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Recognition of viral RNA structures by the cytosolic sensor retinoic acid-inducible gene-I (RIG-I) results in the activation of signaling cascades that culminate with the generation of the type I interferon (IFN) antiviral response. Onset of antiviral and inflammatory responses to viral pathogens necessitates the regulated spatiotemporal recruitment of signaling adapters, kinases and transcriptional proteins to the mitochondrial antiviral signaling protein (MAVS). We previously demonstrated that the serine/threonine kinase IKKε is recruited to the C-terminal region of MAVS following Sendai or vesicular stomatitis virus (VSV) infection, mediated by Lys63-linked polyubiquitination of MAVS at Lys500, resulting in inhibition of downstream IFN signaling (Paz et al, Mol Cell Biol, 2009). In this study, we demonstrate that C-terminus of MAVS harbors a novel TRAF3-binding site in the aa450-468 region of MAVS. A consensus TRAF-interacting motif (TIM), 455-PEENEY-460, within this site is required for TRAF3 binding and activation of IFN antiviral response genes, whereas mutation of the TIM eliminates TRAF3 binding and the downstream IFN response. Reconstitution of MAVS−/− mouse embryo fibroblasts with a construct expressing a TIM-mutated version of MAVS failed to restore the antiviral response or block VSV replication, whereas wild-type MAVS reconstituted antiviral inhibition of VSV replication. Furthermore, recruitment of IKKε to an adjacent C-terminal site (aa 468–540) in MAVS via Lys500 ubiquitination decreased TRAF3 binding and protein stability, thus contributing to IKKε-mediated shutdown of the IFN response. This study demonstrates that MAVS harbors a functional C-terminal TRAF3-binding site that participates in positive and negative regulation of the IFN antiviral response.
To mount an effective antiviral response, the immune system must trigger multiple signaling pathways to promote the production of cytokines and other antiviral factors that collectively suppress viral replication and assembly 1. The most robust antiviral response can be attributed to the induction of type I interferons (IFN) – IFNα and IFNβ. On immunological assault, cellular pattern recognition receptors are rapidly stimulated following the recognition of specific molecular signatures of pathogens, which are often referred to as pathogen-associated molecular patterns 2, 3. In the case of an infection by RNA viruses and some DNA viruses, both the membrane-bound Toll-like receptors (TLRs) and cytosolic sensors, such as retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), have been clearly shown to detect 5′ triphosphate-containing viral RNA structures, generated as a consequence of early virus transcription and/or replication 3, 4. Consequently, detection of viral RNA triggers the rapid activation of various transcription factors, such as NF-κB, AP-1 and members of the IFN regulatory factor (IRF) family (IRF1/3/7), that specifically bind to the IFNβ promoter to stimulate gene expression. Following production, IFNβ engages its cognate receptor and induces a complex intracellular signaling process to specifically activate the expression of multiple IFN-stimulated genes (ISGs), all of which contain a variation of the DNA sequence referred to as IFN-stimulated response element 5.
RLRs including RIG-I, melanoma differentiation factor 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2) reside in the cellular cytoplasm, whereas TLRs are located on the plasma membrane or at the endosomal surface 2, 6, 7, 8, 9. Structurally, all three RLRs contain a DExD-box RNA helicase domain for RNA binding and with the exception of LGP2, also possess a caspase recruitment domain (CARD) that mediates downstream protein-protein interactions. Upon viral RNA recognition, activation of RIG-I promotes self-dimerization and other structural modifications that permit CARD-CARD interaction with the downstream adapter molecule mitochondrial antiviral signaling protein (MAVS; also known as IPS-1/Cardif/VISA) 10, 11, 12, 13. MAVS is composed of multiple motifs including: a C-terminal transmembrane domain (TM), which is essential for targeting to the outer mitochondrial membrane, three TRAF-interacting motifs (TIM), two included in the N-terminal proline-rich region (Pro), and an N-terminal CARD domain. Functionally, the CARD and TM domains appear to be critical for MAVS dimerization and for relaying the signal from RIG-I to downstream adapter molecules 12, 14, 15, 16, 17. The Pro region harbors two distinct TIMs, one located at aa 143-PVQET-147 that binds TRAF2 and TRAF3, and a second TIM located at aa 153-PGENSE-158 that exclusively binds TRAF6 13, 18. An alternate TRAF6-binding site is located in the C-terminus of MAVS at aa 455-PEENEY-460 13, and both N- and C-terminal sites are required for TRAF6-mediated activation of the NF-κB pathway 13. Additionally, MAVS interacts with multiple proteins, including LGP2, TRADD, FADD, RIP1, TRAF5, IKKε, caspase 8/9 and MITA/STING/MYPS/ERIS, TOM70, NLRX1, PCBP2/AIP4, OPTN, PLK1, deubiquitination enzyme A (DUBA), A20, 9, 10, 11, 15, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, all indicating that MAVS functions as a molecular scaffold to which key adapter proteins bind to orchestrate the RIG-I dependent antiviral response.
