title: Importin alpha associates with membranes and participates in nuclear envelope assembly in vitro.
abstract: Importin alpha is well known as an adaptor that functions with Importin beta in the nuclear import of proteins containing specific nuclear localization signals (NLSs). We show here that either an excess or a lack of Importin alpha blocks nuclear envelope (NE) assembly in vitro, and our data suggest that soluble Importin alpha functions in NE assembly in conjunction with NLS-containing partner proteins. Surprisingly, a significant proportion of Importin alpha is found to fractionate with Xenopus egg membranes. We demonstrate that membrane association of Importin alpha is regulated by phosphorylation. Using mutant forms of Importin alpha that either do not bind membranes or are not released from them by phosphorylation, we provide evidence that membrane-associated Importin alpha is required for NE formation. Unlike other functions of Importin alpha, this membrane-associated activity does not require interaction with NLS proteins.
authors.affiliation: European Molecular Biology Laboratory, Meyerhofstrasse, Heidelberg, Germany.
authors.forename: Virginie
authors.initials: V
authors.lastname: Hachet
authors.forename: Thomas
authors.initials: T
authors.lastname: Köcher
authors.forename: Matthias
authors.initials: M
authors.lastname: Wilm
authors.forename: Iain W
authors.initials: IW
authors.lastname: Mattaj
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000595:Amino Acid Sequence; D000818:Animals; D006801:Humans; D008969:Molecular Sequence Data; D009685:Nuclear Envelope / Q000378:metabolism* / Q000648:ultrastructure; D019913:Nuclear Localization Signals; D028861:Nuclear Pore Complex Proteins / Q000378:metabolism; D009865:Oocytes / Q000166:cytology; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D014982:Xenopus laevis; D028901:alpha Karyopherins / Q000235:genetics / Q000378:metabolism*; D028961:beta Karyopherins / Q000378:metabolism
chemical_list: D019913:Nuclear Localization Signals; D028861:Nuclear Pore Complex Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins