PKC eta associates with cyclin E/cdk2/p21 complex, phosphorylates p21 and inhibits cdk2 kinase in keratinocytes

PKC is activated on the cell membrane by phospholipids, thereby transducing signals to intracellular pathways. We provide here another function of PKC , namely, regulating cell cycle by interaction with the cyclin E/cdk2/p21 c omplex. Among the 10 isoforms of PKC, PKC eta is predominantly expressed in squamous cell epithelia and induces terminal differentiation of keratinocy tes, PKC eta that is endogenously expressed or overexpressed was found to a ssociate with the cyclin E/cdk2/p21 complex in keratinocytes of mice and hu mans. Requirement of a possible adaptor protein to the binding was suggeste d by the reconstitution of PKC eta and the cyclin E/cdk2/p21 complex which were prepared from human keratinocytes or Sf9 insect cells. Colocalization of PKC eta with cdk2 and cyclin E was observed in the cytoplasm, particular ly in the perinuclear region. p21 was phosphorylated in the complex in a PK C-activator dependent manner. Association of PKC eta with cdk2 resulted in marked inhibition of cdk2-kinase activity when measured by phosphorylation of Rb, Dominant negative PKC eta associated with the cyclin E/cdk2/p21 comp lex, but caused a little inhibition of cdk2 kinase activity. Among the know n regulatory mechanisms of cdk2 activity, dephosphorylation of Thr160 was d emonstrated.