The insecticidal toxins produced by the bacterium Bacillus thuringiensis are called Cry (for crystal) toxins. The toxins are numbered in a way that uniquely identifies each toxin. Cry3Aa is a toxin that affects beetles and not other species. Why is it so specific?
The answer requires knowledge of the Cry protein. Cry proteins can insert into cell membranes to form pores, channels that allow ions and water to cross the membrane. Mass ion movement can upset the osmotic balance of the cell. In a susceptible insect, the Cry toxin forms pores in the cells of the gut, disrupting the osmotic balance resulting in the swelling and bursting of cells. The cell destruction causes a breach in the insect gut that allow gut bacteria to infect the hemolymph of the insect. The infection can kill the insect.
A Cry toxin alone will not readily insert into membranes. It needs help from the target cell. Cry toxins have 3 domains. Domain I forms the membrane pores. Domains II and III contain a series of loops and other structures that can bind to proteins on the surface of the target cell. It is the binding properties of Domains II and III that determine BT specificity.
Cry3Aa specifically affects some beetles and is toxic to the red flour beetle, Tenebrio molitor. Cells in the gut of the red flour beetle contain proteins called “cadherins”, short for calcium containing proteins responsible for cell adhesion (sticking together). The structure and binding properties of cadherins vary among species. The cadherin of the red flour beetle binds the Domain II of the Cry3Aa toxin in a way that facilitates insertion of the pore forming region into the cell membrane. Insects and species that have cadherins with a different structure do not bind the Cry3Aa and it does not readily insert into the membranes of cells in those species. Thus Cry3Aa can easily insert into the membranes and is quite toxic to red flour beetle. The Cry3Aa does not significantly insert into membranes of non-susceptible species and it is therefore not toxic to bees, butterflies, cows and people.