On the quaternary association of the type III secretion system HrcQB-C protein: Experimental evidence differentiates among the various oligomerization models

Cytosolic sorting platform complexes shuttle type III secretion system effectors to the injectisome in Yersinia enterocolitica

Bacteria use type III secretion injectisomes to inject effector proteins into eukaryotic target cells. Recruitment of effectors to the machinery and the resulting export hierarchy involve the sorting platform. These conserved proteins form pod structures at the cytosolic interface of the injectisome but are also mobile in the cytosol. Photoactivated localization microscopy in Yersinia enterocolitica revealed a direct interaction of the sorting platform proteins SctQ and SctL with effectors in the cytosol of live bacteria. These proteins form larger cytosolic protein complexes involving the ATPase SctN and the membrane connector SctK. The mobility and composition of these mobile pod structures are modulated in the presence of effectors and their chaperones, and upon initiation of secretion, which also increases the number of injectisomes from ~5 to ~18 per bacterium. Our quantitative data support an effector shuttling mechanism, in which sorting platform proteins bind to effectors in the cytosol and deliver the cargo to the export gate at the membrane-bound injectisome.

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.

A Career on Both Sides of the Atlantic: Memoirs of a Molecular Plant Pathologist

This article recounts the experiences that shaped my career as a molecular plant pathologist. It focuses primarily on technical and conceptual developments in molecular phytobacteriology, shares some personal highlights and untold stories that impacted my professional development, and describes the early years of agricultural biotechnology. Writing this article required reflection on events occurring over several decades that were punctuated by a mid-career relocation across the Atlantic. I hope it will still be useful, informative, and enjoyable to read. An extended version of the abstract is provided in the Supplemental Materials , available online.

A dynamic and adaptive network of cytosolic interactions governs protein export by the T3SS injectisome

Many bacteria use a type III secretion system (T3SS) to inject effector proteins into host cells. Selection and export of the effectors is controlled by a set of soluble proteins at the cytosolic interface of the membrane spanning type III secretion ‘injectisome’. Combining fluorescence microscopy, biochemical interaction studies and fluorescence correlation spectroscopy, we show that in live Yersinia enterocolitica bacteria these soluble proteins form complexes both at the injectisome and in the cytosol. Binding to the injectisome stabilizes these cytosolic complexes, whereas the free cytosolic complexes, which include the type III secretion ATPase, constitute a highly dynamic and adaptive network. The extracellular calcium concentration, which triggers activation of the T3SS, directly influences the cytosolic complexes, possibly through the essential component SctK/YscK, revealing a potential mechanism involved in the regulation of type III secretion.

Bacterial type III secretion systems (T3SS) play important roles in pathogenesis. Here, Diepold et al. show the dynamic nature of complexes formed of essential T3SS components in live bacteria, and that extracellular calcium concentrations influence these cytosolic complexes likely via SctK/YscK.

The Structure and Function of Type III Secretion Systems

Type III secretion systems (T3SSs) afford Gram-negative bacteria an intimate means of altering the biology of their eukaryotic hosts--the direct delivery of effector proteins from the bacterial cytoplasm to that of the eukaryote. This incredible biophysical feat is accomplished by nanosyringe "injectisomes," which form a conduit across the three plasma membranes, peptidoglycan layer, and extracellular space that form a barrier to the direct delivery of proteins from bacterium to host. The focus of this chapter is T3SS function at the structural level; we will summarize the core findings that have shaped our understanding of the structure and function of these systems and highlight recent developments in the field. In turn, we describe the T3SS secretory apparatus, consider its engagement with secretion substrates, and discuss the posttranslational regulation of secretory function. Lastly, we close with a discussion of the future prospects for the interrogation of structure-function relationships in the T3SS.

