Possible Insecticidal Mechanisms Mediated by Immune-Response-Related Cry-Binding Proteins in the Midgut Juice of Plutella xylostella and Spodoptera exigua

Cry toxins are insecticidal toxin proteins produced by a spore-forming Gram-positive bacterium Bacillus thuringiensis. Interactions between the Cry toxins and the receptors from midgut brush border membrane vesicles (BBMVs), such as cadherin, alkaline phosphatase, and aminopeptidase, are key steps for the specificity and insecticidal activity of Cry proteins. However, little is known about the midgut juice proteins that may interfere with Cry binding to the receptors. To validate the hypothesis that there exist Cry-binding proteins that can interfere with the insecticidal process of Cry toxins, we applied CrylAbl-coupled Sepharose beads to isolate Cry-binding proteins form midgut juice of Plutella xylostella and Spodoptera exigua. Trypsin-like serine proteases and Dorsal were found to be CrylAbi-binding proteins in the midgut juice of P. xylostella. Peroxidase-C (PDX-C) was found to be the CrylAbl-binding protein in the midgut juice of S. exigua. We proposed possible insecticidal mechanisms of CrylAbl mediated by the two immune-related proteins: Dorsal and PDX-C. Our results suggested that there exist, in the midgut juice, Cry-binding proteins, which are different from BBMV-specific receptors.