Insect tolerance to the crystal toxins Cry1Ac and Cry2Ab is mediated by the binding of monomeric toxin to lipophorin glycolipids causing oligomerization and sequestration reactions

Abstract

Endotoxins from the soil bacterium Bacillus thuringiensis are used worldwide to control insect pests and vectors of diseases. Despite extensive use of the toxins as sprays and in transgenic crops, their mode of action is still not completely known. Here we show that two crystal toxins binding to different glycoprotein receptors have similar glycolipid binding properties. The glycolipid binding domain was identified in a recombinant peptide representing the domain II of the crystal toxin Cry1Ac (M-peptide). The recombinant M-peptide was isolated from bacterial lysates as a mixture of monomers and dimers and formed tetramers upon binding to glycolipid microvesicles from gut tissues and lipid particles from hemolymph plasma. Likewise, when mature toxins and M-peptides where mixed with plasma, these peptides bind to lipid particles and can be separated with lipophorin particles on low-density gradients. When mature toxin and M-peptides are added to lipid particles in increasing amounts, the peptide-particle complexes form higher aggregates that are similar to aggregates formed in low-density gradients in the presence of the toxin. This could indicate that glycolipids on lipid particles are possible targets for toxin monomers in the gut lumen, which upon binding to the glycolipids form tetramers and aggregate particles and thereby sequester the toxin inside the gut lumen before it can interact with receptors on the brush border membrane. The implication is that domain II interacting with glycolipids mediate tolerance to the toxin that is separate from interaction of the toxin with glycoprotein receptors causing toxicity.