Type IV secretion systems: versatility and diversity in function

Abstract

Type IV secretion systems (T4SSs) are large protein complexes which traverse the cell envelope of many bacteria. They contain a channel through which proteins or protein-DNA complexes can be translocated. This translocation is driven by a number of cytoplasmic ATPases which might energize large conformational changes in the translocation complex. The family of T4SSs is very versatile, shown by the great variety of functions among family members. Some T4SSs are used by pathogenic Gram-negative bacteria to translocate a wide variety of virulence factors into the host cell. Other T4SSs are utilized to mediate horizontal gene transfer, an event that greatly facilitates the adaptation to environmental changes and is the basis for the spread of antibiotic resistance among bacteria. Here we review the recent advances in the characterization of the architecture and mechanism of substrate transfer in a few representative T4SSs with a particular focus on their diversity of structure and function.

Fig. 1

Schematic model of the VirB/D4 system of A. tumefaciens and structures of T4SS components determined to date. A. The experimentally predicted locations of VirB/D4 components of A. tumefaciens. Arrows indicate the sequential translocation steps identified during substrate secretion through the VirB/D4 system in A. tumefaciens (Cascales and Christie, 2004). B. Crystal structure of the TrwB hexamer, VirD4 homologue of the E. coli conjugative plasmid R388 (Gomis-Ruth et al., 2001). C. Single subunit of the TrwB hexamer with modelled plausible location of the N-terminal transmembrane domain. D. Crystal structure of the HP5025 hexamer, VirB11 homologue of H. pylori (Savvides et al., 2003). E. Single subunit of the HP0525 hexamer. The N-terminal (NTD) and C-terminal (CTD) domains are indicated. F. Cryo-electron microscopy structure of the T4SS core complex of the E. coli conjugative plasmid pKM101 (Fronzes et al., 2009b), comprising the full-length TraN, TraO and TraF, which correspond to the VirB7, VirB9 and VirB10 homologues of A. tumefaciens respectively. G. Crystal structure of the outer membrane complex, comprising the O-layer (Chandran et al., 2009). The inset shows the characteristic two-helix bundle of TraF that traverses the outer membrane. TraN, TraO and TraF are coloured red, blue and green respectively. H and I. Crystal structures of TraC, the VirB5 homologue encoded by pKM101 (Yeo et al., 2003), and the periplasmic C-terminal domain of VirB8 from B. suis respectively (Terradot et al., 2005). Colour-coding of subunits in (A) is preserved for the individual subunits shown in (B)–(E) and (G)–(I). OM, outer membrane; IM, inner membrane; TMS, transmembrane segment; Cyt., cytosol.

Fig. 2

Genetic organization of T4SSs discussed in detail in this review. Homology to the virB/D4 system is indicated by the text in grey above the schemes, e.g. B4 above a gene means that this gene is homologous to virB4. Genes in blue corresponds to plausible core components, in green ATPases, in yellow plausible surface components, in orange lytic tranglycosylases and in red effector proteins. For L. pneumophila, upper-case gene names = Dot and lower-case gene names = Icm.