Identification of PCNA interacting protein motifs in human DNA polymerase delta

DNA polymerase delta is a highly processive essential replicative DNA polymerase. In humans, Pol delta holoenzyme consists of p125, p50, p68, and p12 subunits and recently, we have shown that p12 exists as a dimer. Extensive biochemical studies suggest that all the subunits of Pol delta interact with the processivity factor proliferating cell nuclear antigen (PCNA) to carry out a pivotal role in genomic DNA replication. While PCNA interaction protein (PIP) motifs in p68, p50 and p12 have been mapped, the PIP in p125, the catalytic subunit of the holoenzyme, remains elusive. Therefore, in this study by using multiple approaches we have conclusively mapped a non-canonical PIP box from residues 999-VGGLLAFA-1008 in p125, which binds to inter domain-connecting loop of PCNA with high affinity. Collectively, including previous studies, we conclude that similar to S. cerevisiae Pol delta, each of the human Pol delta subunits possess motif to interact with PCNA and significantly contribute towards the processive nature of this replicative DNA polymerase.