PKCd is essential for apoptosis, but regulation of the proapoptotic function of this ubiquitous kinase is not well understood. Nuclear translocation of PKCd is necessary and sufficient to induce apoptosis and is mediated via a Cterminal bipartite nuclear localization sequence. However, PKCd is found predominantly in the cytoplasm of nonapoptotic cells, and the apoptotic signal that activates its nuclear translocation is not known. We show that in salivary epithelial cells, phosphorylation at specific tyrosine residues in the N-terminal regulatory domain directs PKCd to the nucleus where it induces apoptosis. Analysis of each tyrosine residue in PKCd by site-directed mutagenesis identified two residues, Y64 and Y155, as essential for nuclear translocation. Suppression of apoptosis correlated with suppressed nuclear localization of the Y-F mutant proteins. Moreover, a phosphomimetic PKCd Y64D/Y155D mutant accumulated in the nucleus in the absence of an apoptotic signal. Forced nuclear accumulation of PKCd-Y64F and Y155F mutant proteins, by attachment of an SV40 nuclear localization sequence, fully reconstituted their ability to induce apoptosis, indicating that tyrosine phosphorylation per se is not required for apoptosis, but for targeting PKCd to the nucleus. We propose that phosphorylation/dephosphorylation of PKCd in the regulatory domain functions as a switch to promote cell survival or cell death.

Background: Nuclear import of protein kinase C␦ is required for DNA-damage-induced apoptosis. Results: c-Src and c-Abl phosphorylate PKC␦ to regulate nuclear import. Tyrosine kinase inhibitors block nuclear translocation of PKC␦ and suppress apoptosis. Conclusion: Tyrosine kinase inhibitors can regulate the pro-apoptotic function of protein kinase C␦. Significance: Tyrosine kinase inhibitors may improve the quality of life in cancer patients receiving radiation therapy.

Background: Tyrosine phosphorylation regulates nuclear translocation of proapoptotic protein kinase C delta (PKC␦). Results: Tyrosine phosphorylation causes a conformational change that exposes the nuclear localization sequence, allowing binding of importin-␣. Conclusion: Nuclear localization of PKC␦ is regulated by access of importin-␣ to the nuclear localization sequence. Significance: Nuclear import of PKC␦, which induces apoptosis, is tightly regulated so as to prevent inappropriate cell death.