title: A systems analysis of importin-{alpha}-{beta} mediated nuclear protein import.
abstract: Importin-beta (Impbeta) is a major transport receptor for Ran-dependent import of nuclear cargo. Impbeta can bind cargo directly or through an adaptor such as Importin-alpha (Impalpha). Factors involved in nuclear transport have been well studied, but systems analysis can offer further insight into regulatory mechanisms. We used computer simulation and real-time assays in intact cells to examine Impalpha-beta-mediated import. The model reflects experimentally determined rates for cargo import and correctly predicts that import is limited principally by Impalpha and Ran, but is also sensitive to NTF2. The model predicts that CAS is not limiting for the initial rate of cargo import and, surprisingly, that increased concentrations of Impbeta and the exchange factor, RCC1, actually inhibit rather than stimulate import. These unexpected predictions were all validated experimentally. The model revealed that inhibition by RCC1 is caused by sequestration of nuclear Ran. Inhibition by Impbeta results from depletion nuclear RanGTP, and, in support of this mechanism, expression of mRFP-Ran reversed the inhibition.
authors.affiliation: Center for Cell Signaling, Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA 22908, USA.
authors.forename: Gregory
authors.initials: G
authors.lastname: Riddick
authors.forename: Ian G
authors.initials: IG
authors.lastname: Macara
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000502:physiology; D018797:Cell Cycle Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism*; D003198:Computer Simulation; D025461:Feedback, Physiological / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D008954:Models, Biological; D009687:Nuclear Proteins / Q000378:metabolism; D029543:Nucleocytoplasmic Transport Proteins / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D013598:Systems Theory; D028901:alpha Karyopherins / Q000378:metabolism*; D028961:beta Karyopherins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000378:metabolism*
chemical_list: D018797:Cell Cycle Proteins; D020662:Guanine Nucleotide Exchange Factors; D009687:Nuclear Proteins; D029543:Nucleocytoplasmic Transport Proteins; C062521:RCC1 protein, human; D028901:alpha Karyopherins; D028961:beta Karyopherins; C087978:karyopherin alpha 2; D020931:ran GTP-Binding Protein