS21 family of insertion sequences (268). Because these elements can transpose (152, 268), it is likely that they provide mobility for the cry genes with which they form typical composite transposons. However, this hypothesis has not been tested experimentally.

Several IS231 variants have been isolated from various B. thuringiensis strains (249, 314, 316) and have been detected in representative strains from well more than half of the known B. thuringiensis serovars (212). In B. thuringiensis subsp. israelensis, an IS231 element (IS231W) is adjacent to the cry11Aa gene (4, 316). Although IS231 elements are frequently associated with cry genes, IS231-related DNA sequences have also been found in strains of B. cereus (190, 212) and Bacillus mycoides (212). In contrast, IS232 has a much smaller range among the organisms surveyed so far, appearing in only 7 of 61 B. thuringiensis serovars (212).

The cry4A gene of the israelensis subspecies is flanked by two repeated sequences in opposite orientations (45). These sequences, designated IS240, display features characteristic of insertion sequences (83). The IS240 transposase is homologous to those of the insertion sequences belonging to the IS6 family. IS240 is widely distributed in B. thuringiensis and is invariably present in known dipteran-active strains (319). Related sequences have also been detected in B. mycoides and B. cereus (212). An IS240 variant has been found upstream of the cry11B gene in the B. thuringiensis subsp. jegathesan (86) and from a plasmid of the dipteran-active strain B. thuringiensis subsp. fukuokaensis (103).

Insertion sequences have been found upstream of the cry1Ca gene (351) and downstream of a cryptic cry2Ab gene (160). These elements encode putative transposases that have significant similarities with the transposase of the IS150 element from Escherichia coli. These potential transposable elements of B. thuringiensis consequently belong to the IS3 family of insertion sequences.

The first transposable element identified in the genus Bacillus was isolated from B. thuringiensis following its spontaneous insertion into a conjugative plasmid transferred from Enterococcus faecalis (217). The genetic and structural characteristics of this transposable element fulfilled the criteria of a Tn element, and it was designated Tn4430 (216). Its transposase is homologous to those of the Tn3 family. In contrast to Tn3, however, the site-specific recombinase that mediates Tn4430 cointegrate resolution is not a resolvase but an integrase (247). Tn4430 is frequently found in the vicinity of genes of the cry1A type in various lepidopteran-active strains (196, 218, 328). However, Tn4430-like sequences have also been detected in several strains of B. cereus (56).

A transposable element designated Tn5401 was isolated from a coleopteran-active B. thuringiensis strain following its spontaneous insertion into a recombinant plasmid (27). Although nucleotide sequence analysis indicates that the structural organization of Tn5401 is similar to that of Tn4430, the transposases and the site-specific recombinases of these transposons are only distantly related (27). Tn4430 and Tn5401 are not known to coexist in any B. thuringiensis strain (27). In B. thuringiensis subsp. tenebrionis, Tn5401 is located just downstream of the cry3Aa gene (3). It is noteworthy that Tn5401 has been successfully used to construct a transposon insertion library in B. thuringiensis (251).

Two open reading frames encoding polypeptides homologous to the transposase and to the resolvase of the Tn3 family of transposons have been identified upstream of the cry16A gene found in Clostridium bifermentans (23, 82). This observation suggests that a Tn element is structurally associated with this cry gene.

Regarding the role of the transposable elements in B. thuringiensis, it is postulated that they are involved in the amplification of the cry genes in the bacterial cell, but this hypothesis has not been clearly tested. A second possible role is one of mediating the transfer of plasmids by a conduction process involving the formation of cointegrate structures between self-conjugative plasmids and chromosomal DNA or nonconjugative plasmids. Indeed, conjugation experiments suggest that Tn4430 mediates the transfer of nonconjugative plasmids by a conduction process (147). Thus, a major adaptive function for these transposable elements may be the horizontal dissemination of genetic material, including cry genes, within the B. cereus-B. thuringiensis species.

A common characteristic of the cry genes is their expression during the stationary phase. Their products generally accumulate in the mother cell compartment to form a crystal inclusion that can account for 20 to 30% of the dry weight of the sporulated cells. The very high level of crystal protein synthesis in B. thuringiensis and its coordination with the stationary phase are controlled by a variety of mechanisms occurring at the transcriptional, posttranscriptional, and posttranslational levels. Agaisse and Lereclus (8) and Baum and Malvar (30) have recently reviewed the regulation of cry gene expression in detail. We present here a broad outline of these regulatory mechanisms.

Sporulation-dependent cry Gene Expression. Extensive studies of the sporulation of B. subtilis have provided detailed information on the complex mechanisms that temporally and spatially control this differentiation process (for reviews, see references 104 and 231). At the transcriptional level, the development of sporulation is controlled by the successive activation of sigma factors, which bind the core RNA polymerase to direct the transcription from sporulation-specific promoters (275). These factors are the primary sigma factor of vegetative cells, ^A, and five factors called ^H, ^F, ^E, ^G, and ^K, which appear in that order in a temporally regulated fashion during development. The ^A and ^H factors are active in the predivisional cell, ^E and ^K are active in the mother cell, and ^F and ^G are active in the forespore.

