The chaperone binding domain of SopE inhibits transport via flagellar and SPI-1 TTSS in the absence of InvB

Type III Secretion in the Melioidosis Pathogen Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative intracellular pathogen and the causative agent of melioidosis, a severe disease of both humans and animals. Melioidosis is an emerging disease which is predicted to be vastly under-reported. Type III Secretion Systems (T3SSs) are critical virulence factors in Gram negative pathogens of plants and animals. The genome of B. pseudomallei encodes three T3SSs. T3SS-1 and -2, of which little is known, are homologous to Hrp2 secretion systems of the plant pathogens Ralstonia and Xanthomonas. T3SS-3 is better characterized and is homologous to the Inv/Mxi-Spa secretion systems of Salmonella spp. and Shigella flexneri, respectively. Upon entry into the host cell, B. pseudomallei requires T3SS-3 for efficient escape from the endosome. T3SS-3 is also required for full virulence in both hamster and murine models of infection. The regulatory cascade which controls T3SS-3 expression and the secretome of T3SS-3 have been described, as well as the effect of mutations of some of the structural proteins. Yet only a few effector proteins have been functionally characterized to date and very little work has been carried out to understand the hierarchy of assembly, secretion and temporal regulation of T3SS-3. This review aims to frame current knowledge of B. pseudomallei T3SSs in the context of other well characterized model T3SSs, particularly those of Salmonella and Shigella.

Type III secretion systems: the bacterial flagellum and the injectisome

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.

The Aeromonas salmonicida subsp. salmonicida exoproteome: determination of the complete repertoire of Type-Three Secretion System effectors and identification of other virulence factors

Background

Aeromonas salmonicida subsp. salmonicida, the etiologic agent of furunculosis, is a major pathogen of fisheries worldwide. Several virulence factors have been described, but the type-three secretion system (T3SS) is recognized as having a major effect on virulence by injecting effectors directly into fish cells. In this study we used high-throughput proteomics to display the differences between in vitro secretome of A. salmonicida wild-type (wt, hypervirulent, JF2267) and T3SS-deficient (isogenic ΔascV, extremely low-virulent, JF2747) strains in exponential and stationary phases of growth.

Results

Results confirmed the secretion of effectors AopH, AexT, AopP and AopO via T3SS, and for the first time demonstrated the impact of T3SS in secretion of Ati2, AopN and ExsE that are known as effectors in other pathogens. Translocators, needle subunits, Ati1, and AscX were also secreted in supernatants (SNs) dependent on T3SS. AopH, Ati2, AexT, AopB and AopD were in the top seven most abundant excreted proteins. EF-G, EF-Tu, DnaK, HtpG, PNPase, PepN and MdeA were moderately secreted in wt SNs and predicted to be putative T3 effectors by bioinformatics. Pta and ASA_P5G088 were increased in wt SNs and T3-associated in other bacteria. Ten conserved cytoplasmic proteins were more abundant in wt SNs than in the ΔascV mutant, but without any clear association to a secretion system. T1-secreted proteins were predominantly found in wt SNs: OmpAI, OmpK40, DegQ, insulinase ASA_0716, hypothetical ASA_0852 and ASA_3619. Presence of T3SS components in pellets was clearly decreased by ascV deletion, while no impact was observed on T1- and T2SS. Our results demonstrated that the ΔascV mutant strain excreted well-described (VapA, AerA, AerB, GCAT, Pla1, PlaC, TagA, Ahe2, GbpA and enolase) and yet uncharacterized potential toxins, adhesins and enzymes as much as or even more than the wt strain. Other putative important virulence factors were not detected.

Conclusions

We demonstrated the whole in vitro secretome and T3SS repertoire of hypervirulent A. salmonicida. Several toxins, adhesins and enzymes that are not part of the T3SS secretome were secreted to a higher extent in the extremely low-virulent ΔascV mutant. All together, our results show the high importance of an intact T3SS to initiate the furunculosis and offer new information about the pathogenesis.

