Binding of TAB2 and TAB3 to K63-linked polyubiquitin chains leads to the activation of TAK1 by an uncertain mechanism. Phosphorylation of TAK1 within the activation loop of the kinase is absolutely required for TAK1 activity. TAB1 is known to augment TAK1 catalytic activity by mediating spontaneous oligomerization and induces autophosphorylation of TAK1 (Kishimoto et al. 2000). The binding of TAB2/3 to polyubiquitinated TRAF6 may facilitate polyubiquitination of TAB2/3 by TRAF6 (Ishitani et al. 2003), which might result in conformational changes within the TAK1 complex that leads to the activation of TAK1. Some biochemical studies revealed that free K63 polyubiquitin chains, which are not conjugated to any cellular protein, can directly activate the TAK1 kinase complex (Xia et al. 2009).