The biochemical activities of the Saccharomyces cerevisiae Pif1 helicase are regulated by its N-terminal domain

PIF1 family helicases represent a highly conserved class of enzymes involved in multiple aspects of genome maintenance. Many PIF1 helicase are multi-domain proteins, but the functions of their non-helicase domains are poorly understood. Here, we characterized how the N-terminal domain (NTD) of the Saccharomyces cerevisiae Pif1 helicase affects its functions both in vivo and in vitro . Removal of the Pif1 NTD alleviated the toxicity associated with Pif1 over-expression in yeast. Biochemically, the N-terminally truncated Pif1 (Pif1ΔN) retained in vitro DNA binding, DNA unwinding, and telomerase regulation activities, but these activities differed markedly from those displayed by full-length recombinant Pif1. However, Pif1ΔN was still able to synergize with the Hrq1 helicase to inhibit telomerase activity in vitro , similar to full-length Pif1. These data impact our understanding of PIF1 helicase evolution and the roles of these enzymes in the maintenance of genome integrity.