Cells constantly synthesize proteins and degrade those that are no longer needed to keep equilibrium. The laureates discovered that this degradation is ATP-dependent and uses a 76-amino-acid peptide called ubiquitin as a tag. When multiple ubiquitin molecules form a poly-ubiquitin chain, a barrel-shaped complex called the proteasome recognizes the target and fragments it. The same mechanism controls the cell cycle, DNA repair and immune responses; its failure can lead to cancer or neurodegenerative diseases. The findings revolutionized molecular biology and drug discovery—proteasome inhibitors are now approved for treating multiple myeloma. By explaining how cells balance “making” and “breaking,” the work fully merited the Nobel Prize.

The ubiquitin-proteasome system (UPS) achieves substrate specificity and catalytic efficiency through a three-enzyme cascade: E1 activating enzymes, E2 conjugating enzymes, and E3 ubiquitin ligases. E1 hydrolyzes ATP to adenylate ubiquitin’s C-terminus, forming a thioester intermediate. Activated ubiquitin is then transferred to an E2, and an E3 ligase brings the E2–ubiquitin and substrate into proximity, forming an isopeptide bond to a substrate lysine. Chains longer than four ubiquitins, often linked via Lys48, constitute a canonical signal for proteasomal recognition. The 19S regulatory particle, an AAA+ ATPase ring, unfolds the tagged protein and translocates it into the 20S core for proteolysis. The UPS governs cyclin turnover during the cell cycle, quantitative control of p53, and generation of peptides for MHC class I antigen presentation. Crosstalk with ubiquitin-like modifiers (SUMO, NEDD8, etc.) forms a sophisticated post-translational modification network. By resolving the paradox of energy-dependent proteolysis, the laureates laid the foundation of modern protein homeostasis research.

Through a set of in vitro assays (1978-83), Ciechanover, Hershko and Rose demonstrated that ATP-dependent proteolysis proceeds via multistep ubiquitin transfer. They purified the heat-stable fraction APF-1, showed covalent conjugation to high-molecular substrates, and introduced the concept that Lys48-linked poly-ubiquitin chains constitute a degradation signal. Subsequent work identified E1 (UBA1), numerous E2s (UBC family) and hundreds of E3 ligases (RING, HECT, RBR), establishing E3s as substrate-specificity factors. The proteasome adopts an α₇β₇β₇α₇ tetradecameric architecture, with N-terminal Thr residues on β-subunits catalyzing peptide bond hydrolysis. UPS malfunction triggers cytotoxicity via ER stress, altered ubiquitin pool, and deubiquitinase imbalance, underpinning cancers, Parkinson’s disease and myopathies. Clinically, 26S inhibitors such as bortezomib and carfilzomib, and E3-hijacking PROTACs, exploit this pathway. UPS cooperates with autophagy in the proteostasis network, and next-generation targets include separase regulation and NEDD8-activating enzyme inhibitors like MLN4924.