TRAF family members are primarily involved in the regulation of inflammation, antiviral responses and apoptosis, and function downstream of the TNF, CD40 and LTβR receptors, to name a few. TRAF proteins play non-overlapping roles in signaling; e.g., TRAF3 uniquely regulates the type I IFN response 33, 34, 35, while both TRAF2 and TRAF6 activate the NF-κB pathway downstream of RIG-I 13. Thus, the RIG-I pathway bifurcates at the level of the TRAFs into two distinct pathways. TRAF2 and TRAF6 activate the classical IKKα/β kinases, and induce phosphorylation of IκBα inhibitor, resulting its proteasomal degradation. Active p65/p50 NF-κB dimers are then released and translocate into the nucleus to activate NF-κB-dependent target genes 36, 37, 38, 39. TRAF3 stimulates the non-canonical IKK-related kinases, TBK1 and IKKε, which induce C-terminal phosphorylation of IRF3/IRF7, leading to IRF3 dimerization, nuclear translocation, DNA binding and activation of IRF-dependent antiviral genes 26, 40, 41, 42, 43.
Cytokine signal transduction is a transient process that is tightly regulated to prevent inappropriate inflammatory or autoimmune responses. Dynamic control of the antiviral response is coordinated at transcriptional levels, as well as by post-translational modifications, such as phosphorylation and ubiquitination. Multiple proteins are involved in the coordination of the RIG-I pathway including A20, DUBA, CYLD, NLRX1, OPTN, PCBP2-AIP4, PSMA7, miR146a, LGP2, NLRC5, gC1qR, FLN29, ISG15, RNF125, Pin1, SIKE and TOM70, to name a few 19, 27, 28, 30, 32, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58. Previously, we demonstrated that IKKε, but not TBK1, was recruited to the MAVS adapter via a Lys63-linked polyubiquitination on Lys500, thus revealing an unexpected function of IKKε in the negative regulation of the inflammatory and antiviral response 15. We also identified Triad3A as an E3 ubiquitin ligase involved in the termination of the IFN response following viral infection, by targeting TRAF3 for K48-linked ubiquitination and proteasomal degradation 59. Recently, Tseng et al. 60 demonstrated that TRAF3 can either positively or negatively control the expression of type I IFN and pro-inflammatory cytokines through distinct types of ubiquitination.
Elucidating the spatiotemporal events involved in the recruitment of adapters, kinases and transcription factors to MAVS is essential for an understanding of the inflammatory and antiviral response. Although TRAF3 is essential for the IFN response, Seth et al. previously demonstrated that removal of the Pro domain from MAVS had no effect on IFN signaling, suggesting the presence of an alternate functional TRAF3-binding site. In this study, we identify a novel TRAF3-binding site in the C-terminus of MAVS (455-PEENEY-460) that is essential for MAVS-mediated antiviral responses. Furthermore, IKKε, recruited to MAVS via K63-mediated ubiquitination of Lys500, decreases the binding and protein stability of TRAF3, thus contributing to negative regulation of the antiviral response.
TRAF3 is an essential regulator of type I IFN expression following viral infection 35, with TRAF3 binding thought to be localized within the Pro region of MAVS 18. However, removal of the Pro region had no effect on type I IFN production 12, implying the presence of another site. Therefore, we initially examined the ability of TRAF3 to interact with either the N-terminal, C-terminal or internal regions of MAVS 12, 13 (Figure 1A). The N-terminal constructs, 1–155 (MAVS aa 1–155); 1–400 (MAVS aa 1–400); or internal construct 157–400 (MAVS aa 157–400) of MAVS were not capable of binding TRAF3 when compared with MAVS wt (Figure 1B, compare lanes 4, 5 and 6 with lane 3), suggesting that the N-terminal or the internal region of MAVS was not capable of interacting with TRAF3 (Figure 1B). Deletion of the first 150 aa of MAVS, as in MAVS aa 151–540, did not reduce TRAF3-MAVS interaction (Figure 1A and and1C,1C, compare lane 4 with 3), further suggesting that N-terminus of MAVS was dispensable for TRAF3 binding. Furthermore, removal of region aa 509–540, which includes the TM domain of MAVS (as in aa 1–508), did not affect binding to TRAF3 (Figure 1C, compare lane 5 with 3), indicating that the TM is not necessary for MAVS-TRAF3 interaction. A series of C-terminal MAVS constructs were used to demonstrate that TRAF3 binds to the C-terminal domain of MAVS: region aa 364–540 (Figure 1D, lane 1) but not region aa 468–540 or aa 503–540 (Figure 1D, lanes 2 and 3) was capable of binding MAVS.
