DIP A 6.3 Cellular regulation via the ubiquitin proteasome pathway

Principal Investigators:

Aaron Ciechanover, Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel

Dieter H. Wolf, The Institute for Biochemistry, University of Stuttgart

Internet: http://www.technion.ac.il
Project duration: 01.01.1999-31.12.2003

 
Aaron Ciechanover

Prof. Ciechanover has been awarded the Nobel Prize for Chemnistry 2004 for his research related to Ubiquitin

Executive summary:

It has emerged as a critically system involved in regulating a broad array of basic cellular processes such as cell division, differentiation, signal transduction, trafficking, and quality control. Not surprisingly, aberrations in the system have been implicated in the pathogenesis of many diseases, certain malignancies, neurodegenerative disorders and pathologies of the inflammatory and immune response among them. Understanding of the underlying mechanisms involved is important for the development of novel, mechanism-based drugs.

To that end, we selected two major proteins that are regulated by the system, (i) the tumor suppressor p53 that plays a major role in inducing apoptosis in DNA-damaged cells, and has been found to be mutated in more then 50% of all tumors, and (ii) the transcriptional regulator NF-fÛB, that is a strong anti-apoptotic factor playing a major role in resistance of malignant cells to apoptosis-inducing therapies like irradiation and chemotherapy. In addition, we selected to study ERAD ¡V Endoplasmic Reticulum Associated Degradation, a major process involved in the cell quality control.

We have identified the structural motives and enzymes involved in signal-transduction NF-kB activation. We have shown that that parenchymal NF-fÛB is an essential coordinator of innate immunity responses, and via its anti-apoptotic activity supports the proliferation of transformed hepatocytes in the liver and serves as a tumor promoter. For p53 we have shown that in normal cells, Mdm2 promotes p53 degradation, whereas p53 becomes immune to Mdm2 in response to oncogenic stress. The resultant p53 accumulation represses further cancer progression. The underlying molecular mechanisms were elucidated. The Mdm2-p53 interaction may serve as a target for the future development of anti-cancer drugs. The ER harbors a quality control system, which senses proteins that fail to fold productively and targets them for destruction by the ubiquitin system (ERAD). This system is involved in the pathogenesis of important diseases such as cystic fibrosis. In our aim to understand ERAD on a molecular basis, we have shown how the system is interconnected with the stress response of the secretory pathway. Furthermore we have identified new components involved in ERAD (Cdc48/p97), and described new concepts for the degradation of soluble and membrane-bound substrates of ERAD.

The ubiquitin system and its various functions.

A. Proteasomal-dependent degradation of cellular proteins. Ubiquitin is activated by the ubiquitin-activating enzyme, E1 (1) followed by its transfer to a ubiquitin-carrier protein (ubiquitin-conjugating enzyme, UBC), E2 (2). E2 transfers the activated ubiquitin moieties to the protein substrate that is bound specifically to a unique ubiquitin ligase E3 (3). Successive conjugation of ubiquitin moieties to one another generates a polyubiquitin chain that serves as the degradation signal for the downstream 26S proteasome (4). The substrate is degraded to short peptides, and free and reusable ubiquitin is released by de-ubiquitinating enzymes (DUBs)(5). B. Mono or oligoubiquitination targets membrane proteins to degradation in the lysosome/vacuole. C. Monoubiquitination or (D) a single modification by a ubiquitin-like (UBL) protein, SUMO for example, can target proteins to different subcellular destinations such as nuclear foci or the nuclear pore complex (NPC). E. Generation of a Lys63-based polyubiquitin chain can activate transcriptional regulators, directly or via activation of intermediate components such as kinases. Ub denotes ubiquitin, K denotes Lys, and S denotes Cys.