Structural basis underlying the metallic-like conductivity of microbial nanowires

Microbial nanowires are electrically conductive proteinaceous pili nanofilaments secreted by \textit{Geobacter sulfurreducens}. In contrast to current biochemical understanding that proteins are insulators, \textit{G. sulfurreducens} pili show organic metallic-like conductivity [1]. Pili also enable direct exchange of electrons among \textit{Geobacter} co-cultures [2]. Site-directed mutagenesis studies revealed that aromatic amino acids confer conductivity to pili [3]. In order to develop a structural understanding of the pili to probe the conduction mechanism at a molecular level, we employed three complementary structural methods -- X-ray microdiffraction using synchrotron radiation, rocking curve X-ray diffraction, and electron diffraction. Studies performed with all these three methods revealed a 3.2 {\AA} periodic spacing in wild-type \textit{G. sulfurreducens} pili, expected for metal-like conductivity and a lack of such spacing in genetically modified non-conductive pili. Notably, both the peak intensity and the conductivity increased 100-fold with lowering the pH from pH 10.5 to pH 2, demonstrating a structure-function correlation in pili. We also reconstructed the three dimensional tertiary structure of pili with homology modeling, which further suggested the 3.2 {\AA} spacing among aromatics associated with metal-like conductivity. \\[4pt] [1] \textit{Nature Nanotechnology}, 6, 573 (2011)\\[0pt] [2] \textit{Science}, 330, 1413 (2010)\\[0pt] [3] \textit{mBio} 4:e00105-13 (2013)