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

Poxviruses are enveloped viruses with a linear, double-stranded DNA genome. Viral DNA synthesis is achieved by a functional DNA polymerase holoenzyme composed of three essential proteins. For vaccinia virus (VACV) these are E9, the catalytic subunit, a family B DNA polymerase, and the heterodimeric processivity factor formed by D4 and A20. The A20 protein links D4 to the catalytic subunit. High-resolution structures have been obtained for the VACV D4 protein in complex with an N-terminal fragment of A20 as well as for E9. In addition, biochemical studies provided evidence that a poxvirus-specific insertion (insert 3) in E9 interacts with the C-terminal residues of A20. Here, we provide solution structures of two different VACV A20 C-terminal constructs containing residues 304-426, fused at their C-terminus to either a BAP (Biotin Acceptor Peptide)-tag or a short peptide containing the helix of E9 insert 3. Together with results from titration studies, these structures shed light on the molecular interface between the catalytic subunit and the processivity factor component A20. The interface comprises hydrophobic residues conserved within the Chordopoxvirinae subfamily. Finally, we constructed a HADDOCK model of the VACV A20(304-426)-E9 complex, which is in excellent accordance with previous experimental data. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Poxviruses are enveloped viruses with a linear, double-stranded DNA genome, and viral DNA synthesis is achieved by a DNA polymerase holoenzyme composed of three essential proteins. The catalytic subunit E9 forms a complex with the heterodimeric processivity factor made up of D4 and A20, which have a specific interaction involving a poxvirus-specific insertion. The interface between E9 and A20 comprises conserved hydrophobic residues and has been structurally characterized, providing insights into their molecular interactions.