Mutagenesis of two surface-exposed loops of the Bacillus thuringiensis CryIC delta-endotoxin affects insecticidal specificity.

Site-directed mutagenesis was used to determine the role of two surface-exposed loops (Gly-317-Phe-320 and Gln-374-Pro-377) in the insecticidal specificity of the Bacillus thuringiensis CryIC delta-endotoxin. Mutant toxins were generated by PCR using degenerate oligonucleotide primers, and expressed in Escherichia coli. More than 50 mutant toxins were screened for toxicity to the lepidopteran Spodoptera frugiperda Sf9 cell line using an in vitro lawn assay. A panel of these mutant toxins, which included toxic and non-toxic variants from both loops, was further screened for activity towards Aedes aegypti larvae. The activity of these mutants to Sf9 cells was quantified more precisely using a cell lysis assay. Three categories of mutants were identified: (1) those non-toxic to either Sf9 cells or Aedes aegypti larvae; (2) those fully toxic to both genera; and (3) those which were only toxic to Sf9 cells. For the first loop, the differential specificity was not restricted to any single residue. In the second loop, two mutant toxins with a Pro-377-->Ala substitution displayed this phenotype. The time dependence of toxicity towards Sf9 cells was examined using the same panel of mutants. All toxic mutants displayed an identical time course to the wild-type toxin, with the exception of the two Pro-377-->Ala mutants of the second loop. These toxins displayed a lower time dependence, no cell death occurring within the first hour of incubation. These results show that the two loops are important determinants of both the activity and specificity of the CryIC delta-endotoxin.