Protein ubiquitination: a regulatory post-translational modification.

The covalent attachment of ubiquitin to a variety of cellular proteins (ubiquitination) is a common post-translational modification in eukaryotic cells. Little is known about the function of these modifications in either the normal or the pathological state. The characteristics of ubiquitination in the nucleus, the cytoplasm, and on the plasma membrane are reviewed and discussed here. Also reported are studies on the enzymes which metabolize ubiquitin, using the ubiquitin-dependent proteolysis system as a model. Four enzymes which specifically recognize ubiquitin and hydrolyze carboxyl-terminal derivatives of ubiquitin have been partially purified from bovine thymus. These are thiol-containing proteases which will also release ubiquitin from ubiquitin-protein conjugates. The presence of these deconjugating enzymes and the proteases in the cytoplasm suggests that there is a partition of conjugates between proteolysis and deconjugation. To study the factors which determine the relative rates of proteolysis versus deconjugation, we have developed a general method of synthesizing large amounts of pure ubiquitin-protein conjugates. The structure/function relationships of ubiquitin have been probed by chemically modifying ubiquitin and examining its activity in the protein degradation system. These studies have identified regions of the ubiquitin molecule which are recognized by the enzymes of the proteolysis system, established that the molecule can be altered and used as a probe of such systems and will guide the design of site-directed mutant ubiquitins in order to more fully define the recognition sites on the ubiquitin molecule. It is likely that studies of these types will lead to an understanding of the molecular interactions required for proper ubiquitin function and allow design of drugs which could be useful in understanding the role of ubiquitination and its importance in normal and pathological states.