Ubiquitin / Proteasome Pathway
The ubiquitin proteasome pathway, conserved from yeast to mammals, is required for the targeted degradation of most short-lived proteins in the eukaryotic cell. Targets include cell cycle regulatory proteins, whose timely destruction is vital for controlled cell division, as well as proteins unable to fold properly within the endoplasmic reticulum.
Ubiquitin modification is an ATP-dependent process carried out by three classes of enzymes. A “ubiquitin activating enzyme” (E1) forms a thio-ester bond with ubiquitin, a highly conserved 76-amino acid protein. This reaction allows subsequent binding of ubiquitin to a “ubiquitin conjugating enzyme” (E2), followed by the formation of an isopeptide bond between the carboxy-terminus of ubiquitin and a lysine residue on the substrate protein. The latter reaction requires a “ubiquitin ligase” (E3). E3 ligases can be single- or multi-subunit enzymes. In some cases, the ubiquitin-binding and substratebinding domains reside on separate polypeptides brought together by adaptor proteins or cullins. Numerous E3 ligases provide specificity in that each can modify only a subset of substrate proteins. Further specificity is achieved by post-translational modification of substrate proteins, including, but not limited to, phosphorylation.
Effects of monoubiquitination include changes in subcellular localization. However, multiple ubiquitination cycles resulting in a polyubiquitin chain are required for targeting a protein to the proteasome for degradation. The multisubunit 26S proteasome recognizes, unfolds, and degrades polyubiquitinated substrates into small peptides. The reaction occurs within the cylindrical core of the proteasome complex, and peptide bond hydrolysis employs a core threonine residue as the catalytic nucleophile.
Recent work has indicated that an additional layer of complexity, in the form of multiubiquitin chain receptors, may lie between the polyubiquitination and degradation steps. These receptors react with a subset of polyubiquitinated substrates, aiding in their recognition by the 26S proteasome, and thereby promoting their degradation.
This pathway is not only important in cellular homeostasis, but also in human disease. Because ubiquitin/proteasome-dependent degradation is often employed in control of the cell division cycle and cell growth, researchers have found that proteasome inhibitors hold some promise of being developed into potential cancer therapeutic agents.
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created September 2004
revised November 2012