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Bromodomain Protein Brd4 Binds to GTPase-Activating SPA-1, Modulating Its Activity and Subcellular Localization

  • Andrea Farina
  • Masakazu Hattori
  • Jun Qin
  • Yoshihiro Nakatani
  • Nagahiro Minato
  • Keiko Ozato
American Society for Microbiology
Publication Date
Oct 01, 2004
  • Biology


Brd4 is a mammalian protein that contains a double bromodomain. It binds to chromatin and regulates cell cycle progression at multiple stages. By immunopurification and mass spectrometry, we identified a Rap GTPase-activating protein (GAP), signal-induced proliferation-associated protein 1 (SPA-1), as a factor that interacts with Brd4. SPA-1 localizes to the cytoplasm and to a lesser degree in the nucleus, while Brd4 resides in the nucleus. Bifluorescence complementation revealed that Brd4 and SPA-1 interact with each other in the nucleus of living cells. Supporting the functional importance of the interaction, Brd4 enhanced Rap GAP activity of SPA-1. Furthermore ectopic expression of SPA-1 and Brd4 redirected subcellular localization of the partner and disrupted normal cell cycle progression. These effects were, however, reversed by coexpression of the two proteins, indicating that a proper balance between Brd4 and SPA-1 in G2 is required for cell division. This work reveals a novel link between Brd4 and a GTPase-dependent mitogenic signaling pathway.

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