Solution Structure of the C-terminal Domain of A20, the Missing Brick for the Characterization of the Interface between Vaccinia Virus DNA Polymerase and its Processivity Factor

•The C-terminal domain of the poxvirus processivity factor A20 shows a new fold.•Poxvirus-specific inserts 0 and 3 of the DNA polymerase E9 are the A20 binding site.•On A20, the interface features a central hydrophobic leucine-binding pocket.•The interface is highly conserved within the chordopoxvirinae subfamily.•Processivity factor binding differs from all other family B DNA polymerases.