Geobacter's PilA pili are periplasmic secretory pseudopili, not the conductive nanowires

Gu, Srikanth, Salazar-Morales, Jain, O'Brien, Yi, Soni, Samatey, Yalcin and Malvankar, "Structure of Geobacter pili reveals secretory rather than nanowire behaviour" (Nature 597, 2021), resolved the structure of Geobacter sulfurreducens pili and found them to be heterodimers of PilA-N and PilA-C that remain periplasmic under nanowire-producing conditions. Functionally, they behave like the pseudopili of a type-II secretion system — machinery that exports the conductive cytochrome filaments (claim-geobacter-conductive-filaments-are-omcs-cytochrome-polymers) rather than acting as the nanowire itself. The structure shows the pili are "loosely attached to cells...demonstrate a substantially lower stability of PilA-N–C compared to T4P filaments," consistent with a transient secretory role rather than a durable extracellular wire.

The electrical evidence points the same way: PilA-N–C filaments are reported as having "a conductivity that is 20,000-fold lower than that of OmcZ nanowires." This inverts the original e-pili model — the pili are not the good conductor; the cytochrome polymers they help secrete are. (OmcZ is reported in this literature as the most conductive of the resolved cytochrome filament types; that ranking was not independently quote-verified in the source session.)

[unverified-quant] The capture also recorded a standalone absolute figure of ~1 mS/cm for PilA-N–C conductivity, but that number arrived via an AI-summarized web fetch of the PMC page rather than confirmed against the paper's own figure/table text; only the ratio (20,000-fold) is directly quoted. Treat the ~1 mS/cm value as unverified pending a primary check. The ratio, not the absolute, is what this note rests on.