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Sporulation-independent cry gene expression. The cry3Aa gene, isolated from the coleopteran-active B. thuringiensis var. tenebrionis, was found to be expressed during vegetative growth, although at a lesser extent than during the stationary phase (95, 252, 339). Analysis of lacZ transcriptional fusions and primer extension experiments indicates that the cry3Aa promoter is weakly but significantly expressed during vegetative growth, is activated from the end of exponential growth until stage II of sporulation (about T₃), and remains active until stage IV of sporulation (about T₇) (10, 324). The cry3Aa promoter, although located unusually far upstream of the start codon (position 558), resembles promoters recognized by the primary sigma factor of vegetative cells, ^A (10). A similar promoter was found 542 bp upstream of the start codon of the cry3Bb gene (30). The expression of cry3Aa is not dependent on sporulation-specific sigma factors either in B. subtilis (7) or in B. thuringiensis (324). Moreover, cry3Aa expression is increased and prolonged in mutant strains unable to initiate sporulation (7, 213, 251, 324). The results indicate that cry3Aa expression is activated by a non-sporulation-dependent mechanism arising during the transition from exponential growth to the stationary phase. The positive effect of mutations preventing the initiation of sporulation suggests that there is an event during sporulation (e.g., the disappearance of ^A in the mother cell) that turns off cry3Aa expression (7, 324).

The stability of mRNA is an important contributor to the high level of toxin production in B. thuringiensis. The half-life of cry mRNA, about 10 min, is at least fivefold greater than the half-life of an average bacterial mRNA (135).

Wong and Chang showed that the putative transcriptional terminator of the cry1Aa gene (a stem-loop structure) acts as a positive retroregulator (416). The fusion of a DNA fragment carrying this terminator with the 3' end of heterologous genes increases the half-life of their transcripts two- to threefold, which in turn increases the expression of their gene products. It has been demonstrated in other systems that the processive activities of 3'-5' exoribonucleases are impeded by 3' stem-loop structures (for a review, see reference 279). It is likely, then, that the cry1Aa transcriptional terminator increases the cry mRNA stability by protecting it from exonucleolytic degradation from the 3' end. Similar terminator sequences, potentially able to form stable stem-loop structures, are found downstream from various cry genes and may contribute to their high-level expression by stabilizing the transcripts. However, alternative processes could determine the rate of mRNA degradation, and the direct involvement of these sequences on mRNA stability has not been tested by deleting them from a cry gene and measuring stability of the message.

Between the cry3Aa promoter, located from positions 560 to 600, and the translational start codon is a region involved at a posttranscriptional level with the accumulation of cry3Aa mRNA as a stable transcript with a 5' end corresponding to nucleotide position 129 (10). Deletion of 60 bp extending from nucleotide positions 189 to 129 has no detectable effect on the expression level or on the position of the 5' end of the transcript (10). It is likely, then, that the initial transcript, begun hundreds of bases upstream, is processed posttranscriptionally.

Insertion of the cry3Aa 5' untranslated region (extending from nucleotides 129 to 12) between the B. subtilis xylA promoter and a lacZ reporter gene increases about 10-fold both the stability of the lacZ fusion mRNA and the production of -galactosidase (9). Deletion and mutation analysis indicate that the sequence required for the stabilizing effect is a perfect Shine-Dalgarno sequence (GAAAGGAGG) mapping at a position between 125 and 117; this sequence has been designated STAB-SD (9). The stability of the cry3Aa mRNA could result from an interaction between the 3' end of 16S rRNA and STAB-SD. The binding of a 30S ribosomal subunit to this sequence may protect the mRNA against 5'-3' ribonuclease activity, resulting in a stable transcript with a 5' end at nucleotide position 129 (i.e., the limit of 30S subunit protection). Potential STAB-SD sequences are also present in similar positions upstream of the cry3Ba, cry3Bb, and cry3Ca genes (96, 200).

Studies have shown that several cry1 genes cloned in E. coli (129) or B. subtilis (344) were able to direct the synthesis of biologically active inclusions, suggesting that the 130- to 140-kDa Cry1 protoxins can spontaneously form crystals. It is generally assumed that the cysteine-rich C-terminal half of the Cry1 protoxins contributes to crystal structure through the formation of disulfide bonds (39). A similar mechanism of protein self-assembly may be responsible for the crystal formation of other 130- to 140-kDa protoxins (e.g., Cry4, Cry5, and Cry7). The cysteine-rich C-terminal region is absent from the 73-kDa Cry3A protoxins. This protein forms a flat, rectangular crystal inclusion in which the polypeptides do not appear to be linked by disulfide bridges (35). Because this protein is able to form identical crystals in both B. thuringiensis and B. subtilis, it is possible that specific host factors are not required for the protein assembly. Analysis of the three-dimensional structure of the Cry3A toxin revealed the presence of four intermolecular salt bridges, which might participate in the formation of the crystal inclusion (222).

Various studies performed with E. coli and B. thuringiensis have demonstrated that crystallization of Cry2A (71 kDa) and Cyt1A (27 kDa) requires the presence of accessory proteins (for recent reviews, see references 8 and 30). These proteins may act at a posttranslational level to stabilize the nascent protoxin molecule and to facilitate crystallization. However, the precise mechanism of their role in crystal formation has not been determined.

Kostichka et al. (192) have reported that a Cry1Ia toxin could be found in the supernatant of B. thuringiensis cultures as a processed polypeptide of 60 kDa. The authors hypothesize that Cry1Ia is an exported protein and therefore interacts with the cellular protein export machinery. Such a characteristic, together with the fact that this toxin is synthesized early in sporulation (192), may have implications for the significance of these toxins in the ecology of B. thuringiensis. Similarly, the Cry16Aa toxin of C. bifermentans seems to be secreted during sporulation (23).