Enhancement of host immune responses by oral vaccination to Salmonella enterica serovar Typhimurium harboring both FliC and FljB flagella

Flagellin, the structural component of the flagellar filament in various motile bacteria, can contribute to the activation of NF-κB and proinflammatory cytokine expression during the innate immune response in host cells. Thus, flagellin proteins represent a particularly attractive target for the development of vaccine candidates. In this study, we investigated the immune response by increasing the flagella number in the iacP mutant strain and the adjuvant activity of the flagellin component FljB of Salmonella enterica serovar Typhimurium. We found that the iacP mutant strain expresses two flagellin proteins (FliC and FljB), which result in increased NF-κB-dependent gene expression in bone marrow derived macrophages. Using an oral immunization mouse model, we observed that the administration of a live attenuated S. typhimurium BRD509 strain expressing the FliC and FljB flagellins induced significantly enhanced flagellin-specific IgG responses in the systemic compartment. The mice immunized with the recombinant attenuated S. typhimurium strain that has two types of flagella were protected from lethal challenge with the Salmonella SL1344 strain. These results indicate that overexpression of flagella in the iacP mutant strain enhance the induction of an antigen-specific immune responses in macrophage cell, and both the FliC and FljB flagellar filament proteins-producing S. typhimurium can induce protective immune responses against salmonellosis.

Molecular characterization of the InvE regulator in the secretion of type III secretion translocases in Salmonella enterica serovar Typhimurium

The type III secretion systems (T3SSs) are exploited by many Gram-negative pathogenic bacteria to deliver a set of effector proteins into the host cytosol during cell entry. The T3SS of Salmonella enterica serovar Typhimurium is composed of more than 20 proteins that constitute the membrane-associated base, the needle and the tip complex at the distal end of the T3SS needle. Membrane docking and piercing between the T3SS and host cells is followed by the secretion of effector proteins. Therefore, a secretion hierarchy among the substrates of the T3SS is required. The secretion of the pore-forming translocase proteins SipB, SipC and SipD is controlled by the T3SS regulator protein, InvE. During an attempt to identify the regions of InvE that are involved in T3SS regulation, it was observed that the secretion of SipB, SipC and SipD was inhibited when the C-terminal 52 amino acids were removed from InvE. In addition, InvE derivatives lacking the N-terminal 30 and 100 residues were unable to secrete translocases into the culture medium. Interestingly, in the absence of the N-terminal 180 residues of InvE, SipD is unstable, resulting in the hypersecretion of SipB. We also found that both the type III secretion signals of SipB and SptP were functionally interchangeable with the first 30 amino acids of InvE, which could allow the secretion of a reporter protein. These results indicate that InvE may have two functional domains responsible for regulating the secretion of translocases: an N-terminal secretion signal and a C-terminal regulatory domain.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

A Comprehensive Subcellular Proteomic Survey of Salmonella Grown under Phagosome-Mimicking versus Standard Laboratory Conditions

Towards developing a systems-level pathobiological understanding of Salmonella enterica, we performed a subcellular proteomic analysis of this pathogen grown under standard laboratory and phagosome-mimicking conditions in vitro. Analysis of proteins from cytoplasmic, inner membrane, periplasmic, and outer membrane fractions yielded coverage of 25% of the theoretical proteome. Confident subcellular location could be assigned to over 1000 proteins, with good agreement between experimentally observed location and predicted/known protein properties. Comparison of protein location under the different environmental conditions provided insight into dynamic protein localization and possible moonlighting (multiple function) activities. Notable examples of dynamic localization were the response regulators of two-component regulatory systems (e.g., ArcB and PhoQ). The DNA-binding protein Dps that is generally regarded as cytoplasmic was significantly enriched in the outer membrane for all growth conditions examined, suggestive of moonlighting activities. These observations imply the existence of unknown transport mechanisms and novel functions for a subset of Salmonella proteins. Overall, this work provides a catalog of experimentally verified subcellular protein locations for Salmonella and a framework for further investigations using computational modeling.

