Investigating The Function Of Conductive Pili In Geobacter Species

Excelectrogens is a special class of microorganisms that can extend the cell respiratory chain to the outside by transfering electrons to external electron accepter, when no lectron accepter can entry the cell. By using artificial electrode as an extracellular electron acceptor, excelectrogens can constitute a MFC(Microbial fuel cell). MFC can converted organics directly into electrical energy, and provide a clean and renewable energy. Playing a role in environmental protection, MFC can degrad organic matter in industrial and domestic wastewater during the production of electricity. Meanwhile, by means of extracellular electron transfer(EET), MFC also contribute to reducing toxic or radioactive metal contaminants, in order to achieve environmental bioremediation.The latest study found that the extracellular electron transference is proposed to be facilitated by filamentous fibers that are referred to as bacterial nanowires in some excelectrogens such as Geobacter sulfurreducens PCA, but the mechanism of pili conductivity is not clear.In this paper, we pridect and indentify the homologous proteins of conductive pilin by using PilFind algorithm and cluster analysis, search sequence characteristics by using multiple sequence alignment program MAFFT to align the pilins of Geobacter, find some pilin relevant motif by using MEME tools, compare the characteristics of protein teritary structure predicted from QUARK. We also explore the evolution of the source of conductive pili by conducting species evolutionary trees and gene phylogenetic tree.Using PilFind to scan the genomes of 9 Geobacter and 2 Pelobacter completely sequenced strains (encoding>42000 proteins), we predict 183 pilin-like proteins. After the cluster analysis, we identity 11 T4a homologous proteins including GSU1496, including five hypothetical proteins and seven short pilins just like G. sulfurreducens PCA. We also find seven homologous genes of GSU1497 by searching the genomes of Geobacter and Pelobacter with PSI-BLAST. After gene linkage analysis of homologous pilins, we found the homologous genes of GSU1496 and GSU1497 in each species has high linear conservativeness. There are some sequence features associated with pili conductivity:some aromatic amino acids (like +1,+24,+27) are highly conserved,but the other residues can vary(like +32,+51,+57). The lysine(+57) in highly variable C-terminal region of pilin may play an important role in pili conuctivity. The secondary structure and the tertiary structure of GSU1497, predicted by PSIPRED and by QUARK respectively, not only resemble that of the C-terminal domain (termed the globular head) of type IV pilins from other bacteria whose PilA proteins consist of a minimum of 140 amino acids, but also fold into an antiparallel four-stranded β-sheet, which is a hallmark of the type Ⅳ pilins. At the end, we discovered a sequence motif of GSU1497 and the homologous genes from all Geobacter species. Construction of species tree and gene tree, we find the short pilins and long pilins of Geobacter and Pelobacter evolved in different branches. G.sulfurreducens.YCA and G.metallireducens GS-15, which are the only two species proved pili conductivity by experiments, evolve in same branch of short plins. We also speculate that Deferribacteres has a closed relationship with Geobacter in pilin evolution.These results suggest that the conductive pilins has a specific sequence features, structual characteristics and evolutionary origins, it will help us to further elucidate the mechanism of pili conductivity.