polyubiquitin-C; contains: ubiquitin (UBC)

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Function

ubiquitin exists either covalently attached to another protein, or free (unanchored)
when covalently bound, it is conjugated to target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys of the ubiquitin (polyubiquitin chains) or a linear polymer linked via the initiator Met of the ubiquitin (linearpolyubiquitin chains)
polyubiquitin chains, when attached to a target protein, have different functions depending on the Lys of the ubiquitin that is linked
Lys-6-linked may be involved in DNA repair
Lys-11-linked is involved in:
- ERAD (endoplasmic reticulum-associated degradation)
- cell-cycle regulation
Lys-29-linked is involved in lysosomal degradation
Lys-33-linked is involved in kinase modification
Lys-48-linked is involved in protein degradation via the proteasome
Lys-63-linked is 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 usually conjugated to Lys of target proteins, however, in rare cases, conjugation to Cys or Ser has been observed
when polyubiquitin is free (unanchored-polyubiquitin), it also has distinct roles, such as in activation of protein kinases, & in signaling

ubiquitin is encoded by 4 different genes
UBA52 & RPS27A genes code for a single copy of ubiquitin fused to the ribosomal proteins L40 & S27a, respectively
UBB & UBC genes code for a polyubiquitin precursor with exact head to tail repeats, the number of repeats differ between species & strains