title: Identification of a protein complex that is required for nuclear protein import and mediates docking of import substrate to distinct nucleoporins.
abstract: We have identified and characterized a 9S protein complex from a Xenopus ovary cytosolic subfraction (fraction A) that constitutes this fraction's activity in recognizing a model nuclear import substrate and docking it at the nuclear pore complex. Because of its function, the complex is termed karyopherin. The 54- and 56-kDa subunits of the complex are termed alpha 1 and alpha 2, respectively, and the 97-kDa subunit is termed beta. In an alternative approach we have identified karyopherin beta from a rat liver cytosolic subfraction A by using immobilized rat nucleoporin Nup98 in a single, affinity-based enrichment step. We have molecularly cloned and sequenced rat karyopherin beta. Comparison with protein sequence data banks showed no significant similarity to other known proteins. Using nitrocellulose-immobilized rat liver nuclear envelope proteins and nuclear import substrate as a ligand, we found Xenopus fraction A-dependent binding to at least three bona fide nucleoporins (Nup214, Nup153, and Nup98) and to a candidate nucleoporin with an estimated molecular mass of 270 kDa. We propose that these nucleoporins function as docking proteins for karyopherin-mediated binding of substrate in a nuclear import/export pathway across the nuclear pore complex.
authors.affiliation: Laboratory of Cell Biology, Rockefeller University, Howard Hughes Medical Institute, New York, NY 10021.
authors.forename: A
authors.initials: A
authors.lastname: Radu
authors.forename: G
authors.initials: G
authors.lastname: Blobel
authors.forename: M S
authors.initials: MS
authors.lastname: Moore
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D002451:Cell Compartmentation; D003001:Cloning, Molecular; D003600:Cytosol / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D009685:Nuclear Envelope / Q000378:metabolism*; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D010053:Ovary; D011485:Protein Binding; D051381:Rats; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014982:Xenopus laevis
chemical_list: D009687:Nuclear Proteins