Structural insights into DNA annealing and recombination by herpesviral DNA-binding proteins ICP8 and BALF2

The DNA-binding proteins of herpesviruses are key factors involved in multiple stages of DNA replication and recombination. The well-studied ICP8 protein of Herpes Simplex Virus 1 binds single-stranded DNA in a cooperative manner and facilitates the annealing of complementary DNA strands in a process that involves the formation of double helical filaments. Moreover, it cooperates with UL12 alkaline nuclease to execute recombination between homologous DNA sequences. In this work, we present the structures of ICP8 and its homolog BALF2 of the Epstein-Barr Virus in complexes with DNA. These structures reveal a conserved conformation of the bound DNA in which several bases are rotated away from the protein, enabling homology search and base-pairing. The crystal structures of ICP8-DNA depict the pairing between two DNA strands, showing the flexibility of base-pairing. In the structure of the double filament of ICP8, we visualize the interactions between protein subunits within and between the antiparallel strands. Finally, we present the first complete structure of a ternary recombinase complex composed of ICP8 and UL12 bound to a model DNA. This structural landscape illustrates the different stages of the DNA recombination process in herpesviruses, shedding light on molecular mechanisms of their replication.