The Amino-Terminal Part of the Needle-Tip Translocator LcrV of Yersinia pseudotuberculosis Is Required for Early Targeting of YopH and In vivo Virulence

Macrophage innate immune responses delineate between defective translocon assemblies produced by Yersinia pseudotuberculosis YopD mutants

Translocon pores formed in the eukaryotic cell membrane by a type III secretion system facilitate the translocation of immune-modulatory effector proteins into the host cell interior. The YopB and YopD proteins produced and secreted by pathogenic Yersinia spp. harboring a virulence plasmid-encoded type III secretion system perform this pore-forming translocator function. We had previously characterized in vitro T3SS function and in vivo pathogenicity of a number of strains encoding sited-directed point mutations in yopD. This resulted in the classification of mutants into three different classes based upon the severity of the phenotypic defects. To investigate the molecular and functional basis for these defects, we explored the effectiveness of RAW 264.7 cell line to respond to infection by representative YopD mutants of all three classes. Signature cytokine profiles could separate the different YopD mutants into distinct categories. The activation and suppression of certain cytokines that function as central innate immune response modulators correlated well with the ability of mutant bacteria to alter anti-phagocytosis and programmed cell death pathways. These analyses demonstrated that sub-optimal translocon pores impact the extent and magnitude of host cell responsiveness, and this limits the capacity of pathogenic Yersinia spp. to fortify against attack by both early and late arms of the host innate immune response.

PopB-PcrV Interactions Are Essential for Pore Formation in the Pseudomonas aeruginosa Type III Secretion System Translocon

The type III secretion system (T3SS) is a syringe-like virulence factor that delivers bacterial proteins directly into the cytoplasm of host cells. An essential component of the system is the translocon, which creates a pore in the host cell membrane through which proteins are injected. In Pseudomonas aeruginosa, the translocation pore is formed by proteins PopB and PopD and attaches to the T3SS needle via the needle tip protein PcrV. The structure and stoichiometry of the multimeric pore are unknown. We took a genetic approach to map contact points within the system by taking advantage of the fact that the translocator proteins of P. aeruginosa and the related Aeromonas hydrophila T3SS are incompatible and cannot be freely exchanged. We created chimeric versions of P. aeruginosa PopB and A. hydrophila AopB to intentionally disrupt and restore protein-protein interactions. We identified a chimeric B-translocator that specifically disrupts an interaction with the needle tip protein. This disruption did not affect membrane insertion of the B-translocator but did prevent formation of the translocation pore, arguing that the needle tip protein drives the formation of the translocation pore. IMPORTANCE Type III secretion systems are integral to the pathogenesis of many Gram-negative bacterial pathogens. A hallmark of these secretion systems is that they deliver effector proteins vectorially into the targeted host cell via a translocation pore. The translocon is crucial for T3SS function, but it has proven difficult to study biochemically and structurally. Here, we used a genetic approach to identify protein-protein contacts among translocator proteins that are important for function. This genetic approach allowed us to specifically break a contact between the translocator PopB and the T3SS needle tip protein PcrV. Breaking this contact allowed us to determine, for the first time, that the needle tip actively participates in the assembly of the translocation pore by the membrane-bound pore-forming translocator proteins. Our study therefore both expands our knowledge of the network of functionally important interactions among translocator proteins and illuminates a new step in the assembly of this critical host cell interface.

Identification of specific sequence motif of YopN of Yersinia pseudotuberculosis required for systemic infection

Type III secretion systems (T3SSs) are tightly regulated key virulence mechanisms shared by many Gram-negative pathogens. YopN, one of the substrates, is also crucial in regulation of expression, secretion and activation of the T3SS of pathogenic Yersinia species. Interestingly, YopN itself is also targeted into host cells but so far no activity or direct role for YopN inside host cells has been described. Recently, we were able show that the central region of YopN is required for efficient translocation of YopH and YopE into host cells. This was also shown to impact the ability of Yersinia to block phagocytosis. One difficulty in studying YopN is to generate mutants that are not impaired in regulation of the T3SS. In this study we extended our previous work and were able to generate specific mutants within the central region of YopN. These mutants were predicted to be crucial for formation of a putative coiled-coil domain (CCD). Similar to the previously described deletion mutant of the central region, these mutants were all impaired in translocation of YopE and YopH. Interestingly, these YopN variants were not translocated into host cells. Importantly, when these mutants were introduced in cis on the virulence plasmid, they retained full regulatory function of T3SS expression and secretion. This allowed us to evaluate one of the mutants, yopN_GAGA, in the systemic mouse infection model. Using in vivo imaging technology we could verify that the mutant was also attenuated in vivo and highly impaired to establish systemic infection.

YopN Is Required for Efficient Effector Translocation and Virulence in Yersinia pseudotuberculosis

Type III secretion systems (T3SSs) are used by various Gram-negative pathogens to subvert the host defense by a host cell contact-dependent mechanism to secrete and translocate virulence effectors. While the effectors differ between pathogens and determine the pathogenic life style, the overall mechanism of secretion and translocation is conserved. T3SSs are regulated at multiple levels, and some secreted substrates have also been shown to function in regulation. In Yersinia, one of the substrates, YopN, has long been known to function in the host cell contact-dependent regulation of the T3SS. Prior to contact, through its interaction with TyeA, YopN blocks secretion. Upon cell contact, TyeA dissociates from YopN, which is secreted by the T3SS, resulting in the induction of the system. YopN has also been shown to be translocated into target cells by a T3SS-dependent mechanism. However, no intracellular function has yet been assigned to YopN. The regulatory role of YopN involves the N-terminal and C-terminal parts, while less is known about the role of the central region of YopN. Here, we constructed different in-frame deletion mutants within the central region. The deletion of amino acids 76 to 181 resulted in an unaltered regulation of Yop expression and secretion but triggered reduced YopE and YopH translocation within the first 30 min after infection. As a consequence, this deletion mutant lost its ability to block phagocytosis by macrophages. In conclusion, we were able to differentiate the function of YopN in translocation and virulence from its function in regulation.

Immunomodulatory Yersinia outer proteins (Yops)-useful tools for bacteria and humans alike

Human-pathogenic Yersinia produce plasmid-encoded Yersinia outer proteins (Yops), which are necessary to down-regulate anti-bacterial responses that constrict bacterial survival in the host. These Yops are effectively translocated directly from the bacterial into the target cell cytosol by the type III secretion system (T3SS). Cell-penetrating peptides (CPPs) in contrast are characterized by their ability to autonomously cross cell membranes and to transport cargo - independent of additional translocation systems. The recent discovery of bacterial cell-penetrating effector proteins (CPEs) - with the prototype being the T3SS effector protein YopM - established a new class of autonomously translocating immunomodulatory proteins. CPEs represent a vast source of potential self-delivering, anti-inflammatory therapeutics. In this review, we give an update on the characteristic features of the plasmid-encoded Yops and, based on recent findings, propose the further development of these proteins for potential therapeutic applications as natural or artificial cell-penetrating forms of Yops might be of value as bacteria-derived biologics.