Structure of the fanconi anaemia monoubiquitin ligase complex

The Fanconi anaemia (FA) pathway repairs DNA damage caused by endogenous and
chemotherapy-induced DNA crosslinks, and responds to replication stress1,2.
Genetic inactivation of this pathway impairs development, prevents blood production
and promotes cancer1,3. The key molecular step in the FA pathway is the
monoubiquitination of a pseudosymmetric heterodimer of FA group I protein
(FANCI)–FA group D2 protein (FANCD2)4,5 by the FA core complex—a megadalton
multiprotein E3 ubiquitin ligase6,7. Monoubiquitinated FANCD2 then recruits
enzymes to remove the DNA crosslink or to stabilize the stalled replication fork. A
molecular structure of the FA core complex would explain how it acts to maintain
genome stability. Here we reconstituted an active, recombinant FA core complex, and
used cryo-electron microscopy and mass spectrometry to determine its structure. The
FA core complex comprises two central dimers of the FA group B protein (FANCB)
and FA-associated protein of 100 kDa (FAAP100) subunits, flanked by two copies of
the RING finger protein, FA complementation group L protein (FANCL). These two
heterotrimers act as a scaffold to assemble the remaining five subunits, resulting in
an extended asymmetric structure. Destabilization of the scaffold would disrupt the
entire complex, resulting in a non-functional FA pathway. Thus, the structure
provides a mechanistic basis for the low numbers of patients with mutations in
FANCB, FANCL and FAAP100. Despite a lack of sequence homology, FANCB and
FAAP100 adopt similar structures. The two FANCL subunits are in different conformations at opposite ends of the complex, suggesting that each FANCL has a
distinct role. This structural and functional asymmetry of RING finger domains may
be a general feature of E3 ligases. The cryo-EM structure of the FA core complex
provides a foundation for a detailed understanding of its E3 ubiquitin ligase activity
and DNA interstrand crosslink repair.