Human Pif1 helicase is a G-quadruplex DNA binding protein with G-quadruplex DNA unwinding activity
Résumé
Pif1 proteins are helicases that in yeast are implicated in the maintenance of genome stability. One activity of Saccharomyces cerevisiae Pif1 is to stabilize DNA sequences that could otherwise form deleterious G-quadruplex (G4) structures by acting as a G4 resolvase. Evidence is presented here that human Pif1 (hPif1, nuclear form) is a G4 DNA binding and resolvase protein and that these activities are properties of the conserved helicase domain (amino acids 206-620 of 641, hPifHD). hPif1 preferentially bound synthetic G4-DNA relative to ssDNA, dsDNA and a partially single-stranded duplex DNA helicase substrate. G4 DNA unwinding but not binding required an extended (<10 nucleotide) 5' ssDNA tail and in competition assays G4 DNA was an ineffective suppressor of helicase activity compared to ssDNA. These data suggest a distinction between the determinants of G4-DNA binding and the ssDNA interactions required for helicase action and that hPif1 may act on G4 substrates by binding alone or as a resolvase. Human Pif1 could therefore have a role in processing G4 structures that arise in the single-stranded nucleic acid intermediates formed during DNA replication and gene expression.
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