ROLES OF UBIQUITINYLATION IN PROTEOLYSIS AND CELLULAR-REGULATION

Most eukaryotic organisms respond to starvation, nutrient deprivation, and/or stress by increasing the rates of intracellular proteolysis. T he amino acids released may be reutilized for synthesis of important p roteins, or directly for the production of energy. This enhanced prote olysis is also required for repair of cellular damage due to environme ntal insults such as heat shock, free radicals, viral infection, or mu tation. Finally, intracellular proteolysis is important in determining the steady-state levels of a wide variety of regulatory proteins, par ticularly those regulating the cell cycle. The ubiquitin-dependent pro teolytic system participates in all of these functions. In spite of it s cytoplasmic localization, this system is selective and acts only on a limited set of substrates. This review discusses the mechanisms of t his selectivity and the potential roles of ubiquitin-dependent proteol ysis.