X-ray images taken at SSRL have revealed how anthrax hijacks important cell machinery to enter and destroy human cells. The results give a clearer picture of how anthrax works and bring researchers closer to developing a therapy against anthrax infection as well as a new cancer therapeutic.

1. One of the three anthrax toxins, PA (the bowling-pin shaped ‘PA83’ in the illustration), binds to a CMG2 receptor on the surface of a human cell. 2. The two other anthrax toxins, LF and EF (oval shapes), stick to PA. The toxins and the receptor are wrapped inside a membrane ball and pulled into the cell. 3. PA transforms its shape and forms a channel through the membrane ball that allows EF and LF to get into the main compartment of the cell and cut off distress calls to the immune system. (Graphic courtesy of Robert Liddington)

Researchers from The Burnham Institute and the National Institute of Allergy and Infectious Diseases used the SSRL data to discover the structure of an anthrax toxin bound with a cell receptor.

Bacillus anthracis, the bacteria that causes anthrax disease, circulates in the bloodstream and produces three toxins. The toxins enter nearby cells and turn off their distress calls to the immune system, like an intruder disabling 911 phone access. Without an immune response, the bacteria keep reproducing and making more toxins. Eventually, the toxins kill the cells.

To get into cells, one of the toxins—protective antigen (PA)—binds to a receptor called CMG2 on the surface of human cells. This triggers the joined pair—to which the two other anthrax toxins are attached—to be wrapped inside a membrane ball and moved inside the cell. PA takes advantage of the receptor and a natural change in the ball’s acidity to change its shape.

"PA makes a big loop, a long hairpin structure that pokes through the membrane of the ball," said Robert Liddington (Burnham Institute), an author on the recent Nature paper describing this work.

Piercing through the ball membrane allows the toxin to form a channel, like a straw in a soda. The other two toxins usurp energy from the cell and use the PA channel to flow out of the ball and into the main part of the cell where they prevent an immune response.

"The toxin is using natural cellular machinery in a very creative way," Liddington said. The new information on how PA recognizes and tricks our cell receptors is potent information for designing anti-anthrax drugs that derail the PA-CMG2 interaction.

PA also binds to a receptor called TEM8 found primarily on the surface of cells lining the blood vessels that supply nutrients to tumors. This scenario for treating cancer involves PA binding to TEM8, allowing one of the toxins, called lethal factor, to enter and kill rapidly proliferating cancer cells. PA toxin injected into blood vessels has already killed tumors in mice. The PA can also enter normal cells via the CMG2 receptor, but the damage is limited because there are no anthrax bacteria pumping out huge doses of toxin. To more precisely target cancer, researchers are looking for differences between TEM8 and CMG2.

"The two receptors are quite similar but probably there are enough differences to engineer anthrax PA molecules to favor binding to the TEM8 receptor," Liddington said.