Site-specific phosphorylation of IkappaBalpha by a novel ubiquitination-dependent protein kinase activity

Cell. 1996 Mar 22;84(6):853-62. doi: 10.1016/s0092-8674(00)81064-8.


Signal-induced activation of the transcription factor NF-kappaB requires specific phosphorylation of the inhibitor IkappaBalpha and its subsequent proteolytic degradation. Phosphorylation of serine residues 32 and 36 targets IkappaBalpha to the ubiquitin (Ub)-proteasome pathway. Here we report the identification of a large, multisubunit kinase (molecular mass approximately 700 kDa) that phosphorylates IkappaBalpha at S32 and S36. Remarkably, the activity of this kinase requires the Ub-activating enzyme (E1), a specific Ub carrier protein (E2) of the Ubc4/Ubc5 family, and Ub. We also show that a ubiquitination event in the kinase complex is a prerequisite for specific phosphorylation of IkappaBalpha. Thus, ubiquitination serves a novel regulatory function that does not involve proteolysis.

MeSH terms

  • Binding Sites / physiology
  • Cytoplasm / enzymology
  • HeLa Cells / enzymology
  • Humans
  • Ligases / metabolism
  • Molecular Weight
  • NF-kappa B / antagonists & inhibitors*
  • Phosphorylation
  • Protein Kinases / metabolism*
  • Serine / metabolism
  • Ubiquitin-Conjugating Enzymes*
  • Ubiquitins / metabolism*
  • Yeasts / genetics


  • NF-kappa B
  • Ubiquitins
  • Serine
  • Ubiquitin-Conjugating Enzymes
  • ubiquitin-conjugating enzyme UBC4
  • Protein Kinases
  • Ligases