Apoptosis, or programmed cell death, is critical for vertebrate development, the clearance of damaged or genetically altered cells and is induced by a variety of exogenous agents including irradiation, chemotherapeutic drugs and cell toxins. Phosphorylation of cellular proteins by protein kinases is widely utilized as a mechanism to relay information within the cell, and signal transduction by this mechanism has been shown to regulate many cellular processes such including proliferation and apoptosis. We are interested in how specific members of the protein kinase C (PKC) family function to modulate apoptosis. As a model we use salivary epithelial cells either in culture, or derived from genetically modified mice that have specific defects in protein kinase C directed signal transduction. Our studies have demonstrated that PKCδ is an early and essential regulator of apoptosis in salivary epithelial cells and that PKCδ may function upstream of the mitochondria as an integrator of diverse death signals (1, 2). Conversely, PKCα activity is required for cell survival, as inhibition of PKCα using a dominant negative PKCα induces apoptosis in salivary epithelial cells (3). Since PKCδ is a ubiquitously expressed kinase, an important question is how the pro-apoptotic function is activated in response to specific signals. Structure-function analysis of PKCδ suggests that activation of this pro-apoptotic function is regulated by multiple mechanisms. Using techniques to localize PKCδ in cells undergoing apoptosis, we have shown that PKCδ translocates to the nucleus, and we have identified a nuclear localization sequence (NLS) in the COOH-terminus of PKCδ (4). Mutations in PKCδ which prevent its nuclear translocation, also inhibit apoptosis, indicating that PKCδ functions in the nucleus to regulate the apoptotic pathway. However, PKCδ is predominantly cytoplasm in the absence of an apoptotic signal, suggesting that nuclear transport must be regulated by an additional mechanism. Our studies indicate that tyrosine phosphorylation of PKCδ on specific residues in the regulatory domain is also necessary for nuclear translocation. Mutation of these residues inhibits both nuclear accumulation of PKCδ and apoptosis, suggesting that in the absence of an apoptotic signal, the regulatory domain of PKCδ functions a cytoplasmic retention signal. While our studies indicate that both tyrosine phosphorylation and the COOH terminal (NLS) are required for apoptotic stimulus induced nuclear import of PKCδ, further studies are needed to decipher how these events are related. Finally, a third level of regulation of the pro-apoptotic activity of PKCδ is evident from studies which show that caspase cleavage of PKCδ occurs in the nucleus of apoptotic cells. Since caspase cleavage is likely a mechanism for amplifying the apoptotic signal, this amplification step can presumably only occur in cells in which PKCδ has been translocated to the nucleus. The identification of both pro- and anti-apoptotic isoforms suggests that PKC may function as a molecular sensor, promoting cell survival under favorable conditions and executing the death of abnormal or damaged cells when needed. Our goal now is to identify nuclear phosphorylation targets of PKCδ and to further understand the mechanism by which PKCδ regulates the apoptotic pathway. Understanding the molecular basis for regulation of apoptosis by PKC isoforms may contribute to the development of therapeutic strategies to treat diseases such as cancer and neurodegenerative disorders.