title: Nuclear protein import: Ran-GTP dissociates the karyopherin alphabeta heterodimer by displacing alpha from an overlapping binding site on beta.
abstract: The alpha subunit of the karyopherin heterodimer functions in recognition of the protein import substrate and the beta subunit serves to dock the trimeric complex to one of many sites on nuclear pore complex fibers. The small GTPase Ran and the Ran interactive protein, p10, function in the release of the docked complex. Repeated cycles of docking and release are thought to concentrate the transport substrate for subsequent diffusion into the nucleus. Ran-GTP dissociates the karyopherin heterodimer and forms a stoichiometric complex with Ran-GTP. Here we report the mapping of karyopherin beta's binding sites both for Ran-GTP and for karyopherin alpha. We discovered that karyopherin beta's binding site for Ran-GTP shows a striking sequence similarity to the cytoplasmic Ran-GTP binding protein, RanBP1. Moreover, we found that Ran-GTP and karyopherin alpha bind to overlapping sites on karyopherin beta. Having a higher affinity to the overlapping site, Ran-GTP displaces karyopherin alpha and binds to karyopherin beta. Competition for overlapping binding sites may be the mechanism by which GTP bound forms of other small GTPases function in corresponding dissociation-association reactions. We also mapped Ran's binding site for karyopherin beta to a cluster of basic residues analogous to those previously shown to constitute karyopherin alpha's binding site to karyopherin beta.
authors.affiliation: Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10021, USA.
authors.forename: J
authors.initials: J
authors.lastname: Moroianu
authors.forename: G
authors.initials: G
authors.lastname: Blobel
authors.forename: A
authors.initials: A
authors.lastname: Radu
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002467:Cell Nucleus / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D046911:Macromolecular Substances; D008961:Models, Structural; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000737:chemistry / Q000378:metabolism*; D010446:Peptide Fragments / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D016133:Polymerase Chain Reaction; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D051381:Rats; D011915:Rats, Inbred BUF; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D028901:alpha Karyopherins; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein
chemical_list: D046911:Macromolecular Substances; D009687:Nuclear Proteins; D010446:Peptide Fragments; D011994:Recombinant Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D020931:ran GTP-Binding Protein