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Mol. Cell. Biol., Apr 1996, 1295-1304, Vol 16, No. 4
J DiDonato, F Mercurio, C Rosette, J Wu-Li, H Suyang, S Ghosh and M Karin
Extracellular stimuli that activate the transcription factor NF-kappaB
cause rapid phosphorylation of the IkappaBalpha inhibitor, which retains
NF-kappaB in the cytoplasm of nonstimulated cells. Phosphorylation of
IkappaBalpha is followed by its rapid degradation, the inhibition of which
prevents NF-kappaB activation. To determine the relationship between these
events, we mapped the inducible phosphorylation sites of IkappaBalpha. We
found that two residues, serines 32 and 36, were phosphorylated in response
to either tumor necrosis factor, interleukin-1, or phorbol ester.
Substitution of either serine blocks or slows down induction of
IkappaBalpha degradation. Substitutions of the homologous sites in
IkappaBbeta, serines 19 and 23, also prevent inducible IkappaBbeta
degradation. We suggest that activation of a single IkappaB kinas e or
closely related IkappaB kinases is the first cr itical step in NF-kappaB
activation. Once phosphorylated, IkappaB is ubiquitinated. Unlike wild-type
IkappaBalpha, the phosphorylation-defective mutants do not undergo
inducible polyubiquitination. As substitution of a conserved lysine residue
slows down the ubiquitination and degradation of IkappaBalpha without
affecting its phosphorylation, polyubiquitination is required for inducible
IkappaB degradation.
Copyright © 1996, American Society for Microbiology
Mapping of the inducible IkappaB phosphorylation sites that signal its ubiquitination and degradation
Department of Pharmacology, Program in Biomedical Science, University of California, San Diego School of Medicine, La Jolla, USA.
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