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

Ribonucleotide reductase consists of two nonidentical proteins, R1 and R2, and catalyzes the rate-limiting step in DNA precursor synthesis: the reduction of ribonucleotides to deoxyribonucleotides. A strictly balanced supply of deoxyribonucleotides is essential for both accurate DNA replication and repair. Therefore, ribonucleotide reductase activity is under exquisite control both transcriptionally and posttranscriptionally. In proliferating mammalian cells, enzyme activity is regulated by control of R2 protein stability. This control, which responds to DNA damage, is effective until cells pass into mitosis. We demonstrate that the mitotic degradation and hence the overall periodicity of R2 protein levels depends on a KEN box sequence, recognized by the Cdh1-anaphase-promoting complex. The mouse R2 protein specifically binds Cdh1 and is polyubiquitinated in an in vitro ubiquitin assay system. Mutating the KEN signal stabilizes the R2 protein during mitosisG(1) in R2 protein-overexpressing cells. The degradation process, which blocks deoxyribonucleotide production during G(1), may be an important mechanism protecting the cell against unscheduled DNA synthesis. The newly discovered p53-induced p53R2 protein that lacks a KEN box may supply deoxyribonucleotides for DNA repair during G(0)G(1).