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  August 1,  2005
Volume 83, Number 31
p. 13


  Toxin Channeling
Research reveals how components of anthrax toxin move into cells

CHANNEL HUNTERS Collier (from left), Krantz, and Melnyk are revealing the workings of anthrax's pore-forming protective antigen.
CHANNEL HUNTERS Collier (from left), Krantz, and Melnyk are revealing the workings of anthrax's pore-forming protective antigen.


Development of drugs to treat anthrax may get a boost from a new report that sheds light on how the toxin is channeled into cells.

The anthrax toxin consists of three components that are nontoxic individually but combine to form toxic complexes. Their entry into target cells begins with endocytosis, forming a membrane-bound structure called an endosome. One of the components, a protein known as protective antigen, forms a heptameric pore in the endosomal membrane. The other two, which are enzymes, traverse that membrane through the pore. The part of the pore that crosses the membrane is a 14-stranded ß-barrel with an inner diameter of about 15 Å--too narrow for folded proteins to slip through. Until now, little has been known about how this pore works.

Whether the pore is simply a static channel or plays an active role in the transport has been a key issue for R. John Collier, a microbiologist at Harvard Medical School who has spent more than 15 years studying how the three components work together. He and coworkers, including postdocs Bryan A. Krantz and Roman A. Melnyk, now have data suggesting the pore plays an active role.

The researchers find that the pore contains a ring of phenylalanine residues--one from each of the seven subunits making up the pore--that is vital for moving the unfolded enzymes across the membrane. This structure, dubbed the -clamp, interacts with hydrophobic portions of the proteins as they unfold to thread themselves through the pore (Science 2005, 309, 777). The -clamp may even help the proteins to unfold in the first place, they suggest.

The researchers propose that the pore functions as a ratchet that enables the unwound leading edge of the protein to move through the channel and makes it easier for the rest of the protein to unfold.

Others have hypothesized that similar structures in other protein pores serve as a gasket to prevent ions and metabolites from leaking out of the cell. The -clamp may function likewise to preserve the pH gradient, Collier says.

Collier doesn't yet know the energy source for the process. Some of the driving force may come from the membrane potential and the pH gradient across the endosomal membrane, he suspects.

In addition to functioning as a chaperone, the -clamp is the binding site for drugs already known to block the channel. "Understanding its existence and eventually its detailed structure may help in devising better drugs against anthrax," Collier says.

Collier and his coworkers continue to try to get a crystal structure of the pore. "It's a membrane protein, and membrane proteins are notoriously difficult to crystallize," he says. "We just haven't found the conditions yet to get crystals of this one."

RATCHET A ring of seven phenylalanine residues (labeled as F427) in the anthrax toxin pore interacts with hydrophobic portions of the other anthrax proteins as they travel through the pore. The cross-section shows the presumed location of this "-clamp" at the top of the ß-barrel stem that crosses the endosomal membrane. The colors indicate different protein domains.

  Chemical & Engineering News
ISSN 0009-2347
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