USP20 is a 914-amino acid protein that shows 59% homology with another DUB, USP33. It contains 4 known domains, an N-terminal Zf UBP domain, a catalytic domain containing conserved histidine and cysteine residues, and two C-terminal DUSP domains.
DUBs are categorised into 5 main groups, ubiquitin-specific proteases (USP), ubiquitin c-terminal hydrolases (UCH), ovarian tumour proteases (OTU), Machado-Joseph disease proteases (MJD), and JAB1/MPN/MOV34 proteases (JAMM/MPN+). The first four groups are cysteine proteases, whereas the last group are Zn metalloproteases. USP20 belongs to the USP group and, like most DUBs, catalyse the breakage of an isopeptide bond between a lysine residue of the target protein and the terminal glycine residue of a ubiquitin protein. This occurs via a conserved cysteine and histidine residue in the catalytic site of the enzyme. The histidine molecule is protonated by the cysteine residue and this allows the cystein residue to undergo a nucleophillic attack on the isopeptide bond, which removes the ubiquitin from the substrate protein.
USP20 deubiquitinates thyronine deiodinase type 2 (D2), an enzyme that converts thyroxine (T4) into active 3,5,3'-triiodothyronine (T3). D2 is ubiquitinated after binding of T4, which signals for the degradation of D2 via the proteasome and also causes an inactivating conformational change of the protein. Deubiquitination by USP20 rescues D2 from degradation and also returns D2 to its active conformation.
USP20 is involved in the recycling of the β2-adrenergic receptor. After agonist stimulation, the receptor is internalised and ubiquitinated. USP20 serves to deubiquitinate the receptor and prevent its degradation by the proteasome. This allows it to be recycled to the cell surface in order to resensitize the cell to signalling molecules.
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