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 also known as TRIKA1 (TRAF6-regulated IKK activator 1)) 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 (Deng et al. 2000, Lamothe et al. 2007). 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.
Lamothe, B, Wu, H, Darnay, BG, Besse, A, Campos, AD, Webster, WK
Deng, L, Pickart, C, Spencer, E, You, J, Wang, C, Slaughter, C, Braun, A, Chen, ZJ, Yang, L
ubiquitin-protein transferase activity of p-T231-CARD9:BCL10:MALT1:TRAF6 oligomers [plasma membrane]
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