Biol. Pharm. Bull. 29(2) 225—233 (2006)

sential role in the survival of multicellular organisms and is observed in a wide variety of physiological situations. Apoptosis is important for biological self-defense, and defects in the ability to undergo apoptosis can lead to various diseases, including cancer, neurodegenerative diseases, and autoimmune diseases. Apoptosis is a well-defined cellular process characterized by morphological changes that include condensation of nuclear chromatin, cell shrinkage, and plasma membrane blebbing. The Fas family cytokine and tumor necrosis factor (TNF) cascades can mediate apoptosis, but recently, many additional apoptotic components have been identified. Using a functional cloning strategy, Deiss et al. identified the deathassociated protein kinase (DAPK) as a positive mediator of apoptosis triggered by interferon-g . The kinase activity of DAPK, which shares homology with myosin light chain kinases, is regulated by calmodulin in a Ca-dependent manner in vitro. Overexpression of DAPK in various cell lines results in cell death, and this death-promoting property strictly depends on the intrinsic kinase activity. Additional isoforms of the DAPK family, including DRP-1, ZIP kinase, DAP kinase-related apoptosis-inducing kinase 1 (DRAK1), and DRAK2 have been identified. These proteins share homology in their primary structures, especially in the catalytic domain. All of these kinases have an apoptosis-promoting activity that is dependent on the kinase activity. We have studied the function of the small Ca -binding protein CHP1 (calcineurin B homologous protein1). CHP1 is a multifunctional regulator of various proteins, including Na /H antiporters, a kinesin-like motor protein, and calcineurin. We identified DRAK2 as a CHP1 binding protein, and found that CHP1 inhibits the kinase activity of DRAK2 in a Ca -dependent manner. DRAK2 is found in thymus, testis, spleen, and brain, tissues in which apoptosis plays an important role. Overexpression of DRAK2 in NIH3T3 cells causes apoptosis like cell death through a mechanism that requires the kinase activity. However, overexpression studies do not necessarily indicate whether DRAK2 causes apoptosis during normal physiological processes. Several components of apoptotic signal transduction pathways, such as DNases, Bcl family members, and caspases, are localized to specific areas of the cell. During UV-induced apoptosis, damaged double stranded DNA signals translocation of the mitochondrial components AIF and Endo-G to the nucleus. UV irradiation also activates cytoplasmic targets, such as the transcriptional factors AP-1 and NF-kB. NF-kB migrates to the nucleus and binds to specific response elements of various genes. Thus, the intracellular localization and translocation of molecules are important elements of apoptosis, and understanding these dynamics will elucidate the regulatory mechanisms of apoptosis. The DAPK family members ZIP kinase and DAP kinase are localized in the nucleus and cytoplasm, respectively. ZIP kinase is located within promyelocytic leukemia oncogenic domains (PODs) and induces apoptosis in response to apoptotic stimuli such as As2O3 or interferon-g. Although we have previously shown that DRAK2 is localized in the nucleus in COS-7 cells and causes apoptosis like cell death in NIH3T3 cells, the correlation between cell death and the intracellular localization of DRAK2 has not been systematically studied. In this study, we examined the correlation between the intracellular localization of DRAK2 and cell death to identify the putative signal transduction pathways involved in apoptotic cell death. We found that the translocation of DRAK is cell type dependent; in certain types of cells, DRAK is spontaneously accumulated in the nucleus, while in other types of cells, it is mainly detected in the cytoplasm. In the case of the former types of cells, ectopically overexpressed DRAK2 induces cell death. In latter types of cells, UV-irradiation causes nuclear translocation of endogenous DRAK2 followed by apoptotic cell death. February 2006 225 Biol. Pharm. Bull. 29(2) 225—233 (2006)