Mode of action of Bacillus thuringiensis Cry and Cyt toxins and their potential for insect control

Abstract

Bacillus thuringiensis Crystal (Cry) and Cytolitic (Cyt) protein families are a diverse group of proteins with activity against insects of different orders--Lepidoptera, Coleoptera, Diptera and also against other invertebrates such as nematodes. Their primary action is to lyse midgut epithelial cells by inserting into the target membrane and forming pores. Among this group of proteins, members of the 3-Domain Cry family are used worldwide for insect control, and their mode of action has been characterized in some detail. Phylogenetic analyses established that the diversity of the 3-Domain Cry family evolved by the independent evolution of the three domains and by swapping of domain III among toxins. Like other pore-forming toxins (PFT) that affect mammals, Cry toxins interact with specific receptors located on the host cell surface and are activated by host proteases following receptor binding resulting in the formation of a pre-pore oligomeric structure that is insertion competent. In contrast, Cyt toxins directly interact with membrane lipids and insert into the membrane. Recent evidence suggests that Cyt synergize or overcome resistance to mosquitocidal-Cry proteins by functioning as a Cry-membrane bound receptor. In this review we summarize recent findings on the mode of action of Cry and Cyt toxins, and compare them to the mode of action of other bacterial PFT. Also, we discuss their use in the control of agricultural insect pests and insect vectors of human diseases.

Figure 1

Three dimensional structures of insecticidal toxins produced by Bacillus thuringiensis Cry1Aa, Cry2Aa, Cry3Aa, Cry3Bb, Cry4Aa, Cry4Bb and Cyt2A.

Figure 2

Relative length of Cry protoxins and position of protease digestion. White boxes represent the protoxin and striped boxes represent the activated toxin. Solid arrows show the amino- and carboxy- terminal cleavage sites of the activated toxins. Doted arrows show the intramolecular cleavages. Cleavage of Cry1A at residue 51 resulted in loss of helix α-1 and pre-pore formation. Cleavage of Cry4B resulted in two fragments of 18 and 46 kDa, while Cry11A resulted in two fragments of 34 and 32 kDa.

Figure 3

Receptor molecules of Cry1A proteins. CADR, cadherin receptor; APN, aminopeptidase-N, ALP, alkaline phosphatase, GCR, 270 kDa glyco-conjugate receptor.

Figure 4

Model of the mode of action of Cry and Cyt toxins. Panel A, sequential interaction of Cry toxins with different receptor molecules in lepidopteran larvae. (1) Solubilization and activation of the toxin; (2). Binding of monomeric Cry toxin to the first receptor (CADR or GCR), conformational change is induced in the toxin and α-helix 1 is cleaved; (3) Oligomer formation; (4) Binding of oligomeric toxin to second receptor (GPI-APN or GPI-ALP), a conformational change occurs and a molten globule state of the toxin is induced; (5) insertion of the oligomeric toxin into lipid rafts and pore formation. Panel B, role of Cyt and Cry toxins in the intoxication of dipteran larvae. (1) Cry and Cyt toxins are solubilized and activated; (2) Cyt toxin inserts into the membrane and Cry toxin binds to receptors located in the membrane (ALP or Cyt toxin); (3) oligomerization of the Cry toxin is induced; (4) oligomer is inserted into the membrane resulting in pore formation.