The hydrophobic TraM protein of pKM101 is required for conjugal transfer and sensitivity to donor-specific bacteriophage

Structure of a VirD4 coupling protein bound to a VirB type IV secretion machinery

Type IV secretion (T4S) systems are versatile bacterial secretion systems mediating transport of protein and/or DNA. T4S systems are generally composed of 11 VirB proteins and 1 VirD protein (VirD4). The VirB1‐11 proteins assemble to form a secretion machinery and a pilus while the VirD4 protein is responsible for substrate recruitment. The structure of VirD4 in isolation is known; however, its structure bound to the VirB1‐11 apparatus has not been determined. Here, we purify a T4S system with VirD4 bound, define the biochemical requirements for complex formation and describe the protein–protein interaction network in which VirD4 is involved. We also solve the structure of this complex by negative stain electron microscopy, demonstrating that two copies of VirD4 dimers locate on both sides of the apparatus, in between the VirB4 ATPases. Given the central role of VirD4 in type IV secretion, our study provides mechanistic insights on a process that mediates the dangerous spread of antibiotic resistance genes among bacterial populations.

Use of chimeric type IV secretion systems to define contributions of outer membrane subassemblies for contact-dependent translocation

Recent studies have shown that conjugation systems of Gram-negative bacteria are composed of distinct inner and outer membrane core complexes (IMCs and OMCCs, respectively). Here, we characterized the OMCC by focusing first on a cap domain that forms a channel across the outer membrane. Strikingly, the OMCC caps of the Escherichia coli pKM101 Tra and Agrobacterium tumefaciens VirB/VirD4 systems are completely dispensable for substrate transfer, but required for formation of conjugative pili. The pKM101 OMCC cap and extended pilus also are dispensable for activation of a Pseudomonas aeruginosa type VI secretion system (T6SS). Chimeric conjugation systems composed of the IMC_pKM101 joined to OMCCs from the A. tumefaciens VirB/VirD4, E. coli R388 Trw, and Bordetella pertussis Ptl systems support conjugative DNA transfer in E. coli and trigger P. aeruginosa T6SS killing, but not pilus production. The A. tumefaciens VirB/VirD4 OMCC, solved by transmission electron microscopy, adopts a cage structure similar to the pKM101 OMCC. The findings establish that OMCCs are highly structurally and functionally conserved - but also intrinsically conformationally flexible - scaffolds for translocation channels. Furthermore, the OMCC cap and a pilus tip protein coregulate pilus extension but are not required for channel assembly or function.

VirB1 orthologs from Brucella suis and pKM101 complement defects of the lytic transglycosylase required for efficient type IV secretion from Agrobacterium tumefaciens

Type IV secretion systems mediate conjugative plasmid transfer as well as the translocation of virulence factors from various gram-negative pathogens to eukaryotic host cells. The translocation apparatus consists of 9 to 12 components, and the components from different organisms are believed to have similar functions. However, orthologs to proteins of the prototypical type IV system, VirB of Agrobacterium tumefaciens, typically share only 15 to 30% identical amino acids, and functional complementation between components of different type IV secretion systems has not been achieved. We here report a heterologous complementation in the case of A. tumefaciens virB1 defects with its orthologs from Brucella suis (VirB1s) and the IncN plasmid pKM101 (TraL). In contrast, expression of the genes encoding the VirB1 orthologs from the IncF plasmid (open reading frame 169) and from the Helicobacter pylori cag pathogenicity island (HP0523) did not complement VirB1 functions. The complementation of VirB1 activity was assessed by T-pilus formation, by tumor formation on wounded plants, by IncQ plasmid transfer, and by IncQ plasmid recipient assay. Replacement of the key active-site Glu residue by Ala abolished the complementation by VirB1 from B. suis and by TraL, demonstrating that heterologous complementation requires an intact lytic transglycosylase active site. In contrast, the VirB1 active-site mutant from A. tumefaciens retained considerable residual activity in various activity assays, implying that this protein exerts additional effects during the type IV secretion process.

