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( Ubiquitin)
Ubiquitin is a highly-conserved regulatory protein that is ubiquitously expressed in eukaryotes. Ubiquitination (or ubiquitylation) refers to the post-translational modification of a protein by the covalent attachment (via an isopeptide bond) of one or more ubiquitin monomers. The most prominent function of ubiquitin is labeling proteins for proteasomal degradation. Besides this function, ubiquitination also controls the stability, function, and intracellular localization of a wide variety of proteins. Ubiquitin (originally, Ubiquitous Immunopoietic Polypeptide) was first identified in 1975 as an 8.5-kDa protein of unknown function expressed universally in living cells. The basic functions of ubiquitin and the components of the ubiquitination pathway were elucidated in the early 1980s in groundbreaking work performed by Aaron Ciechanover, Avram Hershko, and Irwin Rose for which the Nobel Prize in Chemistry was awarded in 2004.[1] The ubiquitylation system was initially characterised as an ATP-dependent proteolytic system present in cellular extracts. A heat-stable polypeptide present in these extracts, ATP-dependent proteolysis factor 1 (APF-1), was found to become covalently attached to the model protein substrate lysozyme in an ATP- and Mg2+-dependent process. Multiple APF-1 molecules were linked to a single substrate molecule by an isopeptide linkage, and conjugates were found to be rapidly degraded with the release of free APF-1. Soon after APF-1-protein conjugation was characterised, APF-1 was identified as ubiquitin. The carboxyl group of the C-terminal glycine residue of ubiquitin (Gly76) was identified as the moiety conjugated to substrate lysine residues. Ubiquitin consists of 76 amino acids and has a molecular mass of about 8.5 kDa. Key features include its C-terminal tail and the Lys residues. It is highly conserved among eukaryotic species Human and yeast ubiquitin share 96% sequence identity. The human ubiquitin sequence is
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