A comparison and analysis of the toxicity and receptor binding properties of Bacillus thuringiensis CryIC ∂‐endotoxin on Spodoptera littoralis and Bombyx mori

The binding of L-[35S]methionine in vivo labelled CryIC toxin to its receptor in brush border membrane vesicle (BBMV's) prepared from Spodoptera littoralis and Bombyx mori was studied. Both insect species were highly susceptible to the CryIC toxin in bioassays, B. mori being 7-fold more sensitive to CryIC than S. littoralis (LC50's of 10 ng/cm2 and 70 ng/cm2, respectively). Competition and direct binding experiments revealed saturable high-affinity binding sites on BBMV's from both insects which had similar binding characteristics for the CryIC toxin (Kd = 10 nM, Bmax = 8 to 9 pmol/mg BBMV's and IC50 = 37 nM for both inspect species). Thus a specific receptor for the CryIC toxin is present in both insect species and the 7-fold greater potency of CryIC towards B. mori is not due to qualitative or quantitative differences in binding affinity or receptor site concentration. Dissociation experiments also indicated that the binding of [35S]CryIC to B. mori BBMV's is partially reversible.