2 DIFFERENT BACILLUS-THURINGIENSIS DELTA-ENDOTOXIN RECEPTORS IN THE MIDGUT BRUSH-BORDER MEMBRANE OF THE EUROPEAN CORN-BORER, OSTRINIA-NUBILALIS (HUBNER) (LEPIDOPTERA, PYRALIDAE)

Binding of three Bacillus thuringiensis insecticidal crystal proteins (ICPs) to the midgut epithelium of Ostrinia nubilalis larvae was chara cterized by performing binding experiments with both isolated brush bo rder membrane vesicles and gut tissue sections. Our results demonstrat e that two independent ICP receptors are present in the brush border o f O. nubilalis gut epithelium. From competition binding experiments pe rformed with I-125-labeled and native ICPs it was concluded that CryIA (b) and CryIA(c) are recognized by the same receptor. An 11-fold-highe r binding affinity of CryIA(b) for this receptor correlated with a 10- fold-higher toxicity of this ICP compared with CryIA(c). The CryIB tox in did not compete for the binding site of CryIA(b) and CryIA(c). Immu nological detection of ingested B. thuringiensis ICPs on gut sections of O. nubilalis larvae revealed binding only along the epithelial brus h border membrane. CryID and CryIE, two ICPs that are not toxic to O. nubilalis, were not bound to the apical microvilli of gut epithelial c ells. In vitro binding experiments performed with native and biotinyla ted ICPs on tissue sections confirmed the correlation between ICP bind ing and toxicity. Moreover, by performing heterologous competition exp eriments with biotinylated and native ICPs, it was confirmed that the CryIB receptor is different from the receptor for CryIA(b) and CryIA(c ). Retention of activated crystal proteins by the peritrophic membrane was not correlated with toxicity. Furthermore, it was demonstrated th at CryIA(b), CryIA(c), and CryIB toxins interact in vitro with the epi thelial microvilli of Malpighian tubules. In addition, CryIA(c) toxin also adheres to the basement membrane of the midgut epithelium.