To further delineate the site of TRAF3 interaction, two different internally deleted MAVS constructs were co-expressed together with TRAF3 – MAVS Δaa 101–450 and MAVS Δaa 101–480. Both constructs lack the Pro region of MAVS, retain the CARD and TM, but differ in the size of the internal deletion that removes the C-terminal TIM motif (455-PEENEY-460; Figure 2A). TRAF3 interacted with MAVS Δaa 101–450 but not with MAVS Δaa 101–480, thus reducing the minimal site of TRAF3 interaction to aa 450–468 (Figure 2B compare lanes 2 and 3). To determine if this interaction was important for the antiviral response, quantitative PCR (qPCR) analysis of various genes induced following viral infection, IFNB1, IFIT1 (ISG56), IP-10, OAS1 and RIG-I, was performed. On the basis of the qPCR results, MAVS Δaa 101–450 induced IFN response genes similarly to MAVS wt, whereas a 50% decrease in gene expression was observed with MAVS Δaa 101–480 (Figure 2C).
The C-terminal TIM of MAVS has been shown to bind TRAF6 13, and although previous studies demonstrated that TRAF6 did not function in IFN signaling 13, 61, we sought to confirm that the activity of MAVS Δaa 101–450 was mediated by TRAF3 and not TRAF6. In TRAF6+/+ and TRAF6−/− MEFs, the MAVS Δaa 101–450 construct induced IFNα4 activity, both in the presence or absence of TRAF6, whereas MAVS Δaa 101–480 did not activate the IFNα4 promoter in either TRAF6+/+ or TRAF6−/− MEFs (Figure 2D), thus ruling out the involvement of TRAF6 in MAVS Δaa 101–450-driven activity. In contrast, when IFN induction was examined in TRAF3−/− MEFs (p100−/− background), the ability of MAVS Δaa 101–450 to initiate IFN transcription was reduced compared with TRAF3+/+ MEFs (Figure 2E). Together, these results demonstrate that MAVS activation of IFN signaling is regulated by TRAF3 binding to the C-terminal TIM of MAVS and does not require TRAF6 or the N-terminal Pro domain of MAVS.
Next, the TIM site located in the C-terminus of MAVS was mutated from 455-PEENEY-460 to 455-AAANEY-460 (referred to as MAVS-Cterm AAA; Figure 3A). Interaction between MAVS wt and TRAF3 wt was readily detected in co-immunoprecipitation (Figure 3B, lane 3), whereas the interaction between TRAF3 and MAVS-Cterm AAA was reduced to undetectable levels (Figure 3B, lane 4). Subsequently, the ability of MAVS wt and MAVS-Cterm AAA to stimulate mRNA expression of various ISGs, IFNB1, IFNA2, IFIT1 (ISG56), IL6, and IP-10, was evaluated in HeLa cells (Figure 3C). Expression of MAVS wt stimulated mRNA levels for all ISGs, whereas MAVS-Cterm AAA did not induce ISG mRNA, or the activity of IFNα4-, IFNβ-, and ISG56- or NF-κB-driven reporter constructs (Figure 3C and and3D).3D). Furthermore, mutation of TRAF3 at amino acids Y440 and Q442 (TRAF3 Y440A/Q442A), residues critical for binding to the TIM of MAVS 18, 59, abrogated the interaction between TRAF3 Y440A/Q442A and MAVS (Figure 3E, lane 5), as well as MAVS-mediated IFNα4 promoter activity (Figure 3F). Thus, mutagenesis of the C-terminal TIM in MAVS or the TIM-interacting motif in TRAF3 eliminated both MAVS-TRAF3 interaction and downstream signaling to IFN response genes.
To further clarify the discrepancy between N- versus C-terminal association of TRAF3, a MAVS expression construct was generated that mutated the N-terminal TIM at aa 143-PVQET-147 to aa 143-AVAEA-147 (referred to as MAVS-Nterm AAA; Supplementary information, Figure S1). Strikingly, mutation of the N-terminal TIM domain did not affect TRAF3 binding to MAVS (Supplementary information, Figure S1, lanes 3 and 4), and furthermore had no effect on IFN signaling, since ISG mRNA levels for IFNB1, IFNA2, IFIT1(ISG56), IL6 and IP-10 remained similar in cells expressing MAVS or MAVS-Nterm AAA (Supplementary information, Figure S1). Additionally, MAVS-Nterm AAA retained the same functional activity as MAVS wt when monitored for IFNβ-, IFNα4-, ISG56- and NF-κB-driven reporter gene activity (Supplementary information, Figure S1). Thus, mutation of the N-terminal TIM did not affect TRAF3 binding or functional activity of MAVS, indicating that the N-terminal TIM does not have a role in TRAF3-mediated IFN induction.
