Identification of PCNA interacting protein motifs in human DNA polymerase delta.

DNA polymerase delta (Poldelta) is a highly processive essential replicative DNA polymerase. In humans, the Poldelta holoenzyme consists of p125, p50, p68, and p12 subunits and recently, we showed that the p12 subunit exists as a dimer. Extensive biochemical studies suggest that all the subunits of Poldelta 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, same 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 motif from residues 999VGGLLAFA1008 in p125, which binds to the inter-domain-connecting loop (IDCL) of PCNA with high affinity. Collectively, including previous studies, we conclude that similar to S. cerevisiae Poldelta, each of the human Poldelta subunits possesses motif to interact with PCNA and significantly contributes towards the processive nature of this replicative DNA polymerase.