Effect of iacP mutation on flagellar phase variation in Salmonella enterica serovar typhimurium strain UK-1

Flagella are surface appendages that are important for bacterial motility and invasion of host cells. Two flagellin subunits in Salmonella enterica serovar Typhimurium, FliC and FljB, are alternatively expressed by a site-specific DNA inversion mechanism called flagellar phase variation. Although this inversion mechanism is understood at the molecular level, the key factor controlling the expression of the two flagellin subunits has not been determined. In this study, we found that a putative acyl carrier protein, IacP, affects flagellar phase variation in S. Typhimurium strain UK-1 under Salmonella pathogenicity island 1 (SPI1)-inducing conditions. Liquid chromatography-mass spectrometry analysis of the secreted proteins from S. Typhimurium determined that the amount of FljB secreted was significantly higher in the iacP mutant strain, a finding confirmed by Western blot analysis. Northern blotting, quantitative PCR, and microarray data showed that the level of FljB in the iacP mutant strain was regulated at the transcriptional level, although the transcription and expression of the fliC gene were independent of IacP. FljB production was abolished by the deletion of the Hin DNA invertase but could be restored by the introduction of a plasmid carrying the hin gene. We also found that in the iacP mutant strain, the orientation of the invertible H segment is in the FljB-expressing phase. Furthermore, electron microscopy observations indicated that the iacP mutant strain had more flagella per cell than the wild-type strain. These results suggest that IacP is associated with flagellar phase switching under SPI1-inducing conditions.

Expanded roles for multicargo and class 1B effector chaperones in type III secretion

Bacterial type III secretion systems (T3SS) are complex protein assemblies that mediate the secretion of protein substrates outside the cell. Type III secretion chaperones (T3SC) are always found associated with T3SS, and they serve in multiple roles to ensure that protein substrates are efficiently targeted for secretion. Bacterial pathogens with T3SS express T3SC proteins that bind effectors, a process important for effector protein delivery into eukaryotic cells during infection. In this minireview, we focus on multicargo and class 1B T3SC that associate with effectors within significant pathogens of animals and plants. As a primary role, multicargo and class 1B T3SC form homodimers and specifically bind different effectors within the cytoplasm, maintaining the effectors in a secretion-competent state. This role makes T3SC initial and central contributors to effector-mediated pathogenesis. Recent findings have greatly expanded our understanding of cellular events linked to multicargo T3SC function. New binding interactions with T3SS components have been reported in different systems, thereby implicating multicargo T3SC in critical roles beyond effector binding. Three notable interactions with the YscN, YscV, and YscQ family members are well represented in the literature. Similar T3SC interactions are reported in the putative related flagellar T3SS, suggesting that secretion mechanisms may be more similar than previously thought. The evidence implicates multicargo and class 1B T3SC in effector binding and stabilization, in addition to T3SS recruitment and docking events.

Discovery of Salmonella virulence factors translocated via outer membrane vesicles to murine macrophages

Salmonella enterica serovar Typhimurium, an intracellular pathogen and leading cause of food-borne illness, encodes a plethora of virulence effectors. Salmonella virulence factors are translocated into host cells and manipulate host cellular activities, providing a more hospitable environment for bacterial proliferation. In this study, we report a new set of virulence factors that is translocated into the host cytoplasm via bacterial outer membrane vesicles (OMV). PagK (or PagK1), PagJ, and STM2585A (or PagK2) are small proteins composed of ∼70 amino acids and have high sequence homology to each other (>85% identity). Salmonella lacking all three homologues was attenuated for virulence in a mouse infection model, suggesting at least partial functional redundancy among the homologues. While each homologue was translocated into the macrophage cytoplasm, their translocation was independent of all three Salmonella gene-encoded type III secretion systems (T3SSs)-Salmonella pathogenicity island 1 (SPI-1) T3SS, SPI-2 T3SS, and the flagellar system. Selected methods, including direct microscopy, demonstrated that the PagK-homologous proteins were secreted through OMV, which were enriched with lipopolysaccharide (LPS) and outer membrane proteins. Vesicles produced by intracellular bacteria also contained lysosome-associated membrane protein 1 (LAMP1), suggesting the possibility of OMV convergence with host cellular components during intracellular trafficking. This study identified novel Salmonella virulence factors secreted via OMV and demonstrated that OMV can function as a vehicle to transfer virulence determinants to the cytoplasm of the infected host cell.

