Determination of dissociation constant of the NFκB p50/p65 heterodimer using fluorescence cross-correlation spectroscopy in the living cell

Two-laser-beam fluorescence cross-correlation spectroscopy (FCCS) is promising technique that provides quantitative information about the interactions of biomolecules. The p50/p65 heterodimer is the most abundant and well understood of the NF kappa B dimers in most cells. However, the quantitative value of affinity, namely the K-d, for the heterodimer in living cells is not known yet. To quantify the heterodimerization of the IPT domain of p50/p65 in the living cell, we used two-laser-beam FCCS. The K-d values of mCherry(2)- and EGFP-fused p50 and p65 were determined to be 0.46 mu M in the cytoplasm and 1.06 mu M in the nucleus of the living cell. These results suggest the different binding affinities of the p50/p65 heterodimer in the cytoplasm and nucleus of the living cell and different complex formation in each region. (C) 2013 Elsevier Inc. All rights reserved.