Assembly and structure of the T3SS

One contribution of 12 to a theme issue 'The bacterial cell envelope'. The flagellum and the injectisome are two of the most complex and fascinating bacterial nanomachines. At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the flagellar filament and the injectisome needle. Recent advances, combining structural biology, cryo-electron tomography, molecular genetics, in vivo imaging, bioinformatics and biophysics, have greatly increased our understanding of the T3SS, especially the structure of its transmembrane and cytosolic components, the transcriptional, post-transcriptional and functional regulation and the remarkable adaptivity of the system. This review aims to integrate these new findings into our current knowledge of the evolution, function, regulation and dynamics of the T3SS, and to highlight commonalities and differences between the two systems, as well as their potential applications.

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“…Further details on the structure of injectisomes and flagella can be obtained from dedicated recent reviews [30,[50][51][52][53].…”

Section: Structural and Functional Homologies Among The T3ss And Othementioning

confidence: 99%

Type III secretion systems: the bacterial flagellum and the injectisome

Diepold

Armitage

2015

Phil. Trans. R. Soc. B

184

175

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“…The needle is capped at its end by the tip complex, which is thought to coordinate the activation of the secretion machine upon contact with target cells (9,10). Because the needle complex can be isolated in a manner suitable for single-particle cryo-electron microscopy, its organization at the quasi-atomic level is known (11)(12)(13)(14)(15)(16)(17)(18). Passage of the secreted proteins through the inner membrane requires the export apparatus, which is composed of a group of poly-topic inner membrane proteins located at the center of the needle complex base (19).…”

mentioning

confidence: 99%

Visualization and characterization of individual type III protein secretion machines in live bacteria

Zhang

Lara‐Tejero

Bewersdorf

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

103

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Type III protein secretion machines have evolved to deliver bacterially encoded effector proteins into eukaryotic cells. Although electron microscopy has provided a detailed view of these machines in isolation or fixed samples, little is known about their organization in live bacteria. Here we report the visualization and characterization of the Salmonella type III secretion machine in live bacteria by 2D and 3D single-molecule switching superresolution microscopy. This approach provided access to transient components of this machine, which previously could not be analyzed. We determined the subcellular distribution of individual machines, the stoichiometry of the different components of this machine in situ, and the spatial distribution of the substrates of this machine before secretion. Furthermore, by visualizing this machine in Salmonella mutants we obtained major insights into the machine's assembly. This study bridges a major resolution gap in the visualization of this nanomachine and may serve as a paradigm for the examination of other bacterially encoded molecular machines.bacterial nanomachines | protein secretion | superresolution microscopy | bacterial pathogenesis | Salmonella virulence T ype III secretion systems are remarkable molecular machines with the capacity to inject multiple bacterial proteins into eukaryotic cells (1, 2). These machines play an essential role in the pathogenic or symbiotic relationships between the bacteria that encode them and their respective hosts. Consequently, type III secretion systems are rapidly emerging as targets for the development of novel therapeutic and prevention strategies with the potential to combat several important infectious diseases (3-5). The components of type III secretion machines are highly conserved across bacterial species although the effector proteins that they deliver are customized for the specific bacteria that harbor them (1, 6, 7). The entire type III secretion machine or injectisome is ∼40 nm in width and ∼150 nm in length and is organized in defined substructures (Fig. 1A). The bestcharacterized substructure is the needle complex, which is a multi-ring cylindrical structure embedded in the bacterial envelope with a needle-like extension that projects several nanometers from the bacterial surface. The needle complex is traversed by a narrow channel that serves as the conduit for the proteins that travel this secretion pathway (8). The needle is capped at its end by the tip complex, which is thought to coordinate the activation of the secretion machine upon contact with target cells (9, 10). Because the needle complex can be isolated in a manner suitable for single-particle cryo-electron microscopy, its organization at the quasi-atomic level is known (11-18). Passage of the secreted proteins through the inner membrane requires the export apparatus, which is composed of a group of poly-topic inner membrane proteins located at the center of the needle complex base (19). Finally, several cytoplasmic proteins form a large complex known as the...

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“…These interactions have driven the evolution of several mechanisms by which they quickly deploy proteinaceous and nucleic acid effectors that manipulate the behaviour of the target organism, often resulting in growth inhibition or death [1][2][3][4][5][6] . Distinct among these mechanisms are the type III, type IV and type VI secretion systems (T3SS, T4SS and T6SS, respectively) that are all capable of transferring proteins, and in the case of the T4SS, protein-DNA complexes, directly into neighbouring cells in a contact-dependent manner [1][2][3] . All three of these systems have been shown to be able to inject virulence factors into eukaryotic hosts, but only the T6SSs have been shown to deliver lethal toxins into bacterial cells 7 .…”

mentioning

confidence: 99%