Abstract
During anaerobic respiration, the bacteria Geobacter sulfurreducens can transfer electrons to extracellular electron accepters through its pilus. G. sulfurreducens pili have been reported to have metallic-like conductivity that is similar to doped organic semiconductors. To study the characteristics and origin of conductive pilin proteins found in the pilus structure, their genetic, structural, and phylogenetic properties were analyzed. The genetic relationships, and conserved structures and sequences that were obtained were used to predict the evolution of the pilins. Homologous genes that encode conductive pilin were found using PilFind and Cluster. Sequence characteristics and protein tertiary structures were analyzed with MAFFT and QUARK, respectively. The origin of conductive pilins was explored by building a phylogenetic tree. Truncation is a characteristic of conductive pilin. The structures of truncated pilins and their accompanying proteins were found to be similar to the N-terminal and C-terminal ends of full-length pilins respectively. The emergence of the truncated pilins can probably be ascribed to the evolutionary pressure of their extracellular electron transporting function. Genes encoding truncated pilins and proteins similar to the C-terminal of full-length pilins, which contain a group of consecutive anti-parallel beta-sheets, are adjacent in bacterial genomes. According to the genetic, structure, and phylogenetic analyses performed in this study, we inferred that the truncated pilins and their accompanying proteins probably evolved from full-length pilins by gene fission through duplication, degeneration, and separation. These findings provide new insights about the molecular mechanisms involved in long-range electron transport along the conductive pili of Geobacter species.
Highlights
Electricigens are microbes that are capable of extracellular electron transfer and electricity production
Because of the environmental and practical significance of extracellular electron transfer along pili and the metallic-like conductivity of Geobacter pili, which has never been reported for other Type IV Pili, understanding the detailed mechanism of this special characteristic is very important
By comparing the truncated and full-length pilin sequences, we found that phenylalanine (F) occupied the +32 site in almost all the full-length pilins, while tyrosine (Y) occupied the +32 site in the truncated pilins; further, Y57, K30, K44, E48, and D53 were not conserved in the full-length pilins, but conserved in the truncated pilins (Figure 1C)
Summary
Electricigens are microbes that are capable of extracellular electron transfer and electricity production These microbes are currently of great interest for their practical applications in global geochemical cycling of metals, minerals, carbon in the environment, bioremediation of contaminants, and microbe-electrode interactions. There are three potential mechanisms for electricigens to transfer electrons to extracellular acceptors: (a) electron transfer via soluble electron-shuttling molecules, such as methylviologen (Lovley, 2006); (b) short-range direct electron transfer via redox-active proteins such as c-type cytochromes (Wrighton et al, 2011); and (c) long-rang electron transport via conductive pili, for Conductive Pilin in Geobacter Species instance the pili of Geobacter sulfurreducens PCA (Reguera et al, 2005). Because of the environmental and practical significance of extracellular electron transfer along pili and the metallic-like conductivity of Geobacter pili, which has never been reported for other Type IV Pili, understanding the detailed mechanism of this special characteristic is very important
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