Structural and mechanistic insight into Holliday-junction dissolution by Topoisomerase IIIα and RMI1

The 'dissolvasome', composed of TopIIIα, BLM and RMI proteins, coordinates DNA-helicase and DNA-topoisomerase activities to resolve double Holliday junctions (dHJs) generated during DNA recombination and repair. The first crystal structure of a human TopIIIα–Rmi1 subcomplex provides insights into how topoisomerase is stimulated to promote dHJ decatenation. Repair of DNA double-strand breaks via homologous recombination can produce double Holliday junctions (dHJs) that require enzymatic separation. Topoisomerase IIIα (TopIIIα) together with RMI1 disentangles the final hemicatenane intermediate obtained once dHJs have converged. How binding of RMI1 to TopIIIα influences it to behave as a hemicatenane dissolvase, rather than as an enzyme that relaxes DNA topology, is unknown. Here, we present the crystal structure of human TopIIIα complexed to the first oligonucleotide-binding domain (OB fold) of RMI1. TopIII assumes a toroidal type 1A topoisomerase fold. RMI1 attaches to the edge of the gate in TopIIIα through which DNA passes. RMI1 projects a 23-residue loop into the TopIIIα gate, thereby influencing the dynamics of its opening and closing. Our results provide a mechanistic rationale for how RMI1 stabilizes TopIIIα-gate opening to enable dissolution and illustrate how binding partners modulate topoisomerase function.