by Marilene Demasi
Protein polyubiquitination was first described as a post-translational modification to direct proteins for degradation. The ubiquitin molecule is covalently bound to the target protein and the polyubiquitin chain is created by successive attachments of ubiquitin through its carboxy-terminal Glycine mainly to Lysine48 (K48) residues of previously conjugated ubiquitin. Proteins tagged with a K48-linked polyubiquitin chain are directed for degradation. However, distinct ubiquitin chains are built up through other Lysine residues from ubiquitin, resulting in distinct structural patterns of ubiquitin complexes, which in most cases are unrelated to target protein degradation.