Molecular Mechanisms of TNFR-associated Factor 6 (TRAF6) Utilization by the Oncogenic Viral Mimic of CD40, Latent Membrane Protein 1 (LMP1)*

Latent membrane protein 1 (LMP1), encoded by Epstein-Barr virus, is required for EBV-mediated B cell transformation and plays a significant role in the development of posttransplant B cell lymphomas. LMP1 has also been implicated in exacerbation of autoimmune diseases such as systemic lupus erythematosus. LMP1 is a constitutively active functional mimic of the tumor necrosis factor receptor superfamily member CD40, utilizing tumor necrosis factor receptor-associated factor (TRAF) adaptor proteins to induce signaling. However, LMP1-mediated B cell activation is amplified and sustained compared with CD40. We have previously shown that LMP1 and CD40 use TRAFs 1, 2, 3, and 5 differently. TRAF6 is important for CD40 signaling, but the role of TRAF6 in LMP1 signaling in B cells is not clear. Although TRAF6 binds directly to CD40, TRAF6 interaction with LMP1 in B cells has not been characterized. Here we tested the hypothesis that TRAF6 is a critical regulator of LMP1 signaling in B cells, either as part of a receptor-associated complex and/or as a cytoplasmic adaptor protein. Using TRAF6-deficient B cells, we determined that TRAF6 was critical for LMP1-mediated B cell activation. Although CD40-mediated TRAF6-dependent signaling does not require the TRAF6 receptor-binding domain, we found that LMP1 signaling required the presence of this domain. Furthermore, TRAF6 was recruited to the LMP1 signaling complex via the TRAF1/2/3/5 binding site within the cytoplasmic domain of LMP1.

LMP1 is a transmembrane (TM) protein consisting of a short cytoplasmic (CY) N-terminal domain, six TM domains, and a long CY C-terminal domain (17,18). The N-terminal domain anchors LMP1 to the plasma membrane and regulates LMP1 processing (17)(18)(19). The TM domains spontaneously self-aggregate and oligomerize within the plasma membrane, resulting in ligand-independent, constitutive activation of signaling terminated by rapid and constant processing of the protein into fragments (18 -20). Two subdomains within the C-terminal domain, C-terminal activating region (CTAR) 1 and CTAR2, are critical for LMP1 signaling (17,19,21). Our lab has shown in both isolated B cells and mice that the C-terminal domain is necessary and sufficient to mediate LMP1 functions in B cells (3,22). Specifically, we have demonstrated that the LMP1 C-terminal CY domain mimics various aspects of LMP1 signaling, including early pathway activation and downstream biological functions of the B cell (3,18,20,(22)(23)(24)(25).
LMP1 is a functional mimic of the tumor necrosis factor receptor superfamily member CD40, an activating receptor constitutively expressed on B cells, macrophages, and dendritic cells (20,26). LMP1 and CD40 signaling result in kinase and NF-B activation and the up-regulation of costimulatory and adhesion molecules (12,18,27). However, LMP1 signals to B cells are amplified and sustained compared with CD40 signals (18,26). Both LMP1 and CD40 lack enzymatic activity (26) and thus use tumor necrosis factor receptor-associated factor (TRAF) adaptor proteins to mediate signaling, but they utilize TRAFs 1, 2, 3, and 5 in distinct and, in some cases, contrasting ways (18,20,28). TRAF3 negatively regulates CD40 signaling but serves as a positive regulator of LMP1 signaling, whereas TRAFs 1 and 2 promote CD40-mediated but not LMP1-mediated JNK and NF-B activation (17, 20, 29 -31). In the absence of TRAF5, CD40-mediated surface molecule up-regulation and IgM production is reduced, but TRAF5 deficiency has no effect on CD40-mediated JNK and NF-B activation (32,33). However, TRAF5 was recently shown to be critical for LMP1-mediated activation of JNK and Akt and the production of IL-6, TNF-␣, and IL-17 (24).
