| Type: | Family | Name: | Ubiquitin |
| Description: | Ubiquitinylation is an ATP-dependent process that involves the action of at least three enzymes: a ubiquitin-activating enzyme (E1, ), a ubiquitin-conjugating enzyme (E2, ), and a ubiquitin ligase (E3, , ), which work sequentially in a cascade. There are many different E3 ligases, which are responsible for the type of ubiquitin chain formed, the specificity of the target protein, and the regulation of the ubiquitinylation process []. Ubiquitinylation is an important regulatory tool that controls the concentration of key signalling proteins, such as those involved in cell cycle control, as well as removing misfolded, damaged or mutant proteins that could be harmful to the cell. Several ubiquitin-like molecules have been discovered, such as Ufm1 (), SUMO1 (), NEDD8, Rad23 (), Elongin B and Parkin (), the latter being involved in Parkinson's disease [].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 []. The terminal glycine in the C-terminal 4-residue tail of ubiquitin can form an isopeptide bond with a lysine residue in the target protein, or with a lysine in another ubiquitin molecule to form a ubiquitin chain that attaches itself to a target protein. Ubiquitin has seven lysine residues, any one of which can be used to link ubiquitin molecules together, resulting in different structures that alter the target protein in different ways. It appears that Lys(11)-, Lys(29) and Lys(48)-linked poly-ubiquitin chains target the protein to the proteasome for degradation, while mono-ubiquitinylated and Lys(6)- or Lys(63)-linked poly-ubiquitin chains signal reversible modifications in protein activity, location or trafficking []. For example, Lys(63)-linked poly-ubiquitinylation is known to be involved in DNA damage tolerance, inflammatory response, protein trafficking and signal transduction through kinase activation []. In addition, the length of the ubiquitin chain alters the fate of the target protein. Regulatory proteins such as transcription factors and histones are frequent targets of ubquitinylation []. | Short Name: | Ubiquitin |
