Eukaryotic-Expressed Human Pif1 Helicase Exhibits Distinctive Binding and Unwinding Characteristics

Abstract

Background: Conserved Pif1 helicases play important roles in maintaining genome stability. Several studies have used prokaryotic-expressed human Pif1 (hPif1) to investigate its biochemical activities. However, the properties have only been revealed to a certain extent owing to difficulties in achieving native-like post-translational maturation. This study aimed to characterize the biochemical properties of the hPif1 helicase domain (hPif1-HD) expressed in insect cells. Methods: The binding characteristics of eukaryotic-expressed hPif1-HD were measured by a fluorescence polarization binding assay. The unwinding kinetics, DNA annealing and strand exchange assays were measured by the stopped-flow fluorescence resonance energy transfer (FRET) assay. Results: The results showed that the eukaryotic-expressed hPif1 exhibited some binding and unwinding preferences different from those in the prokaryotic-expressed protein. Furthermore, several novel biochemical characteristics were discovered, including that this helicase unwound Y-shaped DNA better than overhang DNA, hPif1-HD effectively annealed different substrates, and hPif1 preferentially unwound RNA/DNA heteroduplex over double-stranded DNA (dsDNA). Notably, human telomeric G-quadruplex (G4) was shown to be the best G4 substrate. It was discovered, for the first time, that G4 stimulated hPif1-mediated adjacent dsDNA unwinding. Conclusions: These findings suggest that eukaryotic-expressed hPif1 is more closely related to the binding and unwinding characteristics under physiological conditions, which could potentially expand our understanding of the enzymatic activity and reaction mechanism of hPif1.