Monoubiquitinated FANCI and FANCD2 constitute the ID complex, which forms a sliding clamp on DNA.
Major Finding: Monoubiquitinated FANCI and FANCD2 constitute the ID complex, which forms a sliding clamp on DNA.
Approach: Cryo-electron microscopy revealed the structures of the ubiquitinated and unmodified ID complexes.
Impact: This structural study shows how ubiquitination affects the function of the cancer-linked ID complex.
Some mutations in FANCI or FANCD2, proteins that join to form a dimeric complex called ID that is necessary for the repair of DNA interstrand cross-links (ICL), cause Fanconi anemia, a disease associated with high cancer risk. Monoubiquitination of each protomer in the complex is required for ICL repair, but the mechanism by which this posttranslational modification affects the complex is not known. Using cryo-electron microscopy, Wang and colleagues solved structures of the monoubiquitinated and non-ubiquitinated ID complexes bound to ICL-containing DNA at resolutions of 3.5 and 3.4 Å, respectively. Whereas the FANCI–FANCD2 dimer in the non-ubiquitinated complex resembled an open trough, with DNA positioned at the bottom of the trough, the monoubiquitinated FANCI–FANCD2 dimer completely encircled the DNA. To form the ring, the ubiquitin linked to FANCI and the ubiquitin linked to FANCD2 bind, and FANCI and FANCD2 undergo substantial conformational changes. The hydrophobic patches of each ubiquitin in the complex are sequestered in this configuration, making the previously proposed hypothesis that the role of monoubiquitination of ID is to recruit downstream effectors less likely. Of note, upon monoubiquitination, the FANCI and FANCD2 protomers rotate relative to each other, juxtaposing their respective C-terminal domains to form a zipper-like structure that contains R1285 of FANCI, the mutation of which to a glutamine residue is associated with Fanconi anemia. Biochemical assays revealed that the R1285Q mutation leads to faster deubiquitination of the ID complex by the cognate deubiquitinating enzyme, potentially explaining this mutation's role in disease. Additionally, the DNA nick was not observable in the structure of the monoubiquitinated ID complex, hinting that monoubiquitination may cause the complex to lose specificity for branched DNA, an idea that was supported by biochemical experiments. Thus, this work elucidates the mechanism by which monoubiquitination affects the ID complex: The monoubiquitinated complex may function as a sliding clamp that can move from its initial binding site on DNA to allow nucleases or other factors to access the ICL.
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