Editors' ChoiceHost-Pathogen Interactions

Deactivated by Deamidation

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Sci. Signal.  03 Apr 2012:
Vol. 5, Issue 218, pp. ec101
DOI: 10.1126/scisignal.2003102

Infection by the pathogenic bacterium Shigella flexneri leads to rupture of cells in the intestine, which triggers acute inflammatory host responses that can be mediated by the nuclear factor κB (NF-κB) pathway. Sanada et al. identified Ospl as a bacterial effector protein that suppresses acute inflammatory responses to S. flexneri. The abundance of mRNAs encoding proinflammatory factors such as interleukin-8 (IL-8) and CXC-chemokine ligand 2 (CXCL2) was higher in HeLa cells infected with an ospl-deficient (Δospl) strain than in those infected with a wild-type strain. Cells infected with the Δospl strain also showed increased phosphorylation of inhibitor of NF-κB (IκBα) and increased nuclear translocation of the p65 subunit of NF-κB, which suggested that the activity of the NF-κB pathway was increased. Structural analysis revealed that Ospl is most closely related to the Pseudomonas syringae effector AvrPphB, which is a member of an enzyme superfamily that includes cysteine proteases (such as AvrPphB) and deamidases. Comparison of the structures of Ospl and AvrPphB identified a potential catalytic triad of amino acids (Cys62, His145, and Asp160). Forms of Ospl with alanine substitutions at these positions (C62A, H62A, or D160A) failed to prevent the increase in IκBα phosphorylation and IL8 abundance that occurred in cells infected with Δospl strains. Tumor necrosis factor (TNF)–receptor–associated factor 6 (TRAF6) is an E3 ubiquitin ligase that works with the ubiquitin-conjugated E2 enzymes UBC13 and UEV1A. Self-ubiquitination of TRAF6 was attenuated by the C62A Ospl mutant, and the low abundance of Cxcl2 in ΔospI-infected Traf6–/– mouse embryo fibroblasts was rescued by expression of wild-type TRAF6, but not a form of TRAF6 lacking ubiquitin ligase activity. Mass spectrometric analysis and in vitro assays revealed that Gln100 in UBC13 was deamidated to glutamate by Ospl, but not by the C62A, H62A, or D160A mutants. A form of UBC13 with a mutation that mimicked the deamidated form (Q100E) resulted in reduced self-ubiquitination of TRAF6 in an in vitro assay and decreased activity of an NF-κB reporter gene in cells. Thus, Ospl deamidates UBC13 to limit acute inflammatory host responses mediated by the NF-κB pathway.

T. Sanada, M. Kim, H. Mimuro, M. Suzuki, M. Ogawa, A. Oyama, H. Ashida, T. Kobayashi, T. Koyama, S. Nagai, Y. Shibata, J. Gohda, J.-i. Inoue, T. Mizushima, C. Sasakawa, The Shigella flexneri effector OspI deamidates UBC13 to dampen the inflammatory response. Nature 483, 623–626 (2012). [PubMed]

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