polyubiquitin-B; contains: ubiquitin (UBB)
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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
Structure
Compartment
Notes
- 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