Deubiquitinating enzymes (DUBs), also known as deubiquitinating peptidases, deubiquitinating isopeptidases, deubiquitinases, ubiquitin proteases, ubiquitin hydrolases, ubiquitin isopeptidases, are a large group of proteases that cleave ubiquitin from proteins and other molecules. Ubiquitin is attached to proteins in order to regulate the degradation of proteins via the proteasome and lysosome; coordinate the cellular localisation of proteins; activate and inactivate proteins; and modulate protein-protein interactions. DUBs can reverse these effects by cleaving the peptide or isopeptide bond between ubiquitin and its substrate protein. In humans there are nearly 100 DUB genes, which can be classified into two main classes: cysteine proteases and metalloproteases. The cysteine proteases comprise ubiquitin-specific proteases (USPs), ubiquitin C-terminal hydrolases (UCHs), Machado-Josephin domain proteases (MJDs) and ovarian tumour proteases (OTU). The metalloprotease group contains only the Jab1/Mov34/Mpr1 Pad1 N-terminal+ (MPN+) (JAMM) domain proteases.

In humans there are 102 putative DUB genes, which can be classified into two main classes: cysteine proteases and metalloproteases, consisting of 58 ubiquitin-specific proteases (USPs), 4 ubiquitin C-terminal hydrolases (UCHs), 5 Machado-Josephin domain proteases (MJDs), 14 ovarian tumour proteases (OTU), and 14 Jab1/Mov34/Mpr1 Pad1 N-terminal+ (MPN+) (JAMM) domain-containing genes. 11 of these proteins are predicted to be non-functional, leaving 79 functional enzymes. In yeast, the USPs are known as ubiquitin-specific-processing proteases (UBPs).

There are four main superfamilies of cysteine protease DUBs:

There is also a little known putative group of DUBs called the permutated papain fold peptidases of dsDNA viruses and eukaryote (PPPDEs) superfamily, which, if shown to be bona fide DUBs, would be the fifth in the cysteine protease class.

The Jab1/Mov34/Mpr1 Pad1 N-terminal+ (MPN+) (JAMM) domain superfamily proteins bind zinc and hence are metalloproteases.

DUBs play several roles in the ubiquitin pathway. One of the best characterised functions of DUBs is the removal of monoubiqutin and polyubiquitin chains from proteins. These modifications are a post translational modification (addition to a protein after it has been made) where single ubiquitin proteins or chains of ubiquitin are added to lysines of a substrate protein. These ubiquitin modifications are added to proteins by the ubiquitination machinery; ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s) and ubiquitin ligases (E3s). The end result is ubiquitin bound to lysine residues via an isopeptide bond. Proteins are affected by these modifications in a number of ways: they regulate the degradation of proteins via the proteasome and lysosome; coordinate the cellular localisation of proteins; activate and inactivate proteins; and modulate protein-protein interactions. DUBs play the antagonistic role in this axis by removing these modifications, therefore reversing the fate of the proteins. In addition, a less understood role of DUBs is the cleavage of ubiquitin-like proteins such as SUMO and NEDD8. Some DUBs may have the ability to cleave isopeptide bonds between these proteins and substrate proteins.

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