Two major signaling steps are required for the removal of IkappaB (IkB) alpha an inhibitor of NF-kB: activation of the IkB kinase (IKK) and degradation of the phosphorylated IkB alpha. IKK activation and IkB degradation involve different ubiquitination modes; the former is mediated by K63-ubiquitination and the later by K48-ubiquitination. Mutational analysis of IkB alpha has indicated that K21 and K22 are the primary sites for addition of multiubiquitination chains while K38 and K47 are the secondary sites. In a transesterification reaction the ubiquitin is transferred from the ubiquitin-activating enzyme (E1) to an E2 ubiquitin-conjugating enzyme, which may, in turn, transfer the ubiquitin to an E3 ubiquitin protein ligase. UBE2D2 (UBC4) or UBE2D1 (UBCH5) or CDC34 (UBC3) acts as the E2 and SCF (SKP1-CUL1-F-box)-beta-TRCP complex acts as the E3 ubiquitin ligase (Strack et al. 2000, Wu et al. 2010). beta-TRCP (beta-transducin repeats-containing protein) is the substrate recognition subunit for the SCF-beta-TRCP E3 ubiquitin ligase. beta-TRCP binds specifically to phosphorylated IkB alpha and recruits it to the SCF complex, allowing the associated E2, such as UBC4 and or UBCH5 to ubiquitinate Ikappa B alpha (Baldi et al. 1996, Rodriguez et al. 1996, Scherer et al. 1995, Alkalay et al. 1995).
