Background
Ubiquitin (Ub) is a highly conserved 76 amino-acid protein found throughout eukaryotic cells. A vast number of cellular processes, including targeted protein degradation, cell cycle progression, DNA repair, protein trafficking, inflammatory response, virus budding, and receptor endocytosis, are regulated by Ub-mediated signalling; where the target protein is tagged by single or multi-monomeric Ub (monomeric Ub attached to multiple sites on the substrate) or a polymeric chain of Ubs (Fushman et al., 2010). This post-translational modification is tightly controlled by an enzymatic cascade involving several enzymes (E1, E2, and E3) and occurs through either an isopeptide bond between the C-terminal Glycyl residue of Ub and the epsilon amino group of a Lysyl residue on a target protein or through a peptide bond between the C-terminal Glycyl residue of Ub and the N-terminal amine on a further Ub. In the former (isopeptide bond-linked) case the substrate protein may either be ubiquitin itself - thus leading to the generation of poly-ubiquitin chains - or another target protein (Fushman et al., 2010). Thus, ubiquitin can be attached to a substrate either as a monomer or as a poly-ubiquitin chain. Further - depending on their linkage type (M1, K6, K11, K27, K29, K33, K48 and K63 linked) - the Ub chains can take different structural forms. Chains containing all eight possible Ub linkages have been found in living cells and different ubiquitin chain types may encode different biological signals, allowing this single protein to mediate many diverse functions (Komander 2009; Weeks et al., 2009; Walczak et al., 2012). The functionality of Ub chains is most commonly associated with their attachment to substrate proteins but there is also evidence that they may also play a role in cellular signalling as free chains (Braten et al., 2012). A mass spectrometry-based study found that K48 linkages account for the largest percentage (29%) of all yeast ubiquitin-ubiquitin linkages. The relative abundance of the other linkages were K6 (11%), K11 (28%), K27 (9%), K29 (3%), K33 (4%) and K63 (16%) (Xu et al., 2009). The attachment of K48-linked polyubiquitin chains to proteins is a universal signal for degradation by the proteasome. Ubiquitin chains form distinct structures based on their linkage. K63-linked chains adopt an extended conformation with little interaction between adjacent ubiquitins. In contrast, K11- and K48-linked chains form compact, globular structures with significant ubiquitin-ubiquitin contact (Schaefer et al., 2011). Lys48-linked ubiquitin tetramers are the minimal recognition motif for the proteasome and a K48-ubiquitinated substrate will be degraded within minutes in cells (Thrower et al., 2000).
References
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Fushman D, Walker O (2010) Exploring the linkage dependence of polyubiquitin conformations using molecular modeling. Journal of Molecular Biology 395, 803-814.
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Schaefer JB, Morgan DO (2011) Protein-linked ubiquitin chain structure restricts activity of deubiquitinating enzymes. J Biol Chem 286, 45186-45196.
Thrower JS, Hoffman L, Rechsteiner M, Pickart CM (2000) Recognition of the polyubiquitin proteolytic signal. The EMBO Journal 19, 94-102.
Walczak H, Iwai K, Dikic I (2012) Generation and physiological roles of linear ubiquitin chains. BMC Biol 10, 23.
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