The movement of many transcription factors, kinases and replication factors between the nucleus and cytoplasm is important in regulating their activity1. In some cases, phosphorylation of a protein regulates its entry into the nucleus2; in others, it causes the protein to be exported to the cytoplasm3,4,5,6. The mechanism by which phosphorylation promotes protein export from the nucleus is poorly understood. Here we investigate how the export of the yeast transcription factor Pho4 is regulated in response to changes in phosphate availability. We show that phosphorylation of Pho4 by a nuclear complex of a cyclin with a cyclin-dependent kinase, Pho80–Pho85, triggers its export from the nucleus. We also find that the shuttling receptor used by Pho4 for nuclear export is the importin-β-family member Msn5 (refs 7, 8), which is required for nuclear export of Pho4 in vivo and binds only to phosphorylated Pho4 in the presence of the GTP-bound form of yeast Ran in vitro. Our results reveal a simple mechanism by which phosphorylation can control the nuclear export of a protein.
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We thank I. Herskowitz, J. Li, J. Weissman and members of the O'Shea laboratory for comments on the manuscript; R. Bischoff for the RanQ69L plasmid; M. Lenburg for the Pho85–GFP plasmid; and J. Aitchison, K. Weis, P. Silver, M. Fitzgerald-Hayes, E. Hurt and A. Tartakoff for yeast strains. A.K. is a Fellow of the UCSF Medical Scientist Training Program. N.M.R. and L.S.H. were supported by fellowships from the NIH and E.M.O. was supported by a fellowship from the Jane Coffin Childs Foundation. This work was supported by the David and Lucile Packard Foundation and by an NSF Presidential Faculty Fellowship (E.K.O.).
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Kaffman, A., Rank, N., O'Neill, E. et al. The receptor Msn5 exports the phosphorylated transcription factor Pho4 out of the nucleus. Nature 396, 482–486 (1998) doi:10.1038/24898
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