The human G2 checkpoint control protein hRAD9 is a nuclear phosphoprotein that forms complexes with hRAD1 and hHUS1

Mol Biol Cell. 1999 Jun;10(6):1985-95. doi: 10.1091/mbc.10.6.1985.


Eukaryotic cells actively block entry into mitosis in the presence of DNA damage or incompletely replicated DNA. This response is mediated by signal transduction cascades called cell cycle checkpoints. We show here that the human checkpoint control protein hRAD9 physically associates with two other checkpoint control proteins, hRAD1 and hHUS1. Furthermore, hRAD1 and hHUS1 themselves interact, analogously to their fission yeast homologues Rad1 and Hus1. We also show that hRAD9 is present in multiple phosphorylation forms in vivo. These phosphorylated forms are present in tissue culture cells that have not been exposed to exogenous sources of DNA damage, but it remains possible that endogenous damage or naturally occurring replication intermediates cause the observed phosphorylation. Finally, we show that hRAD9 is a nuclear protein, indicating that in this signal transduction pathway, hRAD9 is physically proximal to the upstream (DNA damage) signal rather than to the downstream, cytoplasmic, cell cycle machinery.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • COS Cells / metabolism
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism*
  • Cell Line
  • DNA-Binding Proteins*
  • Endonucleases / genetics
  • Endonucleases / metabolism*
  • Fungal Proteins / genetics
  • Fungal Proteins / metabolism
  • G2 Phase / physiology*
  • HeLa Cells / metabolism
  • Humans
  • Nuclear Proteins / genetics
  • Nuclear Proteins / metabolism*
  • Phosphoproteins / genetics
  • Phosphoproteins / metabolism
  • Phosphorylation
  • Precipitin Tests
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Schizosaccharomyces pombe Proteins


  • Cell Cycle Proteins
  • DNA-Binding Proteins
  • Fungal Proteins
  • Nuclear Proteins
  • Phosphoproteins
  • Recombinant Proteins
  • Schizosaccharomyces pombe Proteins
  • hus1 protein, S pombe
  • rad9 protein
  • Endonucleases