The Ubiquitin System for Protein Modification and Degradation
<-- (a 3D representation of ubiquitin)
The small, 76-residue, protein is found both as free monomer in eukaryotic cells, and co-valently attached to itself and other proteins. The C-terminus of ubiquitin forms an isopeptide bond with the amino group of a lysine side chain in a target protein. In this way proteins can be covalently modified by the addition of ubiquitin (cf. phosphorylation) which may alter the target protein's function. If a chain of multiple copies of ubiquitin is atached to a target proteins this appears to target the protein for degradation by the large intacellular protease known as the 26S proteasome. However, recent evidence suggests that ubiquitination (or ubiquitinylation - whatever you prefer!) can target proteins for other fates besides degradation by the proteasome.
Ubiquitinylation has been compared to phosphorylation (hence the change in the word) , and indeed the emeging scope and universality of this protein modification suggests this comparison is not fanciful. A great deal of interest is focusing on the multiple roles of ubiquitinylation, not just from the basic science viewpoint, but also because of its importance in disease.
These pages provide some information on the ubiquitin system - including 3D representations of key protein molecules which are part of the ubiquitin system or are found to be ubiquitinated.