Holliday junction branch migration driven by AAA plus ATPase motors

Holliday junctions are key intermediate DNA structures during genetic recombination. One of the first Holliday junction-processing protein complexes to be discovered was the well conserved RuvAB branch migration complex present in bac-teria that mediates an ATP-dependent movement of the Holli-day junction (branch migration). Although the RuvAB complex served as a paradigm for the processing of the Holliday junc-tion, due to technical limitations the detailed structure and underlying mechanism of the RuvAB branch migration com-plex has until now remained unclear. Recently, structures of a reconstituted RuvAB complex actively-processing a Holliday junction were resolved using time-resolved cryo-electron mi-croscopy. These structures showed distinct conformational states at different stages of the migration process. These structures made it possible to propose an integrated model for RuvAB Holliday junction branch migration. Furthermore, they revealed unexpected insights into the highly coordinated and regulated mechanisms of the nucleotide cycle powering sub-strate translocation in the hexameric AAA+ RuvB ATPase. Here, we review these latest advances and describe areas for future research.