Toxic Action of Cry Proteins - When Cry proteins are ingested by insects, they are dissolved in the alkaline juices present in the midgut lumen. The gut pro teases process them hydrolytically to release the core toxic fragments. The toxic fragments are believed to bind to specific high affinity receptors present in the brush border of midgut epithelial cells.

As a result, the brush border membranes develop pores, most likely nonspecific in nature, permitting influx into the epithelial cells of ions and water, which causes their swelling and eventual lysis.

The presence of specific receptors in the midgut epithelium is most likely the chief reason for Cry toxin specificity. The specificity seems to be lost upon reduction of the cysteine residues of the protoxin, but can be restored by reoxidation of these residues.