The next approach to investigate functional consequences of MAVS-TRAF3 interaction was to reconstitute MAVS−/− MEFs, which are unable to mount an antiviral response against RNA virus infection 62, with empty control vector, MAVS wt or MAVS-Cterm AAA expression plasmid and determine the magnitude of the antiviral response. Reconstitution with MAVS wt induced endogenous IFNB1, IFNA4, IFIT1, IL6 and IP-10 mRNA, whereas reconstitution with MAVS Cterm AAA and subsequent vesicular stomatitis virus (VSV) infection reduced ISG expression by 50–80% (Figure 4A). These results were confirmed by monitoring the activity of IFNα4, IFNβ, ISG56 and NF-κB promoters in reconstituted MAVS−/− MEFs; MAVS-Cterm AAA was a poor inducer of all promoters, compared with MAVS wt (Figure 4B).
Next, reconstituted MAVS−/− MEFs were infected with GFP-expressing VSV (designated as GFP-VSV-Δ51) 63, 64 at a MOI of five and virus replication was directly monitored by flow cytometry. MEFs reconstituted with empty vector were > 80% GFP positive and thus highly permissive for virus replication, whereas cells reconstituted with MAVS wt expressed very little GFP (Figure 4C). In MAVS−/− MEFs reconstituted with MAVS-Cterm AAA, the cells were also highly permissive for VSV with > 60% of cells being GFP positive (Figure 4C), indicating that MAVS-Cterm AAA was unable to restore an antiviral state that blocked VSV replication. These results were confirmed by VSV plaque assays with the supernatant from VSV-infected MEFs (Figure 4C). MEFs reconstituted with MAVS wt did not produce any plaques, whereas cells reconstituted with empty vector or MAVS-Cterm AAA produced approximately 3 × 105 p.f.u./ml of VSV (Figure 4D), thus further illustrating the importance of the MAVS C-terminal TRAF3-binding site in generating an IFN-responsive antiviral state.
Because the C-terminal TIM is located adjacent to the IKKε interaction site, we were interested to determine if IKKε cooperatively influenced TRAF3 association with MAVS. MAVS wt and IKKε interaction was readily detected by co-immunoprecipitation (Figure 5A, lane 2), while IKKε and MAVS-Cterm AAA co-expression actually resulted in a modest increase in MAVS-IKKε interaction (Figure 5A, lane 3), suggesting that mutation of the C-terminal TIM facilitated IKKε-MAVS complex formation. To follow this observation, the interaction between a constant amount of MAVS and TRAF3 was examined in the presence of increasing amounts of IKKε. A dose-dependent reduction in the binding between MAVS and TRAF3 was observed with increasing IKKε, suggesting that IKKε binding to MAVS may interfere with TRAF3 binding to MAVS (Figure 5B, lanes 3–7). Furthermore, similar results were obtained when the kinase dead mutant of IKKε (IKKε K38A) was used in the experiment, indicating that the kinase activity of IKKε was not necessary to dislodge TRAF3 from MAVS (Figure 5C, lanes 3–7). Note, however, that the slower migration of MAVS in the presence of IKKε (Figure 5B, lanes 3–7), but not in the presence of IKKεK38A (Figure 5C, lanes 3–7), indicates that MAVS is a target of IKKε-mediated phosphorylation, as previously reported 15.
To confirm that the displacement of TRAF3 occurred via the C-terminus of MAVS, the interaction between a constant amount of C-terminal MAVS construct aa 364–540 and TRAF3 was examined in the presence of increasing amounts of IKKε a dose-dependent reduction in the binding between MAVS aa 364–540 and TRAF3 was observed with increasing IKKε (Figure 5D, lanes 4–7), indicating that IKKε binding to MAVS disrupted TRAF3 binding to the TIM domain located in the C-terminus of MAVS. Of note, the level of TRAF3 protein was also reduced with increasing amounts IKKε or IKKεK38A (Figure 5B-5D, input TRAF3), suggesting a role for the IKKε-MAVS complex in TRAF3 protein turnover.
As demonstrated previously, mutation of MAVS Lys500 to Arg500 abrogated IKKε recruitment to MAVS 15; therefore, we next compared MAVS(K500R)-TRAF3 association in the presence or absence of IKKε. As expected, a fivefold decrease in TRAF3-MAVS binding was observed when IKKε was co-expressed, as compared with MAVS-TRAF3 interaction in the absence of IKKε (Figure 6A, lanes 1, 2). Strikingly, the association between TRAF3-MAVS(K500R) was not affected by increasing amounts of IKKε (Figure 6A, lanes 3, 4), indicating that in the absence of IKKε binding to MAVS(K500R), the MAVS-TRAF3 interaction was not disrupted. This result implies that the recruitment of IKKε to MAVS may disrupt the tethering of the TRAF3-containing IFN signaling complex to the TIM domain.
