Cellular Regulation by Ubiquitin-Dependent Processes

Ubiquitination and deubiquitination: targeting of proteins for degradation by the proteasome

The post-translational modification of proteins by covalent attachment of ubiquitin targets these proteins for degradation by the proteasome. An astounding number of proteins are involved in ubiquitination and deubiquitination of proteins. The pathways are combinatorial, and selectivity of proteolysis will depend strongly on the exact combination of ubiquitinating and deubiquitinating enzymes present at any time. In addition to temporal control, it is likely that these modifications are also regulated spatially. In this review, we discuss the regulation of ubiquitination by enzymes of this pathway and highlight some of the outstanding problems in understanding this regulation.

Ubiquitin-dependent signaling: the role of ubiquitination in the response of cells to their environment

The response of a cell to its external environment requires rapid and significant alteration of protein amount, localization and/or function. This regulation involves a complex combination of processes that control synthesis, localization and degradation. All of these processes must be properly regulated and are often interrelated. Intracellular proteolysis is largely accomplished by the ubiquitin-dependent system and has been shown to be required for growth control, cell cycle regulation, receptor function, development and the stress response. Substrates subject to regulated degradation by this system include cyclins and cyclin-dependent kinase inhibitors, tumor suppressors, transcription factors and cell surface receptors. In addition, proteins that are damaged by oxidation or that are improperly folded or localized are substrates whose degradation by this system often leads to antigen presentation on the surface of the cell in the context of Class I major histocompatibility complex molecules. A very large body of work in the last fifteen years has shown that degradation by this system requires the covalent attachment of a small protein called ubiquitin and that this modification serves to direct target proteins for degradation by a 26S proteolytic particle, the proteasome. Thus, the attachment of the ubiquitin domain is of vital importance in regulating normal growth and differentiation, as well as in defending against cellular damage caused by xenobiotics, environmental insults, infection and mutation. This review focuses on the role of ubiquitination in the cellular signaling pathways that deal with these external influences.