In summary:

➡️ RAD51B complex promotes RAD51 filament assembly on RPA coated DNA
➡️ XRCC3 complex caps filaments & promotes homologous pairing

📄 science.org/doi/10.1126/science.aea1546

Thanks to our collaborators in @boultonlab.bsky.social group also at @crick.ac.uk (8/8)

Additionally, XRCC3 complex capped filaments promote D-loop formation — a key HR intermediate for repairing DSBs and stalled forks.

This shows that capping by the XRCC3 complex directly enhances RAD51’s synaptic activity. (7/8)

XRCC3 remodels the terminal RAD51 N-terminal domain to bind in trans.

This rearrangement is conserved in archaeal paralog RadB and yeast paralog complex Rad55–Rad57, revealing a deeply conserved filament-capping mechanism. (6/8)

Using cryo-EM, single-molecule and biochemical assays ❄️💡, we found these two tetramers functionally differ:

➡️ The RAD51B complex assembles RAD51 filaments in an ATP hydrolysis–coupled, dynamic manner.
➡️ The XRCC3 complex caps the 5′ ends of RAD51 filaments – independent of ATP hydrolysis (5/8)

It is known that the five RAD51 paralogs exist as a tetramer (RAD51B-C-D-XRCC2) and dimer (RAD51C-XRCC3). Our data overturn this paradigm:

➡️ The ‘dimer’ actually exists in a tetrameric XRCC3 complex: XRCC3-RAD51C-RAD51D-XRCC2.
➡️ Therefore, there are two near-identical tetrameric assemblies.

In our previous work, we described the structure-function relationship of the RAD51B-RAD51C-RAD51D-XRCC2 complex (www.nature.com/articles/s41...) in the assembly of RAD51 filaments on RPA-coated ssDNA, which marks double-strand break resected DNA. (3/8)

Homologous recombination repairs DNA double-strand breaks via the RAD51 filament, which searches for homology and drives strand exchange. BRCA2 and the RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, XRCC3) promote filament assembly; however, their mutation causes cancer and Fanconi anaemia. (2/8)