Ubiquitin-40S ribosomal protein S27a (RPS27A)

Ubiquitin: Exists either covalently attached to another protein, or free (unanchored). When covalently bound, it's conjugated to target proteins via an isopeptide bond either as a monomer (monoubiquitin), a polymer linked via different Lys residues of the ubiquitin (polyubiquitin chains) or a linear polymer linked via the initiator Met of the ubiquitin (linear polyubiquitin chains).

Polyubiquitin chains, when attached to a target protein, have different functions depending upon the Lys residue of the ubiquitin that's connected: Lys-6-linked may be involved in DNA repair; Lys-11-linked is involved in ERAD (endoplasmic reticulum-associated degradation) and in 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 resulting in activation of this transcription factor NF-kappa-B. Linear polymer chains formed via attachment by the initiator Met lead to cell signaling.