Biophysical characterization of RelA-p52 NF-κB dimer-A link between the canonical and the non-canonical NF-κB pathway

The NF-κB family consists of the key transcription factors of the NF-κB signaling pathway, well known for its role in innate immune response, organogenesis, and a variety of cellular processes. The five NF-κB subunits-RelA, RelB, c-Rel, p50, and p52-are functional dimers, each of which share a conserved DNA binding domain which contains the dimerization domain (DD) as well. The NF-κB subunits can form 15 potential dimers among themselves of which, RelA-p50 is extensively studied and has largely become synonymous with NF-κB for transcription activation. While various reports have highlighted the importance of NF-κB subunit specificity in the transcription regulation of certain target genes, the dynamic nature of the NF-κB dimer composition is not well understood. In this study, we biophysically characterized six combinatorial dimers from three NF-κB subunits: RelA, p50, and p52, using NMR spectroscopy and differential scanning calorimetry. We show that the dimer composition is dynamic and can readily undergo exchange although at varied rates. Among the six dimers formed, RelA-p52 is found to be the most stable dimer with RelA-RelA being the least. Our results provide a plausible explanation as to why the RelA-p52 heterodimer is active during the later stages of the NF-κB activation and serve as a link between the canonical and non-canonical NF-κB pathway.