Sometimes, transporting electrons from one cell to another is a team effort.
In electroactive bacteria, that team is a group of proteins that shepherds electrons forward, passing them along like a relay baton, so they can penetrate the thick cell envelope comprising multiple layers of membranes that otherwise are not electroconductive. But how these proteins collaborate to achieve this has not been clear.
Cornell researchers have discovered this electron transfer is mediated by CymA proteins' ability to synchronize and form a biomolecular condensate in the inner membrane - something that had not been previously observed in electroactive bacteria. The researchers then demonstrated for the first time that by applying an electrochemical signal to the bacteria, they could manipulate the spatial pattern of the proteins and spur the extracellular process.
The technique could eventually find applications in biotechnologies such as microbial energy conversion, in which electrons need to be shuttled to other cells or electrodes.
The findings were published Feb 17 in Nature Communications. The lead author is former postdoctoral researcher Youngchan Park, now an assistant professor at Indiana University.
The project was led by Peng Chen, the Peter J.W. Debye Professor of Chemistry in the College of Arts and Sciences. It grew out of a collaboration with co-author Buz Barstow, Ph.D. '09, assistant professor of biological and environmental engineering in the College of Agriculture and Life Sciences, that explored how electroactive bacteria interact with semiconducting materials.
That research inspired Chen to delve further into extracellular electron transport in bacteria - specifically Shewanella oneidensis, the most well-known and extensively studied microbe used for electron transport. As a so-called Gram-negative bacteria, S. oneidensis has inner and outer membranes, connected by a kind of buffer zone of periplasm. Those stacked layers are a structural advantage, protecting the bacteria in unfriendly environments - for example, ones rife with heavy metals or antibiotics. However, the membranes, which mostly consist of insulating fatty lipids, aren't electroconductive.
"Electrons have to go through the cell envelope: the inner membrane, outer membrane and the periplasmic space," Chen said. "Now, electron transfer in biology, of course, doesn't just go through solutions. Electrons do not swim through water. Otherwise, they would get short-circuited."
But both membranes and the periplasm contain a secret asset: proteins. Using photoelectrochemistry-fluorescence microscopy, the researchers determined that during extracellular electron transfer, CymA proteins in the inner membrane reorganize in a confined region and drive their electron-transfer partners to do the same in the periplasm.
This type of condensate is a well-documented phenomenon in bacteria and many other types of cells, playing a role in metabolic enzymatic reactions and gene regulation, but it was not known to be important for electron transfer and hadn't been observed in electroactive bacteria, Chen said. The researchers were able to induce the formation of CymA condensate by applying electrochemical signals, which allowed them to quantify the spatiotemporal dynamics of protein reorganization at the single-cell level.
"Many people have applied electrical signals to bacteria, but we discovered that by applying an electrochemical signal to the cell, it can change the spatial pattern of the protein," Chen said. "The pattern initially is homogeneous, and then you condense it. The electrical signal - basically, the electron transfer - will drive the change of a spatial pattern. That's a new thing."
Co-authors also include doctoral student Tianlei Yan; Zhiheng Zhao, Ph.D. '25; former postdoctoral researchers Bing Fu and Muwen Yang; and Farshid Salimijazi, Ph.D. '22.
The work was supported by the National Institutes of Health (NIH). The researchers made use of the Cornell Institute of Biotechnology's Imaging Facility, which is supported by the New York State Stem Cell Science program and the NIH.
