Wild-type p53 regulates human ribonucleotide reductase by protein-protein interaction with p53R2 as well as hRRM2 subunits

Ribonucleotide reductase (RR) plays a key role in the synthesis of DNA and is the only enzyme responsible for the reduction of ribonucleotides to their corresponding deoxyribonucleotides, providing a balanced supply of precursors for DNA synthesis and repair. There are three known human RR subunits, hRRM1, hRRM2, and p53R2, which is encoded by a p53 target gene. It is not clear whether p53 and RR can directly interact at the protein level to regulate DNA repair. It is also not known where deoxyribonucleotides are synthesized in the cell. In coimmunoprecipitation experiments, we found that hRRM2 and p53R2, but not hRRM1, bound to p53 in KB cells, which express wild-type p53. Moreover, in response to UV irradiation, both p53R2 and hRRM2 were released from p53 and shifted to bind hRRM1. Confocal microscopy confirmed the colocalization of p53 with p53R2 and hRRM2 and the translocation of hRRM1, p53R2 and hRRM2 from the cytoplasm to the nucleus after UV treatment. An in vivo RR activity assay showed that the kinetic profile of increased RR activity was consistent with the accumulation of RR subunits in the nucleus. The ability of p53R2 and hRRM2 to shift from binding p53 to hRRM1 in response to UV irradiation was deficient in the presence of mutant p53. Moreover, in cells overexpressing hRRM2, binding of p53R2 to p53 decreased, whereas binding to hRRM1 increased. Our results suggest that wild-type p53 directly interacts with both p53R2 and hRRM2. In response to UV irradiation, p53R2 and hRRM2 dissociate from p53 and p53R2, and hRRM2 and hRRM1 transfer to the nucleus and form an active RR complex to provide dNDPs for DNA repair. Therefore, the direct interaction of p53 with p53R2 and hRRM2 and the nuclear accumulation of RR subunits after UV exposure might play a pivotal role in DNA repair.