Stoichiometric complex formation by proliferating cell nuclear antigen (PCNA) and its interacting protein: purification and crystallization of the DNA polymerase and PCNA monomer mutant complex from Pyrococcus furiosus

Replicative DNA polymerase interacts with processivity factors, the β-­subunit of DNA polymerase III or proliferating cell nuclear antigen (PCNA), in order to function with a long template DNA. The archaeal replicative DNA polymerase from Pyrococcus furiosus interacts with PCNA via its PCNA-interacting protein (PIP) motif at the C-terminus. The PCNA homotrimeric ring contains one PIP interacting site on each monomer and since the ring can accommodate up to three molecules simultaneously, formation of a stable stoichiometric complex of PCNA with its interacting protein has been difficult to control in vitro. A stable complex of the DNA polymerase with PCNA, using a PCNA monomer mutant, has been purified and crystallized. The best ordered crystal diffracted to 3.0 Å resolution using synchrotron radiation. The crystals belong to space group P2₁2₁2, with unit-cell parameters a = 225.3, b = 123.3, c = 91.3 Å.