Geobacter's pili conduct electrons along their length using the rings on aromatic amino acids, according to a study in mBio this week. Contrary to all other known forms of biological electron transport, in which electrons are carried by discrete entities and passed from one to another, Geobacter's pili have a core of aromatic amino acids that turn these hair-like appendages into functioning electron-carrying biological wires, adding credence to a controversial idea in biology.
"It's the aromatic amino acids that make it a wire," says lead author Derek Lovley. Lovley and his colleagues developed a strain of Geobacter, Aro-5, that lacked aromatic amino acids in parts of the pili implicated in electron conduction, replacing them with smaller, non-aromatic amino acids. Without the aromatic amino acids, Lovley says, the pili are nothing more than protein strings. Importantly, Aro-5's pili were complete with OmcS, a multi-heme c-type cytochrome essential for iron oxide reduction that was long suspected of carrying the electrons along the length of the pili. But the presence of working OmcS wasn't enough.
"We showed it's not good enough to just make the string - you've got to make a wire," says Lovley.
Geobacter pili have metallic-like conductivity
G. sulfurreducens respires by removing electrons from organic materials and funneling them to iron oxides or to other microorganisms, much the way humans pull electrons out of organic molecules in food and dump them on oxygen. The bacteria use their pili to reach out to iron oxides or other microbes, transferring the "waste" electrons along the structure to the destination. Geobacter's pili are only 3-5 nanometers wide, but they can be 20 micrometers long, many times longer than the cell itself.
Trafficking in electrons is how all living things breathe, but it is normally carried out by discrete proteins or other molecules that act like containers for shuttling electrons from one place to another. Lovley says earlier results showed the pili in G. sulfurreducens possess metallic-like conductivity, the ability to carry electrons along a continuous structure, a controversial finding.
To investigate how pili accomplish this singular feat, Lovley says they looked to non-biological organic materials that can conduct electricity. "In those synthetic materials, it's aromatic compounds that are responsible for the conductivity. We hypothesized that maybe it's similar in the Geobacter pili. In this case, it would be aromatic amino acids." Aromatic compounds have a highly stable ring-shaped structure made of carbon atoms.
Turning to the pili, Lovley says his group compared the Geobacter pilus protein, PilA with those of other pili-makers, including Pseudomonas, Neisseria and Vibrio species, that aren't capable of long-range electron transport. By comparison, Geobacter lacks a highly conserved N-terminal domain these other sequences retain. Lovley's group guessed that the aromatic amino acids in this area where Geobacter is atypical could be carrying electrons via overlapping pi-pi orbitals.
Using genetic techniques, they developed a strain of Geobacter, Aro-5, that makes pili that lack aromatic amino acids in this key region, then they tested whether these pili could still conduct electricity. They could not. Removing the aromatic amino acids was a bit like taking the copper out of a plastic-covered electrical wire: no copper means no current, and all you're left with is a string.
Removing aromatic amino acids from the pili prevents the bacteria from reducing iron, too, says Lovley, an important point because it adds further proof that Geobacter uses its pili as nanowires for carrying electrons to support respiration.
Where to go from here?
Metal reducers like Geobacter show a lot of promise for use in fuel cells, says Lovley, and by feeding electrons to methanogens, they're an important component of anaerobic digesters that produce methane gas from waste products. Understanding how they shuttle their electrons around and how to optimize the way the pili function could lead to better technologies.
Moving forward, Lovley says his own lab plans to explore the possibilities of biological nanowires, exploring how to make them more or less conductive. "The fact that we were able to make the physical filament, but it's no longer a wire gives us a lot of power for looking at those phenotypes," says Lovley.