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I kappa B alpha can localize in the nucleus but shows no direct transactivation potential

D E Cressman1, R Taub

  • 1Department of Genetics, University of Pennsylvania, School of Medicine, Philadelphia 19104-6145.

Oncogene
|September 1, 1993
PubMed

Insights

Inhibitor of kappa B alpha (I kappa B alpha) can act as a transcriptional activator when in the nucleus, interacting with RelA(p65). This protein, typically cytoplasmic, translocates to the nucleus upon cell stimulation.

Area of Science:

  • Molecular Biology
  • Cellular Biology
  • Transcription Regulation

Background:

  • Inhibitor of kappa B alpha (I kappa B alpha) is known as a cytoplasmic inhibitor of Nuclear Factor kappa B (NF-kappa B) and c-Rel, preventing NF-kappa B nuclear translocation.
  • Some proteins with ankyrin-type repeats function as transcription factors, suggesting potential nuclear roles for I kappa B family members.
  • Previous studies indicate that certain forms of I kappa B can be found in the nucleus, challenging their exclusively cytoplasmic role.

Purpose of the Study:

  • To investigate whether a nuclear form of I kappa B alpha possesses transcriptional activator properties.
  • To identify the specific domain of I kappa B alpha responsible for transactivation.
  • To understand the cellular localization dynamics of I kappa B alpha and its association with NF-kappa B components.

Main Methods:

  • Generation and testing of Gal4-I kappa B alpha fusion proteins for transactivation activity in 3T3 fibroblasts.
  • Domain mapping to pinpoint the region of I kappa B alpha mediating transactivation.
  • Immunofluorescence and cell fractionation studies to determine I kappa B alpha localization under different cellular conditions.
  • Double antibody immunoprecipitation to assess I kappa B alpha association with RelA(p65).

Main Results:

  • Gal4-I kappa B alpha fusion proteins demonstrated strong transactivation of a Gal4 site-containing promoter.
  • The ankyrin repeat domain of I kappa B alpha, not the acidic domain, was identified as crucial for transactivation.
  • Overexpressed I kappa B alpha localized to the nucleus in serum-stimulated cells but remained largely cytoplasmic in serum-deprived cells.
  • I kappa B alpha was found to associate with RelA(p65) in proliferating cells and in nuclear Gal4-I kappa B alpha fusions.
  • The transactivation ability of Gal4-I kappa B alpha is attributed to the activation domain of associated RelA(p65), not direct transactivation by I kappa B alpha itself.

Conclusions:

  • I kappa B alpha can function as a transcriptional activator, mediated by its ankyrin repeat domain and interaction with RelA(p65).
  • Cellular stimulation induces nuclear translocation of I kappa B alpha, enabling its role in transcription.
  • Unlike Bcl-3, I kappa B alpha does not directly transactivate promoters through kappa B sites when bound to RelA(p65).

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