a peptide 76 amino acids in length that can be covalently attached to target lysines either as a monomer (monoubiquitin), as 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). Ubiquitin is encoded by 4 different genes. UBA52 and RPS27A genes code for a single copy of ubiquitin fused to the 60S ribosomal protein L40 (L40), and S27a, respectively. UBB and UBC genes code for a polyubiquitin precursor with exact head to tail repeats, the number of repeats differ between species and strains. Only the 76 amino acids of monoubiquitin product are shown in this entry. At the protein level, it is not possible to determine which of the four genes a given ubiquitin chain was derived from. 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 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 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 residues of target proteins, however, in rare cases, conjugation to Cys or Ser residues has been observed. When polyubiquitin is free (unanchored-polyubiquitin), it also has distinct roles, such as in activation of protein kinases, and in signaling. Ribosomal protein L40 is essential for translation of a subset of cellular transcripts, and especially for cap-dependent translation of vesicular stomatitis virus mRNAs. Hundreds of ubiquitin ligases and hydrolases have been identified, implicating ubiquitin as a major regulatory element in many crucial cellular systems. For better understanding, features described here refer to a single monomer of ubiquitin. Note: This description may include information from UniProtKB.