Bacillus thuringiensis delta-Endotoxin Binding Sites in Two Lepidoptera, Wiseana spp. and Epiphyas postvittana

Proteins from the midguts of the light-brown apple moth Epiphyas postvittana (Walker) and the porina caterpillars Wiseana cervinata (Walker), W. copularis (Meyrick), and W. jocosa (Meyrick) which bind the Bacillus thuringiensis delta-endotoxin Cry1Ba were characterized using Cry1Ba labeled with Bolton and Hunter reagent. A comparison of two iodine labeling techniques on the toxicity of B. thuringiensis delta-endotoxins showed that labeling using chloramine-T substantially decreased Cry1Ba toxicity against the light-brown apple moth E. postvittana, whereas labeling using the Bolton and Hunter reagent had no effect on the toxicity of either Cry1Ac or Cry1Ba to this insect. The characteristics of Cry1Ac binding sites from E. postvittana midguts were determined by competitive binding assays using toxin labeled with 125I by both methods. The difference in values of binding site characteristics found by the two methods was shown to be caused by modification of the toxin by the conditions of chloramine-T labeling. The relationship between number and affinity of the binding sites and the toxicity of the delta-endotoxins is discussed.