We established a novel in vitro phosphorylation and ubiquitination assays of IKB-alpha. Using these assays we have been able to identify the IkB degradation motif and to purify the ubiquitination ligase. Identification of the key enzymes in the IkB-alpha phosphorylation ubiquitination process allowed us to replace many of the components that were initially provided by cell extract with recombinant or immunopurified proteins.