Ubiquitin is a conserved polypeptide unit that plays an important role in the ubiquitin-proteasome pathway. Ubiquitin can be covalently linked to many cellular proteins by the ubiquitination process. This process labels the target protein as a degradation marker for 26S lysosome. Three components are involved in the target protein-ubiquitin conjugation process. Ubiquitin is first activated by forming a thiolester complex with the activation component E1. The activated ubiquitin is subsequently transfered to the ubiquitin-carrier protein E2. E2 is processed to ubiquitin ligase E3 for final delivery to the epsilon-NH2 of the target protein lysine residue (1-3). The ubiquitin-proteasome pathway has been implicated in a wide range of normal biological processes and in disease-related abnormal cases. Several examples (i.e. IkappaB, p53, cdc25a and Bcl-2) have been shown to be targets for the ubiquitination-proteosome process for regulation of cell cycle progression and differentiation, cell stress response and apoptosis (4-7).
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