To address the issue of TRAF3 turnover, total protein immunoblots were performed in cells co-expressing TRAF3 and increasing amounts of IKKε or IKKε K38A in the presence of either MAVS wt or MAVS(K500R) (Figure 6B-6E). A dose-dependent decrease in TRAF3 levels was detected with increasing amounts of IKKε expression (Figure 6B); however, no change in TRAF3 protein level was observed in the presence of MAVS(K500R) (Figure 6C), thus demonstrating that IKKε recruitment to MAVS contributes to the turnover of TRAF3 protein. Furthermore, IKKε kinase activity was not required for TRAF3 turnover, since increasing the amounts of IKKε K38A also diminished TRAF3 levels (Figure 6D). To reinforce that IKKε binding was required for TRAF3 turnover, IKKεK38A was also expressed in the presence of MAVS(K500R); without IKKεK38A binding to MAVS, the level of TRAF3 remained constant (Figure 6E). Altogether, these experiments argue that the formation of a MAVS-IKKε-TRAF3 tripartite complex leads to a disruption in TRAF3 binding and increased turnover of TRAF3 protein, as previously described 62.
To confirm the spatiotemporal events described above in a physiologically relevant context, a kinetic analysis of the MAVS-IKKε-TRAF3 complex was performed in A549 cells infected with SeV (50 HAU/ml; Figure 7). Following immunoprecipitation of endogenous MAVS, RIG-I was detected in the immunoprecipitate as early as 4 h post infection and RIG-I association with MAVS increased to a constant level between 6–24 h. Concomitant with the MAVS-RIG-I interaction, TRAF3 binding was detected, including a TRAF3 band at ~100 kDa, corresponding to a K63-linked ubiquitinated form of TRAF3, as previously described 59 (Figure 7). IKKε was also detected in the immunoprecipitate at 4–8 h (Figure 7, lanes 3–5) 15. Concomitant with IKKε dissociation from MAVS at 10 h was the disappearance of the ~100 kDa TRAF3 band (Figure 7, lane 6). As a measure of IFN signaling activity, the formation of the RIG-I-MAVS-TRAF3 interactome resulted in IRF3 phosphorylation at 4–12 h, as detected by the Ser396 phospho-specific Ab 40, 65. As TRAF3 binding to MAVS decreased, both TRAF3 and IRF3 protein levels also decreased, thus, confirming previous observations that TRAF3 and IRF3 undergo proteasomal degradation following virus infection (Figure 7, lane 8) 66, 67, both of which are hallmarks of the termination of the IFN response.
This study characterizes a novel functional TRAF3-binding site located at the C-terminus of MAVS – the TRAF-interacting motif 455-PEENEY-460. Several lines of evidence support the importance of this site in mediating TRAF3-dependent IFN signaling: (1) binding of TRAF3 was abrogated using MAVS-Cterm AAA mutant (455-AAANEY-460), and TRAF3 binding to the C-terminal TIM was essential for the IFN antiviral response; (2) mutation of the previously characterized N-terminal TIM (143-PVQET-147) did not affect TRAF3 binding or downstream IFN signaling; (3) a non-binding mutant of TRAF3, mutated at amino acids Y440 and Q442 did not interact with the TIM or induce IFN signaling; (4) MAVS-Cterm AAA was unable to reconstitute an antiviral state in MAVS−/− MEFs; and (5) TRAF3 was displaced from MAVS when IKKε was present, a mechanism independent of its kinase activity but dependent on K63 linked ubiquitination of MAVS at Lys500. Interestingly, the stability of TRAF3 was minimally affected when MAVS 364–540 and IKKε were present in our system, although the displacement of TRAF3 from MAVS was readily detected with increasing IKKε. MAVS protein contains multiple regulatory domains and it is likely that the N-terminus of MAVS may also contribute to the stability of TRAF3. These results thus highlight the interplay of C-terminal binding of TRAF3 and IKKε in the shutdown of the IFN response.