The role of TRAF6 in LMP1 signaling in B cells is a significant knowledge gap in the field and is the focus of this study. TRAF6 plays a critical role in CD40 signaling and directly binds to CD40 (34), but the binding site is distinct from that shared by TRAFs 1, 2, 3, and 5 (28,32,35,36). Although TRAF6 has been implicated in LMP1-mediated NF-B and p38 activation, the role of TRAF6 in LMP1-mediated JNK activation is controversial (19,37,38). It is important to note that all of the studies reported to date investigating TRAF6 in LMP1 signaling were performed in mouse embryonic fibroblasts or a human kidney adenovirus-transformed adenocarcinoma cell line, with exogenously overexpressed proteins (19,37,38). One caveat of experiments performed using overexpression systems is that nonphysiologic levels of signaling proteins can lead to aberrant pathway activation, complicating the interpretation of results. Thus, studying signal transduction with proteins expressed at endogenous levels in relevant cell types is crucial. Results of a study of TRAF6 Ϫ/Ϫ mouse embryonic fibroblasts showed that the absence of TRAF6 does not abolish LMP1-mediated p38 activation (19). However, prior studies indicate that TRAF functions can vary depending upon receptor and cell type (39). Thus, we used B cells, the primary target of EBV, in which proteins are expressed at endogenous levels. Additionally, although TRAF6 has been shown to directly associate with CD40, TRAF6 interaction with LMP1 has not been determined (28,32,35,36). Studies have suggested that TRAF6 may associate with the CTAR2 subdomain of LMP1, but there is no direct evidence to support this (19,37,40). It has also been proposed, on the basis of overexpression studies, that the adapter protein TRADD interacts with CTAR2 and TRAF6, linking TRAF6 to CTAR2 (41,42). However, this finding has not been reproduced at normal protein levels in B cells (18,43). Interestingly, not all receptors that employ TRAF6 to promote signaling must directly bind TRAF6. It was recently shown that CD40 can utilize a strictly CY mutant of TRAF6 to mediate a subset of B cell activation signals (29). Similarly, the innate immune Toll-like receptors have been demonstrated to use TRAF6 strictly as a CY adaptor, without direct binding (44).
To understand how LMP1 might employ TRAF6 to promote its effects on B cells, we used TRAF6-deficient mouse B cells expressing a hybrid receptor, in which the LMP1 extracellular and TM domains have been replaced with those of human CD40 (hCD40), to allow control of early LMP1 signaling (18,25,28). This approach allowed us to determine for which LMP1 functions and early signaling pathways TRAF6 may be required. This hybrid molecule has been shown in mouse and human B cell lines, freshly isolated B cells, and mice to accurately represent LMP1-mediated signals to B cells (18,25,28). To address TRAF6 association with LMP1, we used mouse B cells sufficient or deficient in TRAF6, expressing either hCD40LMP1 or variants in which either CTAR1 or CTAR2 was deleted from the LMP1 molecule. Results presented here reveal for the first time that TRAF6 was required for LMP1-mediated B cell activation and associates with the CTAR1 subdomain of LMP1.
Signaling Assays-2 ϫ 10 6 cells were washed in RPMI 1640 medium, resuspended in BCM-10 in 1.5 ml Eppendorf tubes, and rested for 45 min at 37°C. Cells were then stimulated for 10, 15, 30, or 60 min with anti-hCD40 Ab (G28.5, 10 g/ml), 6 M sorbitol (100 l/ml), or anti-mCD40 Ab (HM40.3, 10 g/ml). Whole cell lysates were prepared by pelleting the cells, removing the supernatant, and adding 200 l of 2ϫ SDS-PAGE loading buffer to the cell pellet. Lysates were sonicated using a Branson Sonifier 250 (VWR International) with 20 pulses at 90% duty cycle, output 1.5. Samples were denatured for 10 min at 95°C. NF-B1 activation was determined by measuring protein levels of phosphorylated and total IB␣ (see below), as IB␣ is phosphorylated and then degraded upon activation of the NF-B1 pathway.
Western Blots-Up to 10 l of sample were resolved on 10% SDS-PAGE. Proteins were transferred to Immobilon-P PVDF membranes (Millipore). Membranes were blocked with 10% dry milk in TBST for 1 h, washed in TBST (NaCl, Tris, Tween 20, and H 2 O), and incubated overnight at 4°C with one of the above Abs. Blots were washed in TBST and incubated with secondary Abs for 1 h or overnight and developed using Supersignal West Pico (Pierce). Western blot chemiluminescence was read with an LAS-4000 low-light camera and analyzed with Multi Gauge software (Fujifilm Life Science, Edison, NJ).
CD80 Up-regulation and Flow Cytometry-2.5 ϫ 10 5 cells were stimulated in a 24-well plate in 1 ml BCM-10 for 72 h with 2 g/ml of either MOPC-21 isotype mAb or G28.5 (anti-hCD40 mAb), or 20 g/ml of LPS. Anti-mouse CD16/32 mAb (0.5 mg/ml) was added 10 min prior to staining with FITC-labeled anti-mouse CD80 Ab or FITC-labeled Armenian hamster IgG to block nonspecific binding to B cell Fc␥ receptors. Flow cytometry was performed on a FACScalibur (BD Biosciences). Data were analyzed using FlowJo software (Tree Star, Ashland, OR).
