| Apoptosis, also known as programmed cell death, is critical for embryonic development, immune regulation, and cellular homeostasis of a multi-cellular organism. Failure to control apoptosis can lead to serious diseases that threaten the existence of the organism including cancer, autoimmune diseases, and neurodegenerative disorders.; The process of apoptosis is characterized by a sequential activation of caspases. Proteins of the death domain (DD) superfamily play pivotal roles in this activation by mediating the assembly of oligomeric caspase activating complexes. To elucidate the assembly mechanisms of DDs, we determined the crystal structure of an oligomeric DD complex, which forms the core of the caspase-2 activating complex PIDDosome. The PIDDosome is composed of 3 components, PIDD (the p53-induced protein with a death domain), RAIDD (RIP-associated Ich-1/Ced-3 homologue protein with a death domain) and caspase-2. It is assembled via a DD:DD interaction between RAIDD and PIDD and a CARD:CARD interaction between RAIDD and caspase-2. Our crystal structure of the oligomeric PIDD DD:RAIDD DD complex, mediated by three conserved types of DD interactions, revealed an entirely asymmetric assembly mechanism. we hypothesized that the three types of interactions in this complex may represent most, if not all, modes of interactions in the DD superfamily and may be used to assemble complexes of different stoichiometry.; DNA fragmentation is a hallmark of apoptotic cell death. It is mainly executed by the heterodimeric DNA fragmentation factor complex (DFF40/45, also called CAD/ICAD). DFF40(CAD) is a caspase-activated endonuclease that can cleave naked and chromosomal DNA, whereas DFF45(ICAD) is a specific chaperone and an inhibitor that can suppress the nuclease activity of DFF40. Towards structural elucidation of DNA fragmentation, we purified the DFF40/DFF45 complex using a home-made co-expression system and crystallized a stable fragment of DFF45 and a drosophila homolog of DFF45 known as Drep-3.; Autophagy, another programmed cell death process, is a survival mechanism under nutrient starvation. Recent studies show that it is also used to execute cell death under certain circumstances.; Collectively, these studies provide significant advances to understanding the molecular basis of cell death signaling.; The process of autophagy is dependent on the formation of autophagosome, which are double lipid layer membrane vesicles that sequester cytosolic proteins and organelles non-selectively. Two ubiquitin-like systems are tightly involved in the formation of autophagosomes. As a first step towards understanding autophagy, we expressed and purified several human, mouse proteins that are involved in autophagosome formation. |
