title: Ubiquitin is degraded by the ubiquitin system as a monomer and as part of its conjugated target.
abstract: An important problem concerning regulation of the ubiquitin-proteasome system (UPS) relates to the stability of its own components and the mechanisms of their degradation. It has been demonstrated that monomeric ubiquitin is relatively stable and is probably degraded by the proteasome. It has also been shown that it is destabilized following inactivation of deubiquitinating enzymes, suggesting that failure to release it, results in its concomitant degradation along with its target. Here, we demonstrate that conjugation of monomeric ubiquitin requires both its internal lysines and N-terminal residue. Interestingly however, the degradation of the monomeric species requires also a short C-terminal extension, implying that unlike conjugation, entry into the proteasomal chamber requires a tail that can be generated in the cell via several distinct mechanisms. We further show that accelerated intracellular degradation induced by stress results in depletion of ubiquitin, supporting the notion that ubiquitin is also degraded as part of the chain conjugated to its target substrate.
authors.affiliation: Center for Vascular and Tumor Biology, The Rappaport Faculty of Medicine and Research Institute, Technion-Israel Institute of Technology, Efron Street, Bat Galim, P.O. Box 9649, Haifa 31096, Israel.
authors.forename: Nitzan
authors.initials: N
authors.lastname: Shabek
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Iwai
authors.forename: Aaron
authors.initials: A
authors.lastname: Ciechanover
mesh_terms: D002460:Cell Line; D006367:HeLa Cells; D006801:Humans; D007668:Kidney / Q000378:metabolism*; D008239:Lysine / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex; D008239:Lysine