Crystal structure of the human Polε B-subunit in complex with the C-terminal domain of the catalytic subunit [Protein Structure and Folding]

The eukaryotic B-family DNA polymerases include four members, Polα, Polδ, Polϵ, and Polζ, which share common architectural features, such as the exonuclease/polymerase and C-terminal domains (CTDs) of catalytic subunits bound to indispensable B-subunits, which serve as scaffolds that mediate interactions with other components of the replication machinery. Crystal structures for the B-subunits of Polα and Polδ/Polζ have been reported; the former within the primosome and separately with CTD, and the latter with the N-terminal domain (NTD) of the C-subunit. Here we present the crystal structure of the human Polϵ B-subunit (p59) in complex with CTD of the catalytic subunit (p261C). The structure revealed a well-defined electron density for p261C and the phosphodiesterase (PDE) and oligonucleotide/oligosaccharide-binding domains of p59. However, electron density was missing for the p59 NTD and for the linker connecting it to the PDE domain. Similar to Polα, p261C contains a three-helix bundle in the middle and zinc-binding modules (Zn1 and Zn2) on each side. Intersubunit interactions involving 11 hydrogen bonds and numerous hydrophobic contacts account for stable complex formation with a buried surface area of 3094 Å2. Comparative structural analysis of p59-p261C with the corresponding Polα complex revealed significant differences between the B-subunits and CTDs as well as their interaction interfaces. The B-subunit of Polδ/Polζ also substantially differs from B-subunits of either Polα or Polϵ. This work provides a structural basis to explain biochemical and genetic data on the importance of B-subunit integrity in replisome function in vivo.

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