Analysis of the molecular basis of insecticidal specificity of Bacillus thuringiensis crystal δ-endotoxin

The mechanism of action and receptor binding of a dual-specificity Bacillus thuringiensis var. aizawai ICl delta-endotoxin was studied using insect cell culture. The native protoxin was labelled with 125I, proteolytically activated and the affinity of the resulting preparations for insect cell-membrane proteins was studied by blotting. The active preparations obtained by various treatments had characteristic specificity associated with unique polypeptides, and showed affinity for different membrane proteins. The lepidopteran-specific preparation (trypsin-treated protoxin containing 58 and 55 kDa polypeptides) bound to two membrane proteins in the lepidopteran cells but none in the dipteran cells. The dipteran-specific preparation (protoxin treated sequentially with trypsin and Aedes aegypti gut proteases, containing a 53 kDa polypeptide) bound to a 90 kDa membrane protein in the dipteran (A. aegypti) cells but bound to none in the lepidopteran cells or Drosophila melanogaster cells. The toxicity of trypsin-activated delta-endotoxin was completely inhibited by preincubation with D-glucose, suggesting a role for this carbohydrate in toxin-receptor interaction. The toxicity was also decreased by osmotic protectants to an extent proportional to their viscometric radius. These results support a proposal that initial interaction of toxin with a unique receptor determines the specificity of the toxin, following which cell death occurs by a mechanism of colloid osmotic lysis.