A continuation of the assembly of a G-quadruplex repair complex by the FANCJ DNA helicase

Nucleic acid sequences that are rich in guanines can form G-quadruplex (G4) structures, which can impede DNA replication or repair. The FANCJ helicase plays a role in maintaining genomic stability by facilitating DNA replication through regions of DNA that form G4 structures. This activity has been associated with an AKKQ motif in FANCJ and it is thought that this motif retains the ability to also target 8-oxoguanine-modified G4 (8oxoG4) structures. We hypothesize that the molecular recognition of FANCJ AKKQ to 8oxoG4s is dependent upon the position of DNA damage, sequence composition, and substrate stability. We previously demonstrated that reducing the loop length between guanine tetrads increased the affinity of FANCJ AKKQ when 8-oxoguanines (8oxoGs) were located at the first and fifth positions of the G4 structure. In this work, we use fluorescence spectroscopy to measure the affinities of a FANCJ AKKQ peptide for G4s formed by (GGGT) ₄ , (GGGTT) ₄ , and (TTAGGG) ₄ . G4 conformations and thermal stability were determined via circular dichroism spectroscopy. We substituted 8oxoGs at the first (8oxo1), third (8oxo3), and fifth (8oxo5) positions of each G4 sequence and found that the most destabilized structures were (GGGT) ₄ 8oxo1, (GGGTT) ₄ 8oxo1, and (TTAGGG) ₄ 8oxo5 in KCl. The most destabilized structures in NaCl were (GGGT) ₄ 8oxo1, (GGGTT) ₄ 8oxo5, and (TTAGGG) ₄ 8oxo5. The binding affinities of FANCJ AKKQ differed for the distinct damage positions for (GGGT) ₄ in KCl and for (TTAGGG) ₄ in NaCl. These findings suggest that FANCJ AKKQ targets G4s and 8oxoG4s in a manner dependent upon sequence composition and damage position.