Subcellular targeting regulates the function of caspase-activated protein kinases in apoptosis

Subcellular localization and targeting of proteins play important roles in signal transduction pathways that regulate cell survival and programmed cell death. The regulation of cell survival and cell death requires translocation of many anti- and pro-apoptotic signaling molecules from one subcellular compartment to another. In many cases translocation is triggered by caspase cleavage. Caspase cleavage removes the regulatory domains of the protein kinases MEKK1, Mst-1 and PAK-2 resulting in activation and in relocalization of the catalytic fragments. Caspase-activated MEKK1 translocates from a particulate compartment to the cytosol; caspase-activated Mst-1 and PAK-2 translocate from the cytoplasm to the nucleus. Caspase activation of these protein kinases induces a cell death response. Relocalization of the catalytic fragments to a pro-apoptotic location appears to be required to induce cell death. It is suggested that translocation to a pro-apoptotic location results in phosphorylation of pro-apoptotic substrates. Therefore, these protein kinases could represent novel targets for cancer therapy. Compounds that stimulate cleavage of MEKK1, Mst-1 and PAK-2 or compounds that cause translocation to a pro-apoptotic location could be used to induce cell death of cancer cells.