LIG_TRAF6_MATH_1

The Tumour necrosis factor Receptor-Associated Factors (TRAFs) are a family of seven Ring-type E3 ligases, most of which have a C-terminal MATH domain that interacts with substrates containing the appropriate SLiM (Yamamoto,2021). The family is named for its association with members of the TNFR membrane receptor superfamily (Rothe,1994) but TRAFs also interact with a number of soluble cytosolic proteins. Most TRAF proteins play critical roles in the transmission of signals from the surface receptors leading to the activation of NF-κB gene expression. TRAF6, along with 2 and 5, acts in the canonical activation pathway while TRAF2 and 3 act in the non-canonical pathway (Yamamoto,2021). A striking feature of TRAF6 cell biology is that the sets of interacting proteins may be completely different in the various signalling systems in which it participates (Yamamoto,2021; Chathuranga,2021).

For some but not all TRAFs, the SLiMs are known. The core motif, in the case of TRAF6, is PxExx[DE(Ar)] (where (Ar) is an aromatic residue) (Darnay,1999). TRAF6 interacts with the UBE2N and UBE2V1 heterodimeric E2 ligase which catalyses Lys63-linked polyubiquitin chains (Deng,2000) on substrate proteins docked to the C-terminal MATH (TRAF-C) domain of TRAF6. TRAF6 homotrimerises and some substrates have more than one binding motif in their regions of intrinsically unstructured peptide.

TRAF6 is directly involved in bone development and maintenance (Yamamoto,2021). The activation of NF-κB and NFATc1 is required for the development of the bone resorbing osteoclasts, large multinucleate cells derived from fused monocytes. In precursor cells, binding of the cytokine RANKL (Tnfsf11) to RANK (TNF family receptor Tnfrsf11a) activates the signalling pathway. Trimeric RANK binds TRAF6 trimer using three TRAF6 interacting motifs, enabling cooperative complex formation. This leads to NF-κB expression which together with increased Ca²⁺ signalling then activates NFATc1 expression, which enables osteoclast maturation. TRAF6 knockout mice have bone resorption defects (Lomaga,1999).

TRAF6 is essential for innate immunity signalling in macrophages initiated by Toll-Like Receptors, TLRs (Yamamoto,2021). In the presence of bacterial components such as lipopolysaccharide, surface receptors TLR2, 4, and 6 stimulate the formation of a large oligomeric complex, the MyDDosome, assembled from activated MyD88 and IRAK kinases (Cohen,2020). Endosomal receptors TLR-7 and 9 responding to the presence of nucleic acid (often of viral origin) in these vesicles also stimulate the MyDDosome complex (Yamamoto,2021). Recruitment of TRAF6 by binding motifs in the C-terminal tails of IRAK1, 2, 3 leads to NF-κB activation via the IKK kinase complex.

CD40 (TNFRSF5) is another receptor which signals through TRAF6 (and other TRAFs) in multiple immune cells and other contexts, for example, acting as a coregulator of B cell activation by helper T cells (Yamamoto,2021).

Double-stranded RNA in the cytosol is indicative of viral infection and is detected by RIG-I-like receptors (RLRs) which then induce the mitochondrial surface protein MAVS to self-polymerise (Chathuranga,2021). The resultant MAVS signalosome recruits TRAF6 which participates in NF-κB activation and also IRF-7 activation, the latter inducing type-1 interferon gene expression. DNA in the cytosol is also indicative of pathogen infection: In Teleost fish (but not in mammals), a TRAF6-binding motif in the cytosolic tail of STING, located on the ER, enables similar downstream signalling (de Oliveira Mann,2019).

Of pathogens known to hijack the TRAF6 signalling pathway using motif mimicry, herpesviruses are the most studied. The bovine herpesvirus E3 ligase BICP0 converts the binding motif into a degron and induces K48 polyubiquitination, targeting TRAF6 for destruction by the proteasome (Cao,2019). The T cell-tropic Herpesvirus ateles recruits TRAF6 to stimulate NF-κB signalling (Heinemann,2006). Similarly, HSV U(L)37 also binds TRAF6 for NF-κB stimulation as required for viral replication (Liu,2008).