LA JOLLA, Calif. , October 23, 2001
A collaboration led by Robert C. Liddington, Ph.D., of the Burnham Institute has determined the three-dimensional structure of the anthrax toxin Lethal Factor (LF), a protein required for anthrax to perform its deadly function. The crystal structure of LF facilitates the design of antitoxins that could provide a cure for anthrax at a late stage in infection. This crystal structure will be published in the November 8th print edition of Nature, and on the journal’s website October 23 at 2:00 pm EDT.
Anthrax is a proven agent of biological terrorism. Pulmonary anthrax, in which spores of the anthrax bacteria are inhaled, is typically fatal unless diagnosis is made at an early stage in the disease (when antibiotics such as Ciprofloxacin can provide a complete cure). At late stages in the disease antibiotics can kill the anthrax bacteria, but do not affect the toxin, which is sufficiently concentrated in the bloodstream to induce death.
The crystal structure of LF suggests a blueprint for its undoing. LF is one of the two major toxic proteins produced by anthrax. The other protein, Protective Antigen (PA), is the principal ingredient of the anthrax vaccine currently available. The two proteins comprise anthrax lethal toxin: neither acting alone is lethal, and disabling either one of them disarms anthrax lethality. Separating LF from PA is one way to disable anthrax.
Liddington explained that “We knew from earlier biochemical studies that LF was the key player in death from anthrax, so that determining its three-dimensional structure at atomic resolution could tell us a lot about how it works and thus how to disarm it”.
Blocking LF’s enzyme function is one way to disarm the toxin. LF enters cells and picks out just one kind of protein among the many thousands found in a typical cell. This protein, called “mitogen-activated protein kinase” or “MAPKK” is part of a signaling pathway that instructs the cell to release molecules into the bloodstream that alert the body to bacterial infection. By cutting this line of communication, the bacteria delay the immune response. At a later stage in infection, the immune cells break open, triggering a massive immune response that is too much too late, leading rapidly to death from septic shock.
The crystal structure reported in Nature captures LF bound to its target protein. “The structure tells us how LF recognizes just one protein among thousands, and thus how we could design drugs to block that recognition” says Dr. Liddington.
The knowledge now in hand with Nature’s publication of the anthrax LF makes available vital information for the rational design of a more effective antitoxin against anthrax poisoning. This will involve collaborations with groups at the NIH and USAMRIID. The long-term significance of this work is that it lays the foundation for new research aimed at designing anthrax toxin as a transport system for targeting proteins into cells.
For example, anthrax toxin has been designed as a so-called “targeted toxin” to recognize and destroy HIV-infected cells. Recent studies have also pointed to the utility of anthrax toxin as a cancer drug. Thus anthrax toxin suppresses tumor growth and angiogenesis. “The same pathways that have been developed by anthrax to cause death can be harnessed for beneficial purposes, and our crystal structure will allow us to create designer toxins that can kill a cancer cell more efficiently or deliver a useful protein into a cell that lacks or needs it”, says Dr. Liddington.
Solving the crystal structure of LF was a five-year effort including twelve researchers from several institutions: University of Leicester, U.K., Dana Farber Cancer Institute in Boston, Harvard Medical School, The National Institute of Dental and Craniofacial Research, the University of Michigan Medical School, as well as the Burnham Institute in La Jolla.
This publication was funded in part by grants from the National Institute of Allergy and Infectious Diseases (NIH) and the Medical Research Council (UK), both issued to Dr. Robert Liddington.