In the context of several recent studies, the current observations support a multistep model of regulation of the IFN antiviral response, mediated via TRAF3 interactions with the MAVS scaffold (Figure 8). Sensing of viral RNA structures by RIG-I results in Lys63 ubiquitination of RIG-I by TRIM25, leading to a conformational change in RIG-I that facilitates CARD-CARD domain interactions with MAVS 68 (reviewed in 26). MAVS dimerizes 15, 16, 17 and recruits TRAF3 as an essential step in the activation of the antiviral response (Figure 8A) 12, 18, 35. This study demonstrates the critical role of the C-terminal 455-PEENEY-460 TIM in recruiting TRAF3; and the recruitment requires Lys63-polyubquitination of TRAF3 by an unidentified E3 ubiquitin ligase, or possibly by auto-ubiquitination 26, 59. The interaction of the TRAF3-containing signaling complex (Figure 8, panel A) activates downstream kinases TBK1/IKKε that phosphorylate C-terminal serine residues of latent cytoplasmic IRF3 and IRF7 40, 41, 42. IRF3 and/or IRF7 then dimerize, translocate into the nucleus and bind to specific IRF-response elements in the IFNβ promoter, as well as other type I IFN genes 69. Subsequently, Lys63 polyubiquitination of MAVS at Lys500 leads to the recruitment of IKKε to the C-terminus of MAVS 15, and initiates a termination of the IFN response, by disrupting TRAF3 binding to MAVS (Figure 8, panel B). Kayagaki et al. 31 discovered, through small interfering RNA screen, that the DUBA specifically removed Lys63-linked polyubiquitin chains from TRAF3, resulting in its dissociation from the IFN signaling complex, contributing to the negative regulation of type I IFN response (Figure 8, panel C). De-ubiquitinated TRAF3 is then targeted for Lys48 ubiquitination mediated by the E3 ubiquitin ligase TRIAD3A 59; finally TRAF3 is degraded by a proteasome-dependent mechanism, thus removing an essential adapter involved in the tethering of the IFN signaling complex to MAVS (Figure 8, panel C). TRAF3 undergoes biphasic ubiquitination following viral infection: early after infection, Lys63-linked polyubiquitination contributes to IFN signaling complex formation, while at later times after infection, TRAF3 undergoes Lys48-linked polyubiquitination mediated by TRIAD3A that targets TRAF3 for proteasomal degradation (Figure 8, panel C) 59.
The complexity of the regulatory interactions at the MAVS scaffold is highlighted further by the identification of several other proteins that interact with MAVS, including: TRAF2/5/6, caspase-8/10, RIP1, FADD, TRADD, MITA/STING/MYPS/ERIS, RNF125, REUL, A20, CYLD, Atg5-Atg12, NLRX1, NLRC5, LGP2 (reviewed in 19), proteins that may further contribute to positive or negative regulation of the RIG-I/MAVS pathway. To further add to the complexity of MAVS regulation of signaling, Dixit et al. 70 recently demonstrated that MAVS can localize to cytosolic peroxisome structures, which are formed from the endoplasmic reticulum and are usually involved in cellular metabolism 71. MAVS localization to the peroxisome establishes a distinct population of MAVS involved in the early, transient response against viral pathogens, while mitochondrial MAVS produces a sustained robust antiviral response. Interestingly, peroxisomal MAVS and mitochondrial MAVS may not utilize the same transcription factors to induce antiviral genes. It will be interesting to determine if the adapters tethered to peroxisomal MAVS differ from those associated with mitochondrial MAVS. Recently, Zeng et al. 72 established a cell-free system that will permit further dissection of the pathway and contribute to a better understanding of events leading to the host antiviral and inflammatory responses. In conclusion, we provide evidence that the C-terminal regulatory region of MAVS is involved in positive and negative regulation of the antiviral response through regulatory interactions with TRAF3 and IKKε.
Plasmids encoding FLAG-IKKε, FLAG-IKKεK38A, GFP-IKKε, FLAG-TRAF3, MYC-TRAF3, MYC-MAVS, MYC-MAVS (aa 1–155, aa 1–400, aa 1–580, aa 157–400, aa 151–540, aa 364–540, aa 468–540 and aa 503–540), MYC-MAVS(K500R), FLAG-MAVS Δaa 101–450, FLAG-MAVS Δaa 101–480 have been previously described 14, 15, 29, 40, 59. MAVS multisite point mutants at proline 455 (P), glutamic acid 456 (E), glutamic acid 457 (E) to alanine (A) (MYC-MAVS-Cterm AAA) and proline 143, glutamine 145, threonine 147 to alanine (MYC-MAVS-Nterm AAA), were generated by multisite directed mutagenesis as per manufacturer's instructions (Stratagene, La Jolla, CA, USA). All point mutants were verified by sequencing.
