Structural Basis for the High Conductivity of Microbial Pili as Potential Nanowires

The conductivity of Geobacter sulfurreducens is attributed mainly to its truncated pili, known as microbial nanowires. In this study, we explored the biological factors that limit electron transfer and hence the conductivity of pili, including the types of aromatic residue, distances between aromatic residues, local electrostatic environment around aromatic residues, and percentage of aromatic residues in the pilin subunits that form the pili, as well as the physico-chemical interactions in the pili, by comparing the structures of pili with different conductivities in electricigens. Structures of the Geobacter pili and their mutants were constructed using the symmetric docking module of the Rosetta software. Potential electron transfer pathways in the pili were identified based on Dijkstra's shortest pathway algorithm. We found that the conductivity of full-length pili could be increased when the hydrophobic C-terminal spheres of pilin proteins are truncated. The mutant pili with altered aromatic residues probably have higher conductivity than wild-type, when the interactions between the α-N domains of pilins are enhanced. A larger percentage of aromatic residues in the N-termini of the pilin subunits resulted in higher conductivity of the corresponding pili. These results provide new insights about strategies for synthesizing high electrically conductive nanowires.