Fig. 1

Domain structure and biochemical characteristics of the A20 protein. The typical amino acid sequence of human A20 has been digitally annotated. The A20 protein contains an ovarian tumor (OTU) domain in its amino terminus and seven zinc fingers (ZnF) in its carboxy-terminal end, and each domain has its own characteristics in mediating deubiquitylating (DUB) or ubiquitylating (Ub). The OTU domain of A20 has deubiquitylating enzyme activity, which is mediated by the catalytic residue Cys103 (C103). A20 is capable of binding to ubiquitylated E2 enzymes such as Ubc13 and UbcH5c via OTU and ZnF4. The ZnF4 domain of A20 mediates E3 ubiquitin ligase activity and has K63-linked polyubiquitin-binding affinity. ZnF7 has M1-linked polyubiquitin affinity and competes with other Ub-binding proteins to prevent the degradation of M1-linked chains. A20 also interacts with other protein such as RIP1, TAX1-binding protein 1 (TAX1BP1), and UbcH5a via ZF domain, and with TNF receptor associated factor 6 (TRAF6) via OTU domain. What is more, A20’s ZnF4 and ZnF7 Ub-binding domains are synergistic in regulating Ub-dependent signaling. OTU and ZnF4 domains of A20 complement each other in cells, which is facilitated by dimerization of A20 proteins. A20 can be regulated by posttranscriptional modification. For example, Human A20 is cleaved by MALT1 at Ala439, while mouse A20 is cleaved at the site between ZnF3 and ZnF4. Phosphorylation sites of A20 contains Ser381, Ser480, Ser565, and Thr625, and phosphorylation at Ser381 mediated by IKKβ can improve DUB activity. Additionally, domain-specific mutant mice can be generated by mutating OTU (C103A), ZnF4 (C609A and C612A), or ZnF7 (C764A and C767A) and other specific sites