The activity of TRAF6 is regulated by autoubiquitination. This process, in turn, is regulated by p62. Cells devoid of p62 exhibit low basal TRAF6 polyubiquitination. When p62 is expressed, auto-ubiquitination of TRAF6 is enhanced. The details curated in this event represent the ubiquitination of TRAF6, even as ubiquitin is shown as 1 stoichiometrically.
TRAF6 possesses ubiquitin ligase activity and undergoes K-63-linked auto-ubiquitination after its oligomerization. In the first step, ubiquitin is activated by an E1 ubiquitin activating enzyme. The activated ubiquitin is transferred to a E2 conjugating enzyme (a heterodimer of proteins Ubc13 and Uev1A) forming the E2-Ub thioester. Finally, in the presence of ubiquitin-protein ligase E3 (TRAF6, a RING-domain E3), ubiquitin is attached to the target protein (TRAF6 on residue Lysine 124) through an isopeptide bond between the C-terminus of ubiquitin and the epsilon-amino group of a lysine residue in the target protein. In contrast to K-48-linked ubiquitination that leads to the proteosomal degradation of the target protein, K-63-linked polyubiquitin chains act as a scaffold to assemble protein kinase complexes and mediate their activation through proteosome-independent mechanisms. This K63 polyubiquitinated TRAF6 activates the TAK1 kinase complex.
TRAF6 is crucial for both RIG-I- and MDA5-mediated antiviral responses. The absence of TRAF6 resulted in enhanced viral replication and a significant reduction in the production of type I IFNs and IL6 after infection with RNA virus. Activation of NF-kB and IRF7, but not that of IRF3, was significantly impaired during RIG-like helicases (RLHs) signaling in the absence of TRAF6. TRAF6-induced activation of IRF is likely to be specific for IRF7, while TRAF3 is thought to activate both IRF3 and IRF7. These results strongly suggest that the TRAF6- and TRAF3-dependent pathways are likely to bifurcate at IPS-1, but to converge later at IRF7 in order to co-operatively induce sufficient production of type I IFNs during RLH signaling.
The TRAF6/TAK1 signal activates a canonical IKK complex, resulting in the activation of NF-kB as well as MAPK cascades leading to the activation of AP-1. Although TRAF6/TAK1 has been implicated in Tool like receptor (TLR) mediated cytokine production, the involvement of these molecules in the regulation of type I IFN induction mediated by RIG-I/MDA5 pathway is largely unknown. According to the study done by Yoshida et al RIG-I/IPS-1 pathway requires TRAF6 and MAP3K, MEKK1 to activate NF-kB and MAP Kinases for optimal induction of type I IFNs.
In plasmacytoid dendritic cell induction of type I IFNs critically depends on IFN regulatory factor 7 in TLR7 and 9 signaling (Honda et al 2005). IRF-7, but not IRF3, interacts with MyD88, TRAF6, and IRAKs and translocates to the nucleus upon phosphorylation (Kawai et al 2004; Uematsu et al 2005).
TLR7/8 signaling was shown to induce IRF5 activation along with IRF7 [Schoenemeyer et al 2005], while IRF8 [Tsujimura H et al 2004] and IRF1 were reported to be implicated in TLR9 signaling.
TRAF6 mediates NFkB activation via canonical phosphorylation of IKK complex by TAK1. TRAF6 and TAK1 also regulate MAPK cascades leading to the activation of AP-1.