A complex containing bTrCP recruits Cdc34 to catalyse ubiquitination of IkBa

Activation of transcription factor NF-kappa B is accomplished by degradation of its inhibitor I kappa B alpha, Signal induced phosphorylation of I kappa B alpha on serine 32 and 36 targets the protein for ubiquitination on lysine 21 and 22, Here me use a phosphorylated peptide substrate representing residues 20-43 of I kappa B alpha to investigate requirements for ubiquitination of I kappa B alpha. Phosphorylation dependent polyubiquitination is carried out by a multiprotein complex containing beta TrCP, Skp1 and Cdc53 (Cul1), In the presence of ubiquitin activating enzyme and the protein complex containing beta TrCP, polyubiquitination of I epsilon B alpha peptide mas dependent on the presence of Cdc34, while Ubc5 only stimulated mono- and di-ubiquitination. (C) 1999 Federation of European Biochemical Societies.