A Comparative Study on G-Quadruplex Unfolding Activity of RecQ Helicases

The RecQ family has five member helicases in humans: RecQL1, RecQL2 or WRN, RecQL3 or BLM, RecQL4, and RecQL5. WRN, BLM, and RecQL4 helicases are associated with Werner, Bloom, and Rothmund-Thomson syndromes, respectively. RecQL1 and RecQL5 have not been directly linked with a specific syndrome. Both of WRN and BLM helicases resolve non-canonical DNA structures including the G-quadruplex (GQ), which otherwise result in DNA breaks and genomic instability. Whether RecQL5 also possesses GQ unfolding activity may potentially provide insight about why RecQL5 is not associated with a specific syndrome, assuming GQ structures play a significant role in aforementioned syndromes. We recently demonstrated that BLM unfolds human telomeric GQ (hGQ) in the presence of ATP and also to a lesser extent upon binding to the vicinity of hGQ in the absence of ATP, both under physiologically germane salt and pH conditions. In the current study, GQ unfolding activity of RecQL5 was examined using single molecule FRET using hGQ and other GQ constructs. Our data show that RecQL5 unfolds hGQ at a significantly weaker rate compared to BLM in the presence of ATP, and does not unfold GQ in the absence of ATP. RecQL5-mediated GQ unfolding is strongly dependent on GQ stability, as modulated by salt or GQ structure. Significant differences were observed between the dynamics of BLM and RecQL5 interactions with the DNA, in particular in terms of the ‘reeling’ activity previously observed in several GQ unfolding helicases. In conclusion, RecQL5 is a significantly less efficient GQ destabilizer compared to BLM and WRN, but may assist these helicases in this task. These results are consistent with GQs being significant components of Bloom and Werner syndromes, and might explain why deficiencies in RecQL5 are not directly associated with such a syndrome.