The Role of the Small Export Apparatus Protein, SctS, in the Activity of the Type III Secretion System

Interactions and substrate selectivity within the SctRST complex of the type III secretion system of enteropathogenic Escherichia coli

Many bacterial pathogens employ a protein complex, termed the type III secretion system (T3SS), to inject bacterial effectors into host cells. These effectors manipulate various cellular processes to promote bacterial growth and survival. The T3SS complex adopts a nano-syringe shape that is assembled across the bacterial membranes, with an extracellular needle extending toward the host cell membrane. The assembly of the T3SS is initiated by the association of three proteins, known as SctR, SctS, and SctT, which create an entry portal to the translocation channel within the bacterial inner membrane. Using the T3SS of enteropathogenic Escherichia coli, we investigated, by mutational and functional analyses, the role of two structural construction sites formed within the SctRST complex and revealed that they are mutation-resistant components that are likely to act as seals preventing leakage of ions and metabolites rather than as substrate gates. In addition, we identified two residues in the SctS protein, Pro23, and Lys54, that are critical for the proper activity of the T3SS. We propose that Pro23 is critical for the physical orientation of the SctS transmembrane domains that create the tip of the SctRST complex and for their positioning with regard to other T3SS substructures. Surprisingly, we found that SctS Lys54, which was previously suggested to mediate the SctS self-oligomerization, is critical for T3SS activity due to its essential role in SctS-SctT hetero-interactions.

The Role of the Membrane-Associated Domain of the Export Apparatus Protein, EscV (SctV), in the Activity of the Type III Secretion System

Diarrheal diseases remain a major public health concern worldwide. Many of the causative bacterial pathogens that cause these diseases have a specialized protein complex, the type III secretion system (T3SS), which delivers effector proteins directly into host cells. These effectors manipulate host cell processes for the benefit of the infecting bacteria. The T3SS structure resembles a syringe anchored within the bacterial membrane, projecting toward the host cell membrane. The entry port of the T3SS substrates, called the export apparatus, is formed by five integral membrane proteins. Among the export apparatus proteins, EscV is the largest, and as it forms a nonamer, it constitutes the largest portion of the export apparatus complex. While there are considerable data on the soluble cytoplasmic domain of EscV, our knowledge of its membrane-associated section and its transmembrane domains (TMDs) is still very limited. In this study, using an isolated genetic reporter system, we found that TMD5 and TMD6 of EscV mediate strong self-oligomerization. Substituting these TMDs within the full-length protein with a random hydrophobic sequence resulted in a complete loss of function of the T3SS, further suggesting that the EscV TMD5 and TMD6 sequences have a functional role in addition to their structural role as membrane anchors. As we observed only mild reduction in the ability of the TMD-exchanged variants to integrate into the full or intermediate T3SS complexes, we concluded that EscV TMD5 and TMD6 are not crucial for the global assembly or stability of the T3SS complex but are rather involved in promoting the necessary TMD–TMD interactions within the complex and the overall TMD orientation to allow channel opening for the entry of T3SS substrates.

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.

The Shigella Type III Secretion System: An Overview from Top to Bottom

Shigella comprises four species of human-restricted pathogens causing bacillary dysentery. While Shigella possesses multiple genetic loci contributing to virulence, a type III secretion system (T3SS) is its primary virulence factor. The Shigella T3SS nanomachine consists of four major assemblies: the cytoplasmic sorting platform; the envelope-spanning core/basal body; an exposed needle; and a needle-associated tip complex with associated translocon that is inserted into host cell membranes. The initial subversion of host cell activities is carried out by the effector functions of the invasion plasmid antigen (Ipa) translocator proteins, with the cell ultimately being controlled by dedicated effector proteins that are injected into the host cytoplasm though the translocon. Much of the information now available on the T3SS injectisome has been accumulated through collective studies on the T3SS from three systems, those of Shigella flexneri, Salmonella typhimurium and Yersinia enterocolitica/Yersinia pestis. In this review, we will touch upon the important features of the T3SS injectisome that have come to light because of research in the Shigella and closely related systems. We will also briefly highlight some of the strategies being considered to target the Shigella T3SS for disease prevention.

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.

Specific gene SEN1393 contributes to higher survivability of Salmonella Enteritidis in egg white by regulating sulfate assimilation pathway

Salmonella enterica serovar Enteritidis (S. Enteritidis) presents an excellent capacity to survive in egg white, which is a hostile environment for bacterial growth. To reveal its survival mechanism, this study focuses on the specific gene SEN1393, which has been found to exist only in the genomic sequence of S. Enteritidis. The survival capacity of the deletion mutant strain ΔSEN1393 was proven to be significantly reduced after incubation in egg white. RNA sequencing and RT-qPCR results demonstrate that the expression levels of 19 genes were up-regulated, while the expression levels of 9 genes were down-regulated in egg white. These genes were classified into 6 groups based on their functional categories, namely the sulfate assimilation pathway, arginine biosynthesis, the tricarboxylic acid cycle, the fimbrial protein, the transport and chelation of metal ion, and others (sctT, rhs, and pspG). The strain ΔSEN1393 was deduced to damage FeS cluster enzymes and increase the sulfate and iron requirements, and to reduce bacterial motility and copper homeostasis. Via InterProScan analysis, the gene SEN1393 was speculated to encode a TerB-like and/or DjlA-like protein, and therefore, together with cysJ, possibly reduced the oxidative toxicities resulting from oxyanions such as tellurite, and/or improved CysPUWA conformation to restrain the uptake of the toxic oxyanions. In summary, the gene SEN1393 enabled the higher survival of S. Enteritidis in egg white as compared to other pathogens by regulating the sulfate assimilation pathway.

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.