Molecule of the Month: Anthrax Toxin

Anthrax is a household word, in spite of the fact that anthrax is not a common disease. For humans, anthrax is difficult to contract. It is not transmitted from person to person--it is usually contracted when people come into contact with infected animals or their products. But recently, anthrax has gained the potential to be a major threat through bioterrorism. It is an effective weapon because it forms sturdy spores that may be stored for years, that rapidly lead to lethal infections when inhaled.

Anthrax is caused by an unusually large bacterium, Bacillus anthracis. Once its spores lodge in the skin or in the lungs, it rapidly begins growth and produces a deadly three-part toxin. These toxins are designed for maximum lethality, and are frighteningly effective. Part of the toxin is a delivery mechanism that seeks out cells; another part is a toxic enzyme that rapidly kills the cell. In anthrax toxin, there is one delivery molecule, termed "protective antigen" because of its use in anthrax vaccines (shown on the left from PDB entry 1acc ). It delivers the other two parts, edema factor and lethal factor (center and right, from PDB entries 1k90 and 1jky ), which are the toxic components that attack cells.

These types of multiple-part toxins are quite common in the bacterial world because they are exquisitely effective. Many other examples, such as toxins from the bacteria that cause cholera and whooping cough, may be found in the PDB. The delivery component specifically seeks out cell surfaces and inserts the toxic component where it can do the most damage. The toxic component is far more effective than poisons like cyanide and arsenic. Those poisons attack one-on-one, with a single cyanide molecule poisoning a single protein molecule. But toxic enzymes are compact cell-killing machines. Once inside the cell, they hop from molecule to molecule, destroying each in turn. These molecules are so effective that in some cases a single molecule can kill an entire cell.

The protective antigen is the delivery mechanism of anthrax toxin. The bacteria secrete it as a single chain, as shown on the left from PDB entry 1acc . The protein then finds a cell surface and binds to it. A human protease on the surfaces of cells then clips off a small piece, colored blue here, arming the mechanism. It then combines with six other copies of the protein to form a seven-sided ring, as seen on the right from PDB entry 1tzo . This ring binds firmly to the cell surface and is thought to extend loops into the membrane, forming a pore. The other two parts of the toxin then bind to the ring and are carried into the cell.

The two toxic components of anthrax toxin are both enzymes that attack the signalling functions of the cell. Once they are delivered inside by the protective antigen, they set to work. The edema factor (shown on the left) is an adenyl cyclase enzyme. It takes ATP (in green) and clips off two phosphates, reconnecting the remaining one back in a small loop to form cyclic AMP. Cyclic AMP is an important messenger in cells, often used to relay messages that are sent by hormones. For instance, a rise in cyclic AMP in response to adrenaline can cause an increase in heart rate. Edema factor floods the cell with cyclic AMP, destroying the careful balance normally achieved by hormones.

Lethal factor (shown on the right) attacks at another sensitive spot. It is a very specific protease that makes a cut in several similar mitogen-activated protein kinase kinases. In the illustration, a small segment of this target is shown in green, bound in the active site of the toxin. These kinases are essential steps at the end of another signalling pathway which is important in cell growth and proliferation. The lethal factor destroys this control by disabling one key step in the chain of messsages.