Salmonella pathogenicity island 1 (SPI-1) type III secretion of SopD involves N- and C-terminal signals and direct binding to the InvC ATPase

Salmonella enterica serovar Typhimurium (S. Typhimurium) is an important pathogen and a causative agent of gastroenteritis. During infection, S. Typhimurium assembles molecular-needle complexes termed type III secretion (T3S) systems to translocate effector proteins from the bacterial cytoplasm directly into the host cell. The T3S signals that direct the secretion of effectors still remain enigmatic. SopD is a key T3S effector contributing to the systemic virulence of S. Typhimurium and the development of gastroenteritis. We have scrutinized the distribution of the SopD T3S signals using in silico analysis and a targeted deletion approach. We show that amino acid residues 6–10 act as the N-terminal secretion signal for Salmonella pathogenicity island 1 (SPI-1) T3S. Furthermore, we show that two putative C-terminal helical regions of SopD are essential for its secretion and also help prevent erroneous secretion through the flagellar T3S machinery. In addition, using protein–protein interaction assays, we have identified an association between SopD and the SPI-1 T3S system ATPase, InvC. These findings demonstrate that T3S of SopD involves multiple signals and protein interactions, providing important mechanistic insights into effector protein secretion.

Secretion and translocation signals and DspB/F-binding domains in the type III effector DspA/E of Erwinia amylovora

DspA/E is a type III effector of Erwinia amylovora, the bacterial pathogen that causes fire blight disease in roseaceous plants. This effector is indispensable for disease development, and it is translocated into plant cells. A DspA/E-specific chaperone, DspB/F, is necessary for DspA/E secretion and possibly for its translocation. In this work, DspB/F-binding sites and secretion and translocation signals in the DspA/E protein were determined. Based on yeast two-hybrid assays, DspB/F was found to bind DspA/E within the first 210 amino acids of the protein. Surprisingly, both DspB/F and OrfA, the putative chaperone of Eop1, also interacted with the C-terminal 1059 amino acids of DspA/E; this suggests another chaperone-binding site. Secretion and translocation assays using serial N-terminal lengths of DspA/E fused with the active form of AvrRpt2 revealed that at least the first 109 amino acids, including the first N-terminal chaperone-binding motif and DspB/F, were required for efficient translocation of DspA/E, although the first 35 amino acids were sufficient for its secretion and the presence of DspB/F was not required. These results indicate that secretion and translocation signals are present in the N terminus of DspA/E, and that at least one DspB/F-binding motif is required for efficient translocation into plant cells.

Application of a short, disordered N-terminal flagellin segment, a fully functional flagellar type III export signal, to expression of secreted proteins

Recently, we have demonstrated that the 26-47 segment of Salmonella enterica serovar Typhimurium flagellin is capable of mediating flagellar export. In order to reveal whether other parts of the N-terminal region have any significant influence on secretion, a series of plasmids were constructed containing the lac promoter followed by the 26-47, 2-65, or 2-192 portion of Salmonella flagellin, to which various heterologous proteins of different size were fused (18 constructs overall). Essentially, all three segments could drive protein export; however, the nature of the attached polypeptide also had a significant effect on secretion efficiency. When low export efficiency was observed, it was mainly caused by inclusion body formation. Our data provide strong support for the idea that a short segment within the disordered N-terminal region of axial proteins is recognized by the flagellar type III export machinery. The 26-47 segment of flagellin contains all of the necessary information to direct translocation of attached polypeptide chains. This short (positions 26 to 47) flagellin segment attached to recombinant proteins can be used for secreted protein expression. Certain fusion proteins that are easily degraded within the cells were found to be intact in the medium, implying a potential application of this expression system for proteins with high proteolytic susceptibility.

Functional analysis of the N terminus of the Erwinia amylovora secreted effector DspA/E reveals features required for secretion, translocation, and binding to the chaperone DspB/F

DspA/E is a type III secreted effector protein required for pathogenicity in the apple and pear pathogen Erwinia amylovora, and DspB/F is a small chaperone protein involved in DspA/E secretion. While the secretion and translocation signals of many type III secretion effector proteins in human enteric pathogens have been characterized extensively, relatively little is known about the translocation requirements of many effectors in plant pathogens, including large DspE-like proteins. In this study, we report a functional analysis of the N terminus of DspE. The minimal requirements for secretion, translocation, and chaperone binding were characterized. Translocation assays using an adenylate cyclase (CyaA) reporter indicated that the first 51 amino acids of DspE were sufficient for translocation and that 150 amino acids were required for optimal translocation levels. The minimal translocation signal corresponded with the requirements for secretion into culture media. Mutations of conserved regions in amino acids 2 through 10 and 31 through 40 were found to influence translocation levels of an N-terminal DspE-CyaA fusion. Yeast two-hybrid and in-vitro pull-down assays revealed a chaperone-binding site within amino acids 51 through 100 of DspE and binding to DspF in this region was disrupted by specific mutations. However, neither disruption of the chaperone-binding domain nor deletion of the dspF gene had a significant impact on translocation levels of N-terminal DspE-CyaA fusions. Our results indicate that the minimal translocation signal of DspE is not coincident with the signal for DspF binding and that translocation of the N terminus of DspE is not dependent on the N-terminal DspF-binding domain.

