Structure of Geobacter pili reveals secretory rather than nanowire behaviour.

Extracellular electron transfer by Geobacter species through surface appendages known as microbial nanowires is important in a range of globally important environmental phenomena, as well as for applications in bio-remediation, bioenergy, biofuels and bioelectronics. Since 2005, these nanowires have been thought to be type 4 pili composed solely of the PilA-N protein. However, previous structural analyses have demonstrated that, during extracellular electron transfer, cells do not produce pili but rather nanowires made up of the cytochromes OmcS and OmcZ. Here we show that Geobacter sulfurreducens binds PilA-N to PilA-C to assemble heterodimeric pili, which remain periplasmic under nanowire-producing conditions that require extracellular electron transfer. Cryo-electron microscopy revealed that C-terminal residues of PilA-N stabilize its copolymerization with PilA-C (to form PilA-N-C) through electrostatic and hydrophobic interactions that position PilA-C along the outer surface of the filament. PilA-N-C filaments lack π-stacking of aromatic side chains and show a conductivity that is 20,000-fold lower than that of OmcZ nanowires. In contrast with surface-displayed type 4 pili, PilA-N-C filaments show structure, function and localization akin to those of type 2 secretion pseudopili. The secretion of OmcS and OmcZ nanowires is lost when pilA-N is deleted and restored when PilA-N-C filaments are reconstituted. The substitution of pilA-N with the type 4 pili of other microorganisms also causes a loss of secretion of OmcZ nanowires. As all major phyla of prokaryotes use systems similar to type 4 pili, this nanowire translocation machinery may have a widespread effect in identifying the evolution and prevalence of diverse electron-transferring microorganisms and in determining nanowire assembly architecture for designing synthetic protein nanowires.