SWI5–SFR1 reduces RAD51 recombinase extending units during filament assembly

Homologous recombination is a key pathway for repairing DNA double-strand breaks with high fidelity. The assembly of recombinases on DNA to form nucleoprotein filaments is a crucial and tightly regulated step. This process requires the formation of a stable nucleus, followed by recombinase extension. The dynamic assembly and disassembly of recombinases directly affect recombination progression. Accessory proteins, such as SWI5-SFR1, regulate nucleation and extension steps, modulating filament stability and recombination efficiency. In this study, we extended our investigation to the rapid extension phase of RAD51 filament assembly. We found that mouse SWI5-SFR1 effectively reduces the dissociation probability of mouse RAD51 during filament extension, promoting more uniform filament growth. Step-size analysis revealed that mRAD51 assembles as various oligomeric units, with octamers being the predominant species. This observation reflects both the oligomeric nature and structural preference of mRAD51. In the presence of mSWI5-SFR1, the step-size distribution shifted toward tetramers, indicating that mSWI5-SFR1 modulates the oligomeric state of mRAD51 in solution, thereby facilitating extension and stabilizing DNA binding. Taken together, our findings bridge the gap between the nucleation and extension stages of filament assembly, and propose a comprehensive mechanism for RAD51 filament formation and its regulation by accessory proteins to ensure genome stability.