Signal-induced site-specific phosphorylation targets I kappa B alpha to the ubiquitin-proteasome pathway

The transcription factor NF-kappa B is sequestered in the cytoplasm by the inhibitor protein I kappa B Alpha. Extracellular inducers of NF-kappa B activate signal transduction pathways that result in the phosphorylation and subsequent degradation of I kappa B Alpha. At present, the link between phosphorylation of I kappa B Alpha and its degradation is not understood. In this report we provide evidence that phosphorylation of serine residues 32 and 36 of I kappa B Alpha targets the protein to the ubiquitin-proteasome pathway. I kappa B Alpha is ubiquitinated in vivo and in vitro following phosphorylation, and mutations that abolish phosphorylation and degradation of I kappa B Alpha in vivo prevent ubiquitination in vitro. Ubiquitinated I kappa B Alpha remains associated with NF-kappa B, and the bound I kappa B Alpha is degraded by the 26S Proteasome. Thus, ubiquitination provides a mechanistic link between phosphorylation and degradation of I kappa B Alpha.