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Secrets of Anthrax killer toxin revealed
A microscopic view of the anthrax bacteria is seen in this undated handout photo from the U.S. Army Medical Research and Development Command at Fort Detrick, Md.
TWO GROUPS of researchers have announced key features of how anthrax toxin destroys cells.
In back-to-back papers in the journal Nature, investigators identify how one part of the toxin gets into cells and how another part turns off one of the cell's major internal switches.
The studies also show how at least one molecule can prevent the toxin from destroying cells. Though still in the laboratory stage, these discoveries offer new ways to investigate potential anthrax treatments.
Several types of anthrax exist, but researchers are most concerned with inhalation anthrax, which can occur after a person inhales a large number of bacterial spores.
The spores move to the lungs where they germinate, producing a potent toxin.
``If you do not kill the anthrax bacterium soon after infection, the microbe has time to produce potentially fatal levels of toxin, against which current drugs are not likely to be effective,'' says Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), which funded the two studies.
``These reports greatly increase our understanding of how anthrax toxin destroys cells and offer promising ways to develop treatments for advanced disease by attacking the toxin itself.'' Anthrax toxin has three parts.
Two parts, edema factor (EF) and lethal factor (LF), can destroy cells from the inside or prevent them from working.
A third component, protective antigen (PA), carries EF and LF into the cells.
In the new reports, the researchers asked two critical questions: What molecule on the surface of animal cells does PA use as a doorway, or receptor, for entry; and how does the LF toxin attach to and destroy its intracellular targets.
To answer the first question, the University of Wisconsin's John Young, worked with John Collier, a Harvard University specialist on anthrax toxin. Researchers have long known that anthrax toxin uses an unidentified receptor molecule as a type of Trojan horse, riding the receptor into the interior of a cell. Through genetic analysis, the doctors identified a protein on the surface of animal cells that proved to be the anthrax toxin receptor, which they labelled ATR.
The researchers next identified the region of ATR where the toxin attached. Using this information they then produced a shortened, free-floating version of the receptor that contained the toxin-binding domain.
When they mixed that receptor fragment with rodent cells and anthrax toxin in a test tube, the cells were completely protected from destruction.
``The soluble receptor worked as a decoy or sponge to absorb the toxin before it could attach to the ATR on the cells,'' explains Dr. Young.
``Now that we know what the anthrax receptor looks like, researchers can screen large numbers of smaller molecules to see if they, too, can prevent the toxin from binding ATR and entering cells.''
In the second study, other researchers sought to learn how the LF part of anthrax toxin attaches to its target molecule inside of a person's cells.
After entering cells, LF locates a protein called MAPKK and chews off one of its ends, preventing it from working. Because MAPKK is a key molecular switch that controls a cell's internal communications, its destruction leads to death of the cell.
Robert Liddington, of The Burnham Institute in La Jolla, led an international team of researchers in a study of the three-dimensional structure of LF.
The investigators took X-rays of LF as it attached to MAPKK, uncovering key details of the toxin's surface and how it binds in a specific way to its target protein.
``LF grabs on by means of a long groove in the toxin molecule,'' says Dr. Liddington. ``By understanding how LF attaches to MAPKK, we now have the information needed for rational drug design.''
The ability to construct new anti-toxin compounds based on known features of the protein rather than by randomly screening large numbers of compounds should hasten the development of new drugs to treat anthrax.
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