Immunoprecipitation-Dynal protein G magnetic beads (Invitrogen) were coated with G28.5 or MOPC-21 Ab (10 g/10 l beads) or goat anti-rat IgG followed by anti-mCD40 Ab (1C10) as described previously (29). Abs were conjugated to the beads using disuccinimidyl suberate according to the manufacturer's protocols. Cells (2.0 ϫ 10 7 for cell lines, 1.5 ϫ 10 7 for primary cells) were incubated in 1 ml of BCM-10 with Abcoated beads for either 30 min at 37°C or 45 min at room temperature. Beads and cells were then pelleted and lysed as described previously (29). Bead-bound proteins were resuspended in 2ϫ SDS-PAGE loading buffer and boiled for 10 min at 95°C.

Early LMP1-mediated Signaling Events in TRAF6 Ϫ/Ϫ B Cells-
The use of B cells expressing WT LMP1 is not ideal for studying early LMP1 signaling, as LMP1 signals in a constitutive manner. For this reason, we used mouse B cells stably expressing the chimera hCD40LMP1 (18,22,28). This molecule consists of the extracellular and TM domains of hCD40 and the CY domain of LMP1 (Fig. 1A). LMP1 signaling was induced through the addition of an agonistic anti-hCD40 Ab (G28.5) (18,25,28). Our laboratory has demonstrated that hCD40LMP1 signals in a manner similar to that of WT LMP1 and that the CY C-terminal domain of LMP1 is both necessary and sufficient to promote signaling to B cells (3,18,22,25). It was also demonstrated that the LMP1 CY tail signals similarly in mouse and human B cells, promoting the up-regulation of costimulatory and adhesion molecules (3). To understand how LMP1 uses TRAF6 to promote its effects on B cells, we first examined TRAF6-sufficient (TRAF6 ϩ/ϩ ) and TRAF6-deficient (TRAF6 Ϫ/Ϫ ) mouse B cells, which were created via gene targeting by homologous recombination as described previously (29). As described under "Experimental Procedures," these cell lines were both transfected to stably express hCD40LMP1, which was measured by flow cytometry to pair subclones based on their similar levels of hCD40LMP1 expression. Anti-hCD40 Ab was used to stimulate these cells and induce hCD40LMP1 signaling for 0 -60 min. Stimulation via sorbitol, a sugar that induces osmotic stress (22), was included as a TRAF6-independent control stimulus. Fig. 1, B and C, shows that the absence of TRAF6 abolished the ability of hCD40LMP1 to activate JNK, p38, TAK1, and NF-B1. Impor- tantly, TRAF6 Ϫ/Ϫ cells were not globally incapable of signaling, as demonstrated by the positive response of these cells to stimulation with sorbitol or anti-mCD40 Ab (Fig. 1, B and C). Thus, TRAF6 was required for early LMP1-mediated signaling events.
TRAF6 Requirements for CD80 Up-regulation-Downstream of early signaling pathways, LMP1 induces the up-regulation of costimulatory and adhesion molecules (3,22). Expression of the important costimulatory molecule CD80 on antigen-presenting cells enhances the activation of naïve T cells (52) and is up-regulated by both CD40 and LMP1 (3,22). Previous studies revealed that CD40 does not require the TRAF6 TRAF-C domain to induce CD80 expression in B cells (29). To examine this requirement for LMP1-mediated CD80 up-regulation, TRAF6 Ϫ/Ϫ cells or TRAF6 Ϫ/Ϫ cells transfected to express WT TRAF6 or TRAF6⌬TRAF were cultured in the absence of stimulation or in the presence of an isotype control Ab or anti-hCD40 Ab to induce hCD40LMP1 activation. LPS (a TLR4 agonist) served as a positive control. The absence of TRAF6 abrogated the ability of hCD40LMP1 to induce CD80 expression (Fig. 3). In TRAF6 Ϫ/Ϫ cells, exogenous expression of WT TRAF6, but not similar levels of TRAF6⌬TRAF, induced normal up-regulation of CD80. Although there appears to be a slight shift in CD80 expression when TRAF6 Ϫ/Ϫ cells expressing TRAF6⌬TRAF were stimulated via hCD40LMP1, this change was very modest. Thus, TRAF6⌬TRAF was unable to effectively restore LMP1-mediated CD80 up-regulation. This LMP1-mediated event was revealed to require the TRAF6 TRAF-C domain, suggesting that direct TRAF6 association was needed.
