Activation-induced nuclear translocation of RING3

RING3 is a novel protein kinase linked to human leukaemia. Its Drosophila h omologue female sterile homeotic is a developmental regulator that interact s genetically with trithorax, a human homologue of which is also associated with leukaemia. The RING3 structure contains two mutually related bromodom ains that probably assist in the remodelling of chromatin and thereby affec t 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 localis ed to the nucleus of exponentially growing HeLa cells, it is delocalised th roughout serum-starved fibroblasts, We use immunostaining and confocal micr oscopy to demonstrate that RING3 translocates to the fibroblast nucleus upo n serum stimulation, After translocation, RING3 participates in nuclear pro tein complexes that include E2F proteins; it transactivates the promoters o f several important mammalian cell cycle genes that are dependent on E2F, i ncluding 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 transcripti onal activity of RING3 depends on a monopartite, classical nuclear localisa tion 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.