The functional significance of the RPA- and PCNA-dependent recruitment of Pif1 to DNA

Pif1 family helicases are multifunctional proteins conserved in eukaryotes, from yeast to humans. They are important for the genome maintenance in both nuclei and mitochondria, where they have been implicated in Okazaki fragment processing, replication fork progression and termination, telomerase regulation and DNA repair. While the Pif1 helicase activity is readily detectable on naked nucleic acids in vitro, the in vivo functions rely on recruitment to DNA. We identify the single-stranded DNA binding protein complex RPA as the major recruiter of Pif1 in budding yeast, in addition to the previously reported Pif1-PCNA interaction. The two modes of the Pif1 recruitment act independently during telomerase inhibition, as the mutations in the Pif1 motifs disrupting either of the recruitment pathways act additively. In contrast, both recruitment mechanisms are essential for the replication-related roles of Pif1 at conventional forks and during the repair by break-induced replication. We propose a molecular model where RPA and PCNA provide a double anchoring of Pif1 at replication forks, which is essential for the Pif1 functions related to the fork movement.