Guanine-rich DNA can fold back on itself into four-stranded knots called G-quadruplexes, or G4s — structures that help regulate gene expression and telomere biology but that stall replication forks if left unresolved. The helicase FANCJ is one of the cell’s principal G4-resolving enzymes, and its gene doubles as a Fanconi anemia locus and a hereditary breast-cancer susceptibility gene, since FANCJ operates as a close functional partner of BRCA1. Despite decades of genetic and biochemical work, the structural basis of how FANCJ actually recognizes and dismantles a G4 has been missing.
A team led by Huilin Li at the Van Andel Institute, with collaborators in Japan, used cryo-electron microscopy to visualize human FANCJ bound to a G4-containing DNA substrate, using a non-hydrolyzable ATP analog to trap the enzyme mid-reaction. The structures reveal that FANCJ’s iron-sulfur (Fe-S) cluster domain, long known to be essential for its activity but never structurally tied to substrate binding, makes direct physical contact with the quadruplex. Mutating this Fe-S/G4 interface abolished both binding and unwinding, confirming its central mechanistic role.
The cryo-EM data also captured two distinct conformational states, open and closed, that track the ATP hydrolysis cycle. Cycling between them appears to drive FANCJ’s stepwise translocation along the single-stranded DNA extending from the quadruplex, progressively peeling the four strands apart rather than all at once. This incremental, ratchet-like action helps explain why several cancer-associated mutations clustered in the Fe-S domain selectively cripple G4 unwinding without abolishing FANCJ’s other helicase functions.
The therapeutic implications deserve attention: FANCJ-deficient cells are already known to be hypersensitive to G4-stabilizing drugs such as pyridostatin, a vulnerability conceptually similar to the synthetic lethality PARP inhibitors exploit in BRCA-mutant tumors. Mapping the Fe-S/G4 interface at atomic resolution could help predict which specific FANCJ mutations sensitize a given tumor to that strategy. The main caveat is that the structures rely on one engineered substrate and a non-hydrolyzable ATP mimic rather than the native catalytic cycle, so how well these snapshots generalize to the diverse G4 sequences and crowded conditions of a living cell still needs testing.
PR
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