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Ubiquitin is a small (76 amino acid) protein that is part of a highly conserved protein family in eukaryotes. Ubiquitin is involved in a wide variety of cellular processes via its covalent attachment to target proteins. This process is termed ubiquitylation and is catalyzed by the ubiquitylation cascade.
Ubiquitylation of a protein can alter its activity, stability as well as subcellular localization. These are directly influenced the ubiquitin linkage bound to the target protein. Ubiquitin is attached to a Lys residue within a target protein by four linkages that are principally formed with ubiquitin: mono-, multi-mono as well as poly.
The attachment of multiple ubiquitin moieties to a protein-bound ubiquitin is referred to as poly-ubiquitylation, forming ubiquitin chains on the target protein. Ubiquitin chains can have a distinct structure depending on which Lys (K) residue within ubiquitin is utilized to form the chain. These linkages can take place on K6, K11, K27, K29, K33, K48 or K63 as well as the N-terminus of ubiquitin (Met1). The formation of ubiquitin chains through the attachment of the same linkage gives rise to homotypic chains. The mixing of ubiquitin linkages within a chain forms heterotypic chains.
The attachment of ubiquitin to a substrate protein occurs via a three-step process, known as the ubiquitylation cascade. It is initiated by the ubiquitin-activating enzyme (E1) activating ubiquitin, this is then transferred to a ubiquitin-conjugating enzyme (E2). The substrate for ubiquitylation is recognized by the ubiquitin ligase enzyme (E3), which facilitates the transfer of ubiquitin from the E2 to the target Lys residue. This is a reversible process where ubiquitin chains can be cleaved by the action of deubiquitylases (DUBs).