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Helical protein helps E. coli hang on.

Escherichia coli has proved a versatile bacterium. It appears as various strains, each adapted to where it lives, In the urinary tract, for example, where streams of liquid wash other bacterial invaders away, one strain of E. coli clings to its host with dozens of hairlike appendages called adhesive pili. Now, scientists appear to have found the secret of the pili's tenacity: a helical structure that can unravel under stress (right).

Each pilus, about a micrometer long and 6.8 nanometers thick, consists of 1,000 units of a protein called Papa. Biophysicists Esther Bullitt of the Boston University School of Medicine and Lee Makowski of Florida State University deduced the three-dimensional structure of pili by photographing them with an electron microscope, then reconstructing the images on a computer

As they report in the Jan. 12 Nature, Papa winds tightly into coils that connect to form a closed helix. Three other proteins bind the helix to a sugar on the membrane of an epithelial cell in the urinary tract lining, thus anchoring the E. coli.

The researchers then turned to how this helical structure might help E. coli resist being flushed away. They noticed in micrographs that some of the pili appeared bent and that fine threads connected these partial breaks. The threads, they reasoned, are lengths of Papa in which linked coils had pulled apart. By measuring unraveled pili, the researchers found that these structures unwind to about five times length.

Pili, suggest Bullitt and Makowski, help the bacterium hold fast to its host cell, even through blasts of urine. "If the pili were like a pencil," explains Bullitt, "they would just snap off " Instead, they can stretch slightly and bounce back. Or they can stretch even more, until the bonds between some Papa coils break and the closed helix opens.

More than mere curiosity led the duo to untangle pili structure. They believe their findings may help researchers develop new drugs to treat urinary tract infections -- the cause of 6 million visits to doctors' offices each year For example, scientists might find a molecule that would block the sites on Papa that hold coils of the protein together.
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Title Annotation:Escherichia coli bacteria resists being flushed from tissue
Author:Kaiser, Jocelyn
Publication:Science News
Article Type:Brief Article
Date:Jan 21, 1995
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