Signal-dependent degradation of IκBα is mediated by an inducible destruction box that can be transferred to NF-κB, Bcl-3 or p53

Activation of the transcription factor NF-kappa B in response to a variety of stimuli is governed by the signal-induced proteolytic degradation of NF-kappa B inhibitor proteins, the I kappa Bs. We have investigated the sequence requirements for signal-induced I kappa B alpha phosphorylation and proteolysis by generating chimeric proteins containing discrete sub-regions of I kappa B alpha alpha fused to the I kappa B alpha homologue Bcl-3, the transcription factor NF-kappa B1/p50 and the tumour suppressor protein p53. Using this approach we show that the N-terminal signal response domain (SRD) of I kappa B alpha directs their signal-dependent phosphorylation and degradation when transferred to heterologous proteins. The C-terminal PEST sequence from I kappa B alpha was not essential for induced proteolysis of the chimeric proteins. A deletion analysis conducted on the SRD identified a 25 amino acid sub-domain of I kappa B alpha that is necessary and sufficient for the degradative response in vivo and for recognition by TNF alpha-dependent I kappa B alpha kinase in vitro. The results obtained should prove instrumental in the further characterization of I kappa B-specific kinases, as well as the E2 and E3 enzymes responsible for I kappa B alpha ubiquitination. Furthermore, they suggest a novel strategy for generating conditional mutants, by targetting heterologous proteins for transient elimination by the I kappa B alpha pathway.