Abstract
Fanconi anemia (FA) is a genetically heterogeneous disorder associated with deficiencies in the FA complementation group network. FA complementation group M (FANCM) and FA-associated protein 24 kDa (FAAP24) form a stable complex to anchor the FA core complex to chromatin in repairing DNA interstrand crosslinks. Here, we report the first crystal structure of the C-terminal segment of FANCM in complex with FAAP24. The C-terminal segment of FANCM and FAAP24 both consist of a nuclease domain at the N-terminus and a tandem helix-hairpin-helix (HhH)2 domain at the C-terminus. The FANCM-FAAP24 complex exhibits a similar architecture as that of ApXPF. However, the variations of several key residues and the electrostatic property at the active-site region render a catalytically inactive nuclease domain of FANCM, accounting for the lack of nuclease activity. We also show that the first HhH motif of FAAP24 is a potential binding site for DNA, which plays a critical role in targeting FANCM-FAAP24 to chromatin. These results reveal the mechanistic insights into the functions of FANCM-FAAP24 in DNA repair.
Highlights
Fanconi anemia (FA) is a rare genetic disease characterized by bone marrow failure, developmental defects and increased incidence of cancers [1,2]
As the central event of the FA pathway, the eight upstream FA proteins (FANCA, FANCB, FANCC, FANCE, FANCF, FANCG, FANCL and FANCM) cooperate with five FA-associated proteins (FAAP20, FA-associated protein 24 kDa (FAAP24), FAAP100, MHF1 and MHF2) to form the FA core complex, which acts as an E3 ubiquitin ligase to monoubiquitinate the FANCI-FANCD2 complex and activates the downstream DNA repair pathways [3,4,5,6,7,8,9,10,11,12,13,14]
FANCM can interact with the FANCM-associated histone-fold proteins 1 and 2 (MHF1-MHF2) complex via the region following the helicase domain and with FAAP24 via the C-terminal domains to form two stable complexes to bind different DNA substrates, which are crucial for FANCM to facilitate repair of interstrand crosslinks (ICLs) [3,4,5]
Summary
Fanconi anemia (FA) is a rare genetic disease characterized by bone marrow failure, developmental defects and increased incidence of cancers [1,2]. We obtained these two forms of the FANCM-FAAP24 complex with high purity, stability and homogeneity and confirmed their binding to different types of DNA structures (Figure 1A and Supplementary Figure S1), but failed to obtain any crystals.
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