To date, the structures of three crystal proteinsCry3A (222), Cry1Aa (148), and Cyt2A (223)have been solved by X-ray crystallography. An analysis in the accompanying review demonstrates that Cry3A and Cry1Aa show about 36% amino acid sequence identity (79). This similarity is reflected in their three-dimensional structures; the corresponding domains can virtually be superimposed. Cyt2A, however, shows less than 20% amino acid sequence identity with Cry1Aa and Cry3A, and a similar alignment score would be obtained if the Cyt2A sequence were randomized. Not surprisingly, the Cyt2A structure is radically different from the other two structures. The structures of Cry1Aa, Cry3A, and Cyt2A are compared in Fig. 1.

View larger version (49K): [in this window] [in a new window]   FIG. 1.   Three-dimensional structures of Cry1A, Cry3A, and Cyt2A.

The Cyt toxins, unlike the Cry -endotoxins, are able to lyse a wide range of cell types in vitro (164). Cyt2A consists of a single domain in which two outer layers of alpha-helix wrap around a mixed beta-sheet. Cyt1A is believed to have a similar structure.

Cry3A and Cry1Aa, in contrast to Cyt2A, both possess three domains. Domain I consists of a bundle of seven antiparallel -helices in which helix 5 is encircled by the remaining helices. Domain II consists of three antiparallel -sheets joined in a typical "Greek key" topology, arranged in a so-called -prism fold (330, 343). Domain III consists of two twisted, antiparallel -sheets forming a -sandwich with a "jelly roll" topology.

Höfte and Whiteley (164) drew attention to the five blocks of amino acids conserved among most of the Cry toxins then known. Complete amino acid sequence alignment of the Cry proteins in our data set reveals the same five tracts, or conserved blocks, in most of them (Fig. 2 and 3). Comparison of the carboxyl-terminal halves of sequences with more than 1,000 residues suggests the presence of three additional blocks lying outside the active toxic core.

View larger version (77K): [in this window] [in a new window]   View larger version (55K): [in this window] [in a new window]   View larger version (81K): [in this window] [in a new window]   View larger version (78K): [in this window] [in a new window]   View larger version (62K): [in this window] [in a new window]   FIG. 2.   Amino acid sequence blocks conserved among Cry proteins. For each block, the consensus sequence denotes the positions at which at least 75% of the aligned proteins in the group have an identical or conserved amino acid (indicated by shading). An uppercase letter within the consensus sequence indicates that at least 75% of the residues at that position are identical, while a lowercase letter indicates that at least 75% of the residues are conserved. Conserved amino acids are those that fall into the following groups: a (A, G, S, T, or P); d (D, E, N, or Q); f (F, W, or Y); i (I, L, M, or V); and k (K or R). Highly conserved sequences conform to the consensus sequence at 75% or more of its positions. Variant sequences conform to the consensus sequence of the highly conserved group at 50 to 75% of the positions. Alternate blocks are derived from groups of proteins having a consensus sequence over that sequence block that differs from the corresponding highly conserved sequence at more than half of its positions. Novel sequences have no discernible homology to a conserved block that occupies the same relative position within sequences in the conserved group.

View larger version (35K): [in this window] [in a new window]   FIG. 3.   Positions of conserved blocks among Cry proteins. The cartoon shows the sequence arrangement for each holotype toxin (e.g., Cry1Aa1) having at least one of the conserved blocks defined in the legend to Fig. 2. Sequence blocks are shown as dark gray, light gray, or white to indicate high, moderate, or low degrees of homology, respectively, to the consensus sequence for each conserved block. Variant (var) alternate (alt) are as defined in the legend to Fig. 2. The lengths of each protein and the conserved blocks within them are drawn to scale.

Figure 4 shows an unrooted phylogenetic tree, constructed by an unweighted pair-group method using arithmetic averages algorithm from the multiply aligned Cry and Cyt protein sequences. Five sequence similarity groups are apparent, together with a single outlying sequence (Cry15). The conserved blocks are distributed in a fashion consistent with these similarity groups. The group consisting of Cry1, Cry3, Cry4, Cry7 to Cry10, Cry16, Cry17, Cry19, and Cry20 contains all five of the core blocks. A second group consisting of Cry5, Cry12 to Cry14, and Cry21 contains recognizable homologs of blocks 1, 2, 4, and 5. Block 1 shows more variability within this second group of sequences than within the first. The proteins within this second subgroup also possess a block 2 variant; block 2 sequences show greater sequence similarity within the two groups than between them (Fig. 2). Block 3 is completely absent from this second group of Cry proteins; an unrelated sequence, highly conserved within the second subgroup but absent from the first, lies between blocks 2 and 4. For both groups, when a protein possesses the C-terminal extension, blocks 6, 7, and 8 are invariably present (Fig. 2). Members of a third sequence similarity group, composed of Cry2, Cry11, and Cry18, possess block 1 and a truncated variant of the block 2 core (Fig. 2) but lack convincing homologs of the other conserved blocks (215). An alternating arginine tract not otherwise homologous to block 4 is found near the C terminus of Cry11 and Cry18. A weak homolog of block 5 may also be present among the proteins in this group, but its significance, if any, is uncertain (Fig. 2). The other proteins in the data setCyt1, Cyt2, Cry6, Cry15, and Cry22have no recognizable homologs to the conserved blocks seen in the three groups noted above.

