The interaction between proliferating cell nuclear antigen (PCNA) and DNA polymerase ␦ is essential for processive DNA synthesis during DNA replication/repair; however, the identity of the subunit of DNA polymerase ␦ that directly interacts with PCNA has not been resolved until now. In the present study we have used reciprocal co-immunoprecipitation experiments to determine which of the two subunits of core DNA polymerase ␦, the 125-kDa catalytic subunit or the 50-kDa small subunit, directly

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... eracts with PCNA. We found that PCNA co-immunoprecipitated with human p50, as well as calf thymus DNA polymerase ␦ heterodimer, but not with p125 alone, suggesting that PCNA directly interacts with p50 but not with p125. A PCNA-binding motif, similar to the sliding clamp-binding motif of bacteriophage RB69 DNA polymerase, was identified in the N terminus of p50. A 22-amino acid oligopeptide containing this sequence (MRPFL) was shown to bind PCNA by far Western analysis and to compete with p50 for binding to PCNA in co-immunoprecipitation experiments. The binding of p50 to PCNA was inhibited by p21, suggesting that the two proteins compete for the same binding site on PCNA. These results establish that the interaction of PCNA with DNA polymerase ␦ is mediated through the small subunit of the enzyme. DNA polymerase ␦ (pol ␦), 1 the principal DNA replicase in eukaryotes, also participates in several DNA repair pathways, including nucleotide excision repair, mismatch repair, and long patch base excision repair (1, 2). For both replication and repair functions, pol ␦ requires an accessory protein, the proliferating cell nuclear antigen (PCNA), to carry out highly processive DNA synthesis (3, 4). Crystallographic studies have demonstrated that PCNA forms a homotrimeric ring that encircles double-stranded DNA and functions to tether the DNA polymerase to its template/primer, thus dramatically increasing the processivity of the enzyme (5, 6).