HEK293, Vero and HeLa cells were grown in DMEM supplemented with 10% FBS (Wisent, St-Bruno, Quebec, Canada). A549 cells were cultured in F12K media supplemented with 10% FBS (Wisent). MAVS−/− MEFS (kind gift from Dr Zhijian Chen) and TRAF3−/− MEFs (kind gift from Dr Genhong Cheng) were cultured DMEM 10% FBS (Invitrogen, Carlsbad, CA, USA), supplemented with 1% non-essential amino acids and 1% -glutamine. Culture media and supplements were purchased from Wisent. Where indicated, A549 cells were infected with Sendai virus at 50 HAU/ml (Charles River Lab) in serum free condition supplemented with serum 1 h post infection. Transient transfections were carried out in subconfluent HEK293 cells by calcium phosphate, by Fugene6 (Roche Diagnostics, Mannheim, Germany), in HeLa cells or Lipofectamine 2000 in knockout MEFs (Invitrogen) when indicated, as per manufacturer's recommendations.
Where indicated MAVS−/− cells were infected with 5 m.o.i of VSV (strain Δ51; kind gift from Dr John Bell) in serum free condition supplemented with serum 1 h post infection. Flow cytometry (1 × 104 cells per measurement) was performed with a FACS Calibur (Becton-Dickinson, Mississauga, Ontario, Canada) and analyzed with Cell Quest software and FCS Express version 3 (De Novo Software, Los Angeles, CA) for GFP expression. Plaque assay was done using confluent monolayers of Vero cells in six-well plates were infected with 0.1 ml of serially diluted samples; after 1 h of infection, at 37 °C, medium was removed and replaced with complete medium containing 0.5% methyl cellulose (Sigma-Aldrich, Oakville, Ontario, Canada) for 48 h. Vero cells were fixed in 4% formaldehyde and stained with crystal violet.
Subconfluent HeLa or confluent MAVS−/− MEFs cells were transfected with 20 ng of pRLnull reporter (Renilla luciferase, internal control), 100 ng of pGL3-IFNα4, pGL3-IFNβ, pGL3-ISG56, pGL3-NF-κB, and 100 ng of MYC-MAVS (wt or its mutated or truncated forms) as indicated. As previously described, IFNα4 promoter activity was measured in the presence of exogenous IRF7 at 100 ng 40. At 24 h post transfection, reporter gene activity was measured by Dual-Luciferase reporter assay according to the manufacturer's instructions using GLIOMAX 20/20 luminometer (Promega Corporation, Madison, WI, USA). Three independent experiments were carried out in triplicate. Error bars represent the mean standard deviation for triplicates and analyzed using Prism 5 software.
Whole-cell extracts (WCE, 30–50 μg) were separated in 7.5 or 10% acrylamide gel by SDS-PAGE and transferred to a nitrocellulose membrane (BioRad, Mississauga, ON, Canada). Membranes were blocked for 1 h at room temperature in 5% (v/v) dried milk/0.1% (v/v) Tween-20 in PBS and then were probed with primary antibodies: for 1 h at room temperature or overnight at 4 °C. After washes, membranes were incubated with horse-radish peroxidase-coupled secondary antibody solutions (1 : 3 000 in 5% milk/PBS, KPL, Gaithersburg, MD, USA) for 1 h at room temperature, washed and revealed using ECL reagent (Perkin-Elmer, Waltham, MA, USA) according to manufacturer's instructions.
HEK293 cells were transiently transfected with the indicated expression plasmids. Cells were harvested and immediately lysed in a 1% Triton X-100 lysis buffer (20 mM Tris-HCl, pH 7.5, 150 mM NaCl, 1% Triton X-100, 10% glycerol, 40 mM β-glycerophosphate, 0.1% protease inhibitor cocktail, 1 mM PMSF, 1 mM Na3VO4, 5 mM NaF, 1 mM DTT, 10 mM NEM). Immunoprecipitation was carried out in WCE (500 μg) with 1 μg of anti-MYC (9E10; Santa Cruz Biotechnology, Santa Cruz, CA, USA) or anti-FLAG (Sigma-Aldrich, St Louis, MO, USA) coupled to 50 μl of A/G PLUS-Agarose beads (Santa Cruz Biotechnology) at 4 °C with constant agitation. Following five washes with supplemented lysis buffer, samples were denatured in 2% SDS-loading dye, separated by SDS-PAGE and transferred to a nitrocellulose membrane (BioRad). Co-immunoprecipitated TRAF3 or IKKε was detected by anti-FLAG antibody (Sigma-Aldrich) or by anti-GFP antibody (Roche Diagnostics).