View larger version (66K): [in this window] [in a new window]   FIG. 4.   Sequence similarity groups found among Cry and Cyt proteins. Sequences were aligned by using CLUSTAL W and a phylogenetic tree was constructed by NEIGHBOR as described in the accompanying work (79). The tree was visualized as a radial phylogram by using the TREEVIEW application. The proposed similarity groups are indicated by shading.

The conservation of blocks 1 through 5 is at least consistent with the notion that the proteins within the first subgroup, which includes Cry1 and Cry3, might adopt a similar three-domain tertiary structure. It is possible, too, that the second subgroupCry5, Cry12 to Cry14, and Cry21could possess a variation of the same structural theme. The degree of sequence similarity found in the Cry2, Cry11, and Cry18 group of proteins suggests that a fold similar to that in domain I of Cry3A may be present. Indeed, the crystal structure of Cry2Aa, which has been solved but not yet published (423a), confirms this prediction. Somewhat more surprisingly, Cry2A also possesses second and third domains strikingly similar to those of Cry3A, despite the apparent absence of primary sequence homology between the two proteins over this region.

Block 1 encompasses helix 5 of domain I. As mentioned below (see "Structure-Function Interpretations"), this helix has been implicated in pore formation, a role that might explain its highly conserved nature. The central location of helix 5 within domain I also suggests an essential role in maintaining the structural integrity of the helical bundle.

Block 2 includes helix 7 of domain I and the first -strand of domain II. These two structures comprise the region of contact between the two domains. There are three structurally equivalent salt bridges present between domain I and domain II in Cry1Aa and Cry3A (148); the residues involved lie within block 2. These interactions could be important if domain I changes its orientation relative to the rest of the molecule upon binding of the toxin to its receptor. Alternatively, the salt bridges could be responsible for maintaining the protein in a globular form during solubilization and activation.

Blocks 3, 4, and 5 each lie on one of the three buried strands within domain III. Block 3 contains the last -strand of domain II, a structure involved in interactions between domains I and III. The central two arginines of block 4 may be involved in intermolecular salt bridges affecting crystal or oligomeric aggregation (148, 222). As Grochulski et al. have noted, however, the first and last arginines are solvent exposed (148). These residues have been implicated in channel function (68, 336, 414).

An alternative way of looking at protein families is to examine the relatedness of structural or functional segments independently (47, 378). This type of analysis helped show a correlation between domain II sequence features shared by distantly related toxins and the cross-resistance profile of a diamondback moth mutant (369).

The long hydrophobic and amphipathic helices of domain I suggest that this domain might be responsible for the formation of lytic pores in the intestinal epithelium of the target organism, one of the proposed mechanisms of Cry toxin activity (see "Mechanism of action"). Domain I bears many striking similarities to the pore-forming or membrane-translocating domains of several other bacterial protein toxins, including colicin A, diphtheria toxin, andto a lesser extentPseudomonas exotoxin A (287). The pore-forming domain of colicin A consists of two central alpha-helices (8 and 9) surrounded by eight antiparallel alpha-helices (288). Pore formation is believed to involve insertion of the hydrophobic 8-9 helical hairpin into the membrane (101, 220). Similarly, diphtheria toxin is believed to enter the membrane via a hydrophobic helical hairpin following a pH-induced change in conformation (432). By analogy to these mechanisms, an "umbrella" model has been proposed, in which the Cry proteins also contain a hydrophobic helical hairpin (4-5) that initiates pore formation (222). Schwartz et al. (334) created disulfide bonds within domain I and between domains I and II in order to restrict intramolecular movements. Their results are consistent with the model described above in which helices 4 and 5 insert into the membrane while the rest of domain I flattens out on the membrane surface in an umbrella-like molten globule state. However, the lack of protein structural analysis in this work leaves open the possibility that the disulfide bonds blocked the ability of these mutant proteins to penetrate the membrane.

Similarly, little can be surmised as to the final structure of the lytic pore; a structure involving amphipathic helices (with the hydrophilic faces forming the lumen of the pore) seems the most probable. Given, however, that most domain I helices are largely amphipathic and theoretically long enough to span a membrane, little can be concluded. Even helix 2, which is split by a short nonhelical stretch, could traverse a membrane as part of a channel. Comparison of the Cry3A domain I helices with other known classes of amphipathic helices suggests that many of the helices (in particular 1, 5, and 6) show features characteristic of lytic peptides (378).

In contrast, Hodgman and Ellar (159) have proposed a "penknife" model for pore formation. In this model, based on the similarly named proposal for colicin A insertion (159), the strongly hydrophobic helices 5 and 6, which are joined by a loop at the top of the structure, open in a penknife fashion and insert into the membrane. The remainder of the molecule would remain at the membrane surface or on the receptor. Both the umbrella and penknife models are reviewed and illustrated by Knowles (185).

The surface-exposed loops at the apices of the three -sheets of domain II, because they show similarities to immunoglobin antigen-binding sites, were initially put forward as candidates for involvement in receptor binding. Site-directed mutagenesis and segment swapping experiments, as described under "Mechanism of action," have provided evidence in support of this model. It is interesting to note that domain II has a fold similar to that of the plant lectin jacalin (330). Jacalin is known to bind carbohydrates via the exposed loops at the apex of its -prism fold, whereas at least one Cry protein (Cry1Ac) is believed to recognize carbohydrate moieties on its receptor (188).

