Differential phosphorylation of the signal-responsive domain of I kappa B alpha and I kappa B beta by I kappa B kinases.
NF-kappa B activity is regulated by its association with the inhibitory I kappa B proteins, among which I kappa B alpha and I kappa B beta are the most abundant. I kappa B proteins are widely expressed in different cells and tissues and bind to similar combinations of NF-kappa B proteins. The degradation of I kappa B proteins allows nuclear translocation of NF-kappa B and hence plays a critical role in NF-kappa B activation. Previous studies have demonstrated that, although both I kappa B proteins are phosphorylated by the same I kappa B kinase (IKK) complex, and their ubiquitination and degradation following phosphorylation are carried out by the same ubiquitination/degradation machinery, their kinetics of degradation are quite different. To better understand the underlying mechanism of the differences in degradation kinetics, we have carried out a systematic, comparative analysis of the ability of the IKK catalytic subunits to phosphorylate I kappa B alpha and I kappa B beta. We found that, whereas IKK alpha is a weak kinase for the N-terminal serines of both I kappa B isoforms, IKK beta is an efficient kinase for those residues in I kappa B alpha. However, IKK beta phosphorylates the N-terminal serines of I kappa B beta far less efficiently, thereby providing an explanation for the slower rate of degradation observed for I kappa B beta. Mutational analysis indicated that the regions around the two N-terminal serines collectively influence the relative phosphorylation efficiency, and no individual residue is critical. These findings provide the first systematic analysis of the ability of I kappa B alpha and I kappa B beta to serve as substrates for IKKs and help provide a possible explanation for the differential degradation kinetics of I kappa B alpha and I kappa B beta.
Wu
Chun
C
Section of Immunobiology and the Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
Ghosh
Sankar
S
D000818:Animals; D002384:Catalysis; D002460:Cell Line; D006801:Humans; D020672:I-kappa B Proteins / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D051379:Mice; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation
C083487:I kappa B beta protein; D020672:I-kappa B Proteins