The Bacillus thuringiensis delta-endotoxin Cry1C as a potential bioinsecticide in plants

The Cry1C group of Bacillus thuringiensis delta-endotoxins contains 10 highly homologous members of the Cry1Ca toxin sub-group and additional three members of the Cry1Cb sub-group that differ in domain III sequence. The Cry1Ca bioinsecticidal spectrum encompasses lepidopteran insects that are completely or partially tolerant to the current commercially used Bt crops. Plant-expressed Cry1Ca proteins successfully control specific lepidopteran pests, however, Bt crops expressing Cry1Ca have not been commercialized. This review summarizes the accumulating data in Cry1C research. Multiple sequence alignments of closely related Cry1Ca homologues show that the N-terminal half of the protein, comprising the "active toxin", is less conserved than the C-terminal part, which is involved in the assembly of the toxin-containing crystalline structure during the bacterial sporulation stage. Bioinformatics analyses predict high evolutionary diversity of amino acid residues in the regions identified as toxin-membrane interaction sites. All the three structural domains of Cry1Ca "active toxin" interact in vitro with membrane vesicles produced from epithelial cells of the larval gut. This multi-site-interaction depends on the normal assembly of membrane lipid raft domains, which is disturbed during cell division, when transient Cry1Ca insensitivity is observed. Cry1Ca interaction with the gut epithelial cells involves specific aminopeptidase-N receptors that differ from those described for other Cry1 toxins. The involvement of other membrane components in the interaction remains to be studied. Cry1A-resistant insect pests, such as the Cry1Ac-tolerant mutants of diamondback moth, are sensitive to Cry1Ca, due to the involvement of different genetic loci. Hence pyramiding expression of Cry1Ca and other Cry toxins can broaden the bioinsecticidal spectrum of Bt crops and simultaneously delay the evolution of Cry-resistant insect populations.