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Pumping Iron, Bacteria-style
REBECCA RAWLS
Like the airlock in a spaceship, transporter proteins in the outer membrane of gram-negative bacteria close behind precious iron cations before allowing them to move through the protein into the inner reaches of the cell. That's the picture that emerges from new crystal structures of one type of iron-transporter, known as FecA.
Postdoc Andrew D. Ferguson and professor Johann Deisenhofer of Howard Hughes Medical Institute and the department of biochemistry, University of Texas Southwestern Medical Center, and colleagues have determined the structure of the FecA transporter alone and with its iron substrate bound to it [Science, 295, 1715 (2002)].
Earlier structural studies with a different iron transporter protein showed that, when iron binds to the transporter, it sends a signal through the membrane to recruit an energy-delivering molecule called TonB to the transporter. Energy from TonB is needed to fuel the movement of the iron into the cell against a concentration gradient.
But that structure didn't show how the transporter keeps the iron from leaking back out of the pore. "If no precautions are taken, iron from inside the cell could get back outside again," Deisenhofer explains. "Something has to be done to prevent this, and our structure shows what happens."
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