Ubiquitin is a small regulatory protein that has been found in almost all cells (''ubiquitously'') with nuclei (eukaryotes). It directs proteins to recycling and other functions.
Ubiquitin binds to proteins and labels them for destruction. The ubiquitin tag directs proteins to the proteasome, which is an organelle in the cell that degrades and recycles unneeded proteins.
Ubiquitin tags can also direct proteins to other locations in the cell, where they control other protein and cell mechanisms.
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 at Fox Chase Cancer Center by Aaron Ciechanover, Avram Hershko, and Irwin Rose for which the Nobel Prize in Chemistry was awarded in 2004.
No ubiquitin and ubiquitination machinery are known to exist in prokaryotes. However, ubiquitin is believed to have descended from prokarytotic proteins similar to ThiS or MoaD.
These prokaryotic proteins, despite having little sequence identity (ThiS has 14% identity to ubiquitin), share the same protein fold.
These proteins also share sulfur chemistry with ubiquitin. MoaD, which is involved in molybdenum cofactor biosynthesis, interacts with MoeB, which acts like an E1 ubiquitin-activating enzyme for MoaD, strengthening the link between these prokaryotic proteins and the ubiquitin system.
A similar system exists for ThiS, with its E1-like enzyme ThiF. It is also believed that the ''Saccharomyces cerevisiae'' protein Urm-1, a ubiquitin-related modifier, is a "molecular fossil" that connects the evolutionary relation with the prokaryotic ubiquitin-like molecules and ubiquitin.
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