DNA clamp function of the mono-ubiquitinated Fanconi Anemia FANCI-FANCD2 complex

The FANCI-FANCD2 (ID) complex, mutated in the Fanconi Anemia (FA) cancer predisposition syndrome, is required for the repair of replication forks stalled at DNA interstrand crosslinks (ICL) and related lesions. The FA pathway is activated when two replication forks converge onto an ICL, triggering the mono-ubiquitination of the ID complex. ID mono-ubiquitination is essential for ICL repair by excision, translesion synthesis and homologous recombination, but its function was hitherto unknown. Here, the 3.5 angstrom cryo-EM structure of mono-ubiquitinated ID (IDUb) bound to DNA reveals that it forms a closed ring that encircles the DNA. Compared to the cryo-EM structure of the non-ubiquitinated ID complex bound to ICL DNA, described here as well, mono-ubiquitination triggers a complete re-arrangement of the open, trough-like ID structure through the ubiquitin of one protomer binding to the other protomer in a reciprocal fashion. The structures, in conjunction with biochemical data, indicate the mono-ubiquitinated ID complex looses its preference for ICL and related branched DNA structures, becoming a sliding DNA clamp that can coordinate the subsequent repair reactions. Our findings also reveal how mono-ubiquitination in general can induce an alternate structure with a new function.