Essential interaction between the fission yeast DNA polymerase delta subunit Cdc27 and Pcn1 (PCNA) mediated through a C-terminal p21^Cip1-like PCNA binding motif

Direct interaction between DNA polymerase delta and its processivity factor proliferating cell nuclear antigen (PCNA) is essential for effective replication of the eukaryotic genome, yet the precise manner by which this occurs is unclear. We show that the 54 kDa subunit of DNA polymerase delta from Schizosaccharomyces pombe interacts directly with Pcn1 (PCNA) both in vivo and in vitro. Binding is effected via a short sequence at the C-terminus of Cdc27 with significant similarity to the canonical PCNA binding motif first identified in the mammalian p21(Cip1) protein. This motif is both necessary and sufficient far binding of Pcn1 by Cdc27 int vitro and is essential for Cdc27 function in vivo. We also show that the Pcn binding moth in Cdc27 is distinct from its binding site for Cdc1, the 55 kDa B-subunit of polymerase delta, and present evidence that Cdc27 can bind to Pcn1 and Cdc1 simultaneously. Finally, we show that Cdc27 performs at least two distinct essential functions, one of which is independent of Pcn1 binding.