What they saw when they glimpsed the insides of these quick-frozen bacteria were chemoreceptors arranged in a regular, repeating lattice of hexagonsa structure with six sides and six corners or verticesthat are 12 nanometers apart, center to center. At each of the vertices sit six chemoreceptors, arranged in what scientists call "trimers of dimers," which means there are three sets of two paired receptors in each grouping. The two receptors in each dimer twine around one another, and those dimers then cluster together at one vertex of the hexagon to form a trimer.
"One beauty of this is that we've shown that the receptors cluster in cells in the same way they did in the crystal structure," says Jensen. "In the past, we didn't know if that was an artifact of the crystallography. Now we can see how the pieces fit together in real cells."
The paper also showed that this particular architecture is no single-species fluke. "We looked at 13 different species that cover the whole bacterial kingdom," Jensen says. "The arrays were all the same. This shows us that this structure has been universally conserved, that it's a universal architecture."
And that's important to know, he adds, because it gives scientists a basis for trying to figure out how this sort of architecture leads to the bacteria's sensitivity to chemical cues in its environment and establishes that work using key model systems such as E. coli will be generally applicable.
"Bacterial chemotaxis consists of only a few key components, making it an important model system for all cell signaling pathways," says Briegel. "We need to understand this system first before we can hope to fully understand the more complex eukaryotic signaling systems. Chemotaxis also plays a
|Contact: Lori Oliwenstein|
California Institute of Technology