bEnterovirus 71 (EV71) is a positive-stranded RNA virus which is capable of inhibiting innate immunity. Among virus-encoded proteins, the 3C protein compromises the type I interferon (IFN-I) response mediated by retinoid acid-inducible gene-I (RIG-I) or Toll-like receptor 3 that activates interferon regulatory 3 (IRF3) and IRF7. In the present study, we report that enterovirus 71 downregulates IRF7 through the 3C protein, which inhibits the function of IRF7. When expressed in mammalian cells, the 3C protein mediates cleavage of IRF7 rather than that of IRF3. This process is insensitive to inhibitors of caspase, proteasome, lysosome, and autophagy. H40D substitution in the 3C active site abolishes its activity, whereas R84Q or V154S substitution in the RNA binding motif has no effect. Furthermore, 3C-mediated cleavage occurs at the Q189-S190 junction within the constitutive activation domain of IRF7, resulting in two cleaved IRF7 fragments that are incapable of activating IFN expression. Ectopic expression of wild-type IRF7 limits EV71 replication. On the other hand, expression of the amino-terminal domain of IRF7 enhances EV71 infection, which correlates with its ability to interact with and inhibit IRF3. These results suggest that control of IRF7 by the 3C protein may represent a viral mechanism to escape cellular responses.

Innate immunity provides the first line of host defense against invading microbial pathogens. This defense involves retinoic acidinducible gene-I-like receptors that detect viral RNA and activate the mitochondrial antiviral-signaling (MAVS) protein, an adaptor protein, leading to activation of the innate antiviral immune response. The mechanisms by which the MAVS signalosome assembles on mitochondria are only partially understood. Here, we identify tripartite motif 14 (TRIM14) as a mediator in the immune response against viral infection. TRIM14 localizes to the outer membrane of mitochondria and interacts with MAVS. Upon viral infection, TRIM14 undergoes Lys-63-linked polyubiquitination at Lys-365 and recruits NF-κB essential modulator to the MAVS signalosome, leading to the activation of both the IFN regulatory factor 3 and NF-κB pathways. Knockdown of TRIM14 disrupts the association between NF-κB essential modulator and MAVS and attenuates the antiviral response. Our results indicate that TRIM14 is a component of the mitochondrial antiviral immunity that facilitates the immune response mediated by retinoic acidinducible gene-I-like receptors.A ctivation of the innate immune response involves the detection of pathogen-associated molecular patterns (PAMPs), such as microbial nucleic acids, proteins, lipids, and carbohydrates. PAMPs are recognized by cellular pattern recognition receptors (PRRs), including Toll-like receptors, retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), NOD-like receptors, and C-type lectin receptors. Upon recognition, PRRs trigger a series of signaling events that lead to the induction of type I IFNs and proinflammatory cytokines (1).RLRs such as RIG-I and melanoma differentiation-associated antigen 5 (MDA5) recognize cytosolic viral RNA (2). Upon binding of RNA to the helicase domain, RIG-I or MDA5 undergoes a conformational change (3) and is recruited to the mitochondrial antiviral signaling (MAVS) adaptor. After binding of RIG-1 or MAD5, MAVS recruits various downstream molecules and further activates two kinase complexes: the noncanonical IκB kinases (IKKs) [TANK-binding kinase 1 (TBK1)/IKKi] and the canonical IKK complexes comprised of IKKα, IKK-β, and NF-κB essential modulator (NEMO) (4, 5). The TBK1/IKKi kinases phosphorylate IFN regulatory factor 3/7 (IRF3/7), which translocates to the nucleus and drives the transcription of IFNs (6). The canonical IKKs phosphorylate IκBα, resulting in the ubiquitination and proteasomal degradation of IκBα. NF-κB then is released to the nucleus and stimulates the expression of proinflammatory genes (7).MAVS-deficient mice show abolished virus-triggered induction of IFNs and increased susceptibility to viral infection (8), indicating that MAVS is essential for the innate immune response. MAVS consists of an N-terminal caspase activation and recruitment domain, a proline-rich domain, and a C-terminal transmembrane domain that targets it to the mitochondrial outer membrane (9). It has been suggested that the mitochondrial outer membrane pr...

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