Biochemical functions of Yersinia type III effectors

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Yersinia uses a type III secretion system (TTSS) to deliver six effector proteins into host cells. These six proteins harbor distinct activities that are mimicries of host functions but often have acquired unique biochemical features. The host targets for these effectors appear to be limited to a few key signaling components such as G proteins and kinases, whereas their models of action are diverse and sophisticated. The functions of these effectors are to subvert the host immune defense response, including alterations of the cytoskeleton structure, inhibition of phagocytic clearance, blockage of cytokine production, and induction of apoptosis. These effectors also interfere with communications between the innate and the adaptive immune response, thus aiding the establishment of a systemic infection.

Introduction

The genus Yersinia includes three genetically closely related pathogenic species, Yersiniapestis, Yersiniaenterocolitica, and Yersiniapseudotuberculosis. While Y. pestis is best known as the causative agent of the notorious plague, infection of the latter two in human causes gastroenteritis. The enteropathogenic Yersinia has a strong tropism for lymphoid tissues known as Peyer's patches in the submucosa after penetrating through the M cells in gastrointestinal mucosa. All three Yersinia species have evolved antiphagocytosis mechanism to combat professional and nonprofessional phagocytic cells. Infection also leads to downregulation of the host innate immune signaling pathways. These allow the bacteria to survive and proliferate extracellularly and to establish a systemic infection.

Innate immunity is the first line of defense against invading bacteria pathogens [1]. Upon detection of pathogen components known as pathogen-associated molecular patterns (PAMPs) by a group of sensor proteins referred to pattern-recognition receptors (PRRs), one key response of the host innate immune system is to activate macrophages and dendritic cells to phagocytose and degrade pathogens. The phagocytosis process requires reorganization of actin cytoskeleton structure controlled by Rho family of small GTPases, including RhoA, Rac, and Cdc42. Induction of proinflammatory cytokines such as interleukins and tumor necrosis factor (TNF) is also a crucial component of the innate immune system to eliminate pathogenic bacteria and keep the infection under control. A large body of research has demonstrated the importance of key signal transduction pathways such as mitogen-activated protein kinases (MAPKs) pathway and the nuclear factor-κB (NF-κB) signaling in regulating cytokine production. Activation of the innate immune system can also signal and alarm the adaptive immune system, which mounts responses that further facilitates clearance of the bacteria. Not surprisingly, bacteria pathogens have evolved fine, but diverse, strategies to target and subvert the host innate immune signaling pathways [2]. Yersinia is not an exception in this regard. All the pathogenic species of Yersinia harbor a type III secretion system encoded on a 70-kb virulence plasmid, which functions to thwart the host innate immune function and is essential for Yersinia pathogenicity [3].

The TTSS is a multimeric protein translocation channel that injects effector proteins into host cells. The TTSS is thought to originate from the bacterial flagella export system as homologous structural components exist in both systems. The TTSS is highly conserved in a wide spectrum of Gram-negative bacteria that are pathogenic to animals and plants as well as in symbiotic bacteria like Rhizobium. Effector proteins translocated by the Yersinia TTSS into host cells are referred to the Yersinia outer proteins (Yops). Six Yop effectors (YopH, YpkA/YopO, YopE, YopT, YopJ, and YopM) have been identified, and five of them (except for YopM) have clear biochemical understandings now (Figure 1). The major functions of these Yop effectors during Yersinia pathogenesis are to counteract the host phagocytosis and induce macrophage cell death as well as to suppress the cytokine production [3]. Interestingly, redundant pathogenic functions for different Yop effectors are evident from reported studies. For example, three effectors (YopE, YopT, and YpkA) target Rho GTPases and play a role in altering the cytoskeleton rearrangement processes during phagocytosis. This is probably due to the lack of approaches to dissect functions of individual effectors at a fine temporal and spatial resolution during infection. In addition, animal models that recapitulate the disease process are also needed to better understand functions of these Yops under a more physiological context. In addition to the plasmid-borne TTSS, Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica 1B strains also have chromosome-encoded TTSSs [3]. The chromosomal Ysa TTSS in Y. enterocolitica 1B is shown to be required for efficient colonization in gastrointestinal tissues. Recent proteomic efforts have led to identification of a group of effectors translocated by the Ysa system, which also includes some of Yop effectors such as YopE [4]. However, few of effectors translocated by the chromosomal TTSSs are functionally characterized so far and are not covered in this review.