For endogenous protein interactions, A549 cells were infected or not with SeV as described. At 2, 4, 6 and 8, 10, 12 and 24 h post-infection cells were harvested and lysed as above. Immunoprecipitation was carried out on WCE (1 mg) using an anti-MAVS antibody raised against the C-terminal (rabbit polyclonal, in collaboration with Millipore Corporation Cat: 06-1044; Temecula, CA, USA) for 16 h at 4 °C with constant agitation. Following washes, samples were analyzed by immunoblot. Co-immunoprecipitated RIG-I was detected using an anti-RIG-I antibody raised against the N-terminal (rabbit polyclonal, in collaboration with Millipore Corporation Cat: 06-1040; Temecula, CA, USA) TRAF3 (Santa-Cruz, G-6), IKKε was detected using an anti-IKKε mouse monoclonal antibody (in collaboration with BD Biosciences, San Jose, CA, USA). Other antibodies used in this study include IRF3 ps396 (Millipore Corporation; Temecula, CA, USA); anti-IRF3 antibody raised against the C-terminal (Rabbit polyclonal, in collaboration with Millipore Corporation Cat:06-1045; Temecula, CA, USA); anti-actin (Millipore Corporation; Temecula, CA, USA).
DNase-treated total RNA from HeLa cells and MAVS−/− cells transfected with MYC-MAVS wt, internal deletion constructs or mutated AAA was prepared using the RNeasy kit (Qiagen, Mississauga, ON, Canada). RNA concentration was determined by absorption at 260 nm, and RNA quality was ensured by a 260/280 ratio ≥ 2.0. Total RNA was reverse transcribed with 100 U of Superscript II Plus RNAse H reverse transcriptase using oligo AnCT primers (Gibco BRL Life Technologies, CA, USA). qPCR assays were performed using the SYBR Green I on a Light Cycler apparatus (Roche Diagnostics). Human primers sequences used in this study are as follows: IFNB1forward: 5′-TTGTGCTTCTCCACTACAGC-3′, IFNB1 reverse: 5′-ATCTGAGGTGCCCATGCTAC-3′ IFNA2forward: 5′-CCTGATGAAGGAGGACTCCAT-3′, IFNA2reverse: 5′-AAAAAGGTGAGCTGGCATACG-3′ IFIT1forward:5′-CAACCAAGCAAATGTGAGGA-3′, IFIT1 reverse: 5′-AGGGGAAGCAAAGAAAATGG-3′ IL-6forward: 5′-GGAGACTTGCCTGGTGAAAA-3′, IL-6reverse: 5′-ATCTGAGGTGCCCATGCTAC; IP-10forward: 5′-TCTTCTCACCCTTCTTTTTCATTGT-3′, IP-10reverse: 5′-TTCCTGCAAGCCAATTTTGTC-3′ OAS1forward: 5′-CCTGGTTGTCTTCCTCAGTCC-3′, OAS1reverse: 5′-GTGGAGAACTCGCCCTCTTT-3′ RIG-Iforward: 5′-GCAGAGGCCGGCATGAC-3′, RIG-Ireverse: 5′-TGTAGGTAGGGTCCAGGGTCTTC-3′ ACTINforward: 5′-ACTGGGACGACATGGAGAAAA-3′, ACTINreverse: 5′-GCCACACGCAGCTC-3′. Murine primers sequences used in this study are as follows: IFNBforward: 5′-CACAGCCCTCTCCATCAACT-3′, IFNB reverse: 5′-TCCCACGTCAATCTTTCCTC-3′ IFNA4forward: 5′-GAAGGACAGGAAGGATTTTGGA-3′, IFNA4reverse: 5′-TGAGCCTTCTGGBATCTGTTGGT-3′ IFIT1forward: 5′-CTCTGAAAGTGGAGCCAGAAAAC-3′, IFIT1 reverse: 5′-AAATCTTGGCGATAGGCTACGA-3′ IP-10forward: 5′-AAGTGCTGCCGTCATTTTCT-3′, IP-10reverse: 5′-CACTGGGTAAAGGGGAGTGA-3′ ACTINforward: 5′-CACCAGTTCGCCATGGAT-3′, ACTINreverse: 5′-CCTCGTCACCCACATAGGAG-3′. Murine IL-6primers were obtained from Origene (catalog number: MP206798). PCR efficiency results were obtained from duplicate measurements of two individual cDNA samples. Experiments were performed at least twice. All data are presented as a relative quantification, based on the relative expression of target genes versus ACTIN as reference gene and analyzed using Prism 5 software.
This work was supported by grants from the Canadian Institutes for Health Research, Canadian Cancer Society and the Canadian Foundation for AIDS Research, awarded to JH and grants from L'Agence Nationale de la Recherche contre le SIDA (ANRS), awarded to EM. SP is the recipient of an FRSQ Bourse de Troisième Cycle (Doctorat) and MV was the recipient of a FRM Bourse de Fin de Thèse.
(Supplementary information is linked to the online version of the paper on the Cell Research website.)
N-terminal binding of TRAF3 to MAVS is not required for IFN activation.