How E. Coli Attaches To Host: One Catalyst Protein Greatly Accelerates Activity

Thread-like adhesive organs known as pili enable many pathogens to attach themselves to the infected host’s target cells. The well-known bacterium Escherichia coli, for example, is responsible for more than 80 percent of all infections of the human urinary tract, and uses the numerous pili on its surface to attach itself to the epithelial cells of the urinary passages. The adhesive threads also help the bacteria to penetrate into host cells, where the pathogens can evade attack by antibiotics.

Potent catalyst

A new paper published in Science Express on 8 April 2008 shows that a particular protein called FimD is indispensable for the fast, efficient assembly of the pili. FimD is situated on the bacterium’s outer membrane and acts as a catalyst that greatly accelerates the spontaneous assembly of pilus proteins. The acceleration is so efficient that a single pilus, which can consist of up to 3,000 proteins, is assembled in a few minutes. This process needs no additional energy from the cell. On the other hand, if FimD is absent, very short pilus fragments consisting of only a few pilus components are formed, and this only occurs after hours or days.

However, FimD develops its full effect only if it was in contact with the FimC-FimH complex before being brought into operation. FimH is the protein that is always situated at the tip of a pilus and is responsible for the actual adhesive attachment. FimC is what is known as a “chaperone”, which accompanies the pilus modules to FimD and hands them over there, but is not itself a component of the pilus. FimD is in turn described as an “usher” because it positions in the correct sequence those molecules that are to assemble together.

However, FimD is not just an extremely potent catalyst. The protein also forms a pore in the membrane that is only just big enough to allow the individual protein components of the developing pilus to slip through. It is also possible that FimD is responsible for anchoring the fibril in the cell membrane.

Isolated in high purity

The secret of the successful research: the ETH Zurich researchers led by Senior Research Assistant Mireille Nishiyama succeeded in purifying the FimD protein in a biologically active form so it could develop its catalytic activity fully even in a test tube. It was no easy task for the research leader. She and her group took about a year to obtain FimD in a highly pure and active form. However, thanks to their persistence, the researchers now understand important mechanical details of how the pili are formed.

The study’s first author says the available knowledge provides an opportunity to test numerous substances in high throughput systems, for example those that block the assembly of the pili. Combined with antibiotics, this could firstly combat the bacteria and secondly it could disable their ability to adhere. For example this might provide a more effective way to fight urinary tract infections caused by E. coli.

Copying increases understanding

Supramolecular structures composed of numerous sub-units like the pili of E. coli occur frequently in nature. For example flagellae or even ribosomes consist of a very large number of different proteins. Rudi Glockshuber, Professor of Molecular Biology at the Institute of Molecular Biology & Biophysics, says “As a rule we understand such complex systems only if we can assemble them from purified components in a test tube.” He then stresses not only the medical aspect of this research work but also draws attention to the fact that interest had focused on the mechanistic understanding of the assembly of supramolecular complexes. He says this was a major challenge that functioned very well in this experiment.

Journal reference: Mireille Nishiyama, Takashi Ishikawa, Helene Rechsteiner and Rudi Glockshuber (2008): Reconstitution of Pilus Assembly reveals a Bacterial Outer Membrane Catalyst, Science Express; doi:10.1126/science.1154994