Ubiquitin is expressed as three different precursors: a polymeric head-to-tail concatemer of identical units (polyubiquitin), and two N-terminal ubiquitin moieties, UbL40 and UbS27, that are fused to the ribosomal polypeptides L40 and S27, respectively. Specific endopeptidases cleave these precursor molecules [ PUBMED:15571815 … expand

Ubiquitin is expressed as three different precursors: a polymeric head-to-tail concatemer of identical units (polyubiquitin), and two N-terminal ubiquitin moieties, UbL40 and UbS27, that are fused to the ribosomal polypeptides L40 and S27, respectively. Specific endopeptidases cleave these precursor molecules [ PUBMED:15571815 ] to release ubiquitin moieties that are identical in sequence and contribute to the ubiquitin pool [ PUBMED:16185873 ]. Some organisms express additional ubiquitin fusion proteins [ PUBMED:12729753 ]. Furthermore, there are several ubiquitin-like proteins derived from ubiquitin [ PUBMED:12826404 ].

This entry represents a domain characteristic of ubiquitin (Ub) and ubiquitin-like (Ubl) proteins such as SUMO [ PUBMED:17491593 PUBMED:15479240 ] and Nedd8 [ PUBMED:9857030 ].

Ubiquitin is a protein of 76 amino acid residues, found in all eukaryotic cells and whose sequence is extremely well conserved from protozoan to vertebrates. Ubiquitin acts through its post-translational attachment (ubiquitinylation) to other proteins, where these modifications alter the function, location or trafficking of the protein, or targets it for destruction by the 26S proteasome [ PUBMED:15454246 ].

Ubiquitin is a globular protein, the last four C-terminal residues (Leu-Arg-Gly-Gly) extending from the compact structure to form a 'tail', important for its function. The latter is mediated by the covalent conjugation of ubiquitin to target proteins, by an isopeptide linkage between the C-terminal glycine and the ε-amino group of lysine residues in the target proteins.