Cell 177, 361–369 (2019)

The common soil bacterium Geobacter sulfurreducens forms filamentous appendages that function as nanowires to enable long-range electron transfer during respiration and to exchange electrons with other species. Multiple previous biochemical and imaging studies have proposed that these nanowires were type IV pili consisting of the protein PilA. Using a combination of cryo-EM, mass spectrometry and electronic conductivity on isolated filament proteins, Wang and Gu et al. now identify the nanowires as polymerized filaments of the cytochrome OmcS instead. The heme cofactors in the OmcS nanowires form a continuous chain of parallel-stacked pairs, which optimizes electron transport along the chain and may also enhance stability at the protein–protein interfaces between OmcS units. Although cytochrome polymerization up to tetramers has been observed in other systems, the micrometer scale of the OmcS nanowires, each consisting of hundreds of cytochrome proteins, is unexpected. These nanowire structures shed new light on how bacteria mediate electron transfer during various processes and prompt new investigation into a potential regulatory, rather than structural, role of PilA in this system.

Credit: Elsevier