The -sandwich structure of domain III could play a number of key roles in the biochemistry of the toxin molecule. Li et al. (222) suggest that domain III functions in maintaining the structural integrity of the toxin molecule, perhaps by protecting it from proteolysis within the gut of the target organismbut of course all three domains would have to share this characteristic. From studies in other systems where toxin-receptor interaction leads to pore formation, it is known that -strand structures can participate in receptor binding (11, 71), membrane penetration (283), and ion channel function (241, 242, 427). None of these roles has been ruled out for domain III of Cry proteins; indeed, there is at least some evidence suggesting a role for domain III in receptor binding in certain systems (see "Mechanism of action" below).

Although solving the structure of one of the Cyt toxins has not really clarified their toxic mechanism, the predominantly -sheet structure of Cyt2A suggests a pore based on a -barrel (223). Three of the strands are sufficiently long to span the hydrophobic core of the membrane, and the sheet formed by them shows an amphiphilic or hydrophobic character. Theoretically the number of monomers required to form a barrel of sufficient size would be four to six. Various laboratories (75, 243, 244) have observed that Cyt1A (which is believed to have a common structure with Cyt2A) aggregates on the surface of the target cell but not in solution prior to binding to the cell surface. Using synthetic peptides, Gazit et al. (125) provided further evidence that the Cyt1A toxin self-assembles within the membrane and also identified two -helices (A and C) that appeared to be involved in both membrane interaction and intermolecular assembly. Mathematical modeling hypothesized that Cyt1A exists as a 12-toxin oligomer (243). No receptor-binding motif could be identified in the Cyt2A structure, although the use of monoclonal antibodies has identified a putative cell binding region on Cyt1A (76). Using a number of different biophysical techniques, Butko et al. (55) have also studied the interaction of Cyt1A with lipid membranes. They observed a considerable loosening of the tertiary structure of the toxin upon lipid binding but could find no evidence that the toxin actually enters the membrane. The authors suggest that Cyt1A exerts its effect via a general, detergent-like perturbation of the membrane.

MECHANISM OF ACTION

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General Features

The mechanism of action of the B. thuringiensis Cry proteins involves solubilization of the crystal in the insect midgut, proteolytic processing of the protoxin by midgut proteases, binding of the Cry toxin to midgut receptors, and insertion of the toxin into the apical membrane to create ion channels or pores. Crystals are comprised of protoxins. For the protoxins to become active, a susceptible insect must eat them. For most lepidopterans, protoxins are solubilized under the alkaline conditions of the insect midgut (162). Differences in the extent of solubilization sometimes explain differences in the degree of toxicity among Cry proteins (18, 98). A reduction in solubility is speculated to be one potential mechanism for insect resistance (265). For at least one protein, Cry3A, nicking by chymotrypsin-like enzymes in the midgut may be necessary for solubilization (60).

After solubilization, many protoxins must be processed by insect midgut proteases (203, 379) to become activated toxins. The major proteases of the lepidopteran insect midgut are trypsin-like (204, 270) or chymotrypsin-like (174, 280, 297). The Cry1A protoxins are digested to a 65-kDa toxin protein in a processive manner starting at the C terminus and proceeding toward the 55- to 65-kDa toxic core (69, 73). The carboxy-terminal end of the protoxin, which initially appears to be wound around the toxin in an escargot-like manner, is clipped off processively in 10-kDa sections during processing of the protoxin (74). An interesting and unexpected finding is that DNA is intimately associated with the crystal and appears to play a role in proteolytic processing (38, 76a). The mature Cry1A toxin is cleaved at R28 at the amino-terminal end (277); Cry1Ac, at least, is cleaved at K623 on the carboxy-terminal end (37). Two stages of processing have been detected for Cry1Ia with trypsin or Ostrinia nubilalis midgut proteases: a fully toxic intermediate, with an N terminus at protoxin residue 45 and a C terminus at residue 655 or 659, is further processed to a partially toxic core, with an N terminus clipped to residue 156 (340).

Activated Cry toxins have two known functions, receptor binding and ion channel activity. The activated toxin binds readily to specific receptors on the apical brush border of the midgut microvillae of susceptible insects (161-163). Binding is a two-stage process involving reversible (161, 162) and irreversible (166, 307, 395) steps. The latter steps may involve a tight binding between the toxin and receptor, insertion of the toxin into the apical membrane, or both. It has been generally assumed that irreversible binding is exclusively associated with membrane insertion (166, 307, 395). Certainly the recent report that truncated Cry1Ab molecules containing only domains II and III can still bind to midgut receptors, but only reversibly, supports the notion that irreversible binding requires the insertion of domain I (116). Yet at least some published data is consistent with the notion of tight binding to purified receptors. Tight binding of Cry1Aa and Cry1Ab to purified Manduca sexta aminopeptidase N (APN) has been observed (256), and Cry1Ac may also show some degree of irreversible binding to M. sexta APN. There are likewise indications of irreversible binding for Cry1Ac to purified Lymantria dispar APN (172, 389). Finally, Vadlamudi et al. (385) calculated similar binding constants when toxin bound to brush border membrane vesicles (BBMV) and to nitrocellulose-immobilized receptor (i.e., a ligand blot).

