Abstract

Deubiquitinating enzymes (DUBs) are a family of thiol- and metallo-proteases specialized in the processing of ubiquitin and ubiquitin-like proteins (Amerik AY, Hochstrasser M, 2004).USP7 is a prominent member of the ubiquitin-specific protease (USP) subfamily. It has also been named as Herpesvirus Associated Ubiquitin Specific Protease (HAUSP) since it was first identified as a protein associated with ICP0, a regulatory protein of the herpes simplex virus which directs P53 to proteasomal destruction (Everett et al., 1997). USP7 is involved in the regulation of stress response pathways, epigenetic silencing, cell survival pathway and the progress of infections by DNA viruses(Amerik AY and Hochstrasser M,2004).
USP7 comprises four domains. The N-terminal domain (NTD), which forms an eight-stranded antiparallel beta-sandwitch (Sheng et al., 2005), is a divergent member of the linear motif binding TRAF domain family (PF00917) - see the LIG_TRAF ELM entries. The USP7 NTD is known to interact with peptide ligand motifs in P53 and MDM2, enabling their deubiquitination. The NTD domain can also interact with the EBV protein EBNA1 to block USP7 activity and promote P53 elimination, as part of the mechanism to immortalise B-Cells (Saridakis et al., 2005). Besides the NTD, two C-terminal domains also mediate protein-protein interactions. For example, the HSV ICP0 protein mentioned above binds to one of the C-terminal domains. The USP7 catalytic domain responsible for the deubiquitination of substrate proteins is distinct from these three protein targeting domains.
Structural and mutational binding studies revealed that short peptides in disordered regions of both p53 and MDM2 interact with the same location on the USP7 NTD domain in a mutually exclusive manner. They have related motifs based on a conserved P..S core, but the MDM2 variant binds USP7 with a much higher affinity possibly required for the role of USP7 in regulating p53 destruction by the MDM2 pathway (see LIG_MDM2). The high affinity EBNA site approximates to a P.E..S motif. The Ser is equivalent in both motif classes and is suitable to be regulated by phosphorylation as a PhosphoSer is structurally disallowed in the complex. However proline-directed kinases (MapKs, CDKs, Hipk2 etc.) could not phosphorylate these sites as Pro is structurally disallowed in the position following the Ser.