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Eukaryotic proliferating cell nuclear antigen (PCNA) is an essential replication accessory factor that interacts with a variety of proteins involved in DNA replication and repair. Each monomer of PCNA has an N-terminal domain A and a C-terminal domain B. In the structure of the wild-type PCNA protein, domain A of one monomer interacts with domain B of a neighboring monomer to form a ring-shaped trimer. Glu113 is a conserved residue at the subunit interface in domain A. Two distinct X-ray crystal structures have been determined of a mutant form of PCNA with a substitution at this position (E113G) that has previously been studied because of its effect on translesion synthesis. The first structure was the expected ring-shaped trimer. The second structure was an unanticipated nontrimeric form of the protein. In this nontrimeric form, domain A of one PCNA monomer interacts with domain A of a neighboring monomer, while domain B of this monomer interacts with domain B of a different neighboring monomer. The BB interface is stabilized by an antiparallel β-sheet and appears to be structurally similar to the AB interface observed in the trimeric form of PCNA. The AA interface, in contrast, is primarily stabilized by hydrophobic interactions. Because the E113G substitution is located on this hydrophobic surface, the AA interface should be less favorable in the case of the wild-type protein. This suggests that the side chain of Glu113 promotes trimer formation by destabilizing these possible alternate subunit interactions.

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