Type IV secretion in Gram-negative and Gram-positive bacteria

Abstract

Type IV secretion systems (T4SSs) are versatile multiprotein nanomachines spanning the entire cell envelope in Gram-negative and Gram-positive bacteria. They play important roles through the contact-dependent secretion of effector molecules into eukaryotic hosts and conjugative transfer of mobile DNA elements as well as contact-independent exchange of DNA with the extracellular milieu. In the last few years, many details on the molecular mechanisms of T4SSs have been elucidated. Exciting structures of T4SS complexes from Escherichia coli plasmids R388 and pKM101, Helicobacter pylori and Legionella pneumophila have been solved. The structure of the F-pilus was also reported and surprisingly revealed a filament composed of pilin subunits in 1:1 stoichiometry with phospholipid molecules. Many new T4SSs have been identified and characterized, underscoring the structural and functional diversity of this secretion superfamily. Complex regulatory circuits also have been shown to control T4SS machine production in response to host cell physiological status or a quorum of bacterial recipient cells in the vicinity. Here, we summarize recent advances in our knowledge of 'paradigmatic' and emerging systems, and further explore how new basic insights are aiding in the design of strategies aimed at suppressing T4SS functions in bacterial infections and spread of antimicrobial resistances.

Figure 1. Schematic representation of type IV secretion architecture and functions in bacteria

A. Conjugative T4SSs translocate DNA from the donor bacterium into various recipients, including other bacteria or eukaryotic cells. B. DNA release systems facilitate an exchange of DNA with the extracellular space as well as biofilm formation. C. DNA uptake from the environment proceeds by the ComB T4SS. D. The Xanthomonas citri T4SS can deliver a protein toxin to kill neighboring Gram⁻ bacterial competitors. E. Various pathogenic bacteria and symbionts have evolved T4SSs to deliver effector proteins or DNA–protein complexes into their host (either eukaryotic target cells or protozoan hosts). The T4SSs can either inject their effectors directly into the host cell or secrete them into the medium, thereby exerting remarkably different effects on host cell functions during infection. F. EM reconstructions showing the structure of the plasmid R388 T4SS complex and the core complex. Front view (left) and cut-away front view (right) of the T4SS complex (EMD-2567) comprising the core/outer membrane complex (core/OMC, green), the stalk (grey) and the inner membrane complex (IMC, blue). U-tier, M-tier and L-tier stand for upper, middle and lower tier, respectively. The inner (IM) and outer (OM) membranes are indicated. G. pKM101 core complex (EMD-2232) (top) and truncated core complex lacking the N-terminal part of VirB10 (EMD-2233) (bottom): side view (left) and cut-away side view (right). The bottom right panel shows the superposition of the difference map (between the full-length and the truncated core complex cryo-EM maps) in green, and the cryo-EM structure of the truncated core complex in orange (as in bottom left). The VirB10 N-terminus forms the inner wall of the I-layer and the base. H. Cryo-EM structure of the TraI relaxase-ssDNA complex revealed the molecular basis of DNA unwinding during bacterial conjugation. I. To achieve genetic exchange during bacterial conjugation, two relaxase monomers collaborate, adopting distinct structural conformations to provide the two necessary enzymatic activities for processing the DNA. J. Individual steps are indicated: (1) TraI opens to bind ssDNA and closes to surround DNA entirely during unwinding. (2) DNA binding to transesterase in closed TraI inhibits nicking. (3) DNA splitting by vestigial helicase. This figure was extensively updated from Backert and Meyer (2006), Trokter et al. (2014) and Ilangovan et al. (2017) with permission from CELL Press.