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Guanine-rich nucleic acid sequences can adopt G-quadruplex (G4) structures, which pose barriers to DNA replication and repair. The FANCJ helicase contributes to genome stability by resolving these structures, a function linked to its G4-binding site that features an AKKQ amino acid motif. This site is thought to recognize oxidatively damaged G4, specifically those containing 8-oxoguanine (8oxoG) modifications. We hypothesize that FANCJ AKKQ recognition of 8oxoG-modified G4s (8oxoG4s) depends on the sequence context, the position of the lesion within the G4, and overall structural stability. Using fluorescence spectroscopy, we measured the binding affinities of a FANCJ AKKQ peptide for G4s formed by (GGGT)4, (GGGTT)4, and (TTAGGG)4 sequences. G4 conformation and thermal stability were assessed by circular dichroism spectroscopy. Each sequence was modified to include a single 8oxoG at the first (8oxo1), third (8oxo3), or fifth (8oxo5) guanine position. In potassium chloride (KCl), the most destabilized structures were (GGGT)4 8oxo1, (GGGTT)4 8oxo1, and (TTAGGG)4 8oxo5. In sodium chloride (NaCl), the most destabilized were (GGGT)4 8oxo1, (GGGTT)4 8oxo5, and (TTAGGG)4 8oxo5. FANCJ AKKQ binding affinities varied according to damage position and sequence context, with notable differences for (GGGT)4 in KCl and (TTAGGG)4 in NaCl. These findings support a model in which FANCJ binding to G4 and 8oxoG4 structures is modulated by both the oxidative damage position and the G4 local sequence environment. 

Biolayer interferometry (BLI) is a powerful tool that enables direct observations of protein-G4 interactions in real-time. In this article, we discuss the crucial aspects in conducting a BLI experiment by using the TAR DNA-binding protein (TDP43) and a G4 DNA formed by (GGGGCC)4 as a sample application. We also describe the necessary precautions in designing the DNA substrate and evaluating the signal contributions arising from nonspecific binding interactions. A comprehensive guide is included that details the necessary materials and reagents, experimental procedures, and data analysis methods for researchers who are interested in using BLI for similar studies. The insights provided in this article will allow researchers to harness the potential of BLI and unravel the complexities of protein-G4 interactions with precision and confidence.