Electrically conductive appendages from the anaerobic bacterium Geobacter sulfurreducens were first observed two decades ago, with genetic and biochemical data suggesting that conductive fibres were type IV pili. Recently, an extracellular conductive filament of G. sulfurreducens was found to contain polymerized c-type cytochrome OmcS subunits, not pilin subunits. Here we report that G. sulfurreducens also produces a second, thinner appendage comprised of cytochrome OmcE subunits and solve its structure using cryo-electron microscopy at ~4.3 Å resolution. Although OmcE and OmcS subunits have no overall sequence or structural similarities, upon polymerization both form filaments that share a conserved haem packing arrangement in which haems are coordinated by histidines in adjacent subunits. Unlike OmcS filaments, OmcE filaments are highly glycosylated. In extracellular fractions from G. sulfurreducens, we detected type IV pili comprising PilA-N and -C chains, along with abundant B-DNA. OmcE is the second cytochrome filament to be characterized using structural and biophysical methods. We propose that there is a broad class of conductive bacterial appendages with conserved haem packing (rather than sequence homology) that enable long-distance electron transport to chemicals or other microbial cells.

二十年前首次观察到来自厌氧细菌硫还原地杆菌的导电附属物，遗传和生化数据表明导电纤维是 IV 型菌毛。最近，发现G.sulfurreducens的细胞外传导丝含有聚合的c型细胞色素OmcS亚基，而不是菌毛蛋白亚基。在这里，我们报道G.硫还原菌还产生第二个更薄的附属物，由细胞色素 OmcE 亚基组成，并使用冷冻电子显微镜以约 4.3 Å 的分辨率解析其结构。尽管 OmcE 和 OmcS 亚基没有总体序列或结构相似性，但在聚合后，两者都会形成共享保守血红素堆积排列的细丝，其中血红素由相邻亚基中的组氨酸协调。与 OmcS 丝不同，OmcE 丝高度糖基化。在来自G.sulfurreducens的细胞外级分中，我们检测到包含 PilA-N 和 -C 链以及丰富的 B-DNA 的 IV 型菌毛。OmcE 是第二个使用结构和生物物理方法表征的细胞色素丝。我们认为存在一类具有保守血红素堆积（而不是序列同源性）的导电细菌附属物，能够将电子长距离传输到化学品或其他微生物细胞。