Regulator of Ty1 transposition protein 105 (Rtt105) is an RPA chaperone and is required for genome stability maintenance

Single-stranded DNA (ssDNA) binding protein Replication Protein A (RPA) is essential for protecting ssDNA at replication forks. However, how RPA is guided to DNA replication forks remains unclear. Here, we show that Regulator of Ty1 transposition protein 105 (Rtt105) is an RPA-binding protein and is required for the binding of RPA at replication forks. Cells lacking Rtt105 exhibit extensive genome instability and severe defects in DNA replication. Both the nuclear import of RPA and the extent of genome-wide binding of RPA at replication forks were greatly compromised in rtt105 mutant cells. Mechanistically, Rtt105 is required for the interaction of RPA with Kap95, an importin protein known to be essential for RPA’s nuclear import. Remarkably, Rtt105 strongly enhances RPA binding with ssDNA substrates in vitro. We therefore propose a model in which Rtt105 functions as a RPA chaperone that both escorts RPA during nuclear important and delivers RPA to replication forks. To examine strand bias of Rfa1 in WT and rtt105 deletion mutant cells, we used Enrichment and Sequencing of Protein Associated Nascent DNA (eSPAN) (Yu et al., 2014) for cells at G1 phase and S phase.