Stimulation-dependent IκBα phosphorylation marks the NF-κb inhibitor for degradation via the ubiquitin-proteasome pathway

The nuclear translocation of NF-κB follows the degradation of its inhibitor, IκBα, an event coupled with stimulation-dependent inhibitor phosphorylation. Prevention of the stimulation-dependent phosphorylation of IκBα, either by treating cells with various reagents or by mutagenesis of certain putative IκBα phosphorylation sites, abolishes the inducible degradation of IκBα. Yet, the mechanism coupling the stimulation-induced phosphorylation with the degradation has not been resolved. Recent reports suggest a role for the proteasome in IκBα degradation, but the mode of substrate recognition and the involvement of ubiquitin conjugation as a targeting signal have not been addressed. We show that of the two forms of IκBα recovered from stimulated cells in a complex with RelA and p50, only the newly phosphorylated from, pIκBα, is a substrate for an in vitro reconstituted ubiquitin-proteasome system. Proteolysis requires ATP, ubiquitin, a specific ubiquitin-conjugating enzyme, and other ubiquitin- proteasome components. In vivo, inducible IκBα degradation requires a functional ubiquitin-activating enzyme and is associated with the appearance of high molecular weight adducts of IκBα. Ubiquitin-mediated protein degradation may, therefore, constitute an integral step of a signal transduction process.