Auto-ubiquitination of TRAF6

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具备泛素连接酶活性，并在其寡聚化后经历K-63连接的自身泛素化。首先，泛素由E1泛素激活酶激活。激活后的泛素转移至E2连接酶（由蛋白质Ubc13和Uev1A组成的异源二聚体），形成E2-Ub硫酯。最终，在泛素-蛋白连接酶E3（TRAF6，一种RING结构域E3）的存在下，泛素通过泛素C端与靶蛋白赖氨酸残基的ε-氨基之间的异肽键，附着于靶蛋白（TRAF6上的赖氨酸124位残基）。与导致靶蛋白蛋白酶体降解的K-48连接泛素化相反，K-63连接的多泛素链充当支架，以组装蛋白激酶复合物，并通过蛋白酶体非依赖机制介导其激活。这种K63多泛素化的TRAF6激活TAK1激酶复合物。