Urinary tract infections (UTIs) are the second most common bacteria-borne disease. Respiratory afflictions come in first.
UTIs include both kidney and bladder infections (pyelonephritis and cystitis, respectively). They account for 100,000 hospital admissions and 8 million physician office visits a year in the U.S. alone. A majority of UTI sufferers are women, owing to the anatomical propinquity of their urinary and fecal tracts.
A two-faced microorganism bears responsibility for inflicting urinary tract infections on the human race. It is none other than Escherichia coli, a benign inhabitant of the digestive system, until it attacks and infects kidneys and bladder. In fact, E. coli is in a sense a founder of biotechnology, thanks to its workhorse capacity for microbiological and cloning experimentation.
Far and away the principal pathogenic perpetrator of UTI - by 80 percent - is the same E. coli. Its infective bacterial strains are covered with hair-like spiral tendrils known as pili. These stubby, slender whiskers terminate in subunits that stick to the cells lining the bladder's interior walls.
Microbiologist Scott Hultgren, at Washington University in St. Louis, is senior author of a paper in the current issue of Cell, released online Nov. 15, 2002. Its first author is structural biologist and microbiologist Frederic Sauer, now a postdoctoral fellow at Yale University. Their paper is titled: "Chaperone priming of pilus subunits facilitates a topological transition that drives fiber formation."
"Our finding has shown," Sauer told BioWorld Today, "how the fibers, which consist of thousands of subunits in a row, all look, and how two subunits of the fibers fit together. These subunits are like hot dog buns. Every subunit also has attached to it a floppy tail, or strand, which resembles a floppy hot dog. So every bun has a hot dog nestled into the subunit next to it."
Picture A Hot Dog Squeezed In Its Bun
"What our paper has shown," Sauer continued, "is when the hot dog inserts its tail into the groove on the surface of the subunit, the groove closes, as if squeezed by the sausage. It's as if every subunit contributes its tail, which fits into the neighboring groove, and closes down on it, so you've now got a bunch of subunits in a row. That's what the Cell paper reveals.
"The normal function of the tail is to fit into the groove of the adjacent subunit. We've determined the X-ray crystal structure of the proteins that are required to assemble those hair-like fibers, the pili. These 3-dimensional structures tell us at high resolution how the fibers are assembled. So we know all the surfaces involved and how they're put together. One subunit is about 50 to 70 Angstroms long; the tail 30 to 40 Angstroms in length.
"The goal we're working toward," he recounted, "is to develop small-molecule inhibitors. Now that we know their surfaces, we can design such blockers, so as to prevent their normal assembly. If we don't get a fiber to grow, you won't get an infection. The normal function of the tail is to fit into the groove of the adjacent subunit.
"Now we want to fill the grooves with something that will prevent assembly, and thus prevent infection. How to do that?" Sauer asked rhetorically. "Knowing that the tail exists and how it fits in, we want to make a mimic of the tail - some small compound that would fit into the groove of the subunit. Once that is filled it's no longer available, because the tail of the next subunit can't fit in, so the fiber won't form. We are working toward such a synthetic tail. We haven't tested its functions in vivo yet. This is being done in collaboration with a group in Sweden.
"The chaperone and ushers are two proteins responsible for assembling the fiber," Sauer went on. "The subunit is initially bound by the chaperone, which then fills the subunit groove with its own portion of itself. In other words, now the groove is full, it can't interact with another subunit. So what the chaperone is doing is preventing premature interaction until it gets to the usher."
How Does This Help Drug Or Vaccine Design?
The benefit of the findings for design, Sauer replied, is "knowing what the molecule looks like, the information that there is a groove, how the groove interacts with the tail. Because there are two structures in the paper, one of which has a tail inserted into the groove. That's how we knew that the groove clamped down on the tail, because we saw the interaction by X-ray crystallography. Since we know what the interaction is, we can design an inhibitor of that interaction.
"It turns out," Sauer observed, "that besides E. coli in UTI, there are many different bacteria that assemble these fibers, by mechanisms revealed in our paper. For example, Salmonella, Yersinia - which causes bubonic plague - Proteus mirabilis, Klebsiella - all sorts of urinary tract pathogens that assemble these fibers in this manner. We don't know all the details of how Yersinia uses these fibers, but they do use them. That all needs to be cleared up, but we think that more information is becoming available, on a lot of these diseases.
"And that's one of the bonuses of our study, that an inhibitor might be applicable to other diseases as well as uropathogenic E. coli. Scott Hultgren and others," Sauer concluded, "are actively pursuing these leads."