TRAF6 Association with LMP1-Although no TRAF6 binding motif has been described in the LMP1 CY domain to date, the results presented in Figs. 2 and 3 indicated that TRAF6 associates with the CY domain of LMP1. This domain contains two important subdomains, CTAR1 (amino acids 187-241) and CTAR2 (amino acids 242-386), which mediate the binding of signaling molecules (17)(18)(19)21). Previous studies have shown that CTAR1 and CTAR2 cooperate to mediate LMP1 functions, including activation of early signaling pathways (53). To determine TRAF6 association with LMP1, we used mouse B cell lines stably expressing hCD40LMP1 or mutant hCD40LMP1 molecules described earlier (18,43), expressing only LMP1 CTAR1 (hCD40CTAR1) or only CTAR2 (hCD40CTAR2) (Fig.  4A). An hCD40 molecule lacking nearly all of the CY domain (hCD40⌬55) and shown not to deliver any signals to B cells (47) was included as a control for nonspecific binding. The results presented in Fig. 4B demonstrate that TRAF6 associated with hCD40LMP1 and the CTAR1, but not the CTAR2, subdomain of hCD40LMP1 in B cells.
TRAF6 Association with the TRAF1/2/3/5 Binding Site-The major TRAF1/2/3/5 binding site (TBS) is located within the CTAR1 subdomain of LMP1 (43). Our laboratory previously demonstrated that TRAFs 1, 2, 3, and 5 associate with CTAR1 but not CTAR2 in B cells (18). Although the TRAF6 binding site in CD40 is distinct from the TBS shared by the other TRAFs (32,35,36), no TRAF6 consensus binding site has been described in LMP1. To test whether TRAF6 was recruited to LMP1 via the TBS, we stimulated mouse B cells stably expressing hCD40LMP1 or an hCD40LMP1 receptor with the TBS mutated to disrupt the binding of TRAFs 1, 2, 3, and 5 (hCD40LMP1PQAA1) (43). Disruption of the TBS abolished the ability of hCD40LMP1 to recruit TRAF6 (Fig. 5A). To confirm this important finding in freshly isolated B cells, we took advantage of a new mCD40LMP1 Tg mouse. Our lab previously produced mice that express the mCD40LMP1 Tg on a CD40 Ϫ/Ϫ background so that the endogenous ligand for CD40, CD154, induces signaling through LMP1 only (25). Mice that expressed mCD40LMP1 with a mutated TBS  MARCH 25, 2011 • VOLUME 286 • NUMBER 12 (mCD40LMP1PQAA1 Tg) were developed in the same manner, as described under "Experimental Procedures." Using purified splenic B cells from mCD40 Ϫ/Ϫ mice or mCD40 Ϫ/Ϫ mice expressing mCD40LMP1 or mCD40LMP1PQAA1, we determined TRAF6 association with LMP1. Fig. 5B demonstrates that disruption of the TBS abolished the ability of TRAF6 to associate with LMP1 in freshly isolated mouse B cells, an important confirmation of the results obtained in mouse B cell lines.

