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¹ Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan
² Laboratory of Cell Recognition and Responses, Graduate School of Life Sciences, Tohoku University, Sendai 980-8578, Japan
³ Department of Integrated Genetics, National Institute of Genetics, SOKENDAI, Mishima, Shizuoka 411-8540, Japan
⁴ Laboratory of Biological Signaling, Graduate School of Life Science, University of Hyogo, 3-2-1 Kouto, Kamigori, Ako-gun, Hyogo 678-1297, Japan

Snf2SR, a suppressor of rna1^ts, which is a temperature-sensitive mutation in Schizosaccharomyces pombe RanGAP (GTPase activating protein), possesses both the SNF2 and the helicase domains conserved in the chromatin remodeling SNF2 ATPase/helicase protein family. We have now clarified a function of Snf2SR. Snf2SR indeed showed DNA-stimulated ATPase activity, proving that it is a member of the SNF2 ATPase/helicase family. Consistent with this role, Snf2SR was localized in the nucleus and cell fractionation analysis revealed that Snf2SR was tightly associated with the nuclear matrix. The disruption of snf2SR⁺ was detrimental for a cell proliferation of S. pombe. Snf2SR that did not enhance RanGAP activity by itself, but abolished histone-H3-mediated RanGAP inhibition, as previously reported for the histone H3 methyltransferase, Clr4, another rna1^ts suppressor. In contrast to Clr4, Snf2SR directly bound to the GDP-bound form of the S. pombe Ran homologue Spi1 and enhanced the nucleotide exchange activity of Pim1, the S. pombe RanGEF (guanine nucleotide exchange factor). Over-expression of Spi1-G18V, a Ran GTPase mutant fixed in the GTP-bound form, was lethal to S. pombe snf2SR. Together, our results indicate that Snf2SR is involved in the Ran GTPase cycle in vivo.

^* Correspondence: Email: tnishi{at}molbiol.med.kyushu-u.ac.jp