Bromodomain dimers: A case study of BRD4 and family-wide AlphaFold predictions

Bromodomains are conserved acetyl-lysine reader domains that play a central role in the assembly of transcriptional regulatory complexes. While generally presumed to function as monomers, bromodomain homo-dimers have been identified, and several bromodomain-containing proteins have been linked to biomolecular condensates, where locally elevated concentrations may promote dimerization. Here, we investigated bromodomain dimerization with an integrated approach that combines structural and biophysical measurements with AlphaFold-based predictions across the bromodomain family. Using the second bromodomain (BD2) of BRD4 as a model system, we characterized the thermodynamics and kinetics of its monomer-dimer equilibrium by two-dimensional nuclear magnetic resonance (NMR) lineshape analysis and CPMG relaxation dispersion. We found that the BRD4^BD2 dimer forms transiently with a dissociation constant near 400 μM and a lifetime near 1 ms. Using our NMR-derived restraints, we performed data-driven docking to generate models of the BRD4^BD2 dimer. To assess dimerization propensity across the wider bromodomain family, we leveraged AlphaFold-Multimer and AlphaFold3 to systematically predict homo-dimeric models for all human bromodomains. We identified several predicted dimer architectures, with 15 bromodomain dimers that have higher interface-confidence scores than BRD4^BD2. Overall, our results suggest that weak and reversible dimerization may be more widespread among bromodomains, where it could contribute to function in dynamic transcriptional assemblies.