Figure 3.

The coordinated regulation of the host NF-κB pathway by L. pneumophila effectors via ubiquitin modulation. Upon bacterial infection, the NF-κB family of transcription factors play a central role in controlling the host immune response, such as the production of proinflammatory cytokines, including tumor necrosis factor α (TNFα) and interleukin-1 (IL-1) and the induction of host defense signaling (Li and Verma, 2002). The inhibitor of NF-κB (IκB) suppresses the activity of NF-κB by binding and preventing nuclear translocation. The NF-κB pathway is mainly mediated by the engagement of host immune receptors that can recognize pathogen-associated molecular pattern molecules of extracellular or intracellular bacteria. One of the downstream pathways in which TNFα-receptor-associated factor 6 (TRAF6) is involved, is modulated by a L. pneumophila effector, MavC. TRAF6 is an E3 ubiquitin ligase that functions with the E2 enzyme hetero-dimer complex UBE2N/UBE2V2 (also known as Ubc13/Uev1A) to induce its auto-polyubiquitination and polyubiquitination of other neighboring proteins via Lys63-linked ubiquitin chains (Wang et al., 2012). The Lys63-linked ubiquitin chains of TRAF6 serve as docking sites for TGF-β-activated kinase 1 (TAK1)-binding protein 2 (TAB2). TAB1 and TAB2 promote the recruitment of the serine-threonine kinase TAK1 into the complex to activate the pathway. Activated TAK1 phosphorylates IκB kinase β (IKKβ) to promote the proteasomal degradation of IκB, thereby allowing NF-κB nuclear translocation. TRAF6-mediated Lys63-linked ubiquitin chains are associated with the NF-κB essential modifier (NEMO, also known as IKKγ) in the IKKα/IKKβ/NEMO complex and play a crucial role in IKK signaling. TRAF6 also produces free Lys63-linked polyubiquitin chains, which directly activate TAK1 kinase by binding to TAB2. In this scheme, the inhibitory activity of MavC against UBE2N dampens the NF-κB signaling pathway (including many other signaling branches not shown in this figure) by preventing overall Lys63-linked ubiquitination. MvcA, the metaeffector of MavC, can counteract the activity of MavC in the later stage of infection, presumably to restore cellular conditions that optimize bacterial replication. RavD, the other L. pneumophila effector, functions as a DUB, and contributes to negative regulation of NF-κB signaling. The host multi-subunit E3 ligase LUBAC, composed of HOIL-1L, HOIP, and SHARPIN, catalyzes the formation of Met1-linked linear ubiquitin chains. NEMO harbors a Met1 ubiquitin-specific binding domain that is important for NF-κB signaling (Komander et al., 2009; Rahighi et al., 2009). RavD, which is similar to OTULIN (Keusekotten et al., 2013), cleaves LUBAC-assembled linear ubiquitin chains, thereby inhibiting the host NF-κB response.