In human, together with ubiquitin-conjugating E2-type enzymes UBC13 and UEV1A (also known as UBE2V1), TRAF6 catalyses Lys63-linked ubiquitination. It is believed that auto polyubiquitination and oligomerization of TRAF6 is followed by binding the ubiquitin receptors of TAB2 or TAB3 (TAK1 binding protein 2 and 3), which stimulates phosphorylation and activation of TGF beta-activated kinase 1(TAK1).
TAK1 phosphorylates IKK alpha and IKK beta, which in turn phosphorylate NF-kB inhibitors - IkB and eventually results in IkB degradation and NF-kB translocation to the nucleus. Also TAK1 mediates JNK and p38 MAP kinases activation by phosphorylating MKK4/7 and MKK3/6 respectivly resulting in the activation of many transcription factors.
The role of TRAF6 is somewhat controversial and probably cell type specific. TRAF6 autoubiquitination was found to be dispensable for TRAF6 function to activate TAK1 pathway. These findings are consistent with the new mechanism of TRAF6-mediated NF-kB activation that was suggested by Xia et al. (2009). TRAF6 generates unanchored Lys63-linked polyubiquitin chains that bind to the regulatory subunits of TAK1 (TAB2 or TAB3) and IKK(NEMO), leading to the activation of the kinases.
Xia et al. (2009) demonstrated in vitro that unlike polyubiquitin chains covalently attached to TRAF6 or IRAK, TAB2 and NEMO-associated ubiquitin chains were found to be unanchored and susceptible to N-terminal ubiquitin cleavage. Only K63-linked polyubiquitin chains, but not monomeric ubiquitin, activated TAK1 in a dose-dependent manner. Optimal activation of the IKK complex was achieved using ubiquitin polymers containing both K48 and K63 linkages.
Furthermore, the authors proposed that the TAK1 complexes might be brougt in close proximity by binding several TAB2/3 to a single polyubiquitin chain to facilitate TAK1 kinase trans-phosphorylation. Alternativly, the possibility that polyUb binding promotes allosteric activation of TAK1 complex should be considered (Walsh et al 2008).