br Cellular and target recruitment In Figure
Cellular and target recruitment In Figure 1, we present a simplified classification of the different layers of DUB regulation. The first layer we discuss is that of DUB recruitment factors. Guiding the almost 100 DUBs encoded in the human ACET to their relevant substrates and pathways is crucial for cellular physiology because it insulates DUBs from unwanted interactions and the cell from spurious activity. It can be mediated by distinct regions within the enzyme or by external factors: For instance, the Ubl domain of ubiquitin-specific protease 14 (USP14) recruits it to the proteasome, where its activity is increased 500-fold . The endosomal protein signal transducing adaptor molecule (STAM) recruits the DUBs AMSH (associated molecule with an Src3 homology domain of STAM) and USP8 to the endosome pathway by interacting with an SRC homology 3 (SH3)-binding motif or MIT domain (microtubule interacting and transport), respectively 12, 13. Another pathway that requires proper DUB recruitment is the DNA damage response (DDR). After ultraviolet (UV)-induced DNA damage, monoubiquitylated proliferating cell nuclear antigen (PCNA) mediates signaling that leads to repair. The DUB complex USP1/USP1-associated factor 1 (UAF1) deubiquitylates PCNA after the complex is recruited to the substrate by recruitment factor human ELG1 . BRCA1/BRCA2-containing complex, subunit 36 (BRCC36) is another DUB in the DDR, where it deubiquitylates several proteins as a catalytic subunit of the BRCA1-A complex. In this complex, specialized ubiquitin and small ubiquitin-like modifier (SUMO)-binding domains recruit BRCC36 to sites of damage 15, 16, 17, 18, 19. Finally, also in the nucleus, transcription factor forkhead box K2 (FOXK2) targets the UCH class DUB BAP1 (BRCA1-associated protein 1) to chromatin to facilitate histone H2A deubiquitylation . In NF-κB signaling, ubiquitin conjugation has multiple roles. Different components of the pathway recruit the DUBs USP10 and CYLD (cylindromatosis). The protein MCPIP-1 (monocyte chemotactic protein induced protein 1) recruits USP10, whereas CYLD is recruited by the E3 ligase HOIP (HOIL interacting protein) (Figure 1). Moreover, CYLD contains a B-box domain that promotes its cytoplasmic localization 21, 22. The previous examples illustrated how external proteins can recruit DUBs to relevant pathways or substrates. While DUBs in general can recognize the ubiquitin part of a substrate via their catalytic domains (CDs), sometimes extra specificity is achieved by specialized domains that are present in DUBs themselves. One of the best-studied examples is the DUB USP7. Here, the N-terminal TRAF domain of USP7 binds small peptide motifs in its targets EBNA-1 (Epstein–Barr nuclear antigen 1), p53 and MDM2 (Mouse double minute 2 homolog) to facilitate their deubiquitylation (Figure 1) 23, 24. USP15 uses its DUSP-Ubl domain to recruit and deubiquitylate the E3 ligase BRCA1-associated protein (BRAP) , while the H2A deubiquitinase USP3 requires its intact Zinc finger domain to bind H2A .