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Agrobacterium tumefaciens is a natural genetic engineer that transfers DNA into plants and this is the most frequently applied process for the generation of genetically modified plants. DNA transfer is mediated by a type IV secretion system localized in the cell envelope and extracellular T-pili. We here report the cryo-electron microscopic structures of the T-pilus at 3.2Å resolution and that of the related plasmid pKM101-determined N-pilus at 3Å resolution. Both pili contain a main pilus protein (VirB2 in A. tumefaciens and TraM in pKM101) and phospholipids arranged in a 5-start helical assembly. They contain positively charged amino acids in the pilus lumen and the lipids are positively charged in the T-pilus (phosphatidylcholine) conferring overall positive charge to the lumen. Mutagenesis of the lumen-exposed Arg91 residue in VirB2 resulted in protein destabilization and loss of pilus formation. Our results reveal that different phospholipids can be incorporated into type IV secretion system pili and that the charge of the lumen is of functional importance.

Gram-negative bacteria use membrane-bound type IV secretion systems to assemble pili on the cell surface, followed by cell-cell contact with recipient cells and transfer of plasmid DNA. The process at the cell-cell contact stage of conjugative DNA transfer is not well understood. We here present a biochemical and genetic characterization of the TraC protein that is a minor component of the pili determined by the IncN plasmid pKM101 from Escherichia coli. The cellular and secreted forms of TraC are monomers, TraC preferentially localizes at the cell poles and it is also detected in extracellular membrane vesicles. Purified TraC does not impact the infection with bacteriophages, but we detect binding of TraC to recipient cells and partial complementation of a traC deletion strain by the addition of purified TraC. These results suggest that the protein contributes to conjugation at the cell-cell contact stage.

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