TheShigella flexnerieffector OspG interferes with innate immune responses by targeting ubiquitin-conjugating enzymes

Abstract

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...

Mycobacterium tuberculosis Mce3E Suppresses Host Innate Immune Responses by Targeting ERK1/2 Signaling

“…For example, the Yersinia effector YopJ, which is a ubiquitin-like cysteine protease, targets and downregulates both the NF-kB and MAPK pathways (42). Shigella flexneri effector OspG, which is a protein kinase, targets ubiquitin-conjugated enzymes, thereby affecting phosphoIkBa degradation and subsequent NF-kB activation (43). OspF, another Shigella type III effector, was also shown to inactivate MAPKs, including ERK1/2, JNK, and p38, by irreversibly removing phosphate groups from the phosphothreonine, but not from the phosphotyrosine residue in the activation loop of MAPKs (25).…”

Salmonella Secreted Factor L Deubiquitinase of Salmonella typhimurium Inhibits NF-κB, Suppresses IκBα Ubiquitination and Modulates Innate Immune Responses

“…OspG, a kinase produced by S. flexneri also prevents phospho-I B␣ degradation and subsequent NF-B activation. In accordance with the phenotype of mice infected with ⌬sseL Salmonella, inactivation of OspG increases the host inflammatory response to S. flexneri infection (14).…”

“…For example, AvrA, the bacterial effector from Salmonella, deubiquitinates I B␣ inhibiting NF-B activation (13). The OspG protein kinase of Shigella flexneri has also been shown to bind various ubiquitin-conjugating enzymes belonging to the SCF ␤ -TrCP complex, and prevent phospho-I B␣ degradation (14). Another example of a bacterial virulence factor that interferes with NF-B activation is the YopJ protein expressed in Yersinia species.…”

The Inside Story of Shigella Invasion of Intestinal Epithelial Cells

“…1) (Dong et al 2012). Six T3S effectors have been reported to directly or indirectly inhibit the activation of the proinflammatory transcription factor NF-k. OspF removes phosphothreonine residues on the p38 and Erk2 MAP kinases, preventing their activation (Arbibe et al 2007;Li et al 2007); OspG prevents the ubiquitinylation of IkBa and therefore its degradation, by targeting ubiquitinylated ubiquitin-conjugating enzymes (Kim et al 2005); OspI deamidates Ubc13 to prevent its ubiquitin ligase activity and TRAF-6 signaling (Sanada et al 2012); OspZ is related to the EPEC NleE T3S effector, that prevents NF-k translocation through the cystein methylation of ubiquitin chain-sensing protein (Newton 2010; Zhang et al 2011); the IpaH proteins are E3 ubiquitin ligases. IpaH4.5 ubiquitinates the p65 subunit of NF-kB (Wang et al 2013); IpaH9.8 promotes the polyubiquitination of NEMO/IkB kinase and its degradation (Fig.…”