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Ubiquitin and Ubiquitin-like Proteins in Signal Transduction
Bang, Oksun,Chung, Chin Ha 이화여자대학교 세포신호전달연구센터 2004 고사리 세포신호전달 심포지움 Vol. No.6
Post-translational modification of proteins by ubiquitin(Ub) is a key regulatory cellular mechanism that is involved in a variety of cellular processes, including regulation of intracellular protein breakdown, cell cycle regulation, signal transduction, transcription, and antigen presentation. Ub is a 76-amino acid polypeptide that is highly conserved from yeast to human. It is covalently ligated to a wide variety of target proteins through the action of Ub-activating enzyme(E1), Ub-conjugating enzyme(E2), and Ub protein ligase(E3). The proteins ligated to multiple units of Ub are then degraded by the ATP-dependent, 26S proteasome. Although the role as a tag for protein degradation by the 26S proteasome has been known as a major function of Ub, other functions of ubiquitination has been discovered. For example, mono-ubiquitination is involved in at least three distinct cellular processes, such as histone regulation, endocytosis, and budding of retroviruses from the plasma membrane. A number of other small proteins, so called Ub-like molecules(Ubls), have recently been identified. These proteins are structurally related to Ub, and can be conjugated to various target proteins in a similar manner with Ub. However, covalent attachment of Ub1s does not result in degradation of the modified proteins, but functions in a similar way to mono-ubiquitination. To date, several Ub1s, such as SUMOs, NEDD8, ISG15, ATGs and FUb, have been identified. Of these, the best characterized Ub1 is the mammalian SUMO-1, which can be conjugated to a variety of cellular proteins, such as PML, RanGAP1, and IĸBα. Protein modification by Ub or Ubls is a reversible process, and it has become increasingly obvious that deconjugation of Ub or Ubls plays important roles in regulating the Ub- and Ubl-dependent pathways. Both Ub and Ubls are synthesized as precursor proteins, and therefore need to be processed to yield monomeric Ub and Ub1s by Ub- and Ub1-specific proteases, prior to their conjugation to target proteins. These enzymes also catalyze the removal of Ub or Ubls from Ub- or Ubl-conjugated proteins. Thus, modification of proteins by Ub or Ub1s and its reversal resemble protein phosphorylation and dephosphorylation process catalyzed by protein kinase and phosphatases, respectively, which play the key role in most, if not all, signal transduction pathway. Thus, the studies on Ub- or Ub1-specific proteases should be essential as those on the enzymes involved in their conjugation process. Here, we report recent progress in understanding the physiological function of several Ub- or Ubl-specific proteases.