DISCUSSION
LMP1 is a functional mimic of CD40, yet these receptors use TRAFs 1, 2, 3, and 5 differently (18,20,28). CD40-mediated JNK activation and CD80 up-regulation in B cells are TRAF6dependent but TRAF-C domain-independent (29), whereas in the present study we demonstrate that LMP1-mediated JNK, p38, NF-B1 activation, and CD80 up-regulation required the presence of this TRAF6 domain. Interestingly, previous work showed a decrease in LMP1-mediated CD80 up-regulation in the absence of TRAF3, although there was still residual CD80 up-regulation in TRAF3 Ϫ/Ϫ B cells compared with TRAF6 Ϫ/Ϫ B cells (20). Data in this study show that in the absence of TRAF6 there was no CD80 up-regulation, thus none of the other TRAFs have a redundant role in this process. However, it is possible that TRAF3 and TRAF6 function together to promote CD80 expression, as both TRAFs play a role in this LMP1mediated function. Furthermore, whereas TRAF6 binds to a site on CD40 distinct from the major TBS (28,32,35,36), our results revealed that TRAF6 association required the TBS on LMP1, further highlighting differences between the manner in which LMP1 and CD40 utilize TRAF6. The TRAF-C domain mediates receptor binding to TNFR superfamily members (50,51), and the absence of this domain abolishes TRAF6 binding to members of this superfamily (29,54,55). Interestingly, the TRAF6 TRAF-C domain is not required for several important CD40-mediated TRAF6-dependent events, indicating that CD40 may use TRAF6 as a strictly CY adaptor protein in certain situations (29), similar to TRAF6 use by the Toll-like receptors (44). In LMP1 signaling, the absence of the TRAF6 TRAF-C domain resulted in the abrogation of all tested LMP1-mediated signaling events. Thus, LMP1 likely requires receptor-associated TRAF6 to mediate the effects tested. However, it is possible that the TRAF6 TRAF-C domain is dispensable for some as yet unknown LMP1 functions. The present studies used A20 B cells, selected because they are responsive to CD40 and LMP1-mediated activation signals (20,29). Importantly, subclones completely and specifically deficient in TRAF6 were available (29). However, A20 cells cannot be induced to secrete Ig, and their production of a variety of cytokines and chemokines is not reliably detectable (data not shown). B cell-specific TRAF6 Ϫ/Ϫ mice are not commercially available, and they do not express LMP1 (EBV does not infect mouse B cells). Thus, we cannot exclude the possibility that LMP1 mediates some effects without requiring the TRAF6 TRAF-C domain, but it is clear that LMP1 is much more dependent upon this domain for signaling to B cells than is CD40.
The results presented here showed that TRAF6 associated with the CY domain of hCD40LMP1, specifically the TBS, following stimulation. We cannot simply directly compare the in vivo functions of LMP1 in mCD40LMP1Tg versus mCD40LMP1PQAA1Tg mice because the expression levels of the mCD40LMP1PQAA1 Tg molecule in founder lines are only approximately half the level of expression of mCD40LMP1 WT on B cells of this transgenic mouse (data not shown). This does not affect immunoprecipitation experiments because we adjusted conditions in vitro so that mCD40LMP1 levels were the same for each sample, but this discrepancy in surface levels could affect the interpretation of in vivo characteristics of the mCD40LMP1PQAA1 mouse. Although the mCD40LMP1PQAA1 Tg mice have similar levels of total B cells and B cell subsets compared with the mCD40LMP1 Tg mice, mCD40LMP1PQAA1 mice fail to form spontaneous germinal centers (in contrast to mCD40LMP1 mice), and B cells from mCD40LMP1PQAA1 mice fail to switch from IgM to IgG1 in vitro. 4 FIGURE 4. TRAF6 association with the CTAR subdomains of LMP1. A, schematic representation of hCD40LMP1, hCD40CTAR1, and hCD40CTAR2 receptors. The CY domain of hCD40LMP1 (left) contains two subdomains that allow the binding of signaling complexes: CTAR1 and CTAR2. The hCD40CTAR1 molecule (center) lacks the CTAR2 subdomain, whereas the hCD40CTAR2 molecule (right) lacks the CTAR1 subdomain. B, TRAF6 ϩ/ϩ CH12.LX cells stably expressing hCD40⌬55 (hCD40 lacking most of the CY domain), hCD40LMP1, hCD40CTAR1, or hCD40CTAR2 were stimulated with anti-hCD40 Ab-coated magnetic beads for 0 min (-) or 45 min (ϩ) or with MOPC-21 Ab-coated beads (isotype control, C) for 30 min. hCD40 was immunoprecipitated using the same beads. Western blots show levels of TRAF6 and hCD40 (hCD40⌬55, ϳ35 kDa; hCD40LMP1, ϳ72 kDa; hCD40CTAR1, ϳ48 kDa; hCD40CTAR2, ϳ65 kDa). Data shown are representative of three independent experiments. FIGURE 5. Role of the TRAF binding site in TRAF6 recruitment to hCD40LMP1. A, TRAF6 ϩ/ϩ CH12.LX cells stably expressing hCD40LMP1 or hCD40LMP1 with a mutated TRAF binding site (hCD40LMP1PQAA1) were stimulated with anti-hCD40 Ab-coated beads for 0 min (-) or 30 min (ϩ). hCD40 was immunoprecipitated using the same beads. Western blots show levels of TRAF6 and LMP1. These images were part of the same membrane, but the middle lanes were omitted here for clarity. B, splenic B cells were purified from mCD40 Ϫ/Ϫ , mCD40LMP1 ϩ , or mCD40LMP1PQAA1 ϩ mice (n ϭ 2 per group). Cells were stimulated for 30 min with anti-mCD40 Ab-coated beads. mCD40 was immunoprecipitated using the same beads. Western blots show levels of TRAF2 (control), TRAF6, and LMP1. Data shown are representative of four independent experiments.
