Small molecule inhibitor of the RPA70 N-terminal protein interaction domain discovered using in silico and in vitro methods

The pharmacological suppression of the DNA damage response and DNA repair can increase the therapeutic indices of conventional chemotherapeutics. Replication Protein A (RPA), the major single-stranded DNA binding protein in eukaryotes, is required for DNA replication, DNA repair, DNA recombination, and DNA damage response signaling. Through the use of high-throughput screening of 1500 compounds, we have identified a small molecule inhibitor, 15-carboxy-13-isopropylatis-13-ene-17,18-dioic acid (NSC15520), that inhibited both the binding of Rad9-GST and p53-GST fusion proteins to the RPA N-terminal DNA binding domain (DBD), interactions that are essential for robust DNA damage signaling. NSC15520 competitively inhibited the binding of p53-GST peptide with an IC₅₀ of 10 μM. NSC15520 also inhibited helix destabilization of a duplex DNA (dsDNA) oligonucleotide, an activity dependent on the N-terminal domain of RPA70. NSC15520 did not inhibit RPA from binding single-stranded oligonucleotides, suggesting that the action of this inhibitor is specific for the N-terminal DBD of RPA, and does not bind to DBDs essential for single-strand DNA binding. Computer modeling implicates direct competition between NSC15520 and Rad9 for the same binding surface on RPA. Inhibitors of protein-protein interactions within the N-terminus of RPA are predicted to act synergistically with DNA damaging agents and inhibitors of DNA repair. Novel compounds such as NSC15520 have the potential to serve as chemosensitizing agents.