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 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 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.

• GO:0000187 : activation of MAPK activity
• GO:0031145 : anaphase-promoting complex-dependent proteasomal ubiquitin-dependent protein catabolic process
• GO:0002479 : antigen processing and presentation of exogenous peptide antigen via MHC class I, TAP-dependent
• GO:0006915 : apoptotic process
• GO:0016044 : cellular membrane organization
• GO:0019221 : cytokine-mediated signaling pathway
• GO:0006977 : DNA damage response, signal transduction by p53 class mediator resulting in cell cycle arrest
• GO:0006281 : DNA repair
• GO:0046788 : egress of virus within host cell
• GO:0016197 : endosomal transport
• GO:0007173 : epidermal growth factor receptor signaling pathway
• GO:0008543 : fibroblast growth factor receptor signaling pathway
• GO:0000080 : G1 phase of mitotic cell cycle
• GO:0000082 : G1/S transition of mitotic cell cycle
• GO:0007249 : I-kappaB kinase/NF-kappaB cascade
• GO:0045087 : innate immune response
• GO:0007254 : JNK cascade
• GO:0000216 : M/G1 transition of mitotic cell cycle
• GO:0016071 : mRNA metabolic process
• GO:0002755 : MyD88-dependent toll-like receptor signaling pathway
• GO:0042059 : negative regulation of epidermal growth factor receptor signaling pathway
• GO:0000122 : negative regulation of transcription from RNA polymerase II promoter
• GO:0030512 : negative regulation of transforming growth factor beta receptor signaling pathway
• GO:0032480 : negative regulation of type I interferon production
• GO:0051436 : negative regulation of ubiquitin-protein ligase activity involved in mitotic cell cycle
• GO:0048011 : nerve growth factor receptor signaling pathway
• GO:0007220 : Notch receptor processing
• GO:0007219 : Notch signaling pathway
• GO:0070423 : nucleotide-binding oligomerization domain containing signaling pathway
• GO:0043123 : positive regulation of I-kappaB kinase/NF-kappaB cascade
• GO:0051092 : positive regulation of NF-kappaB transcription factor activity
• GO:0045944 : positive regulation of transcription from RNA polymerase II promoter
• GO:0051437 : positive regulation of ubiquitin-protein ligase activity involved in mitotic cell cycle
• GO:0000209 : protein polyubiquitination
• GO:0042981 : regulation of apoptotic process
• GO:0061418 : regulation of transcription from RNA polymerase II promoter in response to hypoxia
• GO:0000084 : S phase of mitotic cell cycle
• GO:0050852 : T cell receptor signaling pathway
• GO:0008063 : Toll signaling pathway
• GO:0034130 : toll-like receptor 1 signaling pathway
• GO:0034134 : toll-like receptor 2 signaling pathway
• GO:0034138 : toll-like receptor 3 signaling pathway
• GO:0034142 : toll-like receptor 4 signaling pathway
• GO:0006367 : transcription initiation from RNA polymerase II promoter
• GO:0007179 : transforming growth factor beta receptor signaling pathway
• GO:0035666 : TRIF-dependent toll-like receptor signaling pathway

Source: The Gene Ontology Consortium. Gene ontology: tool for the unification of biology. Nat. Genet.. May 2000;25(1):25-9.
World Wide Web URL: http://www.geneontology.org/

Source: Online Mendelian Inheritance in Man, OMIM®. McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University (Baltimore, MD), May, 2012.
World Wide Web URL: http://omim.org/