In humans, the IKKs - IkB kinase (IKK) complex serves as the master regulator for the activation of NF-kB by various stimuli. The IKK complex contains two catalytic subunits, IKK alpha and IKK beta associated with a regulatory subunit, NEMO (IKKgamma). The activation of the IKK complex and the NFkB mediated antiviral response are dependent on the phosphorylation of IKK alpha/beta at its activation loop and the ubiquitination of NEMO [Solt et al 2009; Li et al 2002]. NEMO ubiquitination by TRAF6 is required for optimal activation of IKKalpha/beta; it is unclear if NEMO subunit undergoes K63-linked or linear ubiquitination.
This basic trimolecular complex is referred to as the IKK complex. Each catalytic IKK subunit has an N-terminal kinase domain and leucine zipper (LZ) motifs, a helix-loop-helix (HLH) and a C-terminal NEMO binding domain (NBD). IKK catalytic subunits are dimerized through their LZ motifs.
IKK beta is the major IKK catalytic subunit for NF-kB activation. Phosphorylation in the activation loop of IKK beta requires Ser177 and Ser181 and thus activates the IKK kinase activity, leading to the IkB alpha phosphorylation and NF-kB activation.
RIP1 polyubiquitination was induced upon TNF- or poly(I-C) treatment of the macrophage cell line RAW264.7 and the U373 astrocytoma line (Cusson-Hermance et al 2005). These workers have suggested that RIP1 may use similar mechanisms to induce NF-kB in the TNFR1- and Trif-dependent TLR pathways.
RIP1 modification with Lys-63 polyubiquitin chains was shown to be essential for TNF-induced activation of NF-kB (Ea et al. 2006). It is thought that TRAF family members mediate this Lys63-linked ubiquitination of RIP1 (Wertz et al. 2004, Tada et al 2001, Vallabhapurapu and Karin 2009), which may facilitate recruitment of the TAK1 complex and thus activation of NF-kB. Binding of NEMO to Lys63-linked polyubiquitinated RIP1 is also required in the signaling cascade from the activated receptor to the IKK-mediated NF-kB activation (Wu et al. 2006).