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KEY WORDS: ubiquitin, protein degradation, proteasome, proteolysis
ABSTRACT
The selective degradation of many short-lived proteins in eukaryotic cells is carried out by the ubiquitin system. In this pathway, proteins are targeted for degradation by covalent ligation to ubiquitin, a highly conserved small protein. Ubiquitin-mediated degradation of regulatory proteins plays important roles in the control of numerous processes, including cell-cycle progression, signal transduction, transcriptional regulation, receptor down-regulation, and endocytosis. The ubiquitin system has been implicated in the immune response, development, and programmed cell death. Abnormalities in ubiquitin-mediated processes have been shown to cause pathological conditions, including malignant transformation. In this review we discuss recent information on functions and mechanisms of the ubiquitin system. Since the selectivity of protein degradation is determined mainly at the stage of ligation to ubiquitin, special attention is focused on what we know, and would like to know, about the mode of action of ubiquitin-protein ligation systems and about signals in proteins recognized by these systems.
INTRODUCTION
The past few years have witnessed a dramatic increase in our knowledge of the important functions of ubiquitin-mediated protein degradation in basic biological processes. The selective and programmed degradation of cell-cycle regulatory proteins, such as cyclins, inhibitors of cyclin-dependent kinases, and anaphase inhibitors are essential events in cell-cycle progression. Cell growth and proliferation are further controlled by ubiquitin-mediated degradation of tumor suppressors, protooncogenes, and components of signal transduction systems. The rapid degradation of numerous transcriptional regulators is involved in a variety of signal transduction processes and responses to environmental cues. The ubiquitin system is clearly involved in endocytosis and downregulation of receptors and transporters, as well as in the degradation of resident or abnormal proteins in the endoplasmic reticulum. There are strong indications for roles of the ubiquitin system in development and apoptosis, although the target proteins involved in these cases have not been identified. Dysfunction in several ubiquitin-mediated processes causes pathological conditions, including malignant transformation.
The role of ubiquitin in protein degradation was discovered and the main enzymatic reactions of this system elucidated in biochemical studies in a cellfree system from reticulocytes (reviewed in 1). In this system, proteins are targeted for degradation by covalent ligation to ubiquitin, a 76-amino-acidresidue protein. The biochemical steps in the ubiquitin pathway have been reviewed...