In M. sexta, the Cry1Ab receptor is believed to be a cadherin-like 210-kDa membrane protein (119, 180, 385), while the Cry1Ac and Cry1C receptors have been identified as APN proteins with molecular masses of 120 and 106 kDa, respectively (183, 234, 329). Incorporation of purified 120-kDa APN into planar lipid bilayers catalyzed channel formation by Cry1Aa, Cry1Ac, and Cry1C (335). These receptor assignments can be difficult to reconcile with some ligand blot binding data, however (90, 208). There is also some evidence that domain II from either Cry1Ab or Cry1Ac can promote binding to the larger protein, while domain III of Cry1Ac promotes binding to the presumed APN (91). Alkaline phosphatase has also been proposed to be a Cry1Ac receptor (329). The recent cloning of the putative 210-kDa (386) and 120-kDa (184) Cry1Ac receptors opens exciting possibilities for studies on toxin-receptor interactions. In Heliothis virescens, three aminopeptidases bound to Cry1Ac on toxin affinity columns. One of them, a 170-kDa APN, bound Cry1Aa, Cry1Ab, and Cry1Ac, but not Cry1C or Cry1E. N-Acetylgalactosamine inhibited the binding of Cry1Ac but not that of Cry1Aa or Cry1Ab. The three Cry1A toxins each recognized a high-affinity and a low-affinity binding site on this 170-kDa APN (235). In gypsy moth (L. dispar), the Cry1Ac receptor also seems to be APN, while Cry1Aa and Cry1Ab bind to a 210-kDa brush border membrane vesicle (BBMV) protein (388, 389). In Plutella xylostella (236) and Bombyx mori (425) as well, APN appears to function as a Cry1Ac binding protein. An M. sexta gene encoding a Cry1Ab-binding APN has also been cloned, as has its P. xylostella homolog (92).

Insertion into the apical membrane of the columnar epithelial cells follows the initial receptor-mediated binding, rendering the toxin insensitive to proteases and monoclonal antibodies (415) and inducing ion channels or nonspecific pores in the target membrane. In vitro electrophysiological studies of voltage-clamping of lipid bilayers (338, 348) and sections of whole insect midguts (67, 68, 153, 225, 307) support the functional role of the toxin in pore or ion channel formation. The nature of the ion channel or pore-forming activity of Cry toxins in the insect is still controversial. It is alternatively described as a large lytic pore that is not specific for particular ions (see reference 187 and "Structure-function interpretations") or as an ion-specific channel that disrupts the membrane potential but does not necessarily lyse midgut epithelial cells (see below).

Several recent reviews have considered the mechanism or mode of action of Cry toxins (126, 134, 158, 185, 186, 378, 412, 424). Some of these reviews have presented models for the mode of action. The present review considers the newest primary data on receptor binding and ion channel activity and critically evaluates the extant models.

Soon after methods were developed for preparing insect BBMV (411), BBMV became the subjects of toxin binding studies (323, 413). Several groups were able to correlate a toxin's insect specificity with its affinity for specific receptors on BBMV of susceptible insects (162, 163, 395). In vivo experiments have also confirmed that Cry proteins bind to microvillae in the midgut (49, 93, 426).

A set of in vitro-constructed reciprocal recombinants between Cry1Aa and Cry1Ac (130, 131) provided evidence that insect specificity was localized in the central domain of the toxin for some insects (B. mori and Trichoplusia ni) and the central and C-terminal domains for others (H. virescens). Visser et al. (397) reviewed the use of domain substitutions to locate specificity regions. Van Rie et al. (395) demonstrated that receptor binding correlated with insect specificity, and Lee et al. (209) demonstrated that the specificity and binding domains were colinear for Cry1Aa against B. mori. Examination of the crystal structure of Cry3A (222) suggested a physical basis for receptor binding (see "Toxin structure," above) by the loops of domain II. This suggestion has now been substantiated by site-directed mutagenesis.

Early work by Hoffman et al. (162), Van Rie et al. (395), and others employed competition binding studies to demonstrate a correlation between toxin affinity and insecticidal activity. In a paradoxical finding, however, Wolfersberger (413) observed that Cry1Ab was more active than Cry1Ac against gypsy moth larvae, despite exhibiting a relatively weaker binding affinity. Other examples of this phenomenona lack of correlation between receptor binding affinity and insecticidal activityare now known (123, 327, 395). Liang et al. (224) evaluated binding affinity and dissociation (both reversible and irreversible binding) of Cry1Aa, Cry1Ab, and Cry1Ac with gypsy moth BBMV. While they confirmed that the affinity of Cry1Ab was not directly related to toxin activity, they did observe a direct correlation between the irreversible binding rate and toxicity. Ihara et al. had earlier stressed the importance of considering irreversible binding in explaining the difference in toxicity of Cry1Aa and Cry1Ab to B. mori (166).

Prior to the work of Liang et al. (224), kinetic analysis of Cry toxin-receptor binding relied on the Hill (161) or Scatchard (395) equations that assume a strictly reversible binding:

(1)

where T is a Cry toxin, R is a receptor for this toxin, TR is a toxin that is reversibly bound to the receptor, K_d1 is the dissociation constant k₁ is the on rate, and k₁ is the off rate.

In reality, the toxin becomes irreversibly associated with the apical membrane by insertion (415), giving the following kinetic diagram (224) (including two models for the inserted state of the toxin):

(2)

where T, R, and TR are as described for equation 1; *T is an irreversibly bound toxin, presumably inserted into the membrane but not associated with a receptor; and *TR is an irreversibly bound toxin which is still associated with a receptor.

