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 Dp(53-GFP)=15.4 mu m(2) s(-1), significantly slower than that of GFP alone, D-GFP 41.6 mu m(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.