Visualizing homology search in living cells

Homologous recombination (HR) is a DNA repair process that requires a homologous sequence for the repair of DNA double-stranded breaks. The search for homology is the rate limiting step for HR, but the dynamics of homology search in vivo are currently poorly understood. This is largely due to a lack of tools to visualize homology search in action in living human cells. Here, we show that RAD51 and MND1, two proteins operating in homology search, become visible in long filamentous structures several hours after double-stranded break formation. Using GFP-MND1 we can visualize these filaments in living human cells and find that they are highly dynamic, explore the nuclear space, and can resolve over time. We show that the resolution of these filaments depends on RAD54L, known for its role in RAD51-driven homology search and strand-invasion. In addition, we find that loss of cohesin also inhibits the resolution of these filaments, in accordance with a role for cohesin in HR. Thus, our data suggest that these filaments are visible intermediates of an active DNA repair process, and that MND1-GFP can be used as a tool to study the dynamics of homology search in living human cells.