Specificity of Bacillus thuringiensisδ‐endotoxins

Abstract

To study the molecular basis of differences in the insecticidal spectrum of Bacillus thuringiensisδ-endotoxins, we have performed binding studies with three δ-endotoxins on membrane preparations from larval insect midgut. Conditions for a standard binding assay were established through a detailed study of the binding of ¹²⁵I-labeled Bt2 toxin, a recombinant B. thuringiensisδ-endotoxin, to brush border membrane vesicles of Manduca sexta. The toxins tested (Bt2, Bt3 and Bt73 toxins) are about equally toxic to M. sexta but differ in their toxicity against Heliothis virescens. Equilibrium binding studies revealed saturable, high-affinity binding sites on brush border membrane vesicles of M. sexta and H. virescens. While the affinity of the three toxins was not significantly different on H. virescens vesicles, marked differences in binding site concentration were measured which reflected the differences in in vivo toxicity. Competition experiments revealed heterogeneity in binding sites. For H. virescens, a three-site model was proposed. In M. sexta, one population of binding sites is shared by all three toxins, while another is only recognized by Bt3 toxin. Several other toxins, non-toxic or much less toxic to M. sexta than Bt2 toxin, did not or only marginally displace binding of ¹²⁵I-labeled Bt2 toxin in this insect. No saturable binding of this toxin was observed to membrane preparations from tissues of several non-susceptible organisms. Together, these data provide new evidence that binding to a specific receptor on the membrane of gut epithelial cells is an important determinant with respect to differences in insecticidal spectrum of B. thuringiensis insecticidal crystal proteins.