Regulation Of Apoptotic Signal Transduction Pathways Through Dynamic Protein-Protein Interactions

Proteins are key performers and regulators of the complex events in a living cell. Through interaction with other molecules, the proteins exert various roles which allow a cell to maintain basic metabolic processes, perform specialized roles within an organism, and adapt to a changing environment. Comparing with other forms of molecular interactions, the protein - protein interaction are more specific and dynamic, which ensures the spatial and temporal control of signaling processes in living cells.Apoptosis, as a critical process in homeostasis, is controlled by a diverse range of cell signals. The transduction of signals is achieved by a cascade of protein-protein interactions and activations, and includes a variety of post-translational modifications which regulates the protein interaction networks. The goal of this dissertation research is to investigate the molecular mechanisms by which the protein interaction network is regulated during the process of apoptosis, and how disruption of specific protein-protein interactions may ultimately contribute to tumor development.The first part of this dissertation reports the identification of a novel cleavage site in the caspase-8/Mch5 protein. One of the cleavage products, termed DEDa, was found to disassociate with FADD, and translocate into the nucleus via binding to ERIC1/2 in response to death stimuli. In the nucleus, DEDa could accumulate in the nucleolus, where it interacts with TOPORS and subsequently leads to enhanced transcription of caspase-8 itself. A positive feedback loop is constituted through these protein-protein interactions, which sensitize cells to death receptor mediated apoptosis.Pirh2 is an E3 ligase that target proteins for proteosome-mediated degradation. It best known target is p53, the key tumor suppressor which is found to be mutated in almost half of human cancers. In the second part of this dissertation, we identified that phosphorylation of Pirh2 regulates its interaction with p53, and impairs its E3 ligase activity toward p53. We further implicated a correlation of Pirh2 phosphorylation with tumorigenesis. We also observed a direct interaction between Pirh2 and Keratin 8/18. We further demonstrated that the Pirh2 - Keratin 8/18 association is important for regulating cellular distribution of mitochondria and cell sensitivity to UV-induced apoptosis.Overall, the above results revealed that tight regulation of protein-protein interactions is important for the progression of apoptosis. Studies on this field will help idnetify valuable therapeutic targets for sensitizing cance cells to chemotherapeutic drugs.