PDIP46 (DNA polymerase δ interacting protein 46) is an activating factor for human DNA polymerase δ

PDIP46 (SKAR, POLDIP3) was discovered through its interaction with the p50 subunit of human DNA polymerase delta (Pol delta). Its functions in DNA replication are unknown. PDIP46 associates with Pol delta in cell extracts both by immunochemical and protein separation methods, as well as by ChIP analyses. PDIP46 also interacts with PCNA via multiple copies of a novel PCNA binding motif, the APIMs (AlkB homologue-2 PCNA-Interacting Motif). Sites for both p50 and PCNA binding were mapped to the N-terminal region containing the APIMs. Functional assays for the effects of PDIP46 on Pol delta activity on singly primed ssM13 DNA templates revealed that it is a novel and potent activator of Pol delta. The effects of PDIP46 on Pol delta in primer extension, strand displacement and synthesis through simple hairpin structures reveal a mechanism where PDIP46 facilitates Pol delta 4 synthesis through regions of secondary structure on complex templates. In addition, evidence was obtained that PDIP46 is also capable of exerting its effects by a direct interaction with Pol delta, independent of PCNA. Mutation of the Pol delta and PCNA binding region resulted in a loss of PDIP46 functions. These studies support the view that PDIP46 is a novel accessory protein for Pol delta that is involved in cellular DNA replication. This raises the possibility that altered expression of PDIP46 or its mutation may affect Pol delta functions in vivo, and thereby be a nexus for altered genomic stability.