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Molecular and Cellular Biology, January 2008, p. 487-497, Vol. 28, No. 1
0270-7306/08/$08.00+0     doi:10.1128/MCB.01189-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

A Compartmentalized Phosphorylation/Dephosphorylation System That Regulates U snRNA Export from the Nucleus ^,

Saori Kitao,¹ Alexandra Segref,^2, Juergen Kast,^2, Matthias Wilm,² Iain W. Mattaj,² and Mutsuhito Ohno^1,2*

Institute for Virus Research, Kyoto University, Kyoto 606-8507, and Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Tokyo 102-0075, Japan,¹ European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany²

Received 4 July 2007/ Returned for modification 13 September 2007/ Accepted 12 October 2007

PHAX (phosphorylated adaptor for RNA export) is the key regulator of U snRNA nuclear export in metazoa. Our previous work revealed that PHAX is phosphorylated in the nucleus and is exported as a component of the U snRNA export complex to the cytoplasm, where it is dephosphorylated (M. Ohno, A. Segref, A. Bachi, M. Wilm, and I. W. Mattaj, Cell 101:187-198, 2000). PHAX phosphorylation is essential for export complex assembly, whereas its dephosphorylation causes export complex disassembly. Thus, PHAX is subject to a compartmentalized phosphorylation/dephosphorylation cycle that contributes to transport directionality. However, neither essential PHAX phosphorylation sites nor the modifying enzymes that contribute to the compartmentalized system have been identified. Here, we identify PHAX phosphorylation sites that are necessary and sufficient for U snRNA export. Mutation of the phosphorylation sites inhibited U snRNA export in a dominant-negative way. We also show, by both biochemical and RNA interference knockdown experiments, that the nuclear kinase and the cytoplasmic phosphatase for PHAX are CK2 kinase and protein phosphatase 2A, respectively. Our results reveal the composition of the compartmentalized phosphorylation/dephosphorylation system that regulates U snRNA export. This finding was surprising in that such a specific system for U snRNA export regulation is composed of two such universal regulators, suggesting that this compartmentalized system is used more broadly for gene expression regulation.

Published ahead of print on 29 October 2007.

Supplemental material for this article may be found at http://mcb.asm.org/.

Present address: Center for Neurobiology, University of Hamburg, 20251 Hamburg, Germany.

Present address: The Biomedical Research Centre, 2222 Health Sciences Mall, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.