Phylogenetic analysis of a gene cluster encoding an additional, rhizobial-like type III secretion system that is narrowly distributed among Pseudomonas syringae strains

Background

The central role of Type III secretion systems (T3SS) in bacteria-plant interactions is well established, yet unexpected findings are being uncovered through bacterial genome sequencing. Some Pseudomonas syringae strains possess an uncharacterized cluster of genes encoding putative components of a second T3SS (T3SS-2) in addition to the well characterized Hrc1 T3SS which is associated with disease lesions in host plants and with the triggering of hypersensitive response in non-host plants. The aim of this study is to perform an in silico analysis of T3SS-2, and to compare it with other known T3SSs.

Results

Based on phylogenetic analysis and gene organization comparisons, the T3SS-2 cluster of the P. syringae pv. phaseolicola strain is grouped with a second T3SS found in the pNGR234b plasmid of Rhizobium sp. These additional T3SS gene clusters define a subgroup within the Rhizobium T3SS family. Although, T3SS-2 is not distributed as widely as the Hrc1 T3SS in P. syringae strains, it was found to be constitutively expressed in P. syringae pv phaseolicola through RT-PCR experiments.

Conclusions

The relatedness of the P. syringae T3SS-2 to a second T3SS from the pNGR234b plasmid of Rhizobium sp., member of subgroup II of the rhizobial T3SS family, indicates common ancestry and/or possible horizontal transfer events between these species. Functional analysis and genome sequencing of more rhizobia and P. syringae pathovars may shed light into why these bacteria maintain a second T3SS gene cluster in their genome.

Two translation products of Yersinia yscQ assemble to form a complex essential to type III secretion

The bacterial flagellar C-ring is composed of two essential proteins, FliM and FliN. The smaller protein, FliN, is similar to the C-terminus of the larger protein, FliM, both being composed of SpoA domains. While bacterial type III secretion (T3S) systems encode many proteins in common with the flagellum, they mostly have a single protein in place of FliM and FliN. This protein resembles FliM at its N-terminus and is as large as FliM but is more like FliN at its C-terminal SpoA domain. We have discovered that a FliN-sized cognate indeed exists in the Yersinia T3S system to accompany the FliM-sized cognate. The FliN-sized cognate, YscQ-C, is the product of an internal translation initiation site within the locus encoding the FliM-sized cognate YscQ. Both intact YscQ and YscQ-C were found to be required for T3S, indicating that the internal translation initiation site, which is conserved in some but not all YscQ orthologs, is crucial for function. The crystal structure of YscQ-C revealed a SpoA domain that forms a highly intertwined, domain-swapped homodimer, similar to those observed in FliN and the YscQ ortholog HrcQ(B). A single YscQ-C homodimer associated reversibly with a single molecule of intact YscQ, indicating conformational differences between the SpoA domains of intact YscQ and YscQ-C. A "snap-back" mechanism suggested by the structure can account for this. The 1:2 YscQ-YscQ-C complex is a close mimic of the 1:4 FliM-FliN complex and the likely building block of the putative Yersinia T3S system C-ring.

Playing the "Harp": evolution of our understanding of hrp/hrc genes

With the advent of recombinant DNA techniques, the field of molecular plant pathology witnessed dramatic shifts in the 1970s and 1980s. The new and conventional methodologies of bacterial molecular genetics put bacteria center stage. The discovery in the mid-1980s of the hrp/hrc gene cluster and the subsequent demonstration that it encodes a type III secretion system (T3SS) common to Gram negative bacterial phytopathogens, animal pathogens, and plant symbionts was a landmark in molecular plant pathology. Today, T3SS has earned a central role in our understanding of many fundamental aspects of bacterium-plant interactions and has contributed the important concept of interkingdom transfer of effector proteins determining race-cultivar specificity in plant-bacterium pathosystems. Recent developments in genomics, proteomics, and structural biology enable detailed and comprehensive insights into the functional architecture, evolutionary origin, and distribution of T3SS among bacterial pathogens and support current research efforts to discover novel antivirulence drugs.