Secretion of flagellin by the LEE-encoded type III secretion system of enteropathogenic Escherichia coli

Background

Enteropathogenic Escherichia coli (EPEC) is an attaching and effacing (A/E) pathogen that possesses a type III secretion system (T3SS) encoded within the locus of enterocyte effacement (LEE). The LEE is essential for A/E lesion formation and directs the secretion and translocation of multiple LEE-encoded and non-LEE encoded effector proteins into the cytosol of infected cells. In this study we used proteomics to compare proteins exported to the culture supernatant by wild type EPEC E2348/69, a ΔespADB mutant and a ΔescF T3SS mutant.

Results

We observed that flagellin was consistently and strongly present in the secretome of wild type EPEC and the ΔespADB mutant but present only weakly in the secretome of the ΔescF mutant. Given the ancestral relationship between the flagella export apparatus and virulence associated T3SSs, we investigated whether FliC could utilise the LEE-encoded T3SS for export. In the absence of a functional flagella export apparatus, we showed that FliC could be secreted by the LEE-encoded T3SS and stimulate (Toll-like receptor 5) TLR5 signalling but could not confer motility.

Conclusion

Since the secretion of FliC during A/E lesion formation would presumably be disadvantageous for the pathogen, we propose that virulence associated T3SSs and flagella T3SSs have evolved through a system of chaperones and complex regulatory pathways to be functional at different times to ensure that FliC secretion does not occur during T3SS effector translocation.

Essential role of the SycP chaperone in type III secretion of the YspP effector

The Ysa type III secretion (T3S) system enhances gastrointestinal infection by Yersinia enterocolitica bv. 1B. One effector protein targeted into host cells is YspP, a protein tyrosine phosphatase. It was determined in this study that the secretion of YspP requires a chaperone, SycP. Genetic analysis showed that deletion of sycP completely abolished the secretion of YspP without affecting the secretion of other Ysps by the Ysa T3S system. Analysis of the secretion and translocation signals of YspP defined the first 73 amino acids to form the minimal region of YspP necessary to promote secretion and translocation by the Ysa T3S system. Function of the YspP secretion/translocation signals was dependent on SycP. Curiously, when YspP was constitutively expressed in Y. enterocolitica bv. 1B, it was recognized and secreted by the Ysc T3S system and the flagellar T3S system. In these cases, the first 21 amino acids were sufficient to promote secretion, and while SycP did enhance secretion, it was not essential. However, neither the Ysc T3S system nor the flagellar T3S system translocated YspP into mammalian cells. This supports a model where SycP confers secretion/translocation specificities for YspP by the Ysa T3S system. A series of biochemical approaches further established that SycP specifically interacts with YspP and protected YspP degradation in the cell prior to secretion. Collectively, the evidence suggests that YspP secretion by the Ysa T3S system is a posttranslational event.

Hierarchical effector protein transport by the Salmonella Typhimurium SPI-1 type III secretion system

Background

Type III secretion systems (TTSS) are employed by numerous pathogenic and symbiotic bacteria to inject a cocktail of different “effector proteins” into host cells. These effectors subvert host cell signaling to establish symbiosis or disease.

Methodology/Principal Findings

We have studied the injection of SipA and SptP, two effector proteins of the invasion-associated Salmonella type III secretion system (TTSS-1). SipA and SptP trigger different host cell responses. SipA contributes to triggering actin rearrangements and invasion while SptP reverses the actin rearrangements after the invasion has been completed. Nevertheless, SipA and SptP were both pre-formed and stored in the bacterial cytosol before host cell encounter. By time lapse microscopy, we observed that SipA was injected earlier than SptP. Computer modeling revealed that two assumptions were sufficient to explain this injection hierarchy: a large number of SipA and SptP molecules compete for transport via a limiting number of TTSS; and the TTSS recognize SipA more efficiently than SptP.

Conclusions/Significance

This novel mechanism of hierarchical effector protein injection may serve to avoid functional interference between SipA and SptP. An injection hierarchy of this type may be of general importance, allowing bacteria to precisely time the host cell manipulation by type III effectors.

Sorting of early and late flagellar subunits after docking at the membrane ATPase of the type III export pathway

The bacterial flagellum assembles in a strict order, with structural subunits delivered to the growing flagellum by a type III export pathway. Early rod-and-hook subunits are exported before completion of the hook, at which point a subunit-specificity switch allows export of late filament subunits. This implies that in bacteria with multiple flagella at different stages of assembly, each export pathway can discriminate and sort unchaperoned early and chaperoned late subunits. To establish whether subunit sorting is distinct from subunit transition from the cytosol to the membrane, in particular docking at the membrane-associated FliI ATPase, the pathway was manipulated in vivo. When ATP hydrolysis by the FliI ATPase was disabled and when the pathway was locked into an early export state, both unchaperoned early and chaperoned late subunits stalled and accumulated at the inner membrane. Furthermore, a chaperone that attenuates late subunit export by stalling when docked at the wild-type ATPase also stalled at the ATPase in an early-locked pathway and inhibited export of early subunits in both native and early-locked pathways. These data indicate that the pathways for early and late subunits converge at the FliI ATPase, independent of ATP hydrolysis, before a distinct, separable sorting step. To ascertain the likely signals for sorting, the export of recombinant subunits was assayed. Late filament subunits unable to bind their chaperones were still sorted accurately, but chaperoned late subunits were directed through an early-locked pathway when fused to early subunit N-terminal export signal regions. Furthermore, while an early subunit signal directed export of a heterologous type III export substrate through both native and early-locked pathways, a late subunit signal only directed export via native pathways. These data suggest that subunits are distinguished not by late chaperones but by N-terminal export signals of the subunits themselves.

Minimal YopB and YopD translocator secretion by Yersinia is sufficient for Yop-effector delivery into target cells

Pathogenic Yersinia sp. utilise a common type III secretion system to translocate several anti-host Yop effectors into the cytosol of target eukaryotic cells. The secreted YopB and YopD translocator proteins are essential for this process, forming pores in biological membranes through which the effectors are thought to gain access to the cell interior. The non-secreted cognate chaperone, LcrH, also plays an important role by ensuring pre-secretory stabilisation and efficient secretion of YopB and YopD. This suggests that LcrH-regulated secretion of the translocators could be used by Yersinia to control effector translocation levels. We collected several LcrH mutants impaired in chaperone activity. These poorly bound, stabilised and/or secreted YopB and YopD in vitro. However, these mutants generally maintained stable substrates during a HeLa cell infection and these infected cells were intoxicated by translocated effectors. Surprisingly, this occurred in the absence of detectable YopB- and YopD-dependent pores in eukaryotic membranes. A functional type III translocon must therefore only require minuscule amounts of secreted translocator proteins. Based on these observations, LcrH dependent control of translocation via regulated YopB and YopD secretion would need to be exquisitely tight.

Structure-based mutagenesis of SigE verifies the importance of hydrophobic and electrostatic residues in type III chaperone function

Despite sharing little sequence identity, most type III chaperones display a similar homodimeric structure characterized by negative charges distributed broadly over their entire surface, interspersed with hydrophobic patches. Here we have used SigE from Salmonella as a model for class IA type III chaperones to investigate the role of these surface-exposed residues in chaperone function. SigE is essential for the stability, secretion and translocation of its cognate effector, SopB (SigD). We analysed the effect of mutating nine conserved hydrophobic and electronegative surface-exposed amino acids of SigE on SopB binding, stability, secretion and translocation. Six of these mutations affected some aspect of SigE function (Leu14, Asp20, Leu22, Leu23, Ile25 and Asp51) and three were without effect (Leu54, Glu92 and Glu99). Our results highlight that both hydrophobic and electronegative surfaces are required for the function of SigE and provide an important basis for the prediction of side-chain requirements for other chaperone-effector pairs.