Nuclear localization of the Saccharomyces cerevisiae ribonucleotide reductase small subunit requires a karyopherin and a WD40 repeat protein

Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to the corresponding deoxyribonucleotides and is an essential enzyme for DNA replication and repair. Cells have evolved intricate mechanisms to regulate RNR activity to ensure high fidelity of DNA replication during normal cell-cycle progression and of DNA repair upon genotoxic stress. The RNR holoenzyme is composed of a large subunit R1 (alpha, oligomeric state unknown) and a small subunit R2 (beta(2)). R1 binds substrates and allosteric effectors; R2 contains a diferric-tyrosyl radical [(Fe)(2)-Y-.] cofactor that is required for catalysis. In Saccharomyces cerevisiae, R1 is predominantly localized in the cytoplasm, whereas R2, which is a heterodimer (beta beta') is predominantly in the nucleus. When cells encounter DNA damage or stress during replication, beta beta' is redistributed from the nucleus to the cytoplasm in a checkpoint-dependent manner, resulting in the colocalization of R1 and R2. We have identified two proteins that have an important role in beta beta' nuclear localization: the importin 13 homolog Kap122 and the WD40 repeat protein Wtm1. Deletion of either WTM1 or KAP122 leads to loss of beta beta' nuclear localization. Wtm1 and its paralog Wtm2 are both nuclear proteins that are in the same protein complex with beta beta'. Wtm1 also interacts with Kap122 in vivo and requires Kap122 for its nuclear localization. Our results suggest that Wtm1 acts either as an adaptor to facilitate nuclear import of beta beta' by Kap122 or as an anchor to retain beta beta' in the nucleus.