Nuclear import of the ran exchange factor, RCC1, is mediated by at least two distinct mechanisms.
RCC1, the only known guanine-nucleotide exchange factor for the Ran GTPase, is an approximately 45-kD nuclear protein that can bind chromatin. An important question concerns how RCC1 traverses the nuclear envelope. We now show that nuclear RCC1 is not exported readily in interphase cells and that the import of RCC1 into the nucleoplasm is extremely rapid. Import can proceed by at least two distinct mechanisms. The first is a classic import pathway mediated by basic residues within the NH(2)-terminal domain (NTD) of RCC1. This pathway is dependent upon both a preexisting Ran gradient and energy, and preferentially uses the importin-alpha3 isoform of importin-alpha. The second pathway is not mediated by the NTD of RCC1. This novel pathway does not require importin-alpha or importin-beta or the addition of any other soluble factor in vitro; however, this pathway is saturable and sensitive only to a subset of inhibitors of classical import pathways. Furthermore, the nuclear import of RCC1 does not require a preexisting Ran gradient or energy. We speculate that this second import pathway evolved to ensure that RCC1 never accumulates in the cytoplasm.
Nemergut
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Department of Microbiology, Markey Center for Cell Signaling, University of Virginia, Charlottesville, Virginia 22908, USA. men5w@virginia.edu
Macara
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D000595:Amino Acid Sequence; D001692:Biological Transport; D002451:Cell Compartmentation; D018797:Cell Cycle Proteins; D002463:Cell Membrane Permeability / Q000187:drug effects; D002467:Cell Nucleus / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004072:Digitonin / Q000494:pharmacology; D020662:Guanine Nucleotide Exchange Factors; D006367:HeLa Cells; D006801:Humans; D008954:Models, Biological; D008969:Molecular Sequence Data; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000378:metabolism*
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D020662:Guanine Nucleotide Exchange Factors; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; C062521:RCC1 protein, human; D020931:ran GTP-Binding Protein; D004072:Digitonin