Binding properties of Bacillus thuringiensis Cry4A toxin to the apical microvilli of larval midgut of Culex pipiens

Cry4A is a dipteran-specific delta -endotoxin produced by Bacillus thuringi ensis, and toxic to Culex pipiens (mosquito) larvae. The immunohistochemica l staining of the midgut sections of C. pipiens larvae revealed that Cry4A bound in vitro and in vivo to the microvilli of the epithelial cells of pos terior midgut and gastric caecae. The binding of digoxigenin-labeled Cry4A (DIG-Cry4A) to the apical microvilli was almost abolished in the presence o f excess unlabeled Cry4A, suggesting that the binding of Cry4A to the micro villi was specific. Several Cry4A-specific binding proteins were detected u sing the ligand blotting technique with DIG-Cry4A. Moreover, an insertion a ssay was done, where the binding of DIG-Cry4A to the BBMVs was completely i rreversible and did not compete with excess unlabeled Cry4A. On the basis o f these results, we propose a schematic interpretation for the binding proc ess of Cry4A.