There are several ways in which TRAF6-LMP1 interaction may occur. One possibility is that TRAF6 is recruited to LMP1 via other TRAFs. This mechanism of association has been shown for CD40, as TRAF1 shows very minimal direct binding to CD40 but is largely recruited to the CD40 CY domain by association with TRAF2 (28). It has been demonstrated previously that TRAFs 1 and 2 are not required for LMP1-mediated B cell activation (31), and we confirmed that TRAF1 and TRAF2 were not responsible for mediating recruitment of TRAF6 to LMP1 (supplemental Figs. 1 and 3). TRAFs 1, 2, 3, and 5 were not required for TRAF6 interaction with LMP1. Interestingly, we found that more TRAF6 was recruited to LMP1 in the absence of either TRAF1 or TRAF2, or in the absence of both TRAF1 and TRAF2 (supplemental Figs. 1 and  3). We also found that the absence of TRAF3 had little effect on TRAF6-LMP1 interaction (supplemental Figs. 1 and 3). TRAF5 has been recently reported to be an essential component of LMP1 signaling to B cells both in vitro and in vivo (24). Interestingly, TRAF5 required the presence of TRAF3 to associate with LMP1 (supplemental Fig. 2), indicating that TRAF5 cannot be required for TRAF6 recruitment, as the latter occurred independently of TRAF3 (supplemental Figs. 1 and 3).
TRAF1 showed increased association with LMP1 in the absence of TRAF2 (supplemental Fig. 3), whereas TRAF2 depended on TRAFs 1 and 3 for optimal association with LMP1 (supplemental Fig. 3). Thus, it appears that various TRAFs can compete or cooperate for binding to LMP1, as has been shown for CD40 (31). The enhanced binding of TRAF6 to LMP1 in the absence of TRAFs 1 and/or 2 is consistent with direct binding of TRAF6 to LMP1, although unidentified non-TRAF proteins may also participate. Whereas the structural motif in CD40 responsible for TRAF6 (QEPQEINF) (56) is not present in LMP1, TRAF6 could directly interact with LMP1 via a novel TRAF6 binding site, namely the TBS. This idea has precedent, as it was demonstrated that TRAF2 associates with CD40 via both the originally described TBS (PXQXT) (57) and a second binding site (SXXE) identified later (56). Future studies involving comprehensive structure-function analysis could test whether TRAF6 directly binds to LMP1. However, given the cooperative and competitive interactions seen for LMP1-TRAF binding, such analyses would need to include additional TRAFs in physiologically relevant proportions, as these can clearly influence TRAF6 binding. Simple in vitro assays of interactions between single purified proteins would not accurately model the in vivo situation.
Another possibility is that one or more non-TRAF proteins recruit TRAF6 to LMP1. Future studies could test the possible role of proteins shown to associate with TRAF6, CTAR1, or CD40, including A20, BS69, NEMO, Act1, Malt1, and HOIP (58 -63). A detailed proteomics study could identify any novel proteins involved in TRAF6 recruitment to LMP1.
Finally, TRAF6 could directly bind to LMP1, but additional proteins could enhance the avidity or stability of this association. Examination of these possibilities will be the subject of future studies.
Understanding the exact mechanisms by which LMP1 employs TRAF6 is important for the development of therapies that target the pathogenic effects of LMP1. As LMP1 is rapidly and constantly processed from the cell surface and lacks a typical extracellular domain (18 -20, 53), LMP1 itself may not be an ideal target for therapies. Thus, it would be beneficial to disrupt downstream LMP1 signaling, possibly by targeting TRAF6. As TRAF6 associates with LMP1, it could be feasible to target this interaction while not disrupting key CD40-mediated signals that do not require direct TRAF6 association. As LMP1 does not possess a canonical TRAF6 binding site, it could also be possible to disrupt TRAF6-LMP1 association while leaving its binding to CD40 intact. Although TRAF6 is used by multiple receptors, it is clearly used in different ways. Identifying the unique requirements of TRAF6 in LMP1 signaling adds important new information to our understanding of how the CD40 mimic LMP1 drives B cell pathogenesis.