To better understand how pilins contribute to conduction, Reardon and NMR lead scientist Karl Mueller explored pilin from an electrically conducting bacteria known as Geobacter sulfurreducens.
Previous research on Geobacter's pilin -- PilA -- provided a big hint. PilA required certain spots along its length known as aromatic residues to conduct electricity. Without those aromatic residues where they were, Geobacter had no zip in its pili.
But proteins are like a long string that folds up into a compact three-dimensional shape. Without knowing the shape of pilin, it wasn't clear where the aromatic residues landed in space or how they contributed to electron shuttling.
Hop or Flow?
To find out, the researchers used NMR -- a technology similar to medical MRIs -- at EMSL to picture the shape of PilA.
On its own, PilA looks like a long skinny spring, with a slight kink about halfway up. The aromatic residues, which are bulky anyway, bulge along its length. But the protein by itself isn't enough to reveal how conduction works. Many pilin proteins work together to form a fiber, and Reardon and Mueller only had one.
Nor did the researchers have the whole fiber to put into the NMR instrument. To get more clues, Reardon borrowed the computer image of an assembled fiber from an unrelated species, the bacteria that cause gonorrhea. Gonorrhea's fiber does not conduct electricity nor does its pilin have as many aromatic residues. But its pilin has a similar shape to PilA, so using a computer program, Reardon overlaid PilA on its Gonorrhea cousins.
At this point, the aromatic residues clearly stood out.
"We get clusters of aromatic residues, and they wrap along the wire candy cane style," said Reardon.
But that just raised another question
|Contact: Mary Beckman|
DOE/Pacific Northwest National Laboratory