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A more recent version of this article appeared on May 1, 2008
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Submitted on December 26, 2007
Revised on February 1, 2008
Accepted on February 15, 2008
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-dependent Nuclear Import
Department of Biochemistry I, Faculty of Medicine, Georg-August-University of Göttingen, 37073, Göttingen, Germany
Monitoring Editor: Karsten Weis
In vertebrate cells, the nucleoporin Nup358/RanBP2 is a major component of the filaments that emanate from the nuclear pore complex into the cytoplasm. Nup358 forms a complex with SUMOylated RanGAP1, the GTPase activating protein for Ran. RanGAP1 plays a pivotal role in the establishment of a RanGTP gradient across the nuclear envelope and, hence, in the majority of nucleocytoplasmic transport pathways. Here, we investigate the roles of the Nup358-RanGAP1 complex and of soluble RanGAP1 in nuclear protein transport, combining in vivo and in vitro approaches. Depletion of Nup358 by RNA-interference led to a clear reduction of importin
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-dependent nuclear import of various reporter proteins. In vitro, transport could be partially restored by the addition of importin
, RanBP1 and/or RanGAP1 to the transport reaction. In intact Nup358-depleted cells, overexpression of importin
strongly stimulated nuclear import, demonstrating that the transport receptor is the most rate-limiting factor at reduced Nup358-concentrations. As an alternative approach, we used antibody-inhibition experiments. Antibodies against RanGAP1 inhibited the enzymatic activity of soluble and nuclear pore-associated RanGAP1, as well as nuclear import and export. While export could be fully restored by soluble RanGAP, import was only partially rescued. Together, these data suggest a dual function of the Nup358-RanGAP1 complex as a coordinator of importin
recycling and reformation of novel import complexes.
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