Cryo-EM shows Geobacter's conductive filaments are polymers of the cytochrome OmcS, not PilA pili

electromicrobiologygeobactermicrobial-nanowirescytochromecryo-emextracellular-electron-transfer

Wang, Gu, O'Brien, Yi, Yalcin, Srikanth, Shen, Vu, Ing, Hochbaum, Egelman and Malvankar (Cell, 2019) solved a 3.7 Å cryo-EM structure of the conductive extracellular appendages of wild-type Geobacter sulfurreducens and found they are not pili at all. The structure "surprisingly reveals that, rather than PilA, G. sulfurreducens nanowires are assembled by micrometer-long polymerization of the hexaheme cytochrome OmcS, with hemes packed within ~3.5–6 Å of each other," each subunit contributing six hemes stacked closely along the length of the filament — a heme-to-heme chain that provides a physical path for electron conduction. Directly: "the conductive extracellular appendages of wild-type G. sulfurreducens strain are primarily OmcS filaments."

The paper also disarms the prior model at its foundation, noting that "there has never been any direct evidence that conductive Geobacter extracellular filaments are composed of PilA" — the earlier PilA/e-pili identification rested on indirect inference, not a resolved structure. Derek Lovley, originator of the pili model, is not a co-author (claim-malvankar-lab-overturned-pili-model-without-lovley).

This is a heme-based (redox-hopping) conduction mechanism, mechanistically distinct from the non-redox, metal-like sulfur-ligated nickel wire of cable bacteria — two unrelated evolutionary solutions to the same problem of moving electrons out of a cell, both filed under extracellular electron transfer. A companion follow-up localized the pili themselves to a secretory role (claim-geobacter-pili-are-secretory-pseudopili-not-nanowires).