Yeast Proliferating Cell Nuclear Antigen
Krishna, T. S. R., Kong, X.-P., Gary, S., Burgers, P., and Kuriyan, J. 1994. Crystal structure of the eukaryotic DNA polymerase processivity factor PCNA. Cell 79: 1233-1243.
Following up on the determination of the crystal structure of the bacterial DNA polymerase processivity factor, the structure of the processivity factor (DNA clamp) of eukaryotic DNA polymerase (proliferating cell nuclear antigen, or PCNA) was determined. Despite no sequence similarity between the amino acids of the bacterial and eukaryotic polymerase processivity factors, the chain fold of the two proteins is seen to be strikingly similar. This is particularly striking since each subunit of PCNA is only two-thirds the size of each subunit of the bacterial processivity factor, and yet PCNA is assembled into a ring that is remarkably congruent in its shape and dimensions with the ring formed by the bacterial DNA clamp. This is a consequence of the internal symmetry of these processivity factors. The bacterial ring is made up of two subunits, which each contain three topologically similar domains. PCNA assembles into a trimeric, rather than dimeric, ring in which each subunit contains two rather than three domains. This paper establishes a deep mechanistic similarity between the bacterial and eukaryotic processive DNA polymerases that was not apparent from consideration of the amino acids sequences of the processivity factors alone.

Protein Structure Coordinates: RCSB PDB Code 1PLQ, or get them HERE.