A complex containing betaTrCP recruits Cdc34 to catalyse ubiquitination of IkappaBalpha.

Activation of transcription factor NF-kappaB is accomplished by degradation of its inhibitor IkappaBalpha. Signal induced phosphorylation of IkappaBalpha on serine 32 and 36 targets the protein for ubiquitination on lysine 21 and 22. Here we use a phosphorylated peptide substrate representing residues 20-43 of IkappaBalpha to investigate requirements for ubiquitination of IkappaBalpha. Phosphorylation dependent polyubiquitination is carried out by a multiprotein complex containing betaTrCP, Skp1 and Cdc53 (Cull). In the presence of ubiquitin activating enzyme and the protein complex containing betaTrCP, polyubiquitination of IkappaBalpha peptide was dependent on the presence of Cdc34, while Ubc5 only stimulated mono- and di-ubiquitination.