Bacteria Can't Do Their Thing If They Don't Have Cling; Scientists Open Door To Possible New Treatments For Urinary Tract Infections

"Our findings should lead to new drugs to treat UTIs by blocking the formation of these protein fibers," says study leader Scott J. Hultgren, Ph.D., the Helen Lehbrink Stoever Professor of Molecular Microbiology. "They also should improve our general understanding of how disease-causing bacteria build, fold and secrete proteins that enable them to cause disease."

Hultgren and his laboratory worked in collaboration with Gabriel Waksman, Ph.D., the Roy and Diana Vagelos Professor of Biochemistry and Molecular Biophysics at the School of Medicine, whose laboratory conducted the X-ray crystallography studies showing the structure of the molecules involved in the fiber assembly process. X-ray crystallography reveals the 3-D arrangement of atoms in proteins.

UTIs are the second most common infectious disease in the United States, says Hultgren. Each year they account for 100,000 hospital admissions and 8 million doctor visits. UTIs mainly affect women, about half of whom experience at least one UTI and 20-40 percent of whom develop recurrent infections.

UTIs begin when bacteria gain a foothold on cells lining the kidney or bladder and grow into colonies. They latch onto cells using tiny fibers known as pili. Similar fibers also are produced by bacteria responsible for a variety of gastric, respiratory and other infections.

The fibers are made up of identical individual pieces, or subunits, linked together like plastic snap beads. Earlier work by Hultgren and Waksman found that as each subunit is made within a bacterium, it is joined to another molecule known as a chaperone. Chaperone proteins are found in all living cells and, as their name implies, protect other molecules from trouble. In this case, they shield subunit proteins from interacting with one another at the wrong time and place.

The present study, however, found that the chaperones here also play a key role in fiber assembly. The crystallographic images revealed that each subunit molecule contains a deep groove. The images further showed that an edge of the chaperone molecule fits into this groove and holds it open.

The chaperone-subunit pair then shuttle to a place at the bacterial membrane where pili are assembling. There, the chaperone slips free of the subunit and is replaced by a tail-like strand projecting from another subunit at the base of the growing fiber.

The strand fits into the groove like a hot dog in a bun. With the chaperone no longer holding the groove open, the edge of the "bun" snaps shut around the strand, firmly locking the two subunits together. In this way, the fiber grows longer one "snap bead" at a time.

Discovering that the fibers consist of interlocking tails explains why bacterial pili are so durable and able to resist harsh conditions in the laboratory, says Hultgren.

The researchers now are working to develop a drug that will block the fiber-assembly process. Without pili to help them cling to cells, the bacteria could be swept more readily from the urinary tract and prevented from forming colonies.

"This collaboration is an example of microbiology, biochemistry and structural biology coming together in a beautiful and complementary fashion," says Waksman. "As a result, we now have a much better idea of how bacteria produce pili, and that knowledge may lead to new and better treatments for UTIs and other bacterial diseases."

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Reference: Sauer FG, Pinkner JS, Waksman G, Hultgren SJ. Chaperone priming of pilus subunits facilitates a topological transition that drives fiber formation. Cell, 111(4), 543, Nov. 15, 2002.

Grants from the National Institutes of Health supported this research.