The DNA Binding Activity of p53 Displays Reaction-Diffusion Kinetics

The tumor suppressor protein p53 plays a key role in maintaining the genomic stability of mammalian cells and preventing malignant transformation. In this study, we investigated the intracellular diffusion of a p53-GFP fusion protein using confocal fluorescence recovery after photobleaching. We show that the diffusion of p53-GFP within the nucleus is well described by a mathematical model for diffusion of particles that bind temporarily to a spatially homogeneous immobile structure with binding and release rates k 1 and k 2, respectively. The diffusion constant of p53-GFP was estimated to be D p53-GFP = 15.4 μm 2 s −1, significantly slower than that of GFP alone, D GFP = 41.6 μm 2 s −1. The reaction rates of the binding and unbinding of p53-GFP were estimated as k 1 = 0.3 s −1 and k 2 = 0.4 s −1, respectively, values suggestive of nonspecific binding. Consistent with this finding, the diffusional mobilities of tumor-derived sequence-specific DNA binding mutants of p53 were indistinguishable from that of the wild-type protein. These data are consistent with a model in which, under steady-state conditions, p53 is latent and continuously scans DNA, requiring activation for sequence-specific DNA binding.