Given the irreversible rate component k₂, the reaction cannot reach equilibrium; as the toxin-receptor complex is formed, it is drained away by insertion. Therefore, competition or binding experiments under conditions where insertion can take place (equation 2) do not yield true K_d values (224). Since equilibrium conditions are not obtained, equation 2 should not be considered any more valid for calculation of a classical dissociation constant, K_d, than equation 1. Alternate values, such as the 50% inhibitory concentration (224, 257) or K_com, the so-called competition constant (206, 208, 308, 422), have been used for K_d under these conditions. Under some conditions insertion should not occur, i.e., ligand blotting of ¹²⁵I-labeled Cry1Ac to purified gypsy moth 120-kDa receptor (207) or binding of unlabeled Cry1Ac to purified M. sexta 120-kDa receptor fixed to dextran surfaces in surface plasmon resonance analysis (256). In both cases, the calculated K_d was 100 times that obtained with BBMV, suggesting that the effect of k₂ upon the reversible reaction is considerable. In contrast, competition binding of Cry1Ab to the 210-kDa receptor on a ligand blot differed little from calculated competition binding to M. sexta BBMV (385) or to the cloned 210-kDa receptor expressed in human embryonic 293 cells (386) (708 pM, 1,000 pM, and 1,015 pM, respectively). It may be that the rate of insertion, k₂, is negligible for the 210-kDa receptor, perhaps due to either extremely tight binding to this receptor or a failure to insert.

The prediction that domain II is involved in receptor binding (131, 222) has led to extensive substitution of loop residues in this domain in Cry3A, Cry1A, and Cry1C by mutagenesis (Fig. 5). Data on the effects of mutations in sequences encoding domain II loop regions of selected Cry toxins are summarized in Table 1. Perusal of these data indicates that mutations may have either a negative or positive effect on binding and toxicity and that mutations in different loop regions, sometimes involving the same type of amino acid residue, can have a different effect on binding. Minor changes in binding usually do not have a major effect on toxicity, but a major positive or negative effect has a corresponding positive or negative effect on toxicity. Furthermore, either binding affinity (as measured by competition binding) or irreversible binding may effect toxicity, and for a few mutant proteins one of these parameters may be positive (increased affinity) while the other may be negative (increased dissociation), with an overall negative effect on toxicity. It is apparent that the same mutation in a toxin can have quite different results on different insects. A more complete description of domain II loop mutations is given in a recent review (311).

View larger version (86K): [in this window] [in a new window]   FIG. 5.   Predicted three-dimensional structure of Cry1Ab highlighting the domain II residues shown by mutagenesis to be involved in receptor binding. Domains I (white), II (blue), and III (red) and portions of loops 1 (orange), 2 (yellow), and 3 (violet) and the 8 loop (green) are shown as space-filling molecular structures in the standard presentation (left) and rotated 90° (right).

In summary, the binding picture for domain II is complex. Results clearly suggest that all of the loops of domain II can participate in receptor binding, although perhaps not all at the same time for a given insect or receptor. Different toxins may have the same amino acid sequence in the loops of domain II (e.g., Cry1Ab and Cry1Ac) yet bind to different receptors, at least on ligand blots. The available data seem to show an intriguing similarity between the receptor binding loops of domain II and other known protein-protein epitopes; i.e., a hydrophobic residue capable of tight binding to the receptor is surrounded by hydrophobic or charged residues. Similar interactions have been noted in several other systems (for a general review, see reference 300). A striking demonstration of the importance of a hydrophobic residue in irreversible binding was a series of mutations in F371 of Cry1Ab loop 2 to residues of lower hydrophobicity. This reduction in hydrophobicity was correlated with the gradient of reduced irreversible binding and toxicity (309).

Not included above is a discussion of work on two putative surface loops of domain II of Cry1C (loop 1, ₃₁₇GRNF₃₂₀, and loop 2, ₃₇₄QPWP₃₇₇) (350). This study did not evaluate the effect of mutational alteration of loop residues on binding, but examined cytotoxicity with cultured Spodoptera Sf9 cells and toxicity with Aedes aegypti larvae. The results indicated that specificity differences for Cry1C between Sf9 cells and A. aegypti larvae could be changed radically by single point mutations in the loops. For example, an R-to-I mutation at position 318 (R₃₁₈I) abolished mosquitocidal activity but retained 80% cytotoxicity to Sf9 cells. Likewise, several mutations caused a loss of mosquitocidal activity with only a marginal loss of cytolytic activity against Sf9 cells. Substitutions that altered the charge, such as Q₃₇₄E, completely abolished activity against both cells and mosquito larvae.

Domain III has also been implicated in receptor binding. As mentioned above, several groups (130, 331) have suggested a role for domain III of Cry1Ac in H. virescens specificity. Masson et al. (258) extended the suggestion to include CF-1 cells. Aronson et al. (19) mutated a hypervariable region of domain III (residues 500 to 509) of Cry1Ac. Mutations S₅₀₃A and S₅₀₄A resulted in lower toxicity to M. sexta, with a corresponding decrease in binding to BBMV proteins on ligand blots. Lee et al. (211) analyzed homolog scanning mutants that exchanged domain III between Cry1Aa and Cry1Ac. Hybrid proteins containing the Cry1Aa domain III bound a 210-kDa receptor while hybrid proteins containing the Cry1Ac domain III bound a 120-kDa receptor in gypsy moth. Domain switching experiments have also suggested a role for Cry1Ab domain III in binding to S. exigua (90). Finally, there is one report suggesting a biotin-binding activity for domain III (99), although a role for this activity in receptor binding has not been demonstrated directly.

Mutations in domain I have been shown to affect the ability of the toxin to dissociate from the binding complex. Wu and Aronson (419) created several mutations in domain I of Cry1Ac. The A₉₂D and R₉₃G mutations (at the base of 3) dramatically reduced toxicity to M. sexta. A loss of toxicity by the A₉₂D mutation was also observed in Cry1Aa and Cry1Ab. A series of substitution residues at the 92 and 93 positions revealed that at position 92 only a negatively charged residue caused a loss of toxicity. Any substitution of R₉₃ except the positively charged Lys caused a loss of toxicity. The authors concluded that a positively charged surface is important for toxicity. Chen et al. (67) repeated the mutation at the A₉₂ position in Cry1Ab with A₉₂E. In agreement with Wu and Aronson's result (419), toxicity was almost completely lost. Although competition binding of the mutant toxin to M. sexta was not affected, irreversible binding was severely disrupted. Chen et al. (67) further demonstrated that Y₁₅₃ mutations (at the loop between the bottoms of 4 and 5, on the same surface as A₉₂E) introducing a negative charge had a negative effect on membrane insertion.

In summary, binding studies reveal three types of mutants. Certain mutations in domain II (A mutants) affect competition but not dissociation. Examples are Cry1Ab ₃₆₈RRP₃₇₀ (309) and Cry1Ab loop 3 mutations F₄₄₀A and G₄₃₉A (310). Certain other mutations in domain II (B mutants) affect dissociation but not competition. Examples are Cry1Ab F₃₇₁A (and most other substitutions except Trp) and G₄₃₉A (307). In domain I, certain mutations (C mutants) affect insertion of toxin into the membrane. The distinction between B and C mutants may be arbitrary; it assumes different functions for domains I and II, a point still lacking definitive proof. Examples of C mutants are Cry1Ac A₉₂D or R₉₃G (419) and Cry1Ab A₉₂E or Y₁₅₃D (67). In the above cases, all of these effects were observed in the same toxin (Cry1Ab) and insect (M. sexta) system. Cry3A loop 3 mutants have also been described in which effects on both competition and dissociation were observed (422).

Masson et al. (256) describe differences in off rates for two Cry1Ac toxins that differ in three residues: L₃₆₆F, F₄₃₉S, and a deletion of D₄₄₂. While these differences might be due to other causes, it is interesting that position 366 and positions 439 to 442 occur in loops 2 and 3, respectively. Wells (402) describes human growth hormone mutants in which alanine substitution of positively charged residues affects on rates, and other alanine-scanning mutants in large hydrophobic residues affect off rates. A similar pattern is observed in the Cry toxin mutations of the receptor binding loops. Positive residues may be involved in long-range orientation of the toxin to the receptor, affecting the on rate. In some cases, large hydrophobic residues were involved in tight binding, and their mutants affected the off rate; in other cases, mutations in large hydrophobic residues affected competition binding (that is, on rates).

Considerable work has been reported on the effects of Cry toxins on insect tissue culture cells. Work with CF-1 cells has led to the colloidal osmotic lysis model for the cytolytic activity of Cry toxins (187). This model proposes that an influx of water, along with ions, results in cell swelling and eventually lysis. When exposed to microgram amounts of activated toxin, cells leaked a variety of electrolytes tested, including CrO₄², uridine, and Rb⁺. Under these conditions, then, Cry toxins form a nonspecific pore. Wolfersberger (412) lists the problems that arise from experiments with established cell cultures. The cells are normally maintained at a pH of 6.8not the basic pH found in the lumen of many insect midguts. They lack normal midgut receptors (161) and do not respond as specifically to toxins as does the whole insect (410). They are tolerant to nearly 1,000-fold-greater levels of toxin than insects under physiological conditions (187). From experiments on tissue culture cells it is clear, however, that Cry toxins have a fairly general capacity to insert into membranes and form large, nonspecific pores under certain conditions, including high-toxin concentrations, long incubation times, and relatively low pHs.

Several techniques have been employed to study the ion channel activity of the B. thuringiensis Cry proteins. Harvey and Wolfersberger (153) used electrophysiological analysis of sections of whole midgut of M. sexta to measure short circuit current inhibition (I_SC). The mechanism of I_SC is explained in the excellent review by Wolfersberger (412). Results of recent studies (67, 68), using nanomolar concentrations of toxin, have supported the validity of the voltage clamping technique as an assessment of Cry toxin activity correlating well with bioassays.

Several groups have examined Cry toxin ion channel activity in planar lipid bilayer (PLB) systems. Slatin et al. (348) examined Cry1Ac and Cry3A in PLB membranes of various compositions and found that toxins formed cation-selective channels. Cry1Ac ion channels exhibited multiple opening and closing states (indicating more than one single-channel conductance level or cooperative gating). Cry1Ac channels were commonly 600 pS in size (in 300 mM KCl), while Cry3A formed larger channels of 4,000 pS. Channels did not form at pH 7 but did form at pH 9.7.

In a pivotal paper on Cry protein ion channel activity, Schwartz et al. (338) reported a pH effect on the type and size of ion channels made by Cry1C in PLBs. Under alkaline conditions (pH 9.5), cationic channels of 100 to 200 pS were formed, exhibiting multiple conductance states. Under acidic conditions (pH 6.0), anionic channels of different sizes (8 to 120 pS) were observed. These channels were inhibited by zinc added to the cis chamber, but not to the trans chamber, indicating directionality of the channel. The authors note that behavior