TUMOR-NECROSIS-FACTOR ALPHA-INDUCED PHOSPHORYLATION OF I-KAPPA-B-ALPHA IS A SIGNAL FOR ITS DEGRADATION BUT NOT DISSOCIATION FROM NF-KAPPA-B

Activation of the NF-kappa B/Rel family of transcription factors is re gulated by a cytoplasmic inhibitor, I kappa B alpha, Activity of I kap pa B alpha is in turn modulated by phosphorylation and proteolysis. It has been postulated that phosphorylation of I kappa B alpha leads to its dissociation from NF-kappa B, and free I kappa B alpha is targeted for rapid degradation. However, this phosphorylation-mediated dissoci ation event has not been demonstrated in vivo, We demonstrate that, co ntrary to this hypothesis, phosphorylation of I kappa B alpha induced by tumor necrosis factor alpha in HeLa cells does not induce dissociat ion. We propose a model in which (i) induced phosphorylation of I kapp a B alpha does not result in its dissociation from NF-kappa B, (ii) ph osphorylation of I kappa B alpha serves as a signal for degradation, a nd (iii) degradation of I kappa B alpha occurs while it is still compl exed with NF-kappa B.