I kappa B-alpha inhibits transcription factor NF-kappa B by retaining
it in the cytoplasm. Various stimuli, typically those associated with
stress or pathogens, rapidly inactivate I kappa B-alpha. This liberate
s NF-kappa B to translocate to the nucleus and initiate transcription
of genes important for the defense of the organism. Activation of NF-k
appa B correlates with phosphorylation of I kappa B-alpha and requires
the proteolysis of this inhibitor. When either serine-32 or serine-36
of I kappa B-alpha was mutated, the protein did not undergo signal-in
duced phosphorylation or degradation, and NF-kappa B could not be acti
vated. These results suggest that phosphorylation at one or both of th
ese residues is critical for activation of NF-kappa B.