Exists either covalently attached to anotherprotein, or free (unanchored). When covalently bound, it is conjugatedto target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys residues of theubiquitin (polyubiquitin chains) or a linear polymer linked via theinitiator Met of the ubiquitin (linear polyubiquitin chains). Polyubiquitin chains, when attached to a target protein, have differentfunctions depending on the Lys residue of the ubiquitin that is linked:Lys-6-linked may be involved in DNA repair; Lys-11-linked is involvedin ERAD (endoplasmic reticulum-associated degradation) and in cell-cycle regulation; Lys-29-linked is involved in proteotoxic stressresponse and cell cycle; Lys-33-linked is involved in kinasemodification; Lys-48-linked is involved in protein degradation via theproteasome; Lys-63-linked is involved in endocytosis, DNA-damageresponses as well as in signaling processes leading to activation ofthe transcription factor NF-kappa-B. Linear polymer chains formed viaattachment by the initiator Met lead to cell signaling. Ubiquitin isusually conjugated to Lys residues of target proteins, however, in rarecases, conjugation to Cys or Ser residues has been observed. Whenpolyubiquitin is free (unanchored-polyubiquitin), it also has distinctroles, such as in activation of protein kinases, and in signaling. [1-3]

MQIFVKTLTGKTITLEVEPSDTIENVKAKIQDKEGIPPDQQRLIFAGKQLEDGRTLSDYNIQKESTLHLVLRLRGGAKKRKKKSYTTPKKNKHKRKKVKLAVLKYYKVDENGKISRLRRECPSDECGAGVFMASHFDRHYCGKCCLTYCFNKPEDK