Mammalian p53R2 Protein Forms an Active Ribonucleotide Reductasein Vitro with the R1 Protein, Which Is Expressed Both in Resting Cells in Response to DNA Damage and in Proliferating Cells

Abstract

Recently, a homologue of the small subunit of mammalian ribonucleotide reductase (RNR) was discovered, called p53R2. Unlike the well characterized S phasespecific RNR R2 protein, the new form was induced in response to DNA damage by the p53 protein. Because the R2 protein is specifically degraded in late mitosis and absent in G o /G 1 cells, the induction of the p53R2 protein may explain how resting cells can obtain deoxyribonucleotides for DNA repair. However, no direct demonstration of RNR activity of the p53R2 protein was presented and furthermore, no corresponding RNR large subunit was identified. In this study we show that recombinant, highly purified human and mouse p53R2 proteins contain an iron-tyrosyl free radical center, and both proteins form an active RNR complex with the human and mouse R1 proteins. UV irradiation of serumstarved, G o /G 1 -enriched mouse fibroblasts, stably transformed with an R1 promoter-luciferase reporter gene construct, caused a 3-fold increase in luciferase activity 24 h after irradiation, paralleled by an increase in the levels of R1 protein. Taken together, our data indicate that the R1 protein can function as the normal partner of the p53R2 protein and that an R1-p53R2 complex can supply resting cells with deoxyribonucleotides for DNA repair.Ribonucleotide reductase (RNR) 1 catalyzes de novo synthesis of deoxyribonucleotides from the corresponding ribonucleotides (1). This is the rate-limiting step in DNA precursor synthesis, and it is regulated at many different levels in the mammalian cell: (i) by allosteric control of the activity and specificity of ribonucleotide reductase by nucleoside triphosphate effectors (2), (ii) by the S phase-dependent transcription of the ribonucleotide reductase genes (3), and (iii) by the rapid, proteasomedependent proteolysis of the small subunit in late mitosis (4).Errors in the allosteric control lead to unbalanced dNTP pools, misincorporation of deoxyribonucleotides in DNA, genetic abnormalities or cell death (5). The mammalian ribonucleotide reductase consists of a 1:1 complex of two homodimeric nonidentical subunits called proteins R1 and R2. The 90-kDa R1 protein contains the catalytic site, the binding sites for the allosteric effectors, and redox active disulfides that participate in the reduction of the substrates, while the 45-kDa R2 protein contains an iron center-generated tyrosyl free radical essential for catalysis (6). Transcription of both the R1 and R2 genes is S-phase specific but only the R2 protein shows S-phase-specific expression (3, 7). The levels of the R1 protein are in excess and almost constant during the cell cycle in proliferating cells because of a long half-life of the protein (Ͼ24 h) (8, 9). Therefore, the cell cycle-dependent activity of ribonucleotide reductase is controlled by the synthesis and degradation of the R2 protein. This synthesis starts in early S phase, and then the R2 protein

Mouse ribonucleotide reductase R2 protein: A new target for anaphase-promoting complex-Cdh1-mediated proteolysis

“…Together they can form an active ribonucleotide reductase complex supplying resting cells with dNTPs for DNA repair (9). These results demonstrate that mammalian RNR genes, like the Mec1͞Rad 53 pathway-regulated yeast RNR genes (10,11), are subject to transcriptional regulation after DNA damage.…”

“…The mouse p53R2 protein shows 81% amino acid sequence identity compared with the mouse R2 protein and all of the iron ligands, the tyrosyl free radical, the amino acid residues involved in long-range radical transfer, and the C-terminal heptapeptide, essential for binding to the R1 protein, are conserved (9). The major difference is that the p53R2 protein has a 33-aa residue truncation in the N terminus compared with the R2 protein and therefore lacks the KEN box.…”

Expression of thymidylate synthase in human cells is an early G1 event regulated by CDK4 and p16INK4A but not E2F

“…R2 is rate limiting for the enzymatic activity of RNR and its level is cell cycle dependent (Chabes and Thelander, 2000). However, an analogue of R2, p53R2 has recently been found that is not cell cycle dependent (Guittet et al, 2001;Nordlund and Reichard, 2006). p53R2 can substitute for R2 to form an active RNR enzyme capable of generating deoxyribonucleotides in resting cells or following DNA damage.…”

Corynebacterium ammoniagenes class Ib ribonucleotide reductase: transcriptional regulation of an atypical genomic organization in the nrd cluster

“…Previous studies showed that expression of RNR genes is cell-cycle dependent (Guittet et al, 2001;Jacobson & Fuchs, 1998), being maximal at the beginning of the exponential phase, where increasingly large amounts of dNTPs are required for DNA synthesis and chromosome replication, and decreasing through the late exponential and stationary phases. We have shown that the mRNA levels of C. ammoniagenes nrdE and nrdF are also downregulated from exponential growth to the stationary phase, following the expected transcriptional pattern for RNR genes.…”