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 by Aaron Ciechanover, Avram Hershko, and Irwin Rose for which the Nobel Prize in Chemistry was awarded in 2004.
The ubiquitylation system was initially characterised as an ATP-dependent proteolytic system present in cellular extracts. A heat-stable polypeptide present in these extracts, ATP-dependent proteolysis factor 1 (APF-1), was found to become covalently attached to the model protein substrate lysozyme in an ATP- and Mg2+-dependent process. Multiple APF-1 molecules were linked to a single substrate molecule by an isopeptide linkage, and conjugates were found to be rapidly degraded with the release of free APF-1. Soon after APF-1-protein conjugation was characterised, APF-1 was identified as ubiquitin. The carboxyl group of the C-terminal glycine residue of ubiquitin (Gly76) was identified as the moiety conjugated to substrate lysine residues.
|Number of residues||76|
|Molecular mass||8564.47 Da|
|Isoelectric point (pI)||6.79|
|Gene names||RPS27A (UBA80, UBCEP1), UBA52 (UBCEP2), UBB, UBC|
Ubiquitin is a small protein that occurs in all eukaryotic cells. It performs its myriad functions through conjugation to a large range of target proteins. A variety of different modifications can occur. The ubiquitin protein itself consists of 76 amino acids and has a molecular mass of about 8.5 kDa. Key features include its C-terminal tail and the 7 Lys residues. It is highly conserved among eukaryotic species: Human and yeast ubiquitin share 96% sequence identity. The human ubiquitin sequence in one-letter code (lysine residues in bold):
The process of marking a protein with ubiquitin (ubiquitylation or ubiquitination) consists of a series of steps:
The Anaphase-promoting complex (APC) and the SCF complex (for Skp1-Cullin-F-box protein complex) are two examples of multi-subunit E3s involved in recognition and ubiquitination of specific target proteins for degradation by the proteasome.
Following addition of a single ubiquitin moiety to a protein substrate (monoubiquitination), further ubiquitin molecules can be added to the first, yielding a polyubiquitin chain. In addition, some substrates are modified by addition of ubiquitin molecules to multiple lysine residues in a process termed multiubiquitination. As discussed, ubiquitin possesses a total of 7 lysine residues. Historically the original type of ubiquitin chains identified were those linked via lysine 48. However, more recent work has uncovered a wide variety of linkages involving all possible lysine residues and in addition chains assembled on the N-terminus of a ubiquitin molecule ("linear chains"). Work published in 2007 has demonstrated the formation of branched ubiquitin chains containing multiple linkage types. "Atypical" (non-lysine 48-linked) ubiquitin chains have been discussed in a review by Ikeda & Dikic
The ubiquitination system functions in a wide variety of cellular processes, including:
The most studied polyubiquitin chains - lysine48-linked - target proteins for destruction, a process known as proteolysis. At least four ubiquitin molecules must be attached to a lysine residue on the condemned protein in order for it to be recognised by the 26S-proteasome. The proteasome is a complex, barrel-shaped structure with two chambers, within which proteolysis occurs. Proteins are rapidly degraded into small peptides (usually 3-24 amino acid residues in length). Ubiquitin molecules are cleaved off the protein immediately prior to destruction and are recycled for further use. Although the majority of proteasomal substrates are ubiquitinated, there are examples of non-ubiquitinated proteins being targeted to the proteasome.
Ubiquitin can also mark transmembrane proteins (for example, receptors) for removal from membranes and fulfill several signaling roles within the cell. Cell-surface transmembrane molecules that are tagged with ubiquitin are often monoubiquitinated, and this modification alters the subcellular localization of the protein, often targeting the protein for destruction in lysosomes.
These related molecules have novel functions and influence diverse biological processes. There is also cross-regulation between the various conjugation pathways since some proteins can become modified by more than one UBL, and sometimes even at the same lysine residue. For instance, SUMO modification often acts antagonistically to that of ubiquitination and serves to stabilize protein substrates. Proteins conjugated to UBLs are typically not targeted for degradation by the proteasome, but rather function in diverse regulatory activities. Attachment of UBLs might alter substrate conformation, affect the affinity for ligands or other interacting molecules, alter substrate localization and influence protein stability.
UBLs are structurally similar to ubiquitin and are processed, activated, conjugated and released from conjugates by enzymatic steps that are similar to the corresponding mechanisms for ubiquitin. UBLs are also translated with C-terminal extensions that are processed to expose the invariant C-terminal LRGG. These modifiers have their own specific E1 (activating), E2 (conjugating) and E3 (ligating) enzymes that conjugate the UBLs to intracellular targets. These conjugates can be reversed by UBL-specific isopeptidases that have similar mechanisms to that of the deubiquitinating enzymes.
Note: Ubiquitin is also used to mark paternal mitochondria for destruction during human fertilization.