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Vol. 16, Issue 6, 2960-2971, June 2005

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Nuclear Import of the Stem–Loop Binding Protein and Localization during the Cell Cycle

Judith A. Erkmann ^*  , Eric J. Wagner ^* , Jian Dong ^* , Yanping Zhang , Ulrike Kutay , and William F. Marzluff ^* 

^* Program in Molecular Biology and Biotechnology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; Department of Radiation Oncology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; and Institute of Biochemistry, Swiss Federal Institute of Technology, Zurich, Switzerland

Submitted November 22, 2004; Revised March 15, 2005; Accepted April 1, 2005
Monitoring Editor: Susan Wente

A key factor involved in the processing of histone pre-mRNAs in the nucleus and translation of mature histone mRNAs in the cytoplasm is the stem–loop binding protein (SLBP). In this work, we have investigated SLBP nuclear transport and subcellular localization during the cell cycle. SLBP is predominantly nuclear under steady-state conditions and localizes to the cytoplasm during S phase when histone mRNAs accumulate. Consistently, SLBP mutants that are defective in histone mRNA binding remain nuclear. As assayed in heterokaryons, export of SLBP from the nucleus is dependent on histone mRNA binding, demonstrating that SLBP on its own does not possess any nuclear export signals. We find that SLBP interacts with the import receptors Imp/Imp and Transportin-SR2. Moreover, complexes formed between SLBP and the two import receptors are disrupted by RanGTP. We have further shown that SLBP is imported by both receptors in vitro. Three sequences in SLBP required for Imp/Imp binding were identified. Simultaneous mutation of all three sequences was necessary to abolish SLBP nuclear localization in vivo. In contrast, we were unable to identify an in vivo role for Transportin-SR2 in SLBP nuclear localization. Thus, only the Imp/Imp pathway contributes to SLBP nuclear import in HeLa cells.

Address correspondence to: William F. Marzluff (marzluff{at}med.unc.edu) or Ulrike Kutay (ulrike.kutay{at}bc.biol.ethz.ch).

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