Structural and functional characterization of the VirB5 protein from the type IV secretion system encoded by the conjugative plasmid pKM101

Type IV secretion systems mediate intercellular transfer of macro-molecules via a mechanism ancestrally related to that of bacterial conjugation machineries. TraC of the IncN plasmid pKM101 belongs to the VirB5 family of proteins, an essential component of most type IV secretion systems. Here, we present the structure of TraC. VirB5/TraC is a single domain protein, which consists of a three helix bundle and a loose globular appendage. Structure-based site-directed mutagenesis followed by functional studies indicates that VirB5 proteins participate in protein-protein interactions important for pilus assembly and function.

TraC of IncN plasmid pKM101 associates with membranes and extracellular high-molecular-weight structures in Escherichia coli

Conjugative transfer of IncN plasmid pKM101 is mediated by the TraI-TraII region-encoded transfer machinery components. Similar to the case for the related Agrobacterium tumefaciens T-complex transfer apparatus, this machinery is needed for assembly of pili to initiate cell-to-cell contact preceding DNA transfer. Biochemical and cell biological experiments presented here show extracellular localization of TraC, as suggested by extracellular complementation of TraC-deficient bacteria by helper cells expressing a functional plasmid transfer machinery (S. C. Winans, and G. C. Walker, J. Bacteriol. 161:402-410, 1985). Overexpression of TraC and its export in large amounts into the periplasm of Escherichia coli allowed purification by periplasmic extraction, ammonium sulfate precipitation, and column chromatography. Whereas TraC was soluble in overexpressing strains, it partly associated with the membranes in pKM101-carrying cells, possibly due to protein-protein interactions with other components of the TraI-TraII region-encoded transfer machinery. Membrane association of TraC was reduced in strains carrying pKM101 derivatives with transposon insertions in genes coding for other essential components of the transfer machinery, traM, traB, traD, and traE but not eex, coding for an entry exclusion protein not required for DNA transfer. Cross-linking identified protein-protein interactions of TraC in E. coli carrying pKM101 but not derivatives with transposon insertions in essential tra genes. Interactions with membrane-bound Tra proteins may incorporate TraC into a surface structure, suggested by its removal from the cell by shearing as part of a high-molecular-weight complex. Heterologous expression of TraC in A. tumefaciens partly compensated for the pilus assembly defect in strains deficient for its homolog VirB5, which further supported its role in assembly of conjugative pili. In addition to its association with high-molecular-weight structures, TraC was secreted into the extracellular milieu. Conjugation experiments showed that secreted TraC does not compensate transfer deficiency of TraC-deficient cells, suggesting that extracellular complementation may rely on cell-to-cell transfer of TraC only as part of a bona fide transfer apparatus.

Genetic analysis of the mobilization and leading regions of the IncN plasmids pKM101 and pCU1

The conjugative IncN plasmids pKM101 and pCU1 have previously been shown to contain identical oriT sequences as well as conserved restriction endonuclease cleavage patterns within their tra regions. Complementation analysis and sequence data presented here indicate that these two plasmids encode essentially identical conjugal DNA-processing proteins. This region contains three genes, traI, traJ, and traK, transcribed in the same orientation from a promoter that probably lies within or near the conjugal transfer origin (oriT). Three corresponding proteins were visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and complementation analysis confirmed that this region contains three tra complementation groups. All three proteins resemble proteins of the IncW plasmid R388 and other plasmids thought to have roles in processing of plasmid DNA during conjugation. The hydropathy profile of TraJ suggests a transmembrane topology similar to that of several homologous proteins. Both traK and traI were required for efficient interplasmid site-specific recombination at oriT, while traJ was not required. The leading region of pKM101 contains three genes (stbA, stbB, and stbC), null mutations in which cause elevated levels of plasmid instability. Plasmid instability was observed only in hosts that are proficient in interplasmid recombination, suggesting that this recombination can potentially lead to plasmid loss and that Stb proteins somehow overcome this, possibly via site-specific multimer resolution.

Biochemistry of type IV secretion

In the past year, our knowledge of type IV transporters of Gram-negative bacteria has further expanded. Advances include the discovery of additional members of this family of proteins, increased knowledge of the morphologies of type IV transporters, and a better understanding of the mechanisms by which macromolecules are exported by these systems.