Comparative Analysis of Type IV Pilin in Desulfuromonadales

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.

Structural Basis for the Influence of A1, 5A, and W51W57 Mutations on the Conductivity of the Geobacter sulfurreducens Pili

“…These relationships suggest that aromatic amino acids probably play a decisive role in the magnitude of conductivity. The three aromatic amino acids (F1, F24, and Y27) in each N-terminus of the WT and W51W57 pilin were arranged in a potential electrically conductive geometry in which they were sufficiently close (around 3.5 Å) to account for the experimentally observed metallic-like conductivity of the pili [3,14]. Therefore, the mechanism by which the G. sulfurreducens pilus transfers electrons along pili is attributed, at least in part, to the conjugation of aromatic amino acids (π-π interactions).…”

“…The conductivity of the Geobacter sulfurreducens pilus was increased by the genetic substitution of a tryptophan for each of the two aromatic amino acids (+51 tyrosines and +57 phenylalanines) in PilA (W51W57 mutant) [12], since tryptophan facilitates electron transport more effectively than tyrosines or phenylalanines [10,13]. Additionally, other studies also suggested that aromatic amino acids from different subunits were likely packed closely, resulting in π-π interactions that contribute to pili conductivity [3,8,14]. These results indicated that the mechanism for electron transfer in Geobacter has been attributed to the stacking of aromatic amino acids [3,15].…”

“…Under this condition, homology modeling and molecular dynamics simulation may be useful to achieve this goal [15]. Furthermore, these techniques have been extensively validated by comparison to experimental results in the past [8,14,15,17].…”

Harnessing the power of microbial nanowires

“…Also, unlike other T4a pilins, the gene encoding the precursor of the short pilins ( pilA‐N ) is organized in an operon with a gene ( pilA‐C ) coding for a hypothetical protein (Reguera et al ., 2005; Holmes et al ., 2016). The PilA‐C protein contains domains that could fold as antiparallel β‐sheets, a structural feature found in the globular head of canonical T4a pilins (Shu et al ., 2016). This has led to the suggestion that the genes encoding PilA‐N (pilin precursor) and PilA‐C (hypothetical protein) originated from the duplication of an ancestral full‐length pilin gene that lost the peptide's carboxyl‐ ( pilA‐N ) or amino‐ ( pilA‐C ) terminal regions (Shu et al ., 2016).…”

“…The PilA‐C protein contains domains that could fold as antiparallel β‐sheets, a structural feature found in the globular head of canonical T4a pilins (Shu et al ., 2016). This has led to the suggestion that the genes encoding PilA‐N (pilin precursor) and PilA‐C (hypothetical protein) originated from the duplication of an ancestral full‐length pilin gene that lost the peptide's carboxyl‐ ( pilA‐N ) or amino‐ ( pilA‐C ) terminal regions (Shu et al ., 2016). The need to respire extracellular electron acceptors may indeed have exerted evolutionary pressure on an ancestral full‐length pilin to lose the more insulating β‐strands and acquire a predominantly α‐helical configuration for increased electronic coupling (Feliciano et al ., 2012).…”