Abstract
PKCdelta is essential for apoptosis, but regulation of the proapoptotic function of this ubiquitous kinase is not well understood. Nuclear translocation of PKCdelta is necessary and sufficient to induce apoptosis and is mediated via a C-terminal bipartite nuclear localization sequence. However, PKCdelta 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 PKCdelta to the nucleus where it induces apoptosis. Analysis of each tyrosine residue in PKCdelta 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 PKCdelta Y64D/Y155D mutant accumulated in the nucleus in the absence of an apoptotic signal. Forced nuclear accumulation of PKCdelta-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 PKCdelta to the nucleus. We propose that phosphorylation/dephosphorylation of PKCdelta in the regulatory domain functions as a switch to promote cell survival or cell death.