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Category: Bacterial Pathogenesis
Structure and Activity of the Type VI Secretion System, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781683670285/9781683670278_Chap26-1.gif /docserver/preview/fulltext/10.1128/9781683670285/9781683670278_Chap26-2.gifAbstract:
The type VI secretion system (T6SS) is a multiprotein machine that belongs to the versatile family of contractile injection systems (CISs) ( 1 – 4 ). CISs deliver effectors into target cells using a spring-like mechanism ( 4 – 6 ). Briefly, CISs assemble a needle-like structure, loaded with effectors, wrapped into a sheath built in an extended, metastable conformation ( Fig. 1 ). Contraction of the sheath propels the needle toward the competitor cell. Genomes of Gram-negative bacteria usually encode one or several T6SSs, with an overrepresentation in Proteobacteria and Bacteroidetes ( 8 – 10 ; for a review on the role of T6SS in gut-associated Bacteroidales, see the chapter by Coyne and Comstock [ 7 ]). The broad arsenal of effectors delivered by T6SSs includes antibacterial proteins such as peptidoglycan hydrolases, eukaryotic effectors that act on cell cytoskeleton, and toxins that can target all cell types, such as DNases, phospholipases, and NAD+ hydrolases ( 11 – 14 ). Consequently, the T6SS plays a critical role in reshaping bacterial communities and, directly or indirectly, in pathogenesis ( 15 – 19 ). Destroying bacterial competitors also provides exogenous DNA that can be acquired in naturally competent bacteria and that serves as a reservoir for antibiotic resistance gene spread ( 20 ). This chapter lists the major effector families and summarizes the current knowledge on the assembly and mode of action of the T6SS.
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Schematic representation of the T6SS. The different subunits are labeled, as are the different subcomplexes. IM, inner membrane; OM, outer membrane.
Assembly and mechanism of firing of the T6SS. T6SS biogenesis starts with the positioning and assembly of the membrane complex and the assembly of the BP ( 1 ). The recruitment and docking of the BP on the membrane complex ( 2 ) initiate the TssA-mediated polymerization of the tail tube/sheath tubular structure ( 3 to 5 ), which is stopped when hitting the opposite membrane by the TagA stopper ( 5 ). Sheath contraction propels the tube/spike needle into the target ( 6 ). The ClpV ATPase is recruited to the contracted sheath to recycle sheath subunits ( 6 ). Needle components, and effectors associated with them, are delivered inside the target ( 7 ).
Schematic representation of the mechanisms of effector loading. Effectors are depicted as red circles. Specialized effectors are chimeric needle proteins with extensions encoding the effector. Cargo effectors are independent proteins that associate with needle components (Hcp, VgrG, and PAAR). Binding of cargo effectors to needle components could be direct or mediated by adaptor modules that are independent proteins (adaptors) or extensions of VgrG and PAAR (internal adaptors).