Abstract
PIF1 family helicases promote genome stability during DNA replication and repair, yet the biochemical activity of human PIF1 (hPIF1) remains poorly understood. Here, we directly compare full-length hPIF1 and budding yeast Pif1 (yPif1) and show that the two enzymes operate differently. In bulk assays, hPIF1 displays weak net DNA unwinding because its robust intrinsic single-stranded DNA (ssDNA) annealing activity rapidly reforms duplex DNA behind the helicase motor. Single-molecule magnetic tweezers experiments reveal that hPIF1 is only modestly slower than yPif1 and similarly processive, indicating that poor apparent unwinding primarily reflects rapid reannealing rather than impaired motor activity. Mutagenesis and structural modeling indicate that this functional divergence depends on a yeast-specific insertion within the catalytic domain. Unlike yPif1, hPIF1 fails to stimulate Polδ-dependent displacement-loop (D-loop) extension, arguing that it does not promote DNA unwinding ahead of the migrating bubble. However, similarly to yPif1, hPIF1 likely unwinds newly synthesized DNA coupled with DNA reannealing behind the migrating D-loop. hPIF1 efficiently remodels intramolecular G-quadruplex DNA into fully base-paired dsDNA, whereas yPif1 generates ssDNA products. Together, our data support a model in which hPIF1 acts primarily as a DNA translocase that couples local unwinding with rapid rewinding to clear secondary DNA structures without generating extensive ssDNA.