C-terminus induced asymmetry within a Rad52 homodecamer dictates single-position Rad51 nucleation in homologous recombination.

Homologous recombination (HR) is a pathway for the accurate repair of double-stranded DNA breaks, which are resected to yield single-strand DNA (ssDNA). Rad52 promotes HR by facilitating formation of Rad51 nucleoprotein filaments on ssDNA. Using single-particle cryo-electron microscopy, we show that Rad52 functions as a homodecamer. The N-terminal half of Rad52 is a well-ordered ring, while the C-terminal half is disordered. An intrinsic asymmetry within Rad52 is observed, where one or a few of the C-terminal halves interacts with the ordered N-terminal ring. Within the C-terminal half, we define two conserved charged patches that harbor the Rad51 and RPA interacting motifs. Interactions between these two charged patches promote asymmetry and regulate a two-stage ssDNA binding mechanism. Interestingly, the intrinsic asymmetry allosterically drives Rad51 binding to a single position on the Rad52 ring. We propose a Rad52 catalyzed single-position nucleation model for the formation of pre-synaptic Rad51 filaments in HR.