In this review, I will discuss molecular functions of the six Yop effectors with the emphasis on their biochemical properties (Figure 1) and modulation of their host targets (Figure 2). Comprehensive information about how Yersinia utilizes the activities of those Yops to interfere with various aspects of the host function can be found in a recent review [3].

Section snippets

Dephosphorylation of the focal adhesion complex by the tyrosine phosphatase YopH

YopH was first identified as an essential component of the Yersinia ‘antiphagocytic’ mechanism before it was shown as a TTSS effector [3]. The 51 kDa YopH protein is one of the first enzymes demonstrated to possess protein tyrosine phosphatase (PTPase) activity, and it has the highest specific activity among all the PTPases examined so far. The biochemical properties of YopH have been extensively characterized (Figure 1). YopH contains an N-terminal type III secretion signal (residues 1–17) and

Targeting both the small G protein and trimeric G protein by the Ser/Thr kinase YpkA

YpkA is the first ‘eukaryotic-like’ Ser/Thr kinase identified from pathogenic bacteria [3]. Mutation of YpkA in Y. pseudotuberculosis results in an avirulent strain in mice, suggesting that YpkA is an indispensable virulence determinant. Translocated YpkA is targeted to the inner surface of the plasma membrane in HeLa cells and induces a rounding up of the cell without detachment. Ectopic expression in HeLa cells recapitulates the morphological phenotype, which reveals the disruption of actin

YopE: a prototype of the host-mimicking bacterial RhoGAP family

YopE is one of the first identified Yersinia effectors and is an essential virulence determinant in a mouse infection model [3]. Translocation of YopE into the host cells results in cell rounding up and detachment through depolymerization of actin microfilamentous. Consistently, YopE plays an important role for the antiphagocytic function of Yersinia. Biochemically, YopE is a 23 kDa protein with an N-terminal type III secretion signal followed by a chaperone-binding region and a C-terminal

The cysteine protease YopT cleaves prenylated Rho GTPases

The 36 kDa YopT is the last identified Yop effector with least amounts of secretion compared with other Yops [3]. Translocation of YopT requires the N-terminal type III secretion signal and chaperone-binding domain, and results in a cytotoxic effect, characterized by a cell rounding up phenotype and disruption of actin filamentous structure. An acidic shift of pI of RhoA was observed in infected HeLa cells, suggesting a possible modification of RhoA induced by YopT. Rho GTPases are known to

Ser/Thr acetylation of MKKs and IKK by YopJ prevents their phosphorylation

The 32.5 kDa YopJ (called YopP in Y. enterocolitica) is the primary Yop effector that plays the anti-inflammatory role and the only known one that induces apoptosis in macrophage [28]. Numerous studies have firmly established that YopJ, by inactivating multiple MAPK pathways as well as the NF-κB pathway, inhibits the production of cytokines such as TNFα and IL8. Elucidation of the underlying biochemical mechanism of YopJ function has been an active research topic in the field. The first crucial

The mysterious leucine-rich repeat (LRR) protein YopM

YopM was initially identified in Y. pestis KIM5 strain as a secreted extracellular factor and an essential virulence factor in a mouse model of septicemic plague [3]. YopM was then proposed to bind to thrombin and inhibit thrombin-induced platelet activation during plague pathogenesis. However, YopM was later found to be delivered into the host cells through the Yersinia TTSS. This brings into question the original proposal that YopM inhibits thrombin function as a released extracellular

Conclusions

Yersinia has evolved an array of effectors with diverse biochemical properties (Figure 1). The canonical eukaryotic Ser/Thr kinase and tyrosine phosphatase activities for YpkA and YopH, respectively, probably originate from a horizontal transfer mechanism. Convergent evolution leads to the generation of a bacterial RhoGAP (YopE) and a RhoGDI-like activity (YpkA). Interestingly, the general cysteine protease catalytic folds are also exploited by the bacteria to meet their own need for specific

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

The author apologizes to colleagues whose work could not be cited owing to space limitations. Work in the author's lab is supported by Chinese Ministry of Science and Technology ‘863’ Grant 2005AA210950 and 973 National Basic Research Plan of China 2006CB806502.

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