Mechanism, specificity, and function of FANCD2‐FANCI ubiquitination and deubiquitination

Fanconi anemia (FA) is a rare genetic disorder caused by mutations in any of the currently 22 known FA genes. The products of these genes, along with other FA‐associated proteins, participate in a biochemical pathway, known as the FA pathway. This pathway is responsible for the repair of DNA interstrand cross‐links (ICL) and the maintenance of genomic stability in response to replication stress. At the center of the pathway is the monoubiquitination of two FA proteins, FANCD2 and FANCI, on two specific lysine residues. This is achieved by the combined action of the UBE2T ubiquitin‐conjugating enzyme and a large multicomponent E3 ligase, known as the FA‐core complex. This E2‐E3 pair specifically targets the FANCI‐FANCD2 heterodimer (ID2 complex) for ubiquitination on DNA. Deubiquitination of both FANCD2 and FANCI, which is also critical for ICL repair, is achieved by the USP1‐UAF1 complex. Recent work suggests that FANCD2 ubiquitination transforms the ID2 complex into a sliding DNA clamp. Further ubiquitination on FANCI does not alter this closed‐on‐DNA ID2 conformation. However, the resulting dimonoubiquitinated complex is highly resistant to USP1‐UAF1 deubiquitination. This review will provide an update on recent work focusing on how specificity in FANCD2 ubiquitination and deubiquitination is achieved. Recent findings shedding light to the mechanisms, molecular functions, and biological roles of FANCI/FANCD2 ubiquitination and deubiquitination will be also discussed. Enzymes UBA1 (6.2.1.45), UBE2T (2.3.2.23), FANCL (2.3.2.27), USP1 (3.4.19.12). Dimonoubiquitination of the FANCI‐FANCD2 (ID2) complex is a crucial step in the Fanconi anemia DNA repair pathway. This is achieved by the E2‐ubiquitin‐conjugating enzyme, UBE2T, and a multisubunit ubiquitin E3 ligase, known as the FA‐core complex. FANCD2 ubiquitination results in the ID2 complex clamping on double‐stranded DNA, whereas further ubiquitination on FANCI is required for protecting FANCD2’s ubiquitin from USP1‐UAF1‐mediated deubiquitination.