Bacteria of Shigella spp. are responsible for shigellosis in humans. They use a type III secretion system to inject effector proteins into host cells and induce their entry into epithelial cells or trigger apoptosis in macrophages. We present evidence that the effector OspG is a protein kinase that binds various ubiquitinylated ubiquitin-conjugating enzymes, including UbcH5, which belongs to the stem cell factor SCF ␤-TrCP complex promoting ubiquitination of phosphorylated inhibitor of NF-B type ␣ (phospho-I B␣). Transfection experiments indicated that OspG can prevent phospho-I B␣ degradation and NF-B activation induced by TNF-␣ stimulation. Infection of epithelial cells by the S. flexneri wild-type strain, but not an ospG mutant, led to accumulation of phospho-I B␣, consistent with OspG inhibiting SCF ␤-TrCP activity. Upon infection of ileal loops in rabbits, the ospG mutant induced a stronger inflammatory response than the wild-type strain. This finding indicates that OspG negatively controls the host innate response induced by S. flexneri upon invasion of the epithelium.T he intestinal barrier is endowed with detection and defense mechanisms to achieve tolerance to commensal microorganisms and protection against invading microorganisms (1). Invasion by extracellular and intracellular pathogens is sensed by various signaling pathways converging to activate NF-B, a member of the Rel family of transcription factors involved in the activation of a large number of genes in response to pathogens, stress signals and proinflammatory cytokines (2). Under nonstimulating conditions, NF-B is retained in the cytoplasm through its association with inhibitory proteins (I Bs). A variety of signaling pathways activate I B kinases to phosphorylate I Bs, leading to ubiquitination of phospho-I Bs and their degradation by the proteasome (3), which allows translocation of NF-B to the nucleus, activation of NF-B-regulated genes, and establishment of an inflammatory response.Ubiquitination, resulting in the covalent attachment of the 76-residue ubiquitin to target proteins, involves three sequential steps performed by one ubiquitin-activating enzyme (E1), a limited number of ubiquitin-conjugating enzymes (E2s; also known as Ubc in enzyme designations), and a large number of ubiquitin-ligating enzymes (E3s), respectively (4). Each E3 recognizes a set of substrates and cooperates with one or a few E2s. The E3 complex SCF ␤-TrCP , which promotes ubiquitination of phospho-I B␣, consists of five proteins: the scaffold protein Cullin1, the adaptor protein Skp1, the RING domain protein Roc1, the E2 UbcH5b, and the F box protein ␤-TrCP, which interacts with phospho-I B␣ (5).Bacteria of Shigella spp. are the agent of shigellosis in humans, a disease characterized by the destruction of the colonic epithelium that is responsible for 1 million deaths per year (6). These bacteria use a type III secretion (TTS) system to enter epithelial cells and trigger apoptosis in macrophages (7). TTS systems comprise (i) a secretion apparatus that spans the...

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