Recent bioinformatic analyses identified an operon encoding a potential Type I Secretion System (T1SS) in Treponema pallidum that we hypothesize functions to export key treponemal virulence factors that may contribute to the unique invasiveness and pathogenesis of this spirochete. The membrane fusion protein component (MFP) of T1SSs in other organisms has been shown to play a role in substrate recognition. Hence, the objective of this project is to use the putative MFP, Tp0965, of the potential T. pallidum T1SS to investigate protein-protein interactions with the T. pallidum virulence factor pallilysin (Tp0751) and assess the possibility of the latter being a T1SS substrate. Moreover, protein-protein interactions between Tp0965 and a Treponema phagedenis lysate are investigated with the goal of identifying putative T1SS substrates in this spirochete that could result in the discovery of novel T. pallidum virulence factors via amino acid sequence similarity. Plate-based binding studies and pull-down assays showed a low level of interaction between recombinant Tp0965 and the previously characterized host-component-binding protease, pallilysin, suggesting that the export of this virulence factor could occur via the putative T1SS. Additionally, bioinformatic analyses of the related but cultivable model spirochete T. phagedenis predicted the presence of a potential T1SS homologous to the putative T1SS in T. pallidum. Thus, a more global and unbiased pull-down assay using “bait” Tp0965 and a “prey” T. phagedenis lysate was carried out, followed by mass spectrometric analysis to identify putative novel T1SS substrates with potential homologs in T. pallidum. We successfully identified a T. phagedenis protein, TphBIg, that showed evidence of an interaction with Tp0965. TphBIg seems to possess characteristics of a T1SS substrate suggesting it may be secreted via this system in T. phagedenis. Upon bioinformatic analysis, it was found that TphBIg showed weak amino acid sequence similarity as well as some structural similarity to the T. pallidum protein, Tp0854. Tp0854 is predicted to contain a sialidase and a phosphatase domain with an RTX motif, which is characteristic of some T1SS substrates. Thus, it was hypothesized that if Tp0854 had characteristics of a T1SS, it may interact with Tp0965. Therefore, the phosphatase domain containing the RTX motif was produced recombinantly and plate-based binding studies indeed suggested an interaction with Tp0965, confirming the in silico-predicted interaction. Future experiments to characterize the potential T1SS and substrates in T. pallidum could comprise the functional and structural characterization of the novel putative T1SS substrate, Tp0854. This would include assays to investigate the putative sialidase and phosphatase activities of Tp0854, as well as the identification of Tp0854-Tp0965 interacting sites. Moreover, as a more definite test for T1SS substrate secretion, T. pallidum pallilysin and/or Tp0854 could be expressed heterologously in an E. coli strain harbouring an endogenous T1SS and test for secretion. Similarly, the reconstitution of the T. pallidum putative T1SS in liposomes could be used to further investigate the secretion of pallilysin and/or Tp0854 via this system. Additionally, the optimized unbiased pull-down technique could be further applied to detect more protein-protein interactions within T. pallidum and potentially lead to the identification of more virulence factors that may be secreted via the T1SS. These studies constitute the first investigation of a putative T1SS and substrates within T. pallidum. Thus, insight gained will lead to a better understanding of the mechanisms facilitating T. pallidum host invasion and may reveal new potential vaccine targets to prevent bacterial dissemination and chronic infection. / Graduate

Scrub typhus is a potentially fatal infection that threatens one billion persons in the Asia-Pacific region and is caused by the obligate intracellular bacterium, Orientia tsutsugamushi. How this organism facilitates its intracellular survival and pathogenesis is poorly understood. Intracellular bacterial pathogens utilize the Type 1 (T1SS) or Type 4 secretion system (T4SS) to translocate ankyrin repeat-containing proteins (Anks) into the host cell to modulate host cell processes. The O. tsutsugamushi genome encodes one of the largest known bacterial Ank libraries as well as Type 1 and Type 4 secretion systems (T1SS and T4SS), which are expressed during infection. In silico analyses of the Anks’ C-termini revealed that they possess characteristics of T1SS secretion signals. Escherichia coli expressing a functional T1SS was able to secrete chimeric hemolysin proteins bearing the C-termini of 19 of 20 O. tsutsugamushi Anks. In addition to infecting endothelial cells, O. tsutsugamushi infects professional phagocytes. To better understand why these innate immune cells are unable to eliminate O. tsutsugamushi, we addressed the activity of host NF-κB proinflammatory transcription factor. Screening of O. tsutsugamushi infected cells at an MOI of 1 revealed inhibition of NF-κB nuclear accumulation as early as 8 hours in HeLa and bone-marrow derived macrophage cells. When stimulating infected cells with TNF-α, IκBα degradation still occurs, however NF-κB dependent gene transcription remains downregulated. Immunofluorescence microscopic analysis of TNF-α treated cells ectopically expressing all O. tsutsugamushi Anks revealed that two nuclear trafficking Anks, Ank1 and Ank6, result in a significant decrease in NF-κB nuclear accumulation. Additionally, these Anks also significantly inhibited NF-κB dependent gene transcription. Co-immunoprecipitation experiments revealed that both Anks interact with importin-β1, exportin-1, and the p65 NF-κB subunit. Treating cells with importazole significantly reduces the nuclear accumulation of Ank1 and Ank6. Finally, treating infected cells or cells ectopically expressing Ank1 or Ank6 with leptomycin B resulted in restoration of NF-κB nuclear accumulation. With these data, we propose that O. tsutsugamushi secretes Ank1 and Ank6 to initially interact with importin-β1, which permits their nuclear entry where they then interact with NF-κB and subsequently exportin-1 to prevent NF-κB nuclear accumulation.

The adenylate cyclase toxin (CyaA) of Bordetella pertussis is a 1706-residue protein comprising an amino-terminal adenylate cyclase (AC) domain and a carboxy-terminal Repeat-in-Toxin (RTX) domain. The RTX domain is a hallmark of the family of RTX proteins, which are secreted from the cytosol of Gram-negative bacteria to the cell environment through the Type I Secretion System (T1SS). The RTX domain of CyaA consists of five blocks of RTX nonapetide repeats with a consensus sequence X-(L/I/V)-X-G-G-X-G- X-D. The aim of this work was to determine the role of the RTX domain in biological activities of CyaA and its role in the secretion of the toxin molecule from Bordetella pertussis. Systematic deletion analysis revealed that none of the prepared CyaA constructs was able to translocate its AC domain across the cytoplasmic membrane of host cells and make pores in target membranes. Moreover, deletion of individual RTX repeat blocks resulted in a very low efficacy of secretion of CyaA mutants into cell exterior. These data suggested that structural integrity of the RTX domain of CyaA is essential not only for cytotoxic activities of the toxin molecule but also for its secretion through the T1SS.