Protein kinase Cδ is responsible for constitutive and DNA damage-induced phosphorylation of Rad9

The mammalian homolog of the Schizosaccharomyces pombe Rad9 is involved in checkpoint signaling and the induction of apoptosis. While the mechanisms responsible for the regulation of human Rad9 (hRad9) are not known, hRad9 is subject to hyperphosphorylation in the response of cells to DNA damage. The present results demonstrate that protein kinase Cδ (PKCδ) associates with Rad9 and that DNA damage induces this interaction. PKCδ phosphorylates hRad9 in vitro and in cells exposed to genotoxic agents. The functional significance of the interaction between hRad9 and PKCδ is supported by the finding that activation of PKCδ is necessary for formation of the Rad9-Hus1-Rad1 complex. We also show that PKCδ is required for binding of hRad9 to Bcl-2. In concert with these results, inhibition of PKCδ attenuates Rad9-mediated apoptosis. These findings demonstrate that PKCδ is responsible for the regulation of Rad9 in the Hus1-Rad1 complex and in the apoptotic response to DNA damage.