Structural and dynamic properties of bacterial type IV secretion systems (review)

Abstract

The type IV secretion systems (T4SS) are widely distributed among the gram-negative and -positive bacteria. These systems mediate the transfer of DNA and protein substrates across the cell envelope to bacterial or eukaryotic cells generally through a process requiring direct cell-to-cell contact. Bacteria have evolved T4SS for survival during establishment of pathogenic or symbiotic relationships with eukaryotic hosts. The Agrobacterium tumefaciens VirB/D4 T4SS and related conjugation machines serve as models for detailed mechanistic studies aimed at elucidating the nature of translocation signals, machine assembly pathways and architectures, and the dynamics of substrate translocation. The A. tumefaciens VirB/D4 T4SS are polar-localized organelles composed of a secretion channel and an extracellular T pilus. These T4SS are assembled from 11 or more subunits. whose membrane topologies, intersubunit contacts and, in some cases, 3-dimensional structures are known. Recently, powerful in vivo assays have identified C-terminal translocation signals, defined for the first time the translocation route for a DNA substrate through a type IV secretion channel, and supplied evidence that ATP energy consumption contributes to a late stage of machine morphogenesis. Together, these recent findings describe the mechanics of type IV secretion in unprecedented detail.

Figure 1

Protein interaction networks between type IV secretion subunits. The localization and topologies of the components of the archetypal Agrobacterium tumefaciens VirB/D4 T4SS are represented. Shaded areas correspond to subcomplexes – energy (yellow shading), core (cream), pilus (green), F plasmid-specific (tan) – described in the text. Arrows indicate interactions detected among the subunits by biochemical/structural approaches – (co-immunoprecipitation, co-purification, crystal structure; blue arrow), yeast two–hybrid or peptide linkage (red arrow) and lambda cI repressor fusion or bacterial two hybrid (green arrow). The letter(s) accompanying the arrow denotes the system where the binary interaction was detected (A, A. tumefaciens; B, Bartonella henselae; Hp, Helicobacter pylori; P, Bordetella pertussis; R, Rickettsia sibirica; Xa, Xanthomonas axonopodis pv. citri; W, N, F, X and H, IncW, IncN, IncF, IncX and IncH plasmid incompatibility group conjugation systems). Interactions detected by two-hybrid screens between components localizing in different compartments are not shown, e.g., VirB11-VirB1, VirB11-VirB10 C-terminal domain, VirB11-VirB8 periplasmic domain, VirB11-VirB9. The F-plasmid transfer system Tra/Trb components are probably involved in pilus extension/retraction (F, H, U, W represent TraF, TraH, TraU and TraW; B and I represent TrbB and Trb1). For studies reporting these interactions, see refs. [,,,,,,–,–40,44,46,50,69].

Figure 2

The T-strand translocation pathway, as identified by TrIP. Immunoprecipitation of the VirD2-T-strand transfer intermediate with channel components (dark gray) from wild-type and mutant strains define a temporally- and spatially-ordered transfer pathway (Hatched arrow, blue arrow online). The remaining VirB subunits (light gray) participate in machine biogenesis and/or regulate substrate passage among the channel components [56]. This figure is reproduced in colour in Molecular Membrane Biology online.

Figure 3

Schematic representation of energetic components in macromolecular trafficking systems. Proton–motive force driven machines (upper panel): E. coli Tol-Pal system (left) involved in Group A colicins import and membrane stability, E. coli TonB system (right) required for group B colicins, B12 vitamin and iron-charged siderophores import. The energy is powered by two accessory proteins (TolQR and ExbBD) that probably act as a proton-conducting complex [70]. ATP-driven machines (lower panel): T4SS (left), T2SS (center) required for export of protein or nucleoprotein substrates across the envelope, and f1 filamentous phage extrusion system (right). The T4SS is exemplified by the A. tumefaciens VirB/D4 apparatus, whereas T2SS are exemplified by the Terminal Branch of the General Secretory Pathway (GSP) and the Aeromonas aerophila Exe machine. Subunits that bind or hydrolyze ATP are indicated with an arrow. The crystal or cryo-microscopy protein structures are reproduced from the following references [,,,,–76] with permission from the authors, Elsevier Ed, MacMillan Publishers Ltd, Nature Publishing Group, and the American Society for Biochemistry and Molecular Biology. This figure is reproduced in colour in Molecular Membrane Biology online.