MODULATION OF THE NF-KAPPA B SIGNALING PATHWAY BY THE BACTERIAL TYPE III SECRETION SYSTEM EFFECTORS

The type III secretion system (T3SS) is a bacterial injection system expressed by many Gram-negative bacteria. During the last two decades, the repertoire of T3SS effectors has been greatly explored, and several mechanisms of these effectors have been discovered. The identified host targets of T3SS effectors are involved in different biological events including cytoskeleton rearrangement, cellular signaling, transcription and protein degradation. A/E (attaching and effacing) pathogens including EHEC (Enterohaemorrhaigic E. coli), EPEC (Enteropathogenic E. coli) and C. rodentium (Citrobacter rodentium), a pathogen of mice, inhibit NF-κB transcriptional activity by employing unidentified T3SS effectors. However, the identity of these effector(s) was unknown. In this thesis, my goals were to identify T3SS effectors from attaching and effacing (A/E) pathogens responsible for modulating NF-κB activation and reveal the working mechanism of these identified effectors. In the first project, NleH1 and NleH2, which share C-termini similarity with the S. flexneri T3SS effector OspG, were studied. OspG targets ubiquitin-conjugating enzyme UbcH5 to prevent IκBα degredation, which results in the inhibition of NF-κB activation. We discovered that both NleH1 and NleH2 interact with the N-terminus of ribosomal protein S3 (RPS3) after their translocation into host cells. RPS3 is a non-Rel NF-κB subunit which promotes the DNA binding affinity of NF-κB. We found that NleH1, but not NleH2, blocks the nuclear translocation of RPS3 stimulated by TNF and by bacterial infection. By this process, NleH1 selectively attenuates RPS3-mediated NF-κB dependent gene transcription. In addition, we discovered NleH proteins as Ser/Thr kinases and that kinase activity is critical for the effect of NleH1 on RPS3. By collaborating with Dr. Lenardo's group in National Institute of Allergy and Infectious Diseases (NIAID), we discovered that RPS3 is inducibly associated with and phosphorylated by IKKβ at serine 209 (S209) and that NleH1 efficiently ...