Abstract
RING3 is a novel protein kinase linked to human leukaemia. Its Drosophila homologue female sterile homeotic is a developmental regulator that interacts genetically with trithorax, a human homologue of which is also associated with leukaemia. The RING3 structure contains two mutually related bromodomains that probably assist in the remodelling of chromatin and thereby affect transcription. Consistent with this hypothesis, a RING3-like protein has been identified in the mouse Mediator complex, where it is associated with transcription factors. We show that, whilst RING3 is constitutively localised to the nucleus of exponentially growing HeLa cells, it is delocalised throughout serum-starved fibroblasts. We use immunostaining and confocal microscopy to demonstrate that RING3 translocates to the fibroblast nucleus upon serum stimulation. After translocation, RING3 participates in nuclear protein complexes that include E2F proteins; it transactivates the promoters of several important mammalian cell cycle genes that are dependent on E2F, including dihydrofolate reductase, cyclin D1, cyclin A and cyclin E. We use site-directed mutagenesis of a putative nuclear localisation motif to show that the activation-induced nuclear localisation and consequent transcriptional activity of RING3 depends on a monopartite, classical nuclear localisation sequence. These observations refine and extend the mechanism by which RING3 contributes to E2F-regulated cell cycle progression. Deregulation of this mechanism may be leukaemogenic.