Signal-induced site-specific phosphorylation targets IκBα to the ubiquitin-proteasome pathway

The transcription factor NF-κB is sequestered in the cytoplasm by the inhibitor protein IκBα. Extracellular inducers of NF-κB activate signal transduction pathways that result in the phosphorylation and subsequent degradation of IκBα. At present, the link between phosphorylation of IκBα and its degradation is not understood. In this report we provide evidence that phosphorylation of serine residues 32 and 36 of IκBα targets the protein to the ubiquitin-proteasome pathway. IκBα is ubiquitinated in vivo and in vitro following phosphorylation, and mutations that abolish phosphorylation and degradation of IκBα in vivo prevent ubiquitination in vitro. Ubiquitinated IκBα remains associated with NF-κB, and the bound IκBα is degraded by the 265 proteasome. Thus, ubiquitination provides a mechanistic link between phosphorylation and degradation of IκBα.