The Signal Response of IκBα Is Regulated by Transferable N- and C-Terminal Domains

IκBα retains the transcription factor NF-κB in the cytoplasm, thus inhibiting its function. Various stimuli inactivate IκBα by triggering phosphorylation of the N-terminal residues Ser32 and Ser36. Phosphorylation of both serines is demonstrated directly by phosphopeptide mapping utilizing calpain protease, which cuts approximately 60 residues from the N terminus, and by analysis of mutants lacking one or both serine residues. Phosphorylation is followed by rapid proteolysis, and the liberated NF-κB translocates to the nucleus, where it activates transcription of its target genes. Transfer of the N-terminal domain of IκBα to the ankyrin domain of the related oncoprotein Bcl-3 or to the unrelated protein glutathione S-transferase confers signal-induced phosphorylation on the resulting chimeric proteins. If the C-terminal domain of IκBα is transferred as well, the resulting chimeras exhibit both signal-induced phosphorylation and rapid proteolysis. Thus, the signal response of IκBα is controlled by transferable N-terminal and C-terminal domains.