title: The nuclear transport machinery recognizes nucleoplasmin-histone complexes.
abstract: The nuclear transport of the chromatin remodeling protein nucleoplasmin and chromatin building histones is mediated by importins. Nucleoplasmin (NP) contains a classical bipartite nuclear localization signal (NLS) that is recognized by the importin α/β heterodimer, while histones present multiple NLS-like motifs that are recognized by importin β family members for nuclear targeting. To explore the possibility of a cotransport of histones and their chaperone NP to the nucleus, we have analyzed the assembly of complexes of NP/histones with importins by means of fluorescence anisotropy, centrifugation in sucrose gradients, and isothermal titration calorimetry. Data show that importin α ΔIBB (a truncated form of importin α lacking the autoinhibitory N-terminal domain) and histones (linker, H5, and nucleosomal core, H2AH2B) can simultaneously bind to NP. Analysis of the binding energetics reveals an enthalpy-driven formation of high affinity ternary, NP/Δα/H5 and NP/Δα/H2AH2B, complexes. We find that different amount of importin α molecules can be loaded on NP/histone complexes dependent on the histone type, linker or core, and the amount of bound histones. We further demonstrate that NP/H5 complexes can also incorporate importin α/β, thus forming quaternary NP/histones/α/β complexes that might represent a putative coimport pathway for nuclear import of histones and their chaperone protein NP, enhancing the histone import efficiency.
authors.affiliation: Unidad de Biofísica (CSIC/UPV-EHU), Departamento de Bioquí́mica y Biologí́a Molecular, Universidad del Paí́s Vasco, POB 644, 48080 Bilbao, Spain.
authors.forename: Igor
authors.initials: I
authors.lastname: Arregi
authors.forename: Jorge
authors.initials: J
authors.lastname: Falces
authors.forename: Sonia
authors.initials: S
authors.lastname: Bañuelos
authors.forename: María A
authors.initials: MA
authors.lastname: Urbaneja
authors.forename: Stefka G
authors.initials: SG
authors.lastname: Taneva
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D001693:Biological Transport, Active; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism; D002645:Chickens; D004912:Erythrocytes / Q000378:metabolism; D005454:Fluorescence Polarization; D006657:Histones / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D018832:Molecular Chaperones; D019913:Nuclear Localization Signals; D056489:Nucleoplasmins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D014982:Xenopus laevis; D028901:alpha Karyopherins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D028961:beta Karyopherins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D006657:Histones; D018832:Molecular Chaperones; D019913:Nuclear Localization Signals; D056489:Nucleoplasmins; D028901:alpha Karyopherins; D028961:beta Karyopherins