Killer toxin's punch lies below the belt.

A newly determined three-dimensional structure of the toxic protein that causes botulism shows a surprising twist: A belt of amino acids protects the lethal part of the toxin. Researchers searching for an antidote against the muscle-paralyzing disease may need to revise their strategies.

Although botulism is contracted most often through contaminated food, terrorists have seized upon the toxin as a biological weapon. Produced by the bacterium Clostridium botulinum, the toxin kills by shutting down the muscles needed for breathing. A fatal dose is just 100 billionths of a gram, making the toxin one of the most lethal poisons known.

Bibhuti R. DasGupta of the University of Wisconsin-Madison and his colleagues enlisted the help of researchers at the Lawrence Berkeley (Calif.) National Laboratory to identify the positions of the 1,285 amino acids in the protein. Using X-ray crystallography, they examined one of seven types of the toxin, each produced by a different strain of the bacterium. The structure they deciphered confirms some earlier findings about the toxin, but it also reveals an unexpected feature, the researchers report in the October Nature Structural Biology.

Previous results had shown that the toxin consists of three parts, each playing a separate role in shutting down the nerve cells, or neurons, that control muscles. After one part binds to receptors on a neuron, a second opens up a pore in the cell. A third portion then passes through the pore, breaks away from the rest of the toxin, and interferes with the nerve-signal transmitter called acetylcholine. The new images show what these three parts, or domains, look like, says study coauthor Raymond C. Stevens of Lawrence Berkeley.

Surprisingly, the second domain of the toxin loops around the third domain like a belt, hiding the lethal piece of the protein, Stevens says. The belt conceals the third portion until it enters the cell, making the toxin "like a Trojan horse."

"We had no inkling that the loop was there," says Frank Lebeda of the Army Medical Research Institute of Infectious Diseases at Ft. Detrick in Frederick, Md. Drugs designed to lock onto and disable the toxic part of the protein will have to move the belt out of the way or be small enough to squeeze by.

Currently, no good inhibitors to the toxin exist. Patients are put on artificial respiration and closely monitored for weeks or months until their immune systems clear the toxin. One compound that can be used to treat botulism (SN: 8/2/86, p. 76) "only indirectly alleviates some of the symptoms," Lebeda says. "It temporarily reverses the paralysis."

Interestingly, botulinum toxin itself is being used as a treatment for certain neuromuscular disorders (SN: 1/19/98, p. 42). Injected in tiny, harmless amounts, it quiets muscle spasms.

With the shapes of the six other types of toxin still unknown, Eric A. Johnson of the University of Wisconsin-Madison's Food Research Institute comments that DasGupta's study is "a seminal piece of work, but it's just the beginning."