TI  - Formation of  <prot>p27</prot> - CDK  complexes during the human mitotic cell cycle.
PG  - 135-46
AB  - In eukaryotic cells, the coordinated activation of different
      cyclin-dependent kinases regulates entry into S-phase. In vitro and in
      nonproliferating cells,  <prot>p27</prot>   associates with and inhibits
       cyclin/cycin-dependent kinase (CDK)  holoenzymes containing either <prot>CDK4</prot>,
      <prot>CDK6</prot>, or <prot>CDK2</prot>. Although many different types of proliferating cells
      contain <prot>p27</prot> protein, neither the interactions of  <prot>p27</prot>  with   cyclin / CDK  
      complexes nor the consequences of this interaction during the mitotic
      cycle have been fully explored. We report that, in MANCA cells, the amount
      of <prot>p27</prot> is constant during the cell cycle. In addition,    <prot>p27</prot>    associates with
      three different CDKs:  <prot>CDK2</prot> ,  <prot>CDK4</prot> , and  <prot>CDK6</prot> . Furthermore, the amount of <prot>p27</prot>
      is significantly lower than the amount of <prot>cyclin D3</prot> in these cells. The
      amount of  <prot>CDK4</prot>  and  <prot>CDK6</prot>  associated with   <prot>p27</prot>   does not change in a cell
      cycle-dependent fashion; in contrast, the amount of  <prot>CDK2</prot>  associated with
       <prot>p27</prot>  is lowest in early G1 cells and increases to a maximum in mid-G1
      phase, reaching a steady-state level in late G1-phase cells. After mid-G1
      phase, the amount of each  <prot>p27</prot> / CDK  complex remains constant through the
      remainder of the cell cycle. <prot>p27</prot>-immunoprecipitates contain an Rb-kinase
      activity. The substrate specificity, the expression pattern of this
      kinase, and the ability to deplete 50% of this kinase activity with a
      <prot>CDK6</prot>-specific antibody suggest that the <prot>CDK6</prot> protein mediates, in part,
      the <prot>p27</prot>-associated <prot>Rb</prot>-kinase activity. In contrast,  <prot>p27</prot>  complexes
      containing  <prot>CDK2</prot>  are incapable of phosphorylating<prot> histone H1</prot>. These data
      are consistent with a model wherein  <prot>cyclin D</prot> / CDK  complexes sequester the
      <prot>CDK2</prot>-dependent kinase inhibitory activity of <prot>p27</prot>.
AD  - Program in Molecular Biology, Memorial Sloan-Kettering Cancer Research
