Cystitis clue: XRD reveals antibiotic target

The hairy problem of cystitis

UK scientists have revealed the structure of a complex protein called FimD that acts as an assembly platform for the pili of the bacteria that cause cystitis. The structure of the FimD protein means scientists reveals, for the first time, how these pili "hairs" are assembled from individual protein subunits to complete structures. The work offers up a new target for antibiotic drug design.

Pili are tiny hair-like strands on the outer surface of bacteria that allow them to hook together in groups. In the case of cystitis, adhesive pili allow bacteria to attach themselves to the internal wall of the bladder, this causes bladder cells to engulf the bacteria allowing the pathogen to invade the cells and avoid the chemical onslaught of conventional antibiotics. This means that an infection can lie dormant within the urinary tract and cause recurrent symptoms; a common problem with cystitis.

Now, an international team from University College London and Birkbeck College, the University of York, England, the VIB - Vrije Universiteit Brussels, Belgium, Brookhaven National Laboratory, Upton, Stony Brook University, Stony Brook, New York and Washington University, St Louis, Missouri, USA, explain how bacterial pili are the focus of recent research looking to develop the next generation of antibiotics for cystitis and other conditions. If pili assembly can be deactivated, then the bacteria would be unable to group together to form films on the interior of organs and tissues precluding serious infection. UCL and Birkbeck structural biologist Gabriel Waksman who led the research explains: "Cystitis is one of the most common gram negative bacterial infections; it can also be extremely painful and surprisingly hard to treat, especially repeat infections."

Building up infectious agents

The protein subunits of the pili are produced within the bacteria and initially transported through the inner cell wall. Each subunit is then picked up by a 'chaperone' protein which takes it across to a protein in the outer cell wall called the 'usher'; in the present study, the FimD protein of cystitis-causing bacteria. The usher protein coordinates the ordered assembly, or growth, of the subunits.

"Pili are a prime target for a new breed of antibiotics to target cystitis and other conditions, as without pili bacteria are unable to attach themselves to each other or the walls of human cells, and therefore much less likely to cause serious infections." The structure of FimD provides insights into pilus biogenesis because it unravels the entire mechanism of subunit polymerization and transport across the outer wall of the bacteria," Waksman explains. "Scientists have been working for a number of years to work out how the pili are assembled. This is the first view of the transportation and assembly of pili in action," he adds.

The team's crystal structure is that of the full-length FimD usher bound to the FimC-FimH chaperone-adhesin complex and that of the unbound form of the FimD translocation domain. The structure reveals the FimH to be inserted within the FimD 24-stranded beta-barrel translocation channel. The FimC-FimH component is held in place by interactions with the two carboxy-terminal periplasmic domains of FimD. This binding mode was confirmed by the team using electron paramagnetic resonance spectroscopy in solution. They explain that to accommodate FimH, the usher plug domain is displaced from the barrel lumen to the periplasm, a process that coincides with a marked conformational change in the beta-barrel. The amino-terminal domain of FimD is then ideally placed to catalyse the incorporation of a newly recruited chaperone-subunit complex.

Halting the molecular dance

The team describes the process of pili assembly as one of complex choreography. "The crystal structure of FimD bound to FimC?FimH provides the remarkable view of a protein transporter caught in the act of secreting its cognate substrate," they explain. There is no doubt, they say, that the work will be of crucial importance in designing novel compounds that can disrupt type 1 pilus biogenesis and so prevent cystitis, an infectious disease that afflicts millions of people around the world.

"The next step is screening for inhibitors," Waksman told SpectroscopyNOW. "Such antibiotics would work in preventing recurrence, a real issue with cystitis." He adds that the team is also determining whether such inhibitors might be used to coat catheters as a preventative measure to avoid infection.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.