Bacillus thuringiensis toxins: an overview of their biocidal activity

Abstract

Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

Figure 1

Protein crystals (bipyramidal) mixed with spores from Bt strain H29.3.

Figure 2

Schematic overview of the current nomenclature system used by the Bt Toxin Nomenclature Committee for δ-endotoxins (Cry and Cyt) and secretable (Vip and Sip) toxins [8]. In this example, numbers indicate different Vip proteins changing rank 1 depending of percentage amino acid similarity (for Vip proteins this rank may change to date among Vip1, Vip2, Vip3 and Vip4). The same rule applies for ranks 2, 3 and 4 assigning a different identification digit/letter.

Figure 3

Summarized view showing the known host spectrum of Bt δ-endotoxins (Cry and Cyt) [9,26]. Cry1A-C (separated by hyphen) indicates a group of C3y1A, Cry1B and Cry1C toxins. Cry1B, I (separated by colon) indicates different Cry1B and Cry1I toxins. Semicolons separate groups or individual toxins. Cyt toxins are in red.

Figure 4

Three-dimensional structure of Cry2Aa toxin. This structure from PDB accession number 1I5P [40], is representative of a three-domain toxin produced by Bt. Roman numerals indicate the typical domains of the three-domain Cry proteins: (I), perforating domain; (II), central domain, involved in toxin-receptor interactions; (III), galactose-binding domain, involved in receptor binding and pore formation [7,21,38,39].

Figure 5

Amino acid conserved blocks (1 to 8) among different three-domain Cry proteins (colored boxes) [5]. Green boxes represent the five conserved amino acid blocks first described by Höfte and Whiteley located in the Cry protein toxic core [1]. Red boxes indicate the three additional conserved amino acid blocks found by Schnepf et al. [5]. The protoxin is digested by midgut proteases into a smaller fragment, which is responsible for toxic activity. Three-dimensional structure changes upon proteolytic activation are also depicted for Cry1Ac protoxin: toxic core comprising domains I, II and III (boxed), domain IV (cyan), domain V (blue), domain VI (green), and domain VII (red) [43].

Figure 6

Three-dimensional structure of binary Cry34/Cry35 toxin from PDB accession numbers 4JOX (Cry34) and 4JPO (Cry35) [58]. QxW motifs are depicted as colored residues: red-blue-red and red-cyan-red at positions Q89-K90-W91 and Q133-Q134-W135, respectively, located at the N-terminal domain (gray).

Figure 7

Three-dimensional structure for toxin Cyt1Aa and activated Cyt2Ba monomer from PDB accession numbers 3RON [20] and 2RCI [66], respectively.

Figure 8

Schematic tree showing the distribution and known host spectrum of different secreted Bt toxins namely Sip1 protein, toxic for some Coleoptera [15], Vip1/Vip2 (binary) toxins active against Coleoptera [14] and Hemiptera [75], Vip4 unknown toxicity and host range [8] and Vip3 toxins active against Lepidoptera [13].

Figure 9

Predicted features and conserved domains found in Vip4Aa1 toxin. SP: predicted signal peptide (residues 1–28), PA14: conserved domain (residues 45–179), Binary_toxB: conserved domain (residues 218–631) commonly found in Vip1Aa1 proteins and present in novel Vip4Aa1 toxin.