Investigating the assembly of the bacterial type III secretion system injectisome by in vivo photocrosslinking

Blue Native PAGE Analysis of Bacterial Secretion Complexes

Bacterial protein secretion systems serve to translocate substrate proteins across up to three biological membranes, a task accomplished by hydrophobic, membrane-spanning macromolecular complexes. The overexpression, purification, and biochemical characterization of these complexes is often difficult, thus impeding progress in understading structure and function of these systems. Blue native (BN) polyacrylamide gel electrophoresis (PAGE) allows for the investigation of these transmembrane complexes right from their originating membranes, without the need of long preparative steps, and is amenable to the parallel characterization of a number of samples under near-native conditions. Here, we present protocols for sample preparation, one-dimensional BN PAGE and two-dimensional BN/SDS PAGE, as well as for downstream analysis by staining, immunoblotting, and mass spectrometry on the example of the type III secretion system encoded on Salmonella pathogenicity island 1.

The Contribution of the Predicted Sorting Platform Component HrcQ to Type III Secretion in Xanthomonas campestris pv. vesicatoria Depends on an Internal Translation Start Site

Pathogenicity of the Gram-negative bacterium Xanthomonas campestris pv. vesicatoria depends on a type III secretion (T3S) system which translocates effector proteins into plant cells. T3S systems are conserved in plant- and animal-pathogenic bacteria and consist of at least nine structural core components, which are designated Sct (secretion and cellular translocation) in animal-pathogenic bacteria. Sct proteins are involved in the assembly of the membrane-spanning secretion apparatus which is associated with an extracellular needle structure and a cytoplasmic sorting platform. Components of the sorting platform include the ATPase SctN, its regulator SctL, and pod-like structures at the periphery of the sorting platform consisting of SctQ proteins. Members of the SctQ family form a complex with the C-terminal protein domain, SctQ_C, which is translated as separate protein and likely acts either as a structural component of the sorting platform or as a chaperone for SctQ. The sorting platform has been intensively studied in animal-pathogenic bacteria but has not yet been visualized in plant pathogens. We previously showed that the SctQ homolog HrcQ from X. campestris pv. vesicatoria assembles into complexes which associate with the T3S system and interact with components of the ATPase complex. Here, we report the presence of an internal alternative translation start site in hrcQ leading to the separate synthesis of the C-terminal protein region (HrcQ_C). The analysis of genomic hrcQ mutants showed that HrcQ_C is essential for pathogenicity and T3S. Increased expression levels of hrcQ or the T3S genes, however, compensated the lack of HrcQ_C. Interaction studies and protein analyses suggest that HrcQ_C forms a complex with HrcQ and promotes HrcQ stability. Furthermore, HrcQ_C colocalizes with HrcQ as was shown by fluorescence microscopy, suggesting that it is part of the predicted cytoplasmic sorting platform. In agreement with this finding, HrcQ_C interacts with the inner membrane ring protein HrcD and the SctK-like linker protein HrpB4 which contributes to the docking of the HrcQ complex to the membrane-spanning T3S apparatus. Taken together, our data suggest that HrcQ_C acts as a chaperone for HrcQ and as a structural component of the predicted sorting platform.

Conserved Salt Bridges Facilitate Assembly of the Helical Core Export Apparatus of a Salmonella enterica Type III Secretion System

Virulence-associated type III secretion systems (T3SS) are utilized by Gram negative bacterial pathogens for injection of effector proteins into eukaryotic host cells. The transmembrane export apparatus at the core of T3SS is composed of a unique helical complex of the hydrophobic proteins SctR, SctS, SctT, and SctU. These components comprise a number of highly conserved charged residues within their hydrophobic domains. The structure of the closed state of the core complex SctR₅S₄T₁ revealed that several of these residues form inter- and intramolecular salt bridges, some of which have to be broken for pore opening. Mutagenesis of individual residues was shown to compromise assembly or secretion of both, the virulence-associated and the related flagellar T3SS. However, the exact role of these conserved charged residues in the assembly and function of T3SS remains elusive. Here we performed an in-depth mutagenesis analysis of these residues in the T3SS of Salmonella Typhimurium, coupled to blue native PAGE, in vivo photocrosslinking and luciferase-based secretion assays. Our data show that these conserved salt bridges are not critical for assembly of the respective protein but rather facilitate the incorporation of the following subunit into the assembling complex. Our data also indicate that these conserved charged residues are critical for type III-dependent secretion and reveal a functional link between SctS_E44 and SctT_R204 and the cytoplasmic domain of SctU in gating the T3SS injectisome. Overall, our analysis provides an unprecedented insight into the delicate requirements for the assembly and function of the machinery at the core of T3SS.

HrpB4 from Xanthomonas campestris pv. vesicatoria acts similarly to SctK proteins and promotes the docking of the predicted sorting platform to the type III secretion system

The Gram-negative bacterium Xanthomonas campestris pv. vesicatoria is the causal agent of bacterial spot disease on pepper and tomato plants. Pathogenicity of X. campestris pv. vesicatoria depends on a type III secretion (T3S) system which translocates bacterial effector proteins into plant cells. At least nine membrane-associated and cytoplasmic components of the secretion apparatus are homologous to corresponding Sct (secretion and cellular translocation) proteins from animal pathogens, suggesting a similar structural organisation of T3S systems in different bacterial species. T3S in X. campestris pv. vesicatoria also depends on non-conserved proteins with yet unknown function including the essential pathogenicity factor HrpB4. Here, we show that HrpB4 localises to the cytoplasm and the bacterial membranes and interacts with the cytoplasmic domain of the inner membrane (IM) ring component HrcD and the cytoplasmic HrcQ protein. The analysis of HrpB4 deletion derivatives revealed that deletion of the N- or C-terminal protein region affects the interaction of HrpB4 with HrcQ and HrcD as well as its contribution to pathogenicity. HrcQ is a component of the predicted sorting platform, which was identified in animal pathogens as a dynamic heterooligomeric protein complex and associates with the IM ring via SctK proteins. HrcQ complex formation was previously shown by fluorescent microscopy analysis and depends on the presence of the T3S system. In the present study, we provide experimental evidence that the absence of HrpB4 severely affects the docking of HrcQ complexes to the T3S system but does not significantly interfere with HrcQ complex formation in the bacterial cytoplasm. Taken together, our data suggest that HrpB4 links the predicted cytoplasmic sorting platform to the IM rings of the T3S system.

Molecular Targets and Strategies for Inhibition of the Bacterial Type III Secretion System (T3SS); Inhibitors Directly Binding to T3SS Components

The type III secretion system (T3SS) is a virulence apparatus used by many Gram-negative pathogenic bacteria to cause infections. Pathogens utilizing a T3SS are responsible for millions of infections yearly. Since many T3SS knockout strains are incapable of causing systemic infection, the T3SS has emerged as an attractive anti-virulence target for therapeutic design. The T3SS is a multiprotein molecular syringe that enables pathogens to inject effector proteins into host cells. These effectors modify host cell mechanisms in a variety of ways beneficial to the pathogen. Due to the T3SS’s complex nature, there are numerous ways in which it can be targeted. This review will be focused on the direct targeting of components of the T3SS, including the needle, translocon, basal body, sorting platform, and effector proteins. Inhibitors will be considered a direct inhibitor if they have a binding partner that is a T3SS component, regardless of the inhibitory effect being structural or functional.

Adaptivity and dynamics in type III secretion systems

The type III secretion system is the common core of two bacterial molecular machines: the flagellum and the injectisome. The flagellum is the most widely distributed prokaryotic locomotion device, whereas the injectisome is a syringe-like apparatus for inter-kingdom protein translocation, which is essential for virulence in important human pathogens. The successful concept of the type III secretion system has been modified for different bacterial needs. It can be adapted to changing conditions, and was found to be a dynamic complex constantly exchanging components. In this review, we highlight the flexibility, adaptivity, and dynamic nature of the type III secretion system.

"The structure of the Type III secretion system export gate with CdsO, an ATPase lever arm"

Type III protein secretion systems (T3SS) deliver effector proteins from the Gram-negative bacterial cytoplasm into a eukaryotic host cell through a syringe-like, multi-protein nanomachine. Cytosolic components of T3SS include a portion of the export apparatus, which traverses the inner membrane and features the opening of the secretion channel, and the sorting complex for substrate recognition and for providing the energetics required for protein secretion. Two components critical for efficient effector export are the export gate protein and the ATPase, which are proposed to be linked by the central stalk protein of the ATPase. We present the structure of the soluble export gate homo-nonamer, CdsV, in complex with the central stalk protein, CdsO, of its cognate ATPase, both derived from Chlamydia pneumoniae. This structure defines the interface between these essential T3S proteins and reveals that CdsO engages the periphery of the export gate that may allow the ATPase to catalyze an opening between export gate subunits to allow cargo to enter the export apparatus. We also demonstrate through structure-based mutagenesis of the homologous export gate in Pseudomonas aeruginosa that mutation of this interface disrupts effector secretion. These results provide novel insights into the molecular mechanisms governing active substrate recognition and translocation through a T3SS.

Many pathogenic Gram-negative bacteria utilize T3SS to export virulence factors in a well-regulated manner. Most component proteins of the T3SS are highly structurally conserved, capable of recognizing and secreting diverse effectors, which are recruited to the cytoplasmic sorting complex by chaperones. This work describes the molecular architecture of two essential components of a T3SS, identifies the interface between the components, and establishes the necessity of this interaction for effector secretion.

A Unified Nomenclature for Injectisome-Type Type III Secretion Systems

The independent naming of components of injectisome-type type III secretion systems in different bacterial species has resulted in considerable confusion, impeding accessibility of the literature and hindering communication between scientists of the same field. A unified nomenclature had been proposed by Hueck more than 20 years ago. It found little attention for many years, but usage was sparked again by recent reviews and an international type III secretion meeting in 2016. Here, we propose that the field consistently switches to an extended version of this nomenclature to be no longer lost in translation.