Abstract
Bacteria use type IV secretion (T4S) systems to deliver DNA and protein substrates to a diverse range of prokaryotic and eukaryotic target cells. T4S systems have great impact on human health, as they are a major source of antibiotic resistance spread among bacteria and are central to infection processes of many pathogens. Therefore, deciphering the structure and underlying translocation mechanism of T4S systems is crucial to facilitate development of new drugs. The last five years have witnessed considerable progress in unraveling the structure of T4S system subassemblies, notably that of the T4S system core complex, a large 1 MegaDalton (MDa) structure embedded in the double membrane of Gram-negative bacteria and made of 3 of the 12 T4S system components. However, the recent determination of the structure of -3MDa assembly of 8 of these components has revolutionized our views of T4S system architecture and opened up new avenues of research, which are discussed in this review.
Highlights
Secretion in bacteria is the process by which macromolecules are translocated across the cell envelope
We describe the recent breakthroughs in the structural biology of T4S systems and the proposed mechanisms of action of these complex machines
The expression of VirB2 and VirB5 is, essential [10]. These results suggested that the T4S system might exist in or transition between two states: a secretioncompetent state and a pilus biogenesis-competent state, extending either a short or a long pilus, respectively [26]
Summary
Secretion in bacteria is the process by which macromolecules are translocated across the cell envelope. VirD4 might be located on the side of the T4SS3–10 complex, and if correct, a two-step mechanism of transport (as defined by [48]) must be invoked: the substrate would first be translocated through the IM (1st step) by VirD4 and enter the core/OM complex through the periplasmic side of the complex (2nd step) thereby contacting the VirB6/VirB8 arches, VirB9 ( contactable from the periplasmic side since it forms the outer wall of the core/OM complex), and entering the VirB2 pilus hypothesized here as forming a tube lining the entire VirB10-composed interior of the core/OM complex (Figure 4a). The substrate would go through the IMC of the T4SS3–10 structure, the arches, and access the core/ OM complex through contact with VirB9 and VirB2 as explained above (Figure 4b). It is interesting to speculate that the T4SS3–10 structure might represent a T4S system in its pilus biogenesis mode, but substitution of the VirB4 barrels by VirD4 or formation of mixed VirB4/VirD4 barrels might switch the system to its substrate transfer mode
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