功能
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[Ubiquitin]: Exists either covalently attached to another protein, or free (unanchored). When covalently bound, it is conjuga
ted to target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys residue
s of the ubiquitin (polyubiquitin chains) or a linear polymer linked via the initiator Met of the ubiquitin (linear polyubiqu
itin chains). Polyubiquitin chains, when attached to a target protein, have different functions depending on the Lys residue
of the ubiquitin that is linked: Lys-6-linked may be involved in DNA repair; Lys-11-linked is involved in ERAD (endoplasmic r
eticulum-associated degradation) and in cell- cycle regulation; Lys-29-linked is involved in lysosomal degradation; Lys-33-li
nked is involved in kinase modification; Lys-48-linked is involved in protein degradation via the proteasome; Lys-63-linked i
s involved in endocytosis, DNA-damage responses as well as in signaling processes leading to activation of the transcription
factor NF-kappa-B. Linear polymer chains formed via attachment by the initiator Met lead to cell signaling. Ubiquitin is usua
lly conjugated to Lys residues of target proteins, however, in rare cases, conjugation to Cys or Ser residues has been observ
ed. When polyubiquitin is free (unanchored- polyubiquitin), it also has distinct roles, such